Symposium Organizers
Jun Lou Rice University
Brad Boyce Sandia National Laboratories
Erica Lilleodden GKSS Forschungszentrum
Lei Lu Chinese Academy of Sciences
FF1: Fracture and Fatigue of Nanomaterials
Session Chairs
Monday PM, November 30, 2009
Room 304 (Hynes)
9:30 AM - **FF1.1
In-situ Nanomechanics of the Brittle to Ductile Transition.
William Gerberich 1 , Aaron Beaber 1 , Julia Nowak 2 , Lucas Hale 1 , Yuye Tang 3 , Roberto Ballarini 3 , Fredrik Oestlund 4 , Johann Michler 4
1 Chem. Engng. Mat. Sci., U. of Minnesota, Minneapolis, Minnesota, United States, 2 , Hysitron, Inc., Minneapolis, Minnesota, United States, 3 Civil Engng., U. of Minnesota, Minneapolis, Minnesota, United States, 4 Mechanics of Materials and Nanostructures, EMPA, Swiss Federal Laboratory for Materials, Thun Switzerland
Show AbstractQuantitative attempts at defining fracture toughness levels for the brittle to ductile transition (BDT) of silicon at room temperature are presented. Using a nanoindenter inside both transmission and scanning electron microscopes, both crack initiation and arrest of single crystal silicon <110> and <111> pillars as well as randomly oriented spheres have been observed. Besides toughnesses being up to an order of magnitude greater than bulk silicon, the BDT is decreased to room temperature by simply decreasing size. This ranges between 100 to 300 nm depending on shape and orientation and is a work in progress. Quantification is achieved through utilization of elastic and elastic-plastic finite element solutions with finite deformation. For arrested cracks on {110} planes in silicon columns, solutions using the von Mises criteria clearly show plastic zones reaching the surface of 400 nm diameter columns under compression. This is consistent with dislocation-mediated plasticity providing increased toughness in small volumes. Such a proposed model may also be appropriate to other semiconductor and ceramic crystals.
10:00 AM - FF1.2
Brittle and Ductile Fracture of Gold nanowires: A Quantitative In Situ Transmission Electron Microscopy Study.
Yang Lu 1 , Jianyu Huang 2 , Jun Lou 1
1 Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States, 2 Center for Integrated Nanotechnologies (CINT) , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractMechanical characterization of metallic nanowires has drawn considerable attention for the past decades due to their importance of building reliable nano-electronic devices and predicting the performance of nano-electromechanical-systems (NEMS). Recently, quantitative mechanical measurement of individual nanowires while monitoring their structural evolution (in situ testing) has become an important tool for many researchers. Significant efforts had been made to develop various in situ methods using different loading configurations, such as bending, buckling, indentation and compression to test such nano entities. However, quantitative experiments on metallic nanowires under tensile loading are still quite challenging, especially for nanowires with very small diameters (<10nm). In this paper, we report the usage of a TEM-AFM platform to perform quantitative tensile tests on ultra thin gold nanowires with diameter of 5-10 nanometers inside a transmission electron microscope (TEM). Interestingly, both brittle and ductile fracture modes were observed, and true stress versus strain curves were plotted and discussed. The true breaking strengths were shown to be much higher than bulk gold, which agrees well with theoretical predictions. Corresponding qualitative uni-axial tensile tests were also performed in high resolution TEM (HRTEM) mode to probe the difference between the two fracture mechanisms. More interestingly, the sudden stress drop in the stress-strain curve of gold nanowires failed in ductile mode might directly indicate the occurrence of dislocation nucleation events and onset of plasticity before final fracture. The possible implication of the role played by dislocation nucleation on determination of fracture mode in gold nanowires was discussed.
10:15 AM - FF1.3
Saccharine Effects on Mechanical Behavior of Electrodeposited Ni Thin Films.
Sumit Soni 1 2 , Sean Hearne 2 , Brian Sheldon 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 Center for Integrated Nano-Technologies, Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractThis research presents experimental results of an investigation aimed at understanding grain size driven mechanical processes in electrodeposited Ni thin films, where Saccharine additions are commonly used to improve mechanical properties. Ni films were fabricated using salfamate based electro chemical baths, where it is empirically known that mMol/L concentrations of saccharine will reduce the observed tensile stress in addition to lowering the grain size upto a few nanometer scales. Some previous observations and several theoretical models suggest that Saccharine incorporation results in sulfur segregation at grain boundaries. Since grain boundary formation is also associated with tensile stress evolution, a plausible hypothesis is that saccharine additions are directly altering grain boundary energetics. This suggests that saccharine additions should also have an observable effect on intergranular fracture in these films. To test this prediction, in-situ stress measurements during film growth and fracture testing of these same films were compared. Lithographically patterned substrates were used to produce films with ordered arrays of uniform islands, which improved our understanding of island size effects on stress evolution, and also allowed us to produce a well defined notch along one of the island boundaries (for fracture tests). In-situ uniaxial tensile testing under in a scanning electron microscope (SEM) was then used to obtain the fracture strength of these specimens. This technique also enabled us to capture the real-time microscopic images of deformation while specimens were subjected to the applied load. The observed relationships between residual stress, grain size, and fracture strength were then analyzed with detailed models of both film growth and fracture. This work was supported by the National Science Foundation (Brown University) and the DOE office of Basic Energy Science Center for Integrated Nano-Technology. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
10:30 AM - FF1.4
Water Diffusion and Fracture Behavior in Nano-porous low-k Dielectric Film Stacks.
Han Li 1 , Joost Vlassak 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States
Show AbstractOrganosilicate glasses (OSG) containing nanometer-size pores are leading candidates for use as intra-metal dielectrics in future microelectronics technologies. Compared to their dense counter parts, porous dielectrics are expected to possess a much reduced cohesive strength and poorer adhesion with adjacent layers, and are more prone to the absorption of reactive chemicals during device fabrication. These factors pose significant integration challenges for device fabrication both electrically and mechanically. In this study, we investigate the direct impact of water diffusion on the fracture behavior of film stacks that contain porous OSG coatings. We demonstrate that exposure of the film stacks to water causes significant degradation of the interfacial adhesion energy, but that it has negligible effect on the cohesive fracture energy of the nanoporous OSG layer. Isotope tracer diffusion experiments combined with dynamic secondary ion mass spectroscopy (SIMS) show that water diffuses predominantly along the interfaces, and not through the porous films. This preference of water for the OSG/SiCN interface is attributed to the hydrophilic character of the interface. Indeed, degradation of the cohesive fracture energy is observed if the OSG film stack is exposed to aqueous solutions with organic additives that enhance wetting of the OSG. The adhesion degradation is well described by a simple diffusion/subcritical fracture model.
10:45 AM - FF1.5
An In-situ Electron Microscopy Study of Crack Growth in Nanocrystalline Copper and Copper-Chromium Composite Thin Films.
Seong-Woong Kim 1 , Sharvan Kumar 1
1 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe interaction of an advancing crack with the local microstructure, the modification of the microstructure by the field of the crack-tip, and the subsequent interaction of the crack with the modified microstructure were all observed and documented by conducting straining experiments in-situ in the transmission electron microscope. The materials of choice were i) sub-micron grain size (100-200 nm) Cu produced in the form of 100nm-thick films by vapor deposition., ii) conventional bulk Cu with several micrometer-size grains, and (iii) Cu-Cr composites also produced as thin films by vapor deposition, with the Cr present as evenly-spaced, ~500-micron-sized pillars through the thickness of the film in a matrix of sub-micron grain size Cu. The multi-step procedure adopted to produce these unique microstructures will be described, and preliminary results from the in-situ TEM studies will be presented. Specifically, in the case of the ultra-fine grained Cu which contains a highly faulted microstructure with numerous annealing twins, their ability to resist crack growth will be illustrated. In the case of the Cu-Cr “pillar-composites”, crack interaction with the pillars, cracking of the matrix and interphase-interface cracking were all observed; circumstances under which each is preferred will be discussed.
11:30 AM - **FF1.6
Interaction of Materials Defects with External Length Scales – from Simple to Complex Geometries.
Ralph Spolenak 1
1 Department of Materials, Laboratory for Nanometallurgy, ETH Zurich, Zurich Switzerland
Show AbstractThe microstructure of a material together with the dimensions of the samples determines its mechanical behavior. As many microstructural features such as dislocations, grain-boundaries, Frenkel pairs or stacking fault tetrahedra are on the micron to 100 nanometer length scale changes of materials behavior are expected when also the external length scale enters this regime. This paper presents case studies of scaling in mechanical behavior for various fcc metals (Au, Cu and Al) and some of their alloys as the relationship between typical defect spacing and external dimension is varied. Specific focus is placed on the effect on strength and toughness and the interplay between them. Geometries ranging from thin films over wires and cylinders to porous structures will be discussed. The character of surfaces and interfaces affects time-dependant phenomena as well as materials ductility. Critical relative length scales for changes in deformation mechanisms are presented.
12:00 PM - FF1.7
In-situ Thermal and Lattice-strain Evolution of the Nano-particle-strengthened Alloys Subjected to Low-cycle-fatigue Experiments.
E-Wen Huang 1 , Peter Liaw 1 , Kyle Woods 1 , John Strange 1
1 Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee, United States
Show AbstractIn this study, a constitutive model is developed to describe the thermal and mechanical responses of a nano-particle-strengthened, nickel-base superalloy subjected to cyclic deformation. Under a tension-compression, low-cycle-fatigue experiment, the structural properties of the alloy are investigated using the in-situ neutron-diffraction and the in-situ-temperature measurements simultaneously. The model is based on the macroscopic stress-strain curves incorporating with atomic-scale responses extracted by the evolution of the in-situ neutron-diffraction profiles. A complementary microstructure investigation of the transmission-electron-microscopy is used to compare with the peak-profile evolution of the nano particles and the matrix, respectively, upon cyclic loading. In comparison with the bulk-property evolution, the deformation mechanisms of the nano particles and the matrix under cyclic loading have been characterized by the evolution of the lattice strain, extracted from in-situ neutron-diffraction profile fitting. The cyclic stress-strain responses and temperature variations are found in good agreement with the in-situ neutron-diffraction experimental results.
12:15 PM - FF1.8
Fatigue Enhancement in Nanocrystalline Metals via Grain-Boundary Stabilization.
Henry Padilla 1 , Brad Boyce 1 , Elizabeth Holm 1
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractImprovements in the fatigue lifetime of metals has been known to occur with decreasing grain size, with some of the best performance coming from nanocrystalline metals. Both constant stress amplitude and constant strain amplitude fatigue tests on several Ni alloys reveal that some nanocrystalline microstructures (grain size < 50 nm) are anomalously resistant to fatigue crack initiation. Unlike prior observations in ultrafine grained metals and nanocrystalline metals, the present observed behavior is 'anomalous' because the S-N curve does not scale with the yield strength. While metals typically exhibit Hall-Patch breakdown of the yield strength as the grain size approaches the physical limits of dislocation activity (~5 nm), some nanocrystalline metals actually exhibit enhanced fatigue resistance when the grain size approaches the size of persistent slip band activity (~50-100 nm). When nanocrystalline metals do initiate fatigue cracks, microstructural analysis of failed samples reveals regions of coarsened grains at the initiation site. In all nanocrystalline alloys studied, localized grain coarsening was found at the site of crack initiation. This fatigue-induced coarsening is thought to be a necessary precursor for fatigue crack initiation. Therefore, for these nanocrystalline alloys, grain-boundary stabilization is a pathway towards improved fatigue performance. Interestingly, alloys which are stable against thermally-induced coarsening are not necessarily stable against fatigue-induced coarsening. A Potts model has been developed to understand the role of various microstructural features (particle pinning, solute drag, etc.) on thermally- and mechanically-induced grain growth.* Sandia is a multi-program laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000.
12:30 PM - FF1.9
Wear Properties and Surface Hardness Evolution in Nano-twinned Copper Subjected to Repeated Contact Sliding.
Aparna Singh 1 , Ming Dao 1 , Lei Lu 2 , Subra Suresh 1
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 , Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang China
Show AbstractGrain refinement to about 100 nm and below leads to a significant increase in the strength of nanocrystalline Cu. However, strengthening arising from the nanocrystalline structure also generally leads to a concomitant loss of ductility. Our recent work has shown that the introduction of controlled concentrations of coherent nano-scale twins within the ultrafine grains by recourse to pulsed electrodeposition can impart significant strength to Cu while preserving ductility. In this presentation, we outline the results of our recent investigations of how nano-scale twins influence the tribological characteristics of Cu under conditions of repeated frictional sliding with a diamond indenter. The evolution of friction and damage during repeated sliding contact in ultrafine grained copper with essentially no twins as well as low and high twin densities was systematically and quantitatively studied using a depth-sensing, instrumented indenter with well-controlled sliding capabilities so as to develop a perspective on the effects of twin density on sliding contact fatigue. We have investigated how the sliding friction coefficient of these three materials evolves as a function of the number of passes of the tip over the surface. The amount of material removed by the movement of the indenter tip as a result of repeated frictional sliding is also determined as a function of the number of passes. The collective effects of material strength, structural size scale, wear particle size, surface friction coefficient as well as contact fatigue are quantified through experiments as well as computational simulations. General strategies for designing tribologically resistant surfaces with a good combination of strength, ductility and damage tolerance will be presented.
12:45 PM - FF1.10
Detwinning and Crack Initiation in Fatigued Highly-Aligned Nano-twinned Copper.
Carla Shute 1 , Benjamin Myers 1 , Sujing Xie 1 , Andrea Hodge 2 , Troy Barbee 3 , Julia Weertman 1
1 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 2 Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California, United States, 3 , Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractMonotonic stress-strain tests have shown that nano-twinned Cu has a strength comparable to nanocrystalline Cu with equiaxed grains while exhibiting greatly improved ductility and microstructural stability. However there has been little examination of the properties and failure of nano-twinned material under cyclic deformation. High purity Cu samples produced by magnetron sputtering and containing aligned nano-twins were fatigued to failure in tension-tension loading. The resulting microstructures were examined by FIB, TEM and SEM. It was found that de-twinned regions that form near the surface during cycling are associated with surface depressions that turn into cracks. The process appears rather similar to crack initiation in fatigued UFG Cu, in which the cracks form in the softer slip bands produced by cycling. The S-N curves are closely similar for the two cases: nano-twinned and UFG Cu. Much of this work was performed in the EPIC facility of the NUANCE Center at Northwestern University. It was partially supported by contract DE-AC52-07NA27344 at LLNL.
FF2: Modeling & Simulation of Mechanical Behavior of Nanomaterials
Session Chairs
Erica Lilleodden
Amit Misra
Monday PM, November 30, 2009
Room 304 (Hynes)
2:30 PM - **FF2.1
Multi-scale Modeling of Crack Growth in Ductile Metals.
Alan Needleman 1 , Mathias Wallin 2 , William Curtin 3 , Matti Ristinmaa 2
1 Materials Science and Engineering, University of North Texas, Denton, Texas, United States, 2 Division of Solid Mechanics, Lund University, Lund Sweden, 3 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractThe deformation and fracture of ductile metals typically involves a complex interactions between large numbers of dislocations, grain and phase boundaries and mechanical defects such as micro-cracks. These interactions are strongly influenced by the chemical environment. As a consequence, developing a predictive analysis capability requires modeling phenomena over a range spanning atomic length and time scales (Angstroms and pico-seconds) to crystal or polycrystal length scales (microns, millimeters and larger) and service loading time scales (seconds to years). Some recent work on developing a methodology for coupling a region where plastic flow is described in terms of the collective motion of discrete dislocations with a larger region where plastic flow is characterized in terms of a conventional continuum crystal constitutive relation will be discussed. This method provides a complement to the CADD formulation (Shilkrot, Miller and Curtin, J. Mech. Phys. Solids, 52, 755, 2004) so that, in principle, processes occurring at scales ranging from the atomistic to the continuum can be incorporated in a single analysis. Analyses under both monotonic and cyclic loading conditions will be discussed.
3:00 PM - FF2.2
Hydrogen Assisted Crack Propagation in Aluminum: An Atomistic Study.
Donald Ward 1 , K. Solanki 1
1 Center for Adavanced Vehicular Systems, Mississippi State University, Starkville, Mississippi, United States
Show AbstractLightweight materials have been identified as necessary for the Department of Defense (DoD), Department of Energy (DoE), the Department of Transportation and other governmental agencies and industries. However, one drawback of metallic-lightweight materials, such as aluminum-based alloys, is their susceptibility to fatigue crack propagation that was found to be strongly dependent upon both environment and load ratio. To study the effect of environment, molecular dynamic (MD) calculations were used in conjunction with Monte Carlo (MC) methods to characterize the hydrogen interactions in/around a crack tip, specifically dislocation-hydrogen interaction. Each atomistic simulation began with a MC simulation to introduce hydrogen in the system. A MD simulation then relaxes the initial structure of any high energy configurations. After relaxation the sample was strained, by prescribing a fixed displacement under mode one loading conditions. To account for H diffusion, regardless of the length scale of the crack, following each strain increment a MC simulation was performed to diffuse the H. Following the MC simulation, another MD simulation further strains the sample . This strain-MD-MC-MD iteration was repeated up to the desired deformation or crack tip displacement. The effects of crystal orientation, hydrogen concentration, and temperature are all examined on crack driving forces, such as J-integral, surface energy associated with the Griffith criteria, and the plastic zone size. Finally, to quantify the size scale effect, MD simulations were performed by varying number of atoms.
3:15 PM - FF2.3
Multiscale Materials Modeling Using Coupled Density Functional Theory and Discrete Dislocation Mechanics.
Arun Nair 1 , Derek Warner 1 , Richard Henning 2
1 Civil and Environmental Engineering, Cornell University, Ithaca, New York, United States, 2 Material Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractIncreasing evidence indicates that dislocation nucleation plays a key role in the deformation of nano-structured and nano-dimensioned metals. Yet our understanding of the nucleation process, whether it is from a free surface, grain boundary, or stress concentration, has remained clouded. An important tool for gaining insight into dislocation processes has been atomic-scale computer modeling. However, atomistic simulations of deformation processes have long been plagued by the challenge of accurately and efficiently describing the complexities of multispecies bonding. In the case of metals, this has lead to the majority of the atomistic modeling effort focusing on pure elemental metals in a vacuum, rather than more technologically relevant problems involving alloys with impurities and surface oxides in realistic environments. At the root of the challenge is a trade-off between accuracy and computational expense. At one end of the spectrum lies Kohn-Sham Density Functional Theory (KSDFT). Although KSDFT can produce the interatomic forces originating out of many multi-element bonding situations to reasonable accuracy, its computational expense is severely limiting. Most studies are restricted to less than 1,000 atoms. At the other end of the spectrum lie empirical interatomic potentials. While these are computationally much less expensive, scaling to millions of atoms, they often struggle to accurately capture multi-element bonding.Here we use a concurrent multi-scale approach to address this long-standing challenge. We couple an atomistic region whose forces are calculated via Kohn-Sham Density Functional theory to a continuum region described by linear elasticity. Each domain in the simulation framework is governed by its own energy functional with the constraint that the forces be zero across the domain interface. The KSDFT domain is solved using a plane-wave basis set and pseudo-potentials, while the finite element method is employed for the continuum domain. This approach enables us to examine large simulation cell sizes and thus properly account for the long-range elastic fields associated with key defects such as dislocations. In this talk, we will discuss our application of the above method to study dislocation nucleation from a stress concentration at a crack-tip in aluminum in the presence of various impurities. The competition between dislocation nucleation and its competing mechanism, crack-tip propagation, will be discussed.
3:30 PM - FF2.4
Dislocation Source Strengths and Internal Stress in Nanocrystalline Metals: Predictions from a Fitted Quantized Crystal Plasticity Model.
Peter Anderson 1 , Lin Li 1 , Steven Van Petegem 2 , Helena Van Swygenhoven 2
1 , The Ohio State University, Columbus, Ohio, United States, 2 Materaisl Science and Simulation, Paul Scherrer Institut, Villigen Switzerland
Show AbstractCompared to coarse-grained counterparts, nanocrystalline (nc) metals usually display distinct stress-strain features, such as extremely high strength, an extended micro-to-macro plastic transition regime, and limited ductility [1]. Also, nc Al and Au films are able to recover an abnormally large amount of plastic deformation after unloading [2]. Finally, in situ X-ray diffraction studies of electrodeposited Ni with a 30 nm grain size reveal that at room temperature, the peak broadening observed upon loading is fully reversible upon unloading at room temperature [3]. These features are studied in the context of a quantized crystal plasticity (QCP) model, in which individual grains can spontaneously slip at a critical stress, as observed in molecular dynamics simulations [4]. A consequence is that grains do not accumulate plastic strain incrementally, but rather in discrete jumps.Two key material parameters in the QCP model are the distribution of source strengths for dislocation slip and the distribution of internal stress. Here, we discuss a strategy by which the source strength distribution in nc-Ni can be determined through a comparison of experimental measurements and QCP model predictions of monotonic and cyclic stress-strain response. The model demonstrates how pre-compressive or pre-tensile processing can dramatically affect the monotonic and cyclic properties, particularly at small strain. It also shows how quantized plasticity generates a highly inhomogeneous stress state that can drive large recoverable strain upon unloading, particularly at small strain.[1] Kumar KS, Van Swygenhoven H, Suresh S. Mechanical behavior of nanocrystalline metals and alloys. Acta Materialia 2003;51:5743.[2] Rajagopalan J, Han JH, Saif MTA. Plastic deformation recovery in freestanding nanocrystalline aluminum and gold thin films. Science 2007;315:1831.[3] Budrovic Z, Van Swygenhoven H, Derlet PM, Van Petegem S, Schmitt B. Plastic deformation with reversible peak broadening in nanocrystalline nickel. Science 2004;304:273.[4] Li L, Anderson PM, Lee MG, Bitzek E, Derlet P, Van Swygenhoven H. The stress-strain response of nanocrystalline metals: A quantized crystal plasticity approach. Acta Materialia 2009;57:812.
3:45 PM - FF2.5
A Molecular Dynamics Study of Dislocation Propagation in Nanocrystalline Al containing Oxygen.
Andreas Elsener 1 , Olivier Politano 2 , Peter Derlet 3 , Helena Van Swygenhoven 1
1 NUM/ASQ, PSI, Villigen Switzerland, 2 Institut Carnot de Bourgogne, CNRS-Universite de Bourgogne, Dijon Cedex France, 3 NUM/CMT, PSI, Villigen Switzerland
Show AbstractAtomistic simulation of the mechanical properties of FCC bulk nanocrystalline metals has revealed that grain boundaries can significantly affect the motion of a dislocation as it propagates through a grain. One of the important differences between simulation and experiments in grain boundary dominated metallic structures is the lack of impurities such as oxygen in computational samples. The present work overcomes this limitation by investigating how dislocation propagation is modified when O is present in the grain boundary. To do this a modified variable-charge method [Modell Simul Mater Sci Eng 2008;16:025006] based on the Streitz and Mintmire approach [Phys Rev B 1994;50:11996] is used to efficiently model the variable oxidation state of O in a metallic Al environment. Carefully selected simulations are performed in which a propagating dislocation within a nanocrystalline environment is simulated both with and without O in the nearby surrounding grain boundary network. The athermal stress barrier for the dislocation depinning will be calculated and discussed in terms of recent simulations of O-free Al samples [Acta Materialia 2008;56:4846] .
4:30 PM - FF2.6
Mechanism for Material Transfer in Nanoscale Asperity Contact.
Jun Song 1 , David Srolovitz 2
1 Division of Engineering, Brown University, Providence, Rhode Island, United States, 2 Department of Physics, Yeshiva University, New York, New York, United States
Show AbstractWe perform a series of molecular dynamics simulations of asperity contact and separation in a model metallic system for both symmetric and asymmetric asperity geometries, for loading in different crystalline directions and for systems with different strength of adhesion. We examine contact morphology evolution, force-displacement relations and the amount of material transfer from one surface to the other upon separation with a focus on underlying physical mechanisms that control these. We find that there is a critical work of adhesion, below which no plastic deformation occurs on contact separation and a higher one in which plastic deformation occurs but no material transfer occurs. We interpret these within a model for dislocation nucleation at the crack tip. We observe abrupt changes in the amount of material transferred with increasing work of adhesion that represent thresholds for changes of deformation mechanisms. These depend on the geometry of the contact and the crystallographic orientation relative to the loading direction.
4:45 PM - FF2.7
Responses of Twin and Grain Boundaries at Nanometer Scale to Mechanical Attacks – A Molecular Dynamics Simulation Study.
Lei Yue 1 , Dongyang Li 1 , Hao Zhang 1
1 Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, Canada
Show AbstractFretting is one of main problems that result in failure of electrical contacts in dynamic systems. Fretting results in material loss and introduces high-density dislocations in the contact region, which make the electrical contacts dysfunctional with increased contact resistance, higher interfacial temperature, shortened service life, and accompanied safety issues. Significant efforts have been made to search for high-performance conductive materials having both high electrical conductivity and high wear resistance. It was recently reported that nano-twining in copper considerably increased the strength of this metal. Unlike nanocrystalline copper, which has higher wear resistance but poor conductivity due to enhanced electron scattering at high-density grain boundaries, nano-sized twin boundaries in copper could be a solution to such a dilemma, since their ordered structure should exhibit much lower degree of electron scattering. To better understand the response of twin boundaries to mechanical actions and compare it with grain boundaries at nano-scale, we conducted a computational study on the mechanical behaviors of both twin and grain boundaries using the molecular dynamics technique. Obtained results suggest that although the grain boundary acts as a sink of dislocations, it emits dislocations under stress. The twin boundary is more effective to block the dislocation movement, and its degree of emitting dislocations under stress is considerably lower than that of the grain boundary. These lead the nano-twin boundaries to possess higher resistance to wear attack.
5:00 PM - FF2.8
Twin Instability of Peierls Distortion in Trans-Polyacetylene, a Spontaneous Soliton Induced Actuation Mechanism.
Minghai Li 1 , Xi Lin 1
1 Mechanical Engineering dept, Boston University, Brookline, Massachusetts, United States
Show Abstractwe develop a novel actuation mechanism for 1D conducting polymer chains using a tight-binding model, Su-Schrieffer-Hegger (SSH) model. It is known that 1D conducting polymer chain undergoes two spontaneous conformational relaxations, Peierls dimerization and overall chain contraction. We proved analytically for the first time that overall chain contraction coupled with Peierls dimerization. The doping induced solitonary domain enforces the anti-Peierls dimerization and the corresponding bond length shrink more than that of defect-free neutral chain. The numeric results were performed based on the SSH model, the Peierls-Hubbard model, and the extended Peierls-Hubbard model including “off-diagonal” terms, as well as the first-principles Hartree-Fock calculations show that the overall chain length shrinks at low doping level and expands upon certain doping level, and agree with the experimental data of Sodium-doped trans-Polyacetylene. This provides a possible spontaneous actuation mechanism by doping charges to conducting polymer chains.
5:15 PM - FF2.9
Silk Fiber Mechanics: Multi-scale Modeling of Spider Silk.
Murat Cetinkaya 1 2 , Senbo Xiao 1 3 , Frauke Graeter 1 2 3
1 Protein mechanics and evolution, Max-Planck Institute for Metals Research, Stuttgart, BW, Germany, 2 Bioquant, Heidelberg University, Heidelberg, BW, Germany, 3 , MPG-CAS Partner Institute of Computational Biology, Shanghai China
Show AbstractSilk is an astonishing natural material with its ultimate strength comparable to steel, its toughness higher than of kevlar and its density lower than of cotton and nylon. Silk fibers by silk-worms (Bombyx mori) and spiders (e.g. Nephila clavipes) are also highly extensible. However, the production process of natural silk fibers is quite demanding and synthetic polymers are still far from mimicking the mechanical performance of natural silk fibers. Furthermore, there is limited information regarding the structure and thus the mechanical characteristics of natural silk fibers.Our aim is to quantitatively predict the mechanical properties of spider silk fibers using affordable computational resources. We have developed a bottom-up computational approach to model spider silk fibers and we have managed to bridge atomistic and continuum scale methods [1]. Preliminary results indicate that our method is capable of predicting silk fiber mechanical properties without using any empirical parameters. We have also compared the all-atom simulation results with the ones from continuum scale simulations and showed that our method captures the essence of spider silk mechanics also in quantitative manner. Furthermore, our method could be applied to other polymeric systems that would allow us to determine the most promising candidates for synthetic polymers mimicking spider silk.
5:30 PM - FF2.10
Hierarchical Nanomechanics of Alzheimer's Aβ(1-40) Amyloid Fibrils.
Raffaella Paparcone 1 , Markus Buehler 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractAmyloids show a rather complex mechanical behavior and exceptional properties such as strength, sturdiness and elasticity, in combination with their intriguing ability to self-heal and self-assemble. This is due to the hierarchical architecture and organization across many length-scales, from nano to macro. The elucidation of the missing nanoscale deformation and fracture mechanisms of amyloid materials and their mechanical characterization is crucial for the application of amyloids as building blocks of mechanically resistant bionanomaterials such as in nanowires or protein nanotubes. Here we specifically focus on the Aβ(1-40) amyloid fibril, which is related to the Alzheimer’s disease, showing the characteristic twisted beta-sheet rich fibril geometry. By using large-scale atomistic simulations, we link the biochemical properties, amino acid sequence and atomistic details of amyloid fibrils to their mechanical properties at different levels in the amyloid fibril’s organization. We elucidate associated deformation and failure mechanisms, and identify the molecular basis for the unique properties of amyloids. We report a quantitative comparison of the simulation results with experimental nanomechanical characterization, and discuss effects of loading geometry, fiber orientation and loading rate for different fiber morphologies. The comparison with experimental results shows good agreement of the molecular simulation results.
5:45 PM - FF2.11
Continuum Modeling of Boron Nitride Nanotubes.
Jizhou Song 1 , Jian Wu 2 , Yonggang Huang 2 , Keh-Chih Huang 3
1 , University of Miami, Coral Gables, Florida, United States, 2 , Northwestern University, Evanston, Illinois, United States, 3 , Tsinghua University, Beijing China
Show AbstractBoron nitride nanotubes (BNNTs) possess unique mechanical, thermal, electrical and chemical properties. Their tensile rigidity is comparable to that of carbon nanotubes. They have high thermal conductivity along the nanotube, and good resistance to oxidation at high temperature. Contrary to carbon nanotubes, BNNTs have large band gaps regardless of the chirality and diameter, and are therefore semiconductors.There are two types of continuum studies of BNNTs. One is to model a BNNT as a linear elastic shell [1], with the Young’s modulus and the shell thickness to be fitted by the atomistic simulation results of BNNT tension rigidity and bending rigidity. Such a linear elastic shell theory cannot account for the nonlinear, multi-body atomistic interactions characterized by the interatomic potential [2]. It also neglects the importance effect of BNNT chirality on their mechanical behavior. The other type of continuum studies of BNNTs is to incorporate the nonlinear, multi-body interaction potential for boron nitride into the continuum analysis [3-5]. Such an atomistic-based continuum theory has been used to study the Young’s modulus, stress-strain curve, and Stone-Wales transformation of BNNTs [3-5]. However, this atomistic-based continuum theory is a membrane theory that is not applicable to BNNTs in bending, nor to the instability analysis under compression.We developed a finite-deformation shell theory for single-wall BNNTs based on the interatomic potential in this paper [6]. The theory incorporated the effect of moment and curvature for a curved surface, and accurately accounts for the nonlinear, multi-body atomistic interactions as well as the BNNT chirality. It provides the constitutive relations between stress/moment and strain/curvature in terms of the interatomic potential. The theory is then used to study the buckling of BNTTs under tension and compression.[1]B.I. Yakobson, C.J. Brabec, J. Bernholc, Phys. Rev. Lett. 76, 2511 (1996).[2]Albe K, Möller W and Heinig K H 1997 Radiat. Eff. Defects Solids 141 85-97.[3]Song J, Huang Y, Jiang H, Hwang K C and Yu M F 2006 Int. J. Mech. Sci. 48 1197-1207.[4]Song J, Jiang H, Wu J, Huang Y and Hwang K C 2007 Scripta Mater. 57 571-574.[5]Song J, Wu J, Huang Y, Hwang K C and Jiang H 2008 J. Nanosci. Nanotechnol 8,3774-3780.[6]Song J, Wu J, Huang Y, and Hwang K C 2008 Nanotechnology 19 445705.
Symposium Organizers
Jun Lou Rice University
Brad Boyce Sandia National Laboratories
Erica Lilleodden GKSS Forschungszentrum
Lei Lu Chinese Academy of Sciences
FF3: Mechanical Behavior of Low Dimensional Nanomaterials I
Session Chairs
Tuesday AM, December 01, 2009
Room 304 (Hynes)
9:30 AM - **FF3.1
Dislocations in Nanowires and the Eshelby Twist.
William Nix 1 , David Barnett 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show AbstractRecent experiments involving the vapor-liquid-solid growth of PbS and PbSe nanowires (NWs) [1,2] have shown that chiral branched geometries are sometimes created when the growth of a central NW is catalyzed by a screw dislocation lying along its axis and the orthogonal side branches are catalyzed by metal particles and are dislocation free. The chirality of the structure is caused by the elastic strain of the central screw dislocation, as first described by J.D. Eshelby in 1953. Using the known elastic fields of dislocations in torque-free cylinders, it is easy to show that mixed dislocations of the kind reported by Bierman et al. [1] are unstable and should not have been observed. The known elastic solutions can also be used to examine the stability of multiple screw dislocations in NWs. We find that any number of like-signed screw dislocations can be stable in NWs, counter to initial intuition. The screw dislocations are stabilized by the image torques associated with making the NWs torque-free. While the pitch of some of the observed chiral branched geometries is sometimes greater than that predicted for a single screw dislocation in the central NW, multiple dislocations that might account for such structures have not been observed.1. M.J. Bierman et al., Science, 320, 1060 (2008)2. J. Zhu et al., Nature Nanotechnology, 3, 477 (2008).
10:00 AM - FF3.2
Investigations of Deformation Mechanisms in Ultra High Strength Nanowhiskers.
Daniel Gianola 1 2 , Gunther Richter 3 , Andreas Sedlmayr 1 , Reiner Moenig 1 , Burkhard Roos 4 , Cynthia Volkert 4 , Oliver Kraft 1
1 Institute for Materials Research II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 Materials Science & Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 , Max-Planck Institute for Metals Research, Stuttgart Germany, 4 Institue for Materials Physics, University of Göttingen, Göttingen Germany
Show AbstractMetal nanostructures proposed as fundamental building blocks for nanotechnological devices will often be subject to extreme duress during operation, particularly high mechanical stresses. Investigations of size-dependent deformation have shown that “smaller is stronger” in metals, yet the underlying mechanisms that give rise to this departure from bulk behavior are still elusive. The emerging picture is that plasticity in extremely small volumes is fundamentally different than in large materials; the law of averages gives way to discrete processes that dominate the response. Systematically probing the mechanical response and uncovering the underlying deformation mechanisms of diminishingly small structures at the micro- and nanoscale requires new strategies and approaches that circumvent difficulties associated with handling, gripping, loading, and measuring small specimens. The need for in situ experiments that give a one-to-one correlation between mechanical response and deformation morphology is exacerbated by the fact that electron optics are needed to image and manipulate nanostructures.Here we describe quantitative in situ tensile experiments on quasi-1D nanostructures in a dual-beam scanning electron microscope (SEM) and focused ion beam (FIB). Examples showing results for single-crystalline metallic nanowhiskers, or defect-free wires having diameters between 30 and 300 nm, will be presented in the context of size effects on mechanical behavior. Tensile measurements of individual nanowhiskers reveal strength on the order of the theoretical strength. Plasticity occurs in a highly localized heterogeneous manner, and fracture morphology indicates a size-dependent failure mode. Experiments which introduce defects in the specimens using Ga-ion irradiation during tensile testing are performed to ascertain the influence of defects and their proximity to surfaces on the accommodation of plasticity in small volumes. Transmission electron microscopy (TEM) is also employed to characterize defects in pristine and ion-irradiated nanowhiskers and to correlate with measured tensile behavior.
10:15 AM - FF3.3
In-Situ TEM Studies of Nanomechanics in One-Dimensional Materials.
Reza Shahbazian Yassar 1 , Anjana Ashtana 1 , Hessam Ghasemi 1 , Anahita Pakzad 1 , Kasra Momeni 1 , Yoke Yap 2
1 Mechanical Engineering, Michigan Technology University, Houghton, Michigan, United States, 2 Physics, Michigan Technological University , Houghton, Michigan, United States
Show AbstractOne-dimensional nanomaterials including nanotubes, nanowires, and nanofibers are building blocks for constructing various complex nanodevices. In this work, deformation of individual nanotubes and nanofibers will be performed inside a high-resolution transmission electron microscope (TEM) using a novel piezo-driven atomic force microscope (AFM) and scanning tunneling microscope (STM)–TEM holder. The electrical and mechanical properties of individual nanotubes/nanofibers are obtained from the experimentally recorded I-V and force-displacement curves. Failure and deformation of various nanostructures including ZnO nanowires, BN nanotubes, carbon nanotubes, and cellulose nanocrystals show distinct behavior.
10:30 AM - FF3.4
Ultralow Harmonic Resonance in Zinc Oxide Nanowires.
Nan Yao 1 , David Cohen-Tanugi 1 , Austin Akey 1
1 PRISM, Princeton University, Princeton, New Jersey, United States
Show AbstractZinc Oxide (ZnO) nanowires offer the promise of energy scavenging and precise sensing due to their vibration properties, but their high intrinsic resonance frequency (in the kHz – MHz range) has limited the applications in nanotechnology. In this paper we describe a method for introducing ultralow harmonic resonance frequencies in ZnO nanowires driven through an external field. By using in situ ion implantation, nano-device assembly, signal generation, mechanical measurement, and electron beam characterization, we have achieved resonance at frequencies two orders lower than the natural frequency. Through both experimental investigation and theoretical simulation of electromechanical effects, we show that electric charge imbalance is responsible for the creation of this unprecedented ultralow harmonic resonance behavior in ZnO nanowires.
10:45 AM - FF3.5
Dynamic Behavior of Long-Range Ordered Carbon Nanotubes Chains.
Luigi De Nardo 1 , Chiara Daraio 2
1 Chimica, Materiali e Ingegneria Chimica "G. Natta", Politecnico di Milano, Milano, Milano, Italy, 2 Graduate Aeronautical Laboratories (GALCIT) and Applied Physics, California Institute of Technology, Pasadena, California, United States
Show AbstractSince the Fermi-Pasta-Ulam model was first described, chains of nonlinear oscillators have received an ever-increasing amount of attention in the scientific community. They have been reported in a broad range of physical settings, offering the opportunity to discover new physical phenomena. In this communication, we report on the synthesis and characterization of discrete periodic structures composed of alternating CNT foams and stainless steel cylinders. To assemble our long-range ordered chains, Vertically Aligned CNTs (~50 nm in diameter) have been synthesized via Chemical Vapor Deposition of ferrocene and toluene precursors. Free standing VACNTs forests (800 µm tall) have been partially embedded in poly(dimethylsiloxane) (PDMS) films (50-100 µm thick). This process resulted in the double anchoring of CNT forests between two PDMS films, in which the vertically aligned tubes stand perpendicular to the polymer layers. Obtained PDMS-sandwiched VACNTS showed a super-compressible foam-like hysteretic behavior and a highly nonlinear quasi-static compressive stress-strain response. We subsequently used these structures as building blocks for assembling periodic systems, in which PDMS-embedded VACNTs foams have been alternated to metallic cylinders to form a one dimensional chain of masses (steel cylinders) and springs (CNTs layers), up to 15 elements long. The chains were oriented horizontally to avoid gravitational preload. To study the dynamics of the system, we excited single impulses by controlled impacts of a striker dropped from different heights. Wave propagation at specific locations was measured by calibrated piezosensors embedded in selected cylinders within the chain. We added variable constant static pre-compressive forces to the chain, controlled by means of pulleys supporting different weights, and tested the tunability of the response as a function of the applied static and dynamic load. We report a unique, tunable highly-nonlinear dynamic response of this system; The results showed a dramatic variation of the propagating pulse shape and speed, resulting in the generation of characteristic solitary-like waves when a small amount of static force was added. This work represents the first attempt to assembling CNTs based periodic structures, composed of alternating layers of metallic cylinders and VACNT foams partially embedded in PDMS films; The resulting characteristic dynamics provide a new and interesting platform for acoustic wave control in micro- and nano-structured systems.
11:30 AM - FF3.6
Mechanistic Understanding of the Ductility of Thin Nanocrystalline Metal Films on Polymer Substrates.
Teng Li 1 2 , Zhao Zhang 1 , Benoit Michaux 1
1 Department of Mechanical Engineering, University of Maryland, College Park, Maryland, United States, 2 Maryland NanoCenter, University of Maryland, College Park, Maryland, United States
Show AbstractRecent applications in flexible electronics require that thin metal films grown on polymer substrates be deformable. It has been reported that, when a laminate of a thin metal film (~100 nm thick) on a polymer substrate (10s~100s microns thick) is stretched, the metal film may rupture at strains ranging from 1~2 percent up to 50 percent. The mechanistic understanding of this large variation in the ductility of thin metal films on polymer substrates have been uncertain. Recent experiments have suggested that the rupture strain of a metal film is sensitive to its adhesion to the polymer substrate, and the fracture of the film is formed by a mixture of metal film necking and grain boundary cracking. To reveal the underlying origins for the large variation in ductility, we report a systematic study of the governing mechanisms by considering two completing failure processes: debonding along the interface and cracking along the grain boundaries. We model the interface and the grain boundaries as arrays of non-linear springs, and model the metal and the polymer as elastic-plastic solids. The simulations show that the ductility of the thin metal film is modulated by both the interfacial adhesion and the grain boundary toughness. If the grain boundaries are strong but the interface is weak, interfacial debonding occurs and the thin metal film fractures at a small strain by forming a single neck. If the interface is strong but the grain boundaries are weak, the thin metal film fractures at a small strain by grain boundary cracking. If the interface and the grain boundaries are intermediately strong, the thin metal film fractures at a modest strain by both grain boundary cracking and film necking. If both the interface and the grain boundaries are strong, the thin metal film can deform to a large strain without appreciable necking and grain boundary cracking. The study also explores the effect of grain size on the ductility of thin nanocrystalline metal films on polymer substrates. The simulations depict a deformation map of thin nanocrystalline metal films on polymer substrates, in which the ductility can be quantitatively determined by the interfacial adhesion, grain boundary toughness, and nanocrystalline grain size. Such a map can be used as the guidance for the material design for flexible electronics devices.
11:45 AM - FF3.7
The Effect of Film Thickness on the Strain-to-failure of Polymer-supported Metal Films.
Nanshu Lu 1 2 , Zhigang Suo 1 , Joost Vlassak 1
1 SEAS, Harvard University, Cambridge, Massachusetts, United States, 2 Beckman Institute, University of Illinois at Urbana-Champaign, Urbana-Champaign, Illinois, United States
Show AbstractWe study the strain-to-failure of polyimide-supported Cu films with thickness h varying from 1 μm down to 50 nm. In-situ film electrical resistance is measured as a function of film elongation and is used as an indicator of crack formation in the film. Our results show that while sub-micron films rupture at large elongations by the co-evolution of necking and debonding, films with thickness in the nanometer range fail at much lower strains by grain boundary (GB) decohesion. Moreover, the strain-to-failure of 1 μm-thick films is found to be slightly smaller that of 500 nm-thick films. This is caused by preferred strain localization at a few soft (100) grains in 1 μm-thick films. Using finite element simulations we illustrate that films with lower yield strength exhibit higher strain-to-rupture, independent of film thickness. In summary, we have identified three mechanisms limiting the stretchability of polyimide-bonded Cu films of various thicknesses: intergranular fracture in nano-films, high-strength-induced debonding and necking in sub-micron films, and preferred strain localization at (100) grains in micron films.
12:00 PM - FF3.8
Effects of Substrate Compliance and Yielding on Thin Film Delamination-Tests and Simulations.
Neville Moody 1 , E. David Reedy 2 , Edmundo Corona 2 , Marian Kennedy 3 , Megan Cordill 4 , David Adams 2 , David Bahr 5
1 , Sandia National Laboratories, Livermore, California, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 3 , Clemson University, Clemson, South Carolina, United States, 4 , Erich Schmid Institute, Leoben Austria, 5 , Washington State University, Pullman, Washington, United States
Show AbstractPerformance and reliability are important factors governing the use of emerging thin film compliant substrate devices where compressive stresses can lead to delamination and buckling. However, the effects of substrate compliance on film failure are not well defined especially at sub-micron and nanoscales. We are therefore studying these effects combining compressively stressed thin hard tungsten films on compliant PMMA substrates with simulations employing cohesive zone elements to describe interface fracture. The high compressive film stresses triggered spontaneous buckling accompanied by intense substrate deformation that significantly altered buckle morphologies and fracture energies. In this presentation we will use the results to show how substrate compliance and yielding affect the buckling of thin hard films on compliant substrates and to define a lower bound to seemingly disparate sets of data. This work was supported by Sandia National Laboratories, a Lockheed Martin Company for the USDOE NNSA under Contract DE-AC04 94AL85000.
12:15 PM - FF3.9
Directly Accessing Plasticity and Hardening Energetics in nm-thick Copper Films on Silica by Atomic-level Partitioning of Interfacial Toughness.
Ashutosh Jain 1 , Saurabh Garg 1 , Gopal Pethuraja 1 , Narayanan Ravishankar 1 , Michael Lane 2 , Ganapathiraman Ramanath 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Chemistry Department, Emory and Henry College, Emory, Virginia, United States
Show AbstractInterfacial fracture toughness in thin films and multilayers includes contributions from the cohesive energy to create two new surfaces by debonding, and from plasticity in the ductile layers at the crack tip. Plasticity commences when the applied stress equals the yield stress σy of the ductile material, which depends on film thickness and microstructure. Thin film yield stress is usually determined by combining nanoindentation and theoretical modeling to separate out the substrate effect. Here, we present a direct method to quantify σy for thin film copper through four-point bend interfacial toughness measurements on a model Cu-silica interface tailored with a molecular monolayer (MML). Our recent work has shown that siloxane bridges between organosilane MMLs and silica can be used to toughen Cu-silica interfaces by thermal annealing. Since siloxane bridges are susceptible to hydrolysis, varying the water activity aH2O provides a facile means to tune the interfacial strength, and monitor the onset of plasticity in the ductile Cu overlayer with different thicknesses. Our results capture the film thickness dependence of yield stress, which is in good agreement with values extracted from the modeling of nanoindentation data, but also provide insights into the actual extent of the plastic zone, and its relationship with hardening, in the films at different interfacial bond strengths controlled by varying aH2O. The experimental results are further validated using molecular dynamics simulations and provide a framework that could be adapted for describing interfacial fracture in multilayer interfaces and composites used in a wide variety of applications.
12:30 PM - FF3.10
Time- and Temperature-Dependent Behavior of Au-based Nanocomposite Thin Films.
Kittisun Mongkolsuttirat 1 , Mark McLean 1 , Walter Brown 1 , Richard Vinci 1
1 Materials Science and Engineering, Lehigh University, Bethlehem, Pennsylvania, United States
Show AbstractWe have previously shown that Au thin films reinforced with co-deposited nanoparticles exhibit greater hardness than either pure Au or solid solution strengthened Au films. We have also demonstrated that pure Au films have a surprising capacity for recoverable stress relaxation/creep, and that the rate of relaxation follows an Arrhenius-type temperature dependence. We have now combined these efforts to systematically explore the time and temperature dependent behavior of solid solution and oxide dispersion strengthened Au films. Results from gas-pressure bulge testing will be shown for a temperature range of 20-80 C, a typical range of use in microelectronic and micromechanical devices. The effectiveness of the two strengthening approaches will be discussed in terms of the rates of relaxation, the extents of relaxation, the activation energy of relaxation, and the related relaxation mechanisms.
12:45 PM - FF3.11
Integration of Nano Scale Thin-film Samples with MEMS Actuators during Fabrication.
Mehmet Yilmaz 1 , Jeffrey Kysar 1
1 Department of Mechanical Engineering, Columbia University, New York, New York, United States
Show AbstractRecent experiments have demonstrated that mechanical properties of nanoscale objects are often stochastic in nature. Hence, one can not characterize the mechanical behavior with individual samples; rather one must characterize the distribution of possible values. In this study we discuss the development of methods to integrate a nanoscale mechanical specimen with a MEMS transducer so that many nominally identical specimens to be tested under nominally identical conditions in order to probe the stochastic properties. The current state-of-the-art of the MEMS device for this purpose consists of either a comb-drive or a thermal actuator to apply the required forces, and two differential capacitive sensors to extract the resulting displacements in order to obtain the stress-strain diagram of the tested nanoscale samples. The only drawback to their device is that the nanoscale test specimens must be mounted in the MEMS device using a nanomanipulator and an electron-beam welder after the MEMS device has been microfabricated. The goal of our study is to circumvent the difficulties associated with mounting the nanoscale thin-film specimens into the MEMS device. We do this, in essence, by co-fabrication the thin-film specimens and the MEMS devices. In that way, the nanoscale thin-film mechanical structure of interest is integrated in the microscale actuator during the microfabrication process. The capability of microfabricating the devices and the nanoscale thin-film samples at the same time gives us the opportunity to have many, and nominally identical nanoscale sample-device pairs in a single microfabrication batch while still having the capability of real-time observation of the deformations and extracting the quantitative properties of the nanoscale test samples. Using conventional optical lithography techniques, we are capable of microfabricating almost perfectly aligned integrated sample-device pairs, and geometrically well defined nanometer scale small test samples made from gold. While further reduction in the dimensions is possible, we integrated nanoscale thin-film samples with 100nm thickness, 500nm width, and 7 micrometer length with our MEMS device.
FF4: Mechanical Behavior of Low Dimensional Nanomaterials II
Session Chairs
Tuesday PM, December 01, 2009
Room 304 (Hynes)
2:30 PM - **FF4.1
Quantitative Size Effects in Deformation of Metallic Glasses Pillars.
Jeff De Hosson 1 , C. Chen 1 , Y. Pei 1
1 Applied Physics, Un. of Groningen, Groningen Netherlands
Show AbstractSize effect, or the lack thereof, in deformation of metallic glasses has recently drawn great attention, but with significantly controversial results reported. In this presentation, we show that properly designed quantitative microcompression and microbending tests of metallic glass pillars performed in-situ in a Transmission Electron Microscope using a newly developed Hysitron Picoindenter are capable of revealing various size effects at different size regimes. Micropillars having tip diameters from submicron to sub-100 nm scale are fabricated with FIB (focused ion beam) from two metallic glasses, Zr- and Cu- based respectively. Quantitative in-situ TEM microcompression test of these pillars revealed that all the pillars show predominant shear banding behaviors. However, the characteristics of individual shear banding events are strongly size dependent, correspondingly, the deformation which is shear band nucleation controlled at large size scale becomes shear band propagation controlled with decreasing size. A micromechanical model is proposed to interpret the size dependent shear banding behaviors. However, a final transition in deformation mode from inhomogeneous to fully homogeneous is not observed by microcompression test even to sub-100 nm scale, which is the size limit of FIB method. Nevertheless, we show that, by designing microbending test of the pillars, a fully transition from in deformation mode to fully homogeneous deformation is brought forward to an experimentally accessible size regime (above ~200 nm). A deformation map incorporating these size effects is proposed.
3:00 PM - **FF4.2
The Nature of Deformation Twinning and a Model for its Size Dependence.
Ju Li 1 , Qian Yu 2 , Zhi Wei Shan 3 2 , Lin Xiao 2 , Jun Sun 2 , Xiao Xu Huang 4 , Evan Ma 5 2
1 Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, 2 State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an, ShanXi, China, 3 , Hysitron Incorporated, Minneapolis, Minnesota, United States, 4 Riso National Laboratory for Sustainable Energy, Technical University of Demark, Roskilde Denmark, 5 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractDeformation twinning (DT) [1] to ordinary dislocation plasticity (ODP) is like laser to normal light. In contrast to ODP where inelastic shear activities are far less correlated and have finite slip mean free path, DT is characterized by perfect layer-by-layer correlation and a divergent slip mean free path. A model is presented here based on a generic concept of "stimulated slip", that occurs near "reflectors" whose role is akin to mirrors in a laser resonant chamber, which could be grain boundaries, surfaces, screw dislocations or other defects in a contiguous crystalline volume. This model can explain the Hall-Petch like power-law size scaling of DT flow stress, in polycrystals as well as surface confined pillars. The model is then used to interpret our in situ mechanical experiments on Ti alloy micro- and nano-pillars [2], where DT-to-ODP transition is found to occur below a critical pillar size. ([1] Ogata, Li, Yip, Phys. Rev. B 71 (2005) 224102; [2] Yu, Shan, Li, Xiao, Huang, Sun, Ma, to be published.)
3:30 PM - FF4.3
Length Scale Dependence of Elastic Strain in Metallic Glasses by Molecular Dynamics Simulations.
Uday Vempati 1 , Pavan Valavala 1 , Michael Falk 1 2 3 , Todd Hufnagel 1
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Physics and Astronomy, Johns Hopkins University, Baltimore, Maryland, United States, 3 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractSeveral groups have used x-ray or neutron scattering in situ during loading to study elastic deformation in metallic glasses. Two significant results from these studies are that the apparent elastic modulus is different from that measured by other techniques and that there is an apparent length-scale dependence of the elastic strain, which is smallest in the near-neighbor atomic environment and increases asymptotically over larger distances. These observations suggest a contribution to the elastic deformation of metallic glasses other than the typical bond stretching associated with deformation of crystalline alloys. To investigate these effects, we performed molecular dynamics simulations of binary Lennard-Jones glasses under uniaxial tension and compression. By evaluating pair distribution functions in various directions, we determine the complete strain tensor as well as its dependence on length scale (i.e. distance from an arbitrary central atom). We compare the results of these simulations with our own experimental data, and use insights obtained from the atomistic models to develop a framework for understanding mechanisms of elastic deformation of metallic glasses.
3:45 PM - FF4.4
Size Matters: Nano-scale Plasticity in FIB-less Single Crystals and Nanocrystalline Metals.
Julia Greer 1 , Dongchan Jang 1 , Michael Burek 2 , Ju-Young Kim 1
1 Materials Science, California Institute of Technology, Pasadena, California, United States, 2 Nanotechnology Engineering, University of Waterloo, Waterloo, Ontario, Canada
Show AbstractWe discuss mechanical behavior observed in two distinct material classes: single crystals and nano-crystalline metals with nano-scale dimensions. These nano-pillars range in diameter from 50 nm to 1 micron and are fabricated by E-beam lithography and electroplating, i.e. free from ion damage. Their strengths in uniaxial compression and tension are subsequently measured in a unique in-situ mechanical deformation instrument, SEMentor, comprised of Scanning Electron Microscope (SEM) and Nanoindenter. In a striking deviation from classical mechanics, we observe SMALLER is STRONGER phenomenon in single crystals, manifested by significant increase in strength with size reduction. To the contrary, nano-crystalline materials exhibit SMALLER is SOFTER trend. Unlike in bulk, where plasticity commences smoothly, nano-pillars exhibit numerous discrete events during plastic deformation. These remarkable differences in mechanical response of nano-scale solids under uniaxial compression and tension challenge applicability of conventional plasticity models at the nano-scale. We postulate that they arise from the effects of free surfaces, leading to significant differences in dislocation behavior in crystals and grain-boundary activity in nano-crystalline metals, and serve as the fundamental reasons for observed differences in their plastic deformation. These mechanisms and their effect on the evolved microstructure and the overall mechanical properties will be discussed.
4:30 PM - FF4.5
Modeling of Indentation and Pillar Compression in Nanolayered Al/SiC Composites.
Guanlin Tang 1 , Yu-Lin Shen 1 , Danny Singh 2 , Nikhilesh Chawla 2
1 Mechanical Engineering, University of New Mexico, Albuquerque, New Mexico, United States, 2 School of Materials, Arizona State University, Tempe, Arizona, United States
Show AbstractThe indentation and pillar compression behavior of nanolayered Al/SiC composites is studied numerically. The numerical model mimics the actual material used in our experiments and features the explicit composite structure. Attention is devoted to the evolution of stress and deformation fields in the layered composite during the loading and unloading processes. It is found that the layered composite, consisting of materials with distinctly different mechanical properties, results in unique deformation patterns. Significant tensile stresses can be generated locally along certain directions, which offers a mechanistic rationale for the indentation-induced internal cracking observed experimentally. The unloading process also leads to continued plastic flow in parts of the Al layers, which is unexpected and has implications to the indentation measurement of elastic response of the nanocomposite. In the multilayered micro-pillar structure, deformation is also highly nonuniform under compression. The tapered sidewall was found to have a strong effect on the local and overall deformation response.
4:45 PM - FF4.6
Shock Response of Cu-Nb Nanolayer Composites.
Timothy Germann 1 , Richard Hoagland 1 , Shengnian Luo 1 , Nathan Mara 1 , Amit Misra 1 , Dennis Paisley 1
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Show AbstractLarge-scale classical molecular dynamics (MD) simulations and laser-launched flyer plate experiments have been used to study the shock response of Cu-Nb nanolayered composites. At a layer thickness of 5 nm, the hardness of such metallic multilayers (as measured by quasistatic indentation or compression tests) reaches a maximum due to the difficulty of dislocation transmission across the interfaces. We observe a similar strengthening effect under dynamic shock loading, both in the MD simulations and in post mortem examinations of shock-recovered samples subjected to ~20 GPa shock loading. The MD simulations provide insight into the dislocation nucleation and transmission processes that occur under compression, as well as the subsequent annihilation upon release.
5:00 PM - FF4.7
Effects of Chemistry and Microstructure on the Strengthening Behavior of Nanolayered Films.
Aikaterini Bellou 1 , Nicole Overman 1 , Hussein Zbib 1 , David Bahr 1
1 Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractMultilayer films offer unique possibilities in tailoring material properties for high strength and fatigue resistant operation conditions. The ability to control both the chemistry of the system by choosing the metals that will be used in the stacking sequence and the thickness of the individual layers results in very high strength films. Cu- based composites, a model system that has been of interest for a number of applications, were used to investigate the effects of varying chemistry on the properties of the films. Maintaining the thickness of the individual layers at 20 nm, Cu/Ni, Cu/Nb and Cu/Nb/Ni films were sputtered to a total thickness of approximately 2000 nm. The mechanical properties and especially the strength of the resulting structures were evaluated using both nanoindentation and bulge testing and conclusions were drawn on how the differences in chemistry affect the observed behavior. The hardness values of the films ranged from 1.9 to 3.4 GPa with the higher value corresponding to the Cu/Nb films. Although the Cu/Nb films have the highest hardness in nanoindentation, they have a lower initial yield strength than the other two systems when measured with bulge testing; a higher strain hardening coefficient is responsible for the differences between the initial yielding condition and the subsequent hardness measurements. The addition of a third layer of Ni does not significantly impact the initial yield behavior in bulge testing. A relatively new system of interest, Pt/Mo multilayers, was used to investigate the effects of varying microstructure, by changing layer thickness, on the mechanical behavior of the films. Pt/Mo composites with layer thicknesses varying from 20 to 100 nm were sputtered and their mechanical properties were again evaluated. The hardness values of these films ranged from 3.7 to 6.6 GPa with the higher value corresponding to the thinner layer used. While the general behavior of Pt/Mo is similar to many of the previously studied Cu systems, variations in performance are shown to be impacted by oxidation of the Mo layers. The films appear harder after aging in air, with hardness values increasing up to 18% over a period of three months after deposition. This presentation will discuss both systems in light of comparisons to established models of strengthening, and in particular the effects of alterations of the softer layer on the overall hardness of the nanolaminate.
5:15 PM - FF4.8
Deformation Processes in Nano-scale Al-TiN Metal-ceramic Multilayers.
Dhriti Bhattacharyya 1 , Nathan Mara 2 , Patricia Dickerson 2 , Richard Hoagland 3 , Amit Misra 1
1 MPA-CINT, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States, 2 MST-6, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States, 3 MST-8, Los Alamos National Laboraotry, Los Alamos, New Mexico, United States
Show AbstractNano-scale multilayered Al-TiN composites were deposited using magnetron sputtering technique in two different layer thickness ratios – Al:TiN = 1:1 and Al:TiN = 9:1. The Al layer thickness varied from 2nm to 450 nm. The hardness of the samples was tested by nanoindentation, using a Berkovich tip. In order to measure the stress-strain response of these films, compression tests on Focused Ion Beam (FIB) machined micropillars were conducted with the loading direction perpendicular to the film using a nanoindenter with a flat tip punch. These studies showed remarkable strength in the multilayers, with hardness in excess of 6 GPa at layer thickness of 2 nm and high ductility of up to 14%. Cross-sectional Transmission Electron Microscopy (TEM) was carried out on thin foils extracted using FIB from below the nanoindents and also from the compressed micropillars. The TEM studies indicated that at the smallest of length scales, the TiN layers were co-deforming with the Al layers and showed uncharacteristically high plastic deformation under adequate stresses. These results will be discussed in terms of the fundamental deformation mechanisms in nanoscale multilayered composites.This research is sponsored by DOE, Office of Basic Energy Sciences.
5:30 PM - FF4.9
Uniform and Functionally Graded Oxide-Dispersion Strengthened Nanocomposite Thin Films.
Stephen Farias 1 , Patrick Breysse 2 1 , Chia-Ling Chien 1 3 , Robert Cammarata 1 4
1 Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States, 2 Eberly College of Science, Pennsylvania State University, University Park, Pennsylvania, United States, 3 Physics and Astronomy, Johns Hopkins University, Lexington, Kentucky, United States, 4 Mechanical Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractWe have synthesized nanocomposite materials that are composed of a metallic matrix embedded with a nanoscale oxide-dispersion. Electrochemical deposition involving a novel rotating disk electrode technique is used to produce nanocomposites with a uniform dispersion of alumina nanoparticles in a matrix of Cu and a matrix of FeCo. We are also using the rotating disk electrode method to produce functionally graded nanoparticulate composites. This type of composite involves a multilayer structure composed of a single component metal layer alternately stacked with a metal layer containing a dispersion of the nanoscale oxide particles. The incorporation of a small volume fraction of oxide in these systems results in a significant increase in the strength as measured by nanoindetantion. In the case of the FeCo matrix nanocomposite, the aim is to produce a material with a large enhancement in strength without a significant degradation of the ferromagnetic properties.
5:45 PM - FF4.10
Analysis of the Mechanical Behavior and Dislocation Mechanisms in Trimetallic Nanolaminates.
Firas Akasheh 1 , Cory Overman 2 , Hussein Zbib 2 , David Bahr 2
1 mechanical engineering, Tuskegee University, Tuskegee, Alabama, United States, 2 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractPreviously, plasticity in bimetallic nanolaminates with either coherent or incoherent interfaces was studied. In this work, nanolaminates with trimetallic systems and mixed coherent and incoherent interfaces are studied using dislocation dynamics (DD) along with multiscale models for dislocation-interface interactions. The motivation is to gain insight into the effect of the different layering schemes (layering order and layers’ thicknesses) on the macroscopic strength and toughness of these materials. Stress-strain curves obtained by this analysis show characteristic differences in behavior depending on layer thickness. In the small layer thickness regime (individual layer thickness less than 10 nm), a well defined single yield point is observed, while for the large layer thickness regime (larger than 20 nm) the initial yield is followed by strong hardening and secondary elastic behavior and yield. This behavior can be associated with the observation that super threaders (threaders spanning two layers joined by coherent interfaces) dominate the plastic behavior in the small layer thickness regime. At the microscopic level, the DD analysis indicates a new dislocation mechanism involving cross slip parallel to and caused by the shearing of incoherent interfaces. This mechanism is one contributor to the enhanced ductility of nanolaminates containing incoherent interfaces.
FF5: Poster Session: Mechanical Behavior of Nanomaterials---Experiments and Modeling
Session Chairs
Brad Boyce
Erica Lilleodden
Jun Lou
Lei Lu
Wednesday AM, December 02, 2009
Exhibit Hall D (Hynes)
9:00 PM - FF5.1
In-situ Strain/Stress Measurements during the Compression of GaAs Micropillars by High Resolution EBSD.
Xavier Maeder 1 , William Mook 1 , Christoph Niederberger 1 , Johann Michler 1
1 Laboratory for Mechanics of Materials and Nanostructures, Empa, Thun Switzerland
Show AbstractIn situ EBSD strain measurements have been performed by a cross correlation technique during the compression of a GaAs micropillar inside a high resolution SEM. Such an experiment has so far never been described in the literature and is very promising in that one can follow in detail the internal deformation of the pillar during successive deformation steps. EBSD maps have been taken before, during and after the compression of GaAs micropillars. The EBSD cross-correlation technique provides an angular resolution of 0.01° and strain measurement of 10^-4 with a spatial resolution on the order of 20 nm. With it, the full strain tensor and any lattice rotation can be calculated. The overall stress is compared to the engineering stress determined from the nanoindenter load-displacement data. Due to its high spatial resolution and its strain sensitivity, in situ strain/stress measurements during micromechanical testing is a very promising route for the investigation deformation phenomena on the sub-micron scale.
9:00 PM - FF5.10
Stain Rate and Creep Response of Nanocrystalline FCC Thin Films at Room Temperature.
Nikhil Karanjgaokar 2 , Ioannis Chasiotis 1
2 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 1 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractA comprehensive experimental investigation was carried out to extract the strain-rate dependent mechanical behavior of Au (38-42 nm grain size), Pt (25 nm grain size) and Ni (20 nm grain size) thin films with micron and submicron thicknesses, conducted for the first time at strain rates between 10^-6 - 20 /s. These experiments were possible by full-field strain measurements with a resolution of 25 nm. The microscale tension experiments on nanocrystalline Au films pointed out to a clearly bi-linear increase in the elastic limit and the yield strength with the applied strain rate. This unexpected trend emphasized the significant contribution of room temperature (RT) creep at strain rates between 10^-6 – 10^-4 /s. This realization prompted a series of novel microscale creep experiments. It was found that primary creep begins at a fast rate, on the order of 10-7 /s, lasting for 5-6 hours, thus dominating the mechanical behavior at slow strain rates. Noticeably, prior works focused on the steady-state creep regime which is not as significant as the primary creep. Furthermore, multi-stage creep experiments revealed that the primary creep component decreases with the order of creep cycle, while the steady-state creep remains the same in all creep cycles. This creep response of nanocrystalline metallic films was modeled via a non-linear viscoelastic creep model that captured the effect of applied stress on both primary and steady-state creep.
9:00 PM - FF5.11
Mechanical Properties of Nanostructured hard coating of ZrO2.
Renat Sabirianov 1 2 , Fereydoon Namavar 3 2 , Wai-Ning Mei 1 2 , Xiao Cheng Zeng 4 2
1 Physics, University of Nebraska at Omaha, Omaha, Nebraska, United States, 2 Nebraska Center for Materials and Nanoscience, University of Nebraska, Lincoln, Nebraska, United States, 3 Department of Orthopaedic Surgery and Rehabilitation,, University of Nebraska Medical Center, Omaha, Nebraska, United States, 4 Department of Chemistry University of Nebraska, University of Nebraska , Lincoln, Nebraska, United States
Show AbstractNano-crystalline films of pure cubic ZrO2 (diamond simulant) have been produced by ion beam assisted deposition (IBAD) processes which combine physical vapor deposition with concurrent ion beam bombardment in a high vacuum environment and exhibit superior properties and strong adhesion to the substrate. Oxygen and argon gases are used as source materials to generate energetic ions to produce these coatings with differential nanoscale (7 to 70 nm grain size) characteristics that affect the wettability, roughness, mechanical and optical properties of the coating. The nanostructurally stabilized chemically pure cubic phase has been shown to possess hardness as high as 16 GPa and a bulk modulus of 235 GPa. We examine the mechanical properties and the phase stability in zirconia nanoparticles using first principle electronic structure method. The elastic constants of the bulk systems were calculated for monoclinic, tetragonal and cubic phases. We find that calculated bulk modulus of cubic phase (237GPa) agrees well with the measured values, while that of monoclinic (189GPa) or tetragonal (155GPa) are considerably lower. We observe considerable relaxation of lattice in the monoclinic phase near the surface. This effect combined with surface tension and possibly vacancies in nanostructures are sources of stability of cubic zirconia at nanoscale. We performed calculation of the rough nanocrystalline surface by forming hillocks on (111) textured surface mimicking the structures observed in AFM measurements. Due to the symmetry of surface termination the substantial inhomogeneity of charge distribution is observed near surface. Oxygen atoms with low coordination provide locally negative charge sites on the surface. Mechanisms responsible for the stability of the cubic phase of zirconia and evolution of its mechanical properties at nanoscale are discussed.
9:00 PM - FF5.12
The Mechanical Properties and Structure of Silica Nanowires via Simulations.
Lilian Davila 1 , Valerie Leppert 1 , Eduardo Bringa 2
1 School of Engineering, University of California Merced, Merced, California, United States, 2 , Universidad Nacional de Cuyo, Mendoza Argentina
Show AbstractInorganic nanostructures such as nanosprings, nanowires and nanorings are important morphologies of great scientific interest for future technological progress. We have focused our work on the nature and properties of silica nanowires. Nanowires have useful mechanical, electrical and optical properties that could make them useful in small-scale sensing and micro-system applications. We have performed large-scale molecular dynamics (MD) simulations to study the nature and mechanical properties of amorphous silica nanowires. The behavior of non-crystalline silica nanowires is studied using empirical interatomic potentials developed by Feuston and Garofalini. We have applied MD simulations to study the response of the silica nanowires to elevated compressive loads. We have centered our studies on the nanostructural changes occurring in the material and the correlation between the medium-range order (~10 nm), through the characteristic ring distribution of this material. Several glassy nanowires ranging in diameter from ~1.4 nm to ~14 nm are investigated. We also derived the elastic modulus of the nanowires from the stress-strain curves and found a distinctive dependence on nanowire diameter. For a nanowire length of ~14 nm and a diameter of ~4 nm, we do not observe any change in the amorphous structure up to 36% uniaxial compression because the nanowires expand laterally to accommodate uniaxial stresses. A longer nanowire, with length of ~57.3 nm and diameter of ~4 nm, shows a buckling instability and reduced strength at similar strain conditions. In both cases, the ring size distribution reveals the glassy nanostructures remain essentially unaffected at elevated compressions. The ring structure and Young’s modulus for thicker nanowires, with diameters above ~6 nm and lengths of ~14 nm, increasingly resemble those typical of the bulk material. Results are compared with recent experimental findings and theoretical predictions. This investigation contributes to an understanding of the nature of silica nanowires and their mechanical properties, influencing structure-dependent applications and design of nanoscale devices, with implications in nanotechnology, materials science, photonics and medicine.
9:00 PM - FF5.13
Stiffness of Frictional Contact of Dissimilar Solids.
Jinhaeng Lee 1 , Yanfei Gao 1 2 , Allan Bower 3 , George Pharr 1 4
1 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Division of Engineering, Brown University, Providence, Rhode Island, United States, 4 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe classic Sneddon relationship of the normal contact stiffness and the contact size is valid for axisymmetric, frictionless contact, in which the two contacting solids are approximated by elastic half-spaces. Deviation from this result critically affects the accuracy of load and displacement sensing indentation techniques like nanoindentation. This work gives a thorough numerical and analytical investigation of the corrections to Sneddon’s solution when the finite Coulomb friction exists between an elastic half-space and a flat-ended rigid punch with circular or noncircular contact shape. Two issues requiring great care in the finite element simulations are meshing near the contact edge and how friction is implemented. Although the stick or slip zone sizes are quite different between the penalty and Lagrangian methods, the calculated contact stiffnesses are almost the same, and can be considerably larger than those predicted by Sneddon’s solution. Because of linearity of the contact problem, the stiffness correction factor is found to be a function of the friction coefficient, Poisson’s ratio, and the contact shape, but independent of the contact size. For circular punch contact, the numerical solutions agree remarkably well with a previous analytical solution. For non-circular punch contact, the results can be represented using the analogy between contact problems and bi-material fracture mechanics. The correction factor is a multiplicative factor of that for circular contact, and the multiplier only depends on the punch end shape but not on the friction coefficient or Poisson’s ratio. For non-rigid indenters, the dependence of the stiffness on Poisson’s ratio can be replaced by that on Dundurs parameters.
9:00 PM - FF5.14
Transition Pathway Analysis of Homogeneous Dislocation Nucleation in a Perfect Silicon Crystal.
Hasan Saeed 1 , Satoshi Izumi 1 , Shotaro Hara 1 , Shinsuke Sakai 1
1 Department of Mechanical Engineering, The University of Tokyo, Tokyo Japan
Show AbstractNucleation of dislocations in solid materials is directly related to their key mechanical properties of ductility and ideal strength. This nucleation, which occurs either from homogeneous or from heterogeneous sources, is a classical example of stress mediated, thermally activated transitions, and is well-defined within the framework of the Transition State Theory (TST). Therefore, even at stresses lower than the athermal threshold, where the activation barrier for the transition disappears, there is a wide range of stresses for which dislocation nucleation is feasible provided thermal energy sufficient to overcome the activation barrier is available.Owing to the high strain rates resulting directly from the limited simulation times accessible with current computational resources, dynamic atomistic simulation techniques such as Molecular Dynamics (MD) are ill-equipped to access thermally activated transitions. It is so because at such high strain rates, the stresses driving the transition typically exceed the athermal threshold, resulting in a complex mix of multiple simultaneous phenomena (phase transformations, interaction of dislocations, etc); making it impossible to focus on nucleation under low stress conditions. We use the Nudged Elastic Band (NEB) method, which is an efficient transition pathway search algorithm, to examine homogeneous dislocation nucleation in a perfect Silicon crystal. We focus on homogeneous nucleation for two reasons: Firstly, the athermal stress for homogeneous nucleation is effectively the ideal strength of the material, and hence holds considerable academic interest. Secondly, it serves as a useful reference for comparison between results for different materials (comparison with homogeneous nucleation in a perfect Copper crystal); and between different configurations of the same material (comparison with heterogeneous nucleation from a sharp corner in Silicon). Input configurations are provided to the NEB algorithm by artificially introducing a shuffle set dislocation loop into the otherwise perfect crystal. We determine the minimum energy path and the saddle point configuration corresponding to a range of stress values. This gives the relationship between the activation energy and the resolved shear stress for homogeneous dislocation nucleation in Silicon. The process of converging onto the correct solution is an iterative one, and the quality of input configurations is critical. The results obtained are compared with results for other nucleation cases as reported in literature.We show that for a range of resolved shear stress values (typically for stresses near the athermal threshold), the dislocation embryo is far from perfect, and the critical nucleate transitions to an in-plane shear perturbation where the shear displacement of most particles is considerably less than the Burger’s vector. With decreasing resolved shear stress values, the embryo approaches that of a perfect dislocation.
9:00 PM - FF5.15
Delamination Mechanics of a Rectangular Film in the Presence of Long-range Intersurface Forces.
Guangxu Li 1 , Kai-tak Wan 1
1 Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractThin film adhesion is ubiquitous in biological systems and microelectronics. A new model is constructed to portray delamination of a clamped rectangular planar film from a rigid substrate in the presence of long-range intersurface forces. The testing configuration is similar to the classical “pull-off” test. The applied load is balanced by the disjoining pressure or intersurface forces with range, y, and magnitude, p, at the membrane-punch interface, and mechanical equilibrium is maintained throughout the loading-unloading and adhesion-delamination processes. The interfacial attraction is taken to be uniform and confined to the cohesive zone behind the delamination edge. With a fixed adhesion energy, γ = p y, we consider the specific behavior for an ideal zero force range interaction with y = 0 and p = ∞ (JKR limit), the transition with finite y and p (JKR-DMT transition), to an ideal infinite force range with y = ∞ and p = 0 (DMT limit). The DMT-JKR transition is of particular interests when the intersurface forces are involved potential involving electrostatic, van der Waals, steric and DLVO type of double-layer interactions. The delamination trajectory for a thin flexible membrane under stretching as in our earlier solution is distinctly different from a thick and stiff film under bending, especially in terms of the “pull-in” when membrane jumps into contact and “pinch-off” when membrane snaps. The adhesion-delamination mechanics is derived from first principles to relate the measureable quantities: applied load (F), substrate displacement (w0), contact width (l), and membrane deformed profile w(x). Simultaneous measurements of these quantities allow one to determine the γ, p and y. Predication from the theory is rigorously compared with finite element analysis showing consistency. The model is crucial in determining behavior of moveable membranes in MEMS/NEMS, thin tissues, cell adhesion and aggregation.
9:00 PM - FF5.18
In-situ X-ray Diffraction during Synthesis of Nanoporous Gold.
Steven Van Petegem 1 , Stefan Brandstetter 1 , Robert Maass 1 , Andrea Hodge 2 3 , Juergen Biener 3 , Helena Van Swygenhoven 1
1 NUM/ASQ, PSI, Villigen Switzerland, 2 Aerospace and Mechanical Engineering Department, University of Southern California, Los Angeles, California, United States, 3 Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, Livermore, California, United States
Show AbstractRecently nanoporous metals synthesized by selective dealloying of binary alloys have attracted considerable attention because to their possible application as sensors or actuators. These materials exhibit a sponge-like structure with a pore size distribution on the nanometer length scale. Special attention has been given to the Ag-Au model system because it can be synthesized with a wide range of ligaments sizes and densities. Furthermore interesting mechanical properties have been reported such as a size dependent Young’s modulus and strength.Although large progress is made in understanding the dealloying process and corresponding formation of pores, little is known about the evolving microstructure of the Ag-Au alloy and the final nanoporous Au matrix. In this work we present a comprehensive x-ray diffraction study of nanoporous gold, including in-situ x-ray diffraction during synthesis and ex-situ Laue micro-diffraction. We find that during synthesis the ligament sizes continuously increase with time, even when the dissolving process has finished. Furthermore most grains develop initially in-plane tensile stresses, which are partly relaxed during further dealloying. Post dealloying microdiffraction experiments indicate that the crystal structure of the grains is very well preserved. No indication for the formation of additional boundaries could be found. On the other hand, when the surface is treated with focused ion beam milling significant surface damage was revealed (Nanoletters 9 (2009) 1158).
9:00 PM - FF5.19
Nano-indentation Study of Three-dimensional Periodic Nanoframes.
Jae-Hwang Lee 1 2 , Steven Kooi 2 , Edwin Thomas 1 2
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 2 , Institute for Soldier Nanotechnologies, MIT, Cambridge, Massachusetts, United States
Show AbstractThree-dimensional (3D) nano-frames are now available by self-assembly, two-photon writing and interference lithography. Among these techniques, interference lithography opens a way to achieve large-area 3D nanoframes with relative ease. The uniqueness of the 3D nanoframes made by the interference lithography is that the structure is defined by few Fourier components, which results in a smooth geometrical surface; i.e. the gradient of the surface is mathematically continuous. And the inherent smoothness of the nanoframe would make the structure be more favorable for mechanical applications as it minimizes possible stress concentrations.Here, we demonstrate mechanical properties of the 3D nanoframes of epoxy and of glassy carbon. The epoxy nanoframe is fabricated by 4-beam interference lithography using a Nd:YAG pulsed laser, and subsequently carbonized to produce a glassy carbon nanoframe. Each type of nanoframe is examined by a nanoindentation tool (TriboIndenter, Hysitron Inc.) to determine the elastic moduli of the structures as a function of volume fraction. To confirm the experimental results of volume-fraction-dependent moduli, numerical simulations using a finite element method are employed. Furthermore, we also estimate the energy lost due to plastic deformation of the nanoframe in a loading step. Finally, by comparison study, we demonstrate how the large differences between the bulk elastic properties of epoxy and glassy carbon affect the mechanical properties of the nanoframes.
9:00 PM - FF5.2
Length Scale of Thermodynamic, Kinetic, and Mechanical Stabilities of Metallic Glass.
Hiroki Ushida 1 , Shigenobu Ogata 1 , Hajime Kimizuka 1
1 Department of Mechanical Science and Bioengineering, Osaka University, Osaka Japan
Show AbstractMetallic glasses (MG) are one of the promising structural materials because of their high elastic limit, high toughness, and high corrosion resistance. In these days, MGs are begun to apply to the nanostructures such as nanowires and nanopillers. An understanding of thermodynamic and kinetic stabilities of MG is essential for explaining and predicting the glass forming ability and the mechanical properties, e.g., yield strength and toughness. We should note that yield strength of nanometer-sized MG is controlled by the nucleation of local shear deformation, while the yield strength of millimeter-sized MG is dominated by the shear band propagation property. A number of analyses have been done on macroscopic thermodynamic and kinetic stabilities of MG, e.g., glass forming ability analysis, critical cooling rate analysis, by means of experimental observations and atomistic modeling. In addition, the local atomic structural orderings (LASO), e.g., short range order (SRO) and middle range order (MRO) are also actively analyzed by using atomistic modeling. However, unfortunately relations between the local thermodynamic and kinetic stabilities and the LASO in view of its length scale and energetics of the local atomic structures have not been fully discussed, even though the fact of the nonexistence of the long range order in MG strongly suggests that the local stabilities are governing the stabilities of whole MG. In the present study, we firstly analyze thermodynamic and kinetic stabilities of local regions in the CuZr bulk MG at finite temperature (300K-600K) and its region size dependency using an atomistic modeling based on the metadynamics. This method reproduces the free energy profile of configuration space for the atomic structures in the local region surrounded by bulk MG and thus, allows to find all the free energy minima corresponding to metastable atomic structures of the local region and to estimate activation energies between the metastable atomic structures. From these analyses, we find the stabilities of whole MG are determined by the stabilities of the atomic structure in ~10nm sized local region. This result suggests that the length scale of LASO is also ~10nm. We secondly analyze local strength of MG which can be defined a maximum stress against a local uniform deformation using newly developed local strain constrained molecular dynamics. The local strength varies with the size of the region applied uniform deformation. We find the weakest size is of ~10nm. This size is conceptually corresponding to the size of the shear transformation zone (STZ). We proposed a concept of length scale of thermodynamic, kinetic and mechanical stability of MG. This brings the fundamental understanding of mechanical properties of MG nanostructures.
9:00 PM - FF5.22
Reduction of Residual Stress in Density Modulated Thin Films.
Arif Alagoz 1 , Toh-Ming Lu 2 , Tansel Karabacak 1
1 Department of Applied Science, University of Arkansas at Little Rock, Little Rock, Arkansas, United States, 2 Department of Physics, Applied Physics, and Astronomy, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractControl of residual stress in thin films is critical in obtaining high mechanical quality coatings without cracking, buckling, or delamination. In this work, we present a simple and effective method of residual stress reduction in sputter deposited thin films by stacking low and high material density layers of the same material. This multilayer density modulated film is formed by successively changing working gas pressure between high and low values, which results in columnar nanostructured and dense continuous layers, respectively. In order to investigate the evolution of residual stress in density modulated thin films, we deposited ruthenium films using a DC magnetron sputtering system at alternating argon pressures of 20 and 2 mTorr. Wafer’s radius of curvature was measured to calculate the intrinsic thin film stress of multilayer ruthenium coatings as a function of total film thickness by changing the number of high density and low density layers. Morphology and crystal structure of the thin films are also investigated by scanning electron microscopy (SEM) and x-ray diffraction (XRD), respectively. It is shown that low material density nanostructured layers act as compliant layers and significantly reduce the total film stress mainly originating from high density layers under high compressive stress. By engineering the film density, we were able to reduce film stress more than one order of magnitude compared to the conventional dense films produced at low working gas pressures. Due to their low stress and enhanced mechanical stability, we were able to grow these density modulated films to much higher thicknesses without suffering from buckling. Compressive stress in high density regions of our multilayer films is believed to be relaxed by deformation of low density layers through the formation of nanocracks. It is proposed that nanocrack propagation in thin film is localized in nanostructured layers avoiding the buckling or delamination of the whole film.
9:00 PM - FF5.23
Size Dependence of the Mechanical Properties of Au Nanoparticles.
Leila Costelle 1 , Vladimir Tuboltsev 1 , Jyrki Raisanen 1
1 Department of Physics, Division of Materials Physics, University of Helsinki, Helsinki Finland
Show AbstractRecent experiments on nanoscale materials, including nanowires and nanobelts, have revealed that when structures are reduced in extent to nanoscopic proportions, they exhibit enhanced strength. Being able to synthesize these structures reliably and understand their behavior is a necessary prerequisite for their successful application as future building blocks.In this work, a simple top-down fabrication method by conventional EBL is proposed to fabricate gold nanobelts with effective diameters ranging from 10 to 250 nm. AFM based force spectroscopy approaches were used to investigate the size-dependent mechanical properties of nanostructures. Size-dependant effects appear when the volume of a nanostructure is so small that surface effects start to be relevant. This happens usually at the 10nm size range. Information from the force-displacement curves, combined with some models are used to compute the reduced modulus of elasticity of the nanostructures.
9:00 PM - FF5.24
Nanoindentation-induced Phase Transformations in Silicon at Elevated Temperatures.
Simon Ruffell 1 , David Munoz-Paniagua 2 , Jim Williams 1 , Jodie Bradby 1 , Peter Norton 3
1 , Australian National University, Canberra, Australian Capital Territory, Australia, 2 , National Institute for Nanotechnology, Edmonton, Alberta, Canada, 3 Department of Chemistry, University of Western Ontario, London, Ontario, Canada
Show AbstractDuring nanoindentation amorphous (a-Si) and crystalline silicon (c-Si) have been shown to undergo pressure-induced phase transformations. During loading the starting silicon transforms to a metallic phase (Si-II) at a pressure of ~12 GPa. During unloading this phase further transforms to either a-Si or a mixture of polycrystalline high pressure phases (Si-III and Si-XII). The latter phases are favoured by slow unloading and are formed through a nucleation and growth process. The formation of these phases is accompanied by a so-called pop-out event during unloading. Very rapid unloading favours the formation of a-Si from Si-II.The nanoindentation-induced phase transformation behaviour at elevated temperatures (25 to 150 °C), for both a-Si and c-Si matrices, has been studied by analysis of load/unload curves and Raman micro-spectroscopy. Nucleation of Si-III/Si-XII on unloading is greatly enhanced with increasing temperature. For example, at temperatures >75 °C Si-III/Si-XII has a 100% probability of being formed with unload rates 2 orders of magnitude greater than those typically required to form Si-III/Si-XII at room-temperature in a-Si. At the highest temperatures the phases form in a continuous fashion without a pop-out event. We conclude that elevated temperatures enhance the nucleation of Si-III and Si-XII during unloading and the final composition of the phase transformed zone is also dependent on the thermal stability of the phases in their respective matrices.
9:00 PM - FF5.25
Nonlinear Large Deformation of Diamond Crystals during Indentation by DFT-Based FEM.
Naohiro Toda 2 , Hajime Kimizuka 1 , Shigenobu Ogata 1 , Yuan Zhong 3 , Ting Zhu 3
2 Super Hard Materials Development Department, Sumitomo Electric Hardmetal Corp., Itami, Hyogo, Japan, 1 Department of Mechanical Science and Bioengineering, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan, 3 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractWe have developed a new framework of a finite-element method (FEM) analysis, with a constitutive relation based on density functional theory (DFT), as an efficient method to characterize the nonlinear elastic deformation of diamond crystal. In the present method, the stress-strain relations are obtained during FEM analysis on the fly based on the plane-wave-based DFT total-energy calculations and their numerical database is simultaneously constructed, which enables us to obtain high-precision stress without any empirical parameters even under finite strained conditions. The database significantly improves the total computational efficiency without loss of accuracy. Once the stress-strain database is constructed, total computational cost of DFT-based FEM analysis is reduced by 99 % and above. Using our DFT-based FEM, we conducted the (111)<11-2> shear-deformation tests of diamond crystal under various stress conditions, to examine the external-stress dependence of its deformation characteristics. The each obtained stress-strain relation exhibits nonlinear elastic behavior, and the stress reaches the maximum value and then rapidly decreases at large deformation. This fact indicates that the body begins to undergo permanent deformation, and the stress at this point is defined as the ideal maximum shear stress. It is also noted that the present results and the direct DFT results agree very well each other. Thus our method has potential to estimate nonlinear elastic behavior and ideal strength of crystal under various external stress conditions. The effectiveness of the present method is demonstrated through numerical simulations for the load-displacement response during indentation of diamond single crystal (DSC). Calculations are performed for indentation on the (111), (110) and (100) surfaces of DSC. During the indentation, the shear stress in small volumes beneath the indenter can achieve the theoretical limit of a perfect crystal. The ensuing nonlinear elastic instability can trigger homogeneous slip and/or cracking inside the crystal, which is at the onset of permanent deformation. We quantify the critical conditions of the permanent deformation occurring, including critical indentation load, instability occurring location, critical stress and instability mode at the instability point.
9:00 PM - FF5.26
Atomistic Mechanisms in Fracture of Graphene Sheets under Peeling Loading.
Dipanjan Sen 1 , Markus Buehler 2
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractThe manufacturing and separation of individual graphene sheets is of great interest due to their exceptional mechanical and electronic properties. Peeling of individual or few layers of graphene sheets from adhesive substrates can be a promising method of obtaining nanometer-width graphene sheets. Here we present atomistic simulations using the ReaxFF reactive force field, focused on mechanical peeling of graphene sheets from different adhesive surfaces. We observe formation of tapered sheet fragments, and compare to predictions from continuum theory of peeling of elastic thin films. We observe a dependence of the angle of taper on the adhesive strength of the substrate, and asymmetric tearing shapes for large adhesions. We also study effect of number of graphene layers and their bending rigidity on the tearing shape. Such varying-width tapered graphene sheets can have interesting properties for molecular electronics applications.
9:00 PM - FF5.27
Formation of Nanostructures in Face Centered Cubic Metal Powder Particles Impacted at High Velocities.
Yu Zou 1 , Eric Irissou 2 , Jean-Gabriel Legoux 2 , Stephen Yue 1 , Jerzy Szpunar 1
1 Mining and Materials Engineering, McGill University, Montreal, Quebec, Canada, 2 Industrial Materials Institute (IMI), National Research Council Canada (NRC), Boucherville, Quebec, Canada
Show AbstractMicron-sized pure copper, nickel and aluminum powder particles were deposited at supersonic velocities on steel substrates, respectively, using the method of cold gas spraying. Microstructural evolutions of the particles were investigated using electron backscatter diffraction and transmission electron microscopy. We found nanocrystals in the interfacial regions of both nickel and aluminum particles. Moreover, nanotwins were observed in the impacted copper and nickel particles. Formation of these nanostructures is explained by the deformation/recovery mechanisms in the complex thermo-mechanical history of metal powder particles during kinetic impact process.
9:00 PM - FF5.28
Mechanical Characterization of the Cement Line in Bovine Bone through Nanoindentation.
Timothy Montalbano 1 , Gang Feng 1
1 Mechanical Engineering, Villanova University, Villanova, Pennsylvania, United States
Show AbstractDepth sensing nanoindentation, back-scattered electron microscopy, and energy dispersive X-ray spectroscopy (EDS) were used to characterize a bovine bone at different lamellae in an osteon with the focus on the cement line, namely the osteon boundary. Knowing the mechanical behavior of the cement line can help explain bone mechanics and the role of osteons in crack-stopping mechanisms, which was not well understood in the literature. This investigation can also broaden the knowledge and provide clarification on mechanical properties of bone lamellae at the sub-micron scale. Back-scattered electron microscopy and EDS analysis were performed to establish whether successive lamellae are distinguished merely by their orientation or if they are a function of mineral content as well. In order to study the lamella property variance at different distances from the Haversian canal, several lamellae within an osteon and the cement line were indented with multiple indentations on each individual lamella, and then an analysis of covariance was carried out for the indentation results. Although bone is visco-elastoplastic, the time-dependent effect was separated from the indentation data using a specific data analysis method, so that the instantaneous elastic modulus and hardness could be measured correctly. Furthermore, strain-rate dependence of mechanical properties of each lamella was investigated, which was not well studied in the literature. Finally, the viscoelastic properties, such as the creep compliance and relaxation modulus, would be discussed based on indentation creep and relaxation data.
9:00 PM - FF5.3
Twining and Slip Activity in Magnesium <11-20> Single Crystal.
Gyu Seok Kim 1 2 , Sangbong Yi 1 , Yuanding Huang 1 , Norbert Huber 1 , Marc Fivel 2 , Erica Lilleodden 1
1 Institute of Materials Research, GKSS Research Center, Geesthacht Germany, 2 Science et Ingénierie des Materiaux et Procédés, (SIMaP-GPM2), Grenoble Institute of Technology, Grenoble France
Show AbstractUniaxial μ-compression tests have been performed on single crystal Mg with <11-20> compression direction, an orientation unfavorable for basal slip. Results show that the early stages of deformation proceed via dislocation plasticity until a critical point is reached at which twinning occurs. Twinning leads to a reorientation of the crystal favorable for basal slip, typically with the <2-1-1-3> aligned with the compression direction. Basal slip is then readily activated within the twin and is indicated by a large strain burst in the stress-strain response. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures.
9:00 PM - FF5.30
Rougness of Fracture Surfaces in a Model Nanosilicate Gel Determined from Magnetic Resonance Imaging.
Henrik Hemmen 1 , Eduardo de Azevedo 2 , Christian Nielsen 1 , Knut Magnus 1 , Ricardo de Souza 3 , Mario Engelsberg 3 , Jon Otto Fossum 1
1 Physics, Norwegian University of Science and Technology, Trondheim Norway, 2 Programa de Pós-Graduação em Ciência de Materiais , Universidade Federal de Pernambuco, Recife Brazil, 3 Departamento de Física, Universidade Federal de Pernambuco, Recife Brazil
Show AbstractIn this work we have determined the roughness of fractures in soft transparent nanosilicate (clay) gels, which were prepared by dispersing the synthetic clay Laponite RD in de-ionized water. A novel way of identifying roughness exponents through the use of Magnetic Resonance Imaging has been developed. By recording the 1H signal from vertical MRI slices of the clay gel, a mapping of the clay–air interface is possible. Conventional methods of extracting height profiles for roughness determination, such as laser- or stylus profilometry are unsuited for this kind of gel, due to the gels transparency and softness. From the one-dimensional MRI height profiles, the roughness exponent was calculated using established numerical methods (see e.g. [1]). Fracture surfaces were obtained by a controlled removal of filter paper attached to the surface of the gel, a method similar to the peeling method described in [2]. For fracture surfaces created with Mode-I fracturing, the roughness exponent has been found to be 0.56 (±0.05), with no observable dependency between fracturing speed and roughness. On the other hand, when combining Mode-I and Mode-II fracturing, a velocity-dependent roughness in the direction perpendicular to the fracture propagation direction is revealed. We argue that these observations can be accounted for by considering the shear thinning behaviour of Laponite gels. [1] Bouchaud E. (1997). Scaling properties of cracks. J. Phys. Cond. Mat. 9, 4319-4344.[2] Tanaka Y., Fukao K., Miyamoto Y. (2000). Fracture energy of gels. Eur. J. Phys. E 3, 395–401.
9:00 PM - FF5.31
Effect of Alloying Elements on the Elastic Properties of γ-Ni and γ'-Ni3Al from First-principles Calculations.
Yunjiang Wang 1 , Chongyu Wang 1 2
1 Physics Department, Tsinghua University, Beijing China, 2 , The International Center for Materials Physics, Chinese Academy of Sciences, Shengyang China
Show AbstractThe effect of alloying elements Ta, Mo, W, Cr, Re, Ru, Co, and Ir on the elastic properties of both γ-Ni and γ'-Ni3Alis studied by first-principles method. Results for latticeproperties, elastic moduli and the ductile/brittle behaviors are all presented. Our calculated values agree well with the existing experimental observations. Results show all the additions decrease the lattice misfit between γ and γ' phases. Different alloying elements are found to have different effect on the elastic moduli of γ-Ni. Whereas all the alloying elements slightly increase the moduli of γ'-Ni3Al expect Co. Both of the two phases are becoming more brittle with alloying elements, but Cois excepted. The electronic structures of γ' phase alloyedwith different elements are provided as example to elucidate the different strengthening mechanisms. The directional covalent-like bonding between alloying element and the host atom is responsible for the increase in moduli and brittleness.
9:00 PM - FF5.32
Application of Strip Bending Test to Mechanical Characterization of Transfer-printed Metallic Film with Nano-scale Thickness.
Jae-Hyun Kim 1 , Hyun-Joo Choi 1 , Sun-Ah Song 1 , Hak-Joo Lee 1 , Byung-Ik Choi 1
1 Nano-Mechanics, Korea Institute of Machinery and Materials, Daejeon Korea (the Republic of)
Show AbstractTransfer printing is one of the promising solutions for manufacturing flexible devices with high performance. In the transfer printing, a soft stamp is utilized to move nano-thick objects from a donor substrate to a receiver substrate. One of the key issues in the transfer printing is the characterization of adhesion among the stamp, the substrates, and the objects. It is very important to control the adhesion to improve the yield of the transfer printing process. From the point of reliability, we need to pay attention to another issue. The transferred objects experience external stress or strain during the transfer printing process, and this can cause permanent or critical damage to the objects. For the reliability evaluation and design, it is necessary to characterize the mechanical behaviors of the transferred objects, and use them for device simulation and design. In this study, we propose a technique based on strip bending test for characterization of transferred objects. Using the strip bending test, it is possible to characterize both the adhesion and the mechanical properties of the transferred objects. For the specimen fabrication, Au thin film thinner than 200 nm is patterned into strips using photolithography, and they are then transferred to a silicon substrate with a rectangular trench. The transferred strips are suspended over the trench like bridges. We measure the mechanical properties by applying a transverse load to a bridge. By measuring the critical force for detaching the bridge from the substrate, we can also characterize the adhesion between the Au bridge and Si substrate in a form of interfacial fracture toughness. The measured results can be utilized as design parameters for transfer process and the devices with transferred Au thin films.
9:00 PM - FF5.34
Strength of Nanotubes, Filaments and Nanowires from Sonication Induced Scission.
Yan Yan Huang 1 , Tuomas Knowles 1 , Eugene Terentjev 1
1 Department of Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractWe propose a simple model to describe the cavitation-induced breakage of mesoscale filaments during their sonication in solution. The model predicts a limiting length below which scission no longer occurs. This characteristic length is a function of the tensile strength and diameter of the filament, as well as the solvent viscosity and cavitation parameters. We show that the model predicts accurately experimental results for materials ranging from carbon nanotubes to protein fibrils, and discuss the use of sonication-induced breakage as a probe for the strength of nanostructures.
9:00 PM - FF5.35
Simulation of Plastic Deformation in Nanoparticles.
Yoshiaki Kogure 1 , Masao Doyama 1 , Tadatoshi Nozaki 1
1 , Teikyo University of Science and Technology, Uenohara, Yamanashi, Japan
Show AbstractRecently, varieties of nanoparticles has been developed and widely been used as industrial materials. The nanoparticles show peculiar and superior character in mechanical, thermal and electrical properties. For example, fluids containing anoparticles show strongly enhanced thermal conductivity, which is called ‘nanofluid’. Morphology and plastic deformation of metallic nanoparticles have been simulated by means of molecular dynamics simulation. The embedded atom method potential for copper is used to express the interaction of atoms. About 37000 atoms are contained in a simulation sample. As an initial condition, atomic systems are kept in high temperature molten states (1800 K), and a system is quenched to 0 K and relaxed to make a particle of amorphous state. The particle of polycrystalline state is also produced by slowly cooling the molten particle. The crystalline and amorphous structure are evaluated by calculating the radial distribution function. The particles are deformed by shear stresses and the change of atomistic structure and the displacement of atoms are investigated. Change of the mean potential energy and the material density by the deformation are also calculated for crystalline and amorphous particles,
9:00 PM - FF5.36
On Effective Indenters Used in Nanoindentation Data Analysis.
Guanghui Fu 1 , Tiesheng Cao 2 , Ling Cao 1
1 , LC Dental, Fremont, California, United States, 2 Ultrasonographic Diagnostics, Fourth Military Medical University, Xi'an China
Show AbstractNanoindentation experiments have become a commonly used technique to investigate mechanical properties of thin films and small volumes of materials. Effective indenter concept was introduced by Pharr and Bolshakov to explain nanoindentation unloading curves. The shape of the effective indenter is approximated by power-law fit. The mechanical properties of the material are obtained through the effective indenter shape function and the fundamental relation. There is no investigation on the effect of the approximation on the nanoindentation data analysis, which is the focus of the presented study. In this paper, contact stiffness is derived without any approximation. It shows that the approximation method does lead to significant errors in the fundamental relation, and overestimates the contact stiffness by 57% when uniform pressure distribution is assumed.
9:00 PM - FF5.37
Modeling Adhesion Contact between a Compliant Cylinder and a Rigid Substrate.
Jiayi Shi 1 , Sinan Mueftue 1 , Kai-tak Wan 1
1 Mechanical and Industrial Engineering, Northeastern University, Boston, Massachusetts, United States
Show AbstractAdhesion of a biological cell to an apposing cell or a bio-substrate is essential in life-sciences, as it is the key to many physiological functions. When two cells adhere, mechanical deformation is inevitably coupled with the intersurface interactions and deformed membrane geometry exhibited by the encapsulating membranes. Classic adhesion theories, including Johnson-Kendall-Roberts (JKR), Derjaguin-Muller-Toporov (DMT) and Dugdale-Barenblatt-Maugis (DBM) mostly account for contact behavior between solid-solid spheres, but are inadequate for cell-cell and cell-substrate adhesion. The fact is that mechanical deformation of a cell is dominated by membrane bending and membrane stretching rather than a solid continuum. This paper aims to investigate the mechanical deformation of an ideal cylindrical shell in the presence of intersurface interactions with a planar rigid substrate using finite element method. A Lennard-Jones like potential is introduced to mimic the convoluted surface force potentials such as electrostatic and van der Waals. Without loss of generality, two essential variables, namely, surface force range and magnitude, are allowed to vary. The highly nonlinear problem is numerically solved by the Newton-Raphson method to generate the deformed membrane profiles, the adhesion-delamination trajectory and mechanical responses. Relations between external load, contact area, vertical displacement, and internal pressure to achieve mechanical equilibrium and the associated membrane stress and strain are derived.
9:00 PM - FF5.38
The Effects of Alloying Components on the Adhesion Strength of the Oxides on Carbon Steels.
Chaewon Song 1 , Seungmok Cho 2 , Yoonhee Kang 3 , Jongsub Lee 3 , Jinwoo Park 1
1 Dep. of Materials Science and Engineering, Yonsei university, Seoul Korea (the Republic of), 2 Materials Science and Technology Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 3 Joining Research Group, Technical Research Laboratory, POSCO, Pohang Korea (the Republic of)
Show AbstractThis work presents a framework to predict the adhesion strength of the oxides formed on various carbon steels during hot-rolling process. The oxides have been known to deteriorate the quality of the final products of carbon steels such as the weldability and formability. Hence, significant research efforts have been made to develop the physical or chemical de-scaling techniques and controlling the adhesion strength of oxides to steels at the initial oxidation step is known to be critical for the successful de-scaling. However, currently there is no theoretical understanding about the factors determining the adhesion strength of various Fe-oxides on the steel surfaces. In this study, oxides are formed on various carbon steels with different alloying compositions under the same oxidation atmosphere. Phase identification of various Fe oxides are analyzed by high resolution transmission electron microscopy (HR-TEM). The adhesion strength of the oxides on the steels is measured by micro-hardness and wear tests. Based on the lattice mismatches between oxides and steels measured by HR TEM, the interfacial adhesion is predicted and compared with the measurement results. The predictions based on the lattice mismatches are found to agree well with the test results. As the oxide phases formed at the interfaces are determined by the compositions of alloying elements as well as carbon, the compositions are considered as the major factor that determines the adhesion strength of the oxides on steels. Hence, to find out the most affecting alloying components, thermodynamic calculations using ThermoCalc® Software are done and the results are verified experimentally.
9:00 PM - FF5.39
Mechanical Properties of Colloidal Silica Particles after MeV Ion-Induced Shape Tailoring.
Juan-Carlos Cheang-Wong 1 , Ulises Morales 1
1 Instituto de Fisica, Universidad Nacional Autonoma de Mexico, Mexico, D.F., Mexico
Show AbstractColloidal silica particles are being intensively studied due to their potential applications in catalysis, intelligent materials, optoelectronic devices and coating technology. The properties of these SiO2 particles depend on their size, size distribution and shape, which in turn determine the different roles they can play as electronic substrates, electrical and thermal insulators, photonic bandgap crystals, masks for lithographic nanopatterning, etc, in technologically expected nanodevices. Ion irradiation induces damage and structural changes in solids due to energy losses of multi-MeV heavy ions via ionization events and atomic collisions occurring in the near-surface region of the irradiated sample. Indeed, it has been observed that amorphous glassy materials like silicon dioxide can undergo extreme deformations under exposure to high-energy beams. This ion-beam induced anisotropic deformation of amorphous materials such as silica has been observed in the case of SiO2 films on Si substrates as well as in colloidal silica particles.For this work, spherical submicrometer-sized silica particles were prepared by the Stöber process, from a reaction mixture containing tetraethoxysilane, ammonia and ethanol, and deposited into silicon wafers. Monodisperse spherical particles were obtained with a narrow size distribution. In order to tailor the particle shape, the samples were irradiated at room temperature with 8 MeV Si ions and fluences up to 5×1015 Si/cm2, following an angle of 45° with respect to the sample surface. After the Si irradiation the as-prepared spherical silica particles turned into ellipsoidal particles, as a result of the increase of the particle dimension perpendicular to the ion beam and a decrease in the direction parallel to the ion beam. This effect increases with the ion energy and fluence, and depends on the electronic energy loss of the impinging ion. The size and shape of the silica particles were determined by SEM. The effect of the particle deformation on the mechanical properties was studied mainly by hardness measurements and will be discussed in this paper.
9:00 PM - FF5.4
An In Situ Scanning Electron Microscopy Study of Size Dependent Mechanical Behaviors of Metallic Nanowires.
Cheng Peng 1 , Yang Lu 1 , Yogeeswaran Ganesan 1 , Yongjie Zhan 1 , Jun Lou 1
1 Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States
Show AbstractMetallic nanowires are of great technological importance due to their current and potential applications in miniaturized electronic, optical, thermal and electromechanical systems. It is thus crucial to acquire a thorough understanding of their mechanical properties at comparable length scales. In addition to the technological driving force, one-dimensional metallic materials provide a unique opportunity to investigate fundamental mechanisms in materials science governing the origin and transitions of size dependent mechanical behavior for metals. This talk presents some of our recent efforts to study the size dependent mechanical behavior of metallic nanowires. We have developed a simple micro-device that allows in situ quantitative mechanical characterization of metallic nanowires, in scanning electron microscope (SEM) chamber equipped with a quantitative nanoindenter. The unique design of this device makes it possible to convert compression from nanoindentation to uni-axial tension at the sample stages. Finite element analysis (FEA) is employed to model the device behavior under mechanical loading and compared with experiments. Also in this work, Ni, Cu and Au nanowires with different diameters ranging from tens of nanometers to hundreds of nanometers were fabricated by template-assisted electro-chemical deposition and hydrothermal synthesis. The morphology and microstructure of these nano-entities were studied using SEM and TEM. Finally, main results on size effects in deformation and fracture behavior of Ni, Cu and Au nanowires will be discussed.
9:00 PM - FF5.40
Micromechanical Testing of Nanostructured NbTiNi Hydrogen Permeation Membranes.
Tetsuya Kusuno 1 , Yusuke Shimada 1 , Mitsuhiro Matuda 1 , Masaaki Otsu 1 , Kazuki Takashima 1 , Minoru Nishida 2 , Kazuhiro Ishikawa 3 , Kiyoshi Aoki 3
1 , Kumamoto University, Kumamoto Japan, 2 , Kyushu University, FUKUOKA Japan, 3 , Kitami Institute of Technology, HOKKAIDO Japan
Show AbstractNb-TiNi alloys consisting of the bcc-(Nb,Ti) and the B2-TiNi are one of the candidate materials for hydrogen permeation materials. The amount of hydrogen permeation depends on the thickness of materials, and liquid quenched ribbons of alloys are promising as a high performance membrane. Therefore the mechanical properties of membrane are important to ensure the reliability and durability of membrane. The microstructure of the alloy also affects the mechanical properties. In the present work, the micro mechanical tests have been carried out for melt-spun Nb-TiNi thin film consisting of amorphous and nanocrystalline phase. The relation between mechanical properties and microstructural changes due to heat treatments in the melt-spun film has been also discussed. Nb-TiNi alloy thin film was prepared by melt-spun technique, and was then heat-treated at 873K-1023K. Micro-sized cantilever specimens with dimensions of 10×10×50 μm3 were prepared by focused ion beam (FIB) machining. Notches with a depth of 5 μm were also introduced into the micro-sized specimens by FIB machining. Fracture tests were carried out using a mechanical testing machine for micro-sized specimens, which we have developed. In addition, Microstructures were observed by TEM. The fracture toughness values (KQ) of as melt-spun sample, which consists of crystallized area in an amorphous matrix was 3.3MPam1/2. Fracture toughness increased with increasing anneal temperature of above 873K. This may be due to the nanocrystallization (the grain size is 50-100nm) of the amorphous phase and the formation of aggregates which are composed of B2-TiNi and bcc-Nb(Ti) with the cube-on-cube orientation relationship. The KQ values of the sample annealed at 1023K for 36ks was 6.7MPam1/2, the grain size of which was 150-200nm. These results suggest that the KQ values for Nb-TiNi alloy thin film increase with increasing the grain size, which is related to annealing temperature.
9:00 PM - FF5.41
Size Effects and Deformation Mechanisms in Multi-layer Metallic Composites with Coherent and Incoherent Interfaces.
Ioannis Mastorakos 1 , Hussein Zbib 1
1 School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington, United States
Show AbstractWe employ Molecular Dynamic simulations to investigate the layer thickness effect on the deformation behavior of trimetallic nanoscale multi-layer metallic composites under biaxial loading conditions. Two special cases are studied: (a) Cu/Ni/Nb and (b) Cu/Ni/Cu/Nb with both coherent and incoherent interfaces. The layer thickness of Cu and Ni is kept constant to 5nm, and the layer thickness of Nb varies from 1nm – 5nm. The perfect structure is loaded biaxially to a strain level that is high enough to nucleate dislocations, followed by complete unloading, and resulting in a dislocation structure in the unloaded configuration. Subsequently, the unloaded structure, which is populated with dislocation, is loaded again biaxially up to an effective strain of 20%. The produced stress – strain curves are then compared with the stress – strain curves of the Cu/Ni and Cu/Nb bilayers in order to investigate the role of Nb layer thickness on the overall deformation behavior of the trilayer. Additionally, the option of depositing Sn (substitutional) atoms on the Cu/Nb interface is studied in an attempt to discuss how the weak incoherent interface can be made stronger.
9:00 PM - FF5.45
Mechanical Properties of the Nanocrystalline High Pressure Phases of Silicon Si-III and Si-XII.
Bianca Haberl 1 , Jodie Bradby 1 , Simon Ruffell 1 , Naoki Fujisawa 1 , Jim Williams 1
1 Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractIt is well known that the diamond-cubic form of silicon, Si-I, undergoes a phase transformation to a metallic phase, Si-II, when pressurized to ∼11 GPa in a diamond-anvil cell. On pressure release this Si-II undergoes further phase transformation to the crystalline phases Si-XII and Si-III, which in turn re-transform completely to Si-II upon re-pressurization. The formation of a metallic Si phase has also been observed at similar pressures under indentation loading. Depending on the unloading conditions, nanocrystalline high pressure phases can be nucleated under the indentation tip. However, cyclic loading experiments to moderate load levels have reported elastic behavior after the initial formation of the high pressure phases. Thus, it was concluded that the high pressure phases might either have a higher indentation hardness than Si-I or that the high pressure phases created by indentation do not re-transform to Si-II when pressurized. The reasons for this different behavior are unclear and this study addresses this issue. A relatively large volume (9 μm diameter extending approximately 500 nm below the surface) of the high pressure phases (a mixture of Si-III and Si-XII phases) is created by indentation using a microscale spherical tip. This volume is then probed for its mechanical properties with a small Berkovich tip. The indentation behavior of these high pressure phases is compared to the indentation behavior of Si-I, bulk as well as nanocrystalline material, whereby the nanocrystalline material itself was created from the high pressure phases by thermal annealing. Indentation data as well as Raman microspectroscopy and cross-sectional transmission electron microscopy reveal that the nanocrystalline silicon (Si-I) phase transforms in a similar manner to single crystal silicon. Interestingly, the high pressure phases (Si-III/Si-XII) seem to deform plastically at moderate loads via slip along the grain boundaries rather than through phase transformation. Additionally, the indentation hardness of these different materials was determined by two different methods, by instrumented indentation according to the method of Oliver and Pharr and by directly scanning the projected area of the residual indent impression with an atomic force microscope. A lower indentation hardness of the nanocrystalline silicon compared to bulk material is found, whereas the high pressure phases exhibit a higher indentation hardness than either.
9:00 PM - FF5.6
Effect of Oxygen and Hydrogen Concentration on Nanoindentation-induced Phase Transformations in Amorphous Silicon.
Simon Ruffell 1 , Jyotsna Vedi 1 , Jodie Bradby 1 , Jim Williams 1
1 , Australian National University, Canberra, Australian Capital Territory, Australia
Show AbstractDuring nanoindentation, silicon undergoes a series of pressure-induced phase transformations. On loading, diamond cubic Si-I transforms to a metallic phase (Si-II) at a pressure of ~11 GPa. During unloading Si-II further phase transforms to either amorphous silicon (a-Si) or a mixture of high pressure polycrystalline phases (Si-III and Si-XII) depending on unloading rate. “Pure” a-Si formed by ion-implantation undergoes these pressure-induced phase transformations. However, chemical vapour deposited a-Si does not. One possibility for this is the high impurity content in such films.The effect of the local oxygen and hydrogen concentration in ion-implanted amorphous Si (a-Si) on nanoindentation-induced phase transformations has been investigated. Implantation of O and H into the a-Si films has been used to controllably introduce an approximately constant concentration of each species, ranging from ~1018 to ~1021 cm-3, over the depth range of the phase transformed zones. Nanoindentation was performed under conditions that ensure a phase transformed zone composed completely of Si-III/XII in the nominally O and H-free a-Si. The effect of the local concentration has been investigated by analysis of the unloading curves, Raman micro-spectroscopy, and cross-sectional transmission electron microscopy (XTEM). The formation of Si-III/XII from Si-II on unloading is suppressed with increasing oxygen and hydrogen concentration, favouring a greater volume of a-Si within the zones. Raman and XTEM verify that the volume of Si-III/XII decreases with increasing O and H concentration. With the smaller volumes of Si-III/XII, the pop-out normally observed on load versus penetration depth curves during unloading decreases in magnitude, becoming more kink-like and is barely discernable at high concentrations of oxygen and hydrogen. The probability of forming any high pressure phases is reduced from 1 to ~0.1 for concentrations of ~1021 cm-3. We suggest that the bonding of O and H with Si reduces the formation of Si-III/XII during unloading through a similar mechanism to that of oxygen-retarded solid phase crystallization of a-Si. In the case of H, the formation of gas bubbles may also play a role. These findings open up the possibility to engineer subsurface nanostructures composed of the high pressure crystalline phases by a combination of selective area implantation and tailoring of impurity concentration-depth profiles.
9:00 PM - FF5.7
Mechanomutable Carbon Nanotube Arrays.
Steven Cranford 1 , Markus Buehler 1
1 Department of Civil and Environmental Engineeirng, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractHere we present atomistic-based multi-scale simulation studies of a magnetically active array of carbon nanotubes to illustrate the concept of mechanomutability. We show that by applying external fields, it is possible to change the nanostructure and to induce a desired mechanical response. Direct numerical simulations are reported that illustrate this concept via mechanical testing through nanoindentation. The purpose of simulating nanoindentation was to depict a commonly applied means of physical mechanical testing. In addition, the mechanical properties of nanotube arrays are difficult to predict a priori due to interaction of bundles and the indenter, complementing the stochastic element of molecular dynamics. Specifically, we show that the contact stiffness of an array of carbon nanotubes can be changed reversibly from approximately 73 MPa to 910 MPa due to the application of an external field. We implement a hierarchical approach using coarse grained molecular modeling to develop a framework that can successfully collaborate atomistic theory and simulations with material synthesis and physical experimentation. This method can further be implemented to investigate the multiscale dynamics of other molecular systems, such as protein and polymer composites at length scales beyond the capacity of traditional molecular dynamics. A fluid and synergistic multiscale foundation is required to bridge the gap between atomistic and continuum-level modeling, to exploit the unique properties of nanomechanical behavior, and facilitate the progress of developing novel mechanomutable structural materials.
9:00 PM - FF5.9
Dislocation Dynamics Simulations of Metal Nano- and Micro-Imprinting.
Yunhe Zhang 1 , Erik Van der Giessen 2 , Lucia Nicola 1
1 Materials Science and Engineering, Delft University of Technology, Delft Netherlands, 2 Department of Applied Physics, University of Groningen, Groningen Netherlands
Show AbstractMetal nano- and micro-imprinting is of great technological interest due to its current as well as its potential applications in miniaturized systems. The objective of this study is to investigate numerically the capability of metal films to retain imprints when indented by an array of rigid bodies of various shapes, size and spacings. The challenge originates from the size dependent plastic properties at (sub-) micron size scales, which causes a non-trivial interaction of the plastic zones underneath the indenters [1]. At the length scale of interest for miniaturized devices, conventional finite element simulations based on classical continuum plasticity fail in predicting localized stresses and deformations. The approach used in this study is 2D discrete dislocation plasticity [2], where plasticity in the metal film originates from the collective motion of discrete dislocations. The discreteness of dislocations, with an evolving density, is the key element for size dependent plasticity, giving rise to a large deviation of submicron-structure behavior from that of bulk metal. For instance, large number of dislocations gliding out the metal free surface during loading leave surface steps that are comparable in size to the depth of the final imprint. Dislocations are modeled as line singularities in an otherwise isotropic linear elastic medium. Constitutive rules are supplied for the glide of dislocations as well as their generation, annihilation and pinning at point obstacles. The simulations track the evolution of the dislocation structure during loading, unloading and relaxation and provides an accurate description of the residual stresses after imprinting.[1] L. Nicola, A. F. Bower, K.-S. Kim, A. Needleman, and E. Van der Giessen, Phil. Mag. 88 Vol 82-94 3713-3729 (2008)[2] E. Van der Giessen and A. Needleman, Modelling Simul. Mater. Sci. Eng. 3 689-735 (1995)
9:00 PM - FF5: PosterI
FF5.20 Transferred to FF2.11
Show Abstract
Symposium Organizers
Jun Lou Rice University
Brad Boyce Sandia National Laboratories
Erica Lilleodden GKSS Forschungszentrum
Lei Lu Chinese Academy of Sciences
FF6/DD6: Joint Session: Nanomechanics for MEMS/NEMS Applications
Session Chairs
Brad Boyce
Henry Padilla
Kevin Turner
Wednesday AM, December 02, 2009
Room 304 (Hynes)
9:30 AM - **FF6.1/DD6.1
Measuring and Mapping Mechanical Properties with Nano-Scale Resolution.
Robert Cook 1
1 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractDevices based on micro- and nano-electromechanical systems (MEMS and NEMS) offer enormous potential for economic and quality of life improvements, with application length scales varying from infrastructure sensors that ensure the safety of buildings to biological actuators that manipulate individual cells. A critical element in achieving such advances is the ability to measure and manipulate the mechanical properties of materials and devices with nano-scale resolution. This is particularly so as nanomaterials, such as molecularly-engineered structures, nanogranular films, or multifunctional nanowires or nanotubes are integrated into MEMS and NEMS; all with mechanical properties very different from their bulk analogs, if they exist. This presentation describes contact probe-based methods developed to measure the elastic, plastic, and fracture properties of materials at the nanoscale. Emphasis is placed on the quantification of modulus, yield stress, strength, and toughness of nanomaterials required by MEMS and NEMS developers to optimize device fabrication and performance. Electron- and optical-based methods are also described that permit strain mapping with nanoscale resolution. The interrelationship between strain and properties for nanomaterials, generated by their large surface to volume ratios, and the opportunities for innovative MEMS and NEMS devices are outlined.
10:00 AM - FF6.2/DD6.2
Imperfection and Size Dependent Ductility of Thin Freestanding Metallic Films.
Thomas Pardoen 1 , Charles Brugger 1 , Michael Coulombier 1 , Alexandre Boe 1 , Marie-Stephane Colla 1 , Joris Proost 1 , Thierry Massart 2 , Jean-Pierre Raskin 1
1 , Université catholique de Louvain, Louvain-la-Neuve Belgium, 2 , Université libre de Bruxelles, Brussels Belgium
Show AbstractThe resistance of metallic thin films to necking and plastic localization is an important issue in applications such as flexible electronics, thin coatings on deformable substrates, and several types of MEMS structures. The resistance to necking is controlled by the strain hardening capacity, strain rate sensitivity, and presence of imperfections. Recent experimental results obtained using a novel concept of nanomechanical lab-on-chip on aluminium and palladium thin film samples with thicknesses ranging between 50 and 500 nm and widths ranging between 1 and 6 micrometers demonstrate several types of size effects affecting the ductility. Both the thickness and width control the strain hardening capacity of the film, hence its resistance to plastic localization. Furthermore, a strong statistical dependence of the ductility on the overall size of the sample, related to the distribution of material and geometrical imperfections, is observed and rationalized using a Weibull type analysis.An advanced strain gradient plasticity model, based on the Fleck-Hutchinson theory, with evolving higher order boundary conditions at the grain boundaries has been set up to investigate the size dependent plastic response of these thin films. The model has been implemented in a 2-D finite element code within a finite strain setting. The origins of the size effects result from the presence of grain boundaries and oxide layers on the surface. Specific boundary conditions are applied on these two types of interfaces to simulate an impenetrable behaviour at low stress and dislocation transmission/nucleation at higher stress. Motivated by the experimental data, the model is used to investigate separately the effects of grain size, sample thickness, width, and imperfection, combined to the properties of the grain boundaries, on the strain hardening capacity and uniform elongation.
10:15 AM - FF6.3/DD6.3
Improved In-situ On-chip Microelectromechanical Uniaxial Tensile Tester.
Siddharth Hazra 1 , Jack Beuth 1 , Maarten de Boer 2
1 Mechanical engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractIn spite of being brittle, polycrystalline silicon (polysilicon) is used extensively in microelectromechanical systems design due to its high strength (1-3 GPa). Fracture is a primary concern in assessing the reliability of such devices. Due to the limitations posed by their small size, it has been difficult to extract fracture strength measurements with significant sample sizes; data sets reported in literature are usually limited to <40 measurements. Recently, we have demonstrated a high-throughput on-chip uniaxial microtensile tester. It is fabricated using the Sandia SUMMiT V™ process, and is viable for extracting statistically significant fracture-strength data sets for a single layer of structural polysilicon (poly3). It uses thermal actuation and a prehensile gripping mechanism to load a polysilicon tensile specimen to failure. We measured the tensile bar misalignment to be below 0.2° for both the in-plane and out-of-plane orientations while straining. Being in-situ, it requires less than ten minutes to extract a strength measurement. However, there remained some ambiguity as to whether the strength was obtained from a failure of the gripping mechanism or the tensile specimen. Therefore, we have designed and fabricated devices with an improved gripper geometry. In this paper we shall report fracture strengths measured using this redesigned gripping mechanism and a larger data set (~100 measurements) along with relevant statistical inferences from the sample. Additionally, we assess the strength of other structural polysilicon layers (poly21 and poly4 in the SUMMiT V™ process) using a similar experimental setup. Here, a larger out-of-plane misalignment is expected, and we use interferometry, finite element analysis and fractography to quantitatively assess its effect on fracture strength.
10:45 AM - FF6.5/DD6.5
Size-dependent Mechanical Properties of Polymer-nanowires Fabricated by Two-photon Lithography.
Satoru Shoji 1 , Sana Nakanishi 1 , Tomoki Hamano 1 , Satoshi Kawata 1 2
1 Department of Applied Physics, Osaka University, Osaka Japan, 2 Nanophotonics Laboratory, RIKEN, Saitama Japan
Show AbstractTwo-photon lithography allows us to fabricate arbitrary three-dimensional structures with micro/nano-spatial resolution. The intrinsic three-dimensional spatial resolution of two-photon lithography is a promising tool for developing a variety of novel photonic and mechanical nano-devices. In addition, two-photon lithography makes it possible to study the properties of polymers of micro/nano-meter dimension [1-4]. In this presentation, we show the evidence that the elasticity and the transition temperature of polymers start to show size-dependent characteristics when the size of the polymer decreases down to a few hundreds of nanometers. We fabricated free-standing polymer nano-wires in the shape of coil spring by two-photon lithography, and measured the elasticity of the nano-wires by applying a mechanical tension onto the springs through laser trapping technique under temperature control. From the stretching length of the spring generated by a certain optical force, the spring constant of the spring and the shear modulus of polymer were calculated from simple Hook’s law. The material we used is a compound of methyl-methacrylate, dipentaerythritol hexaacrylate, benzil, and 2-benzyl-2-(dimethylamino)-4’-morpholino-butyrophenone. We scanned a light spot of a Ti:Sapphire laser at 780 nm with a pulse width of 80 fs focused by a 1.4 NA microscope objective to shape the polymer nano-wires suspended by a thick pillar. We observed that the elasticity of the polymer, which is usually an invariable coefficient, changes according to the thickness of the polymer wire. Furthermore, we changed the temperature of the entire polymer structures from -20 to 40 °C with measuring the elasticity of the springs. We observed phase transition of polymer wires with a rapid change of the share modulus, which also shows a size-dependent behavior. Recently, the similar experimental results, i.e. the drop of the glass transition temperature, were also reported in thin polymer nano-films. Our result is a clear evidence of such a nano size-effect of mechanical properties in polymers confirmed from free-standing polymer nanostructures. [1]S. Nakanishi, H. Yoshikawa, S. Shoji, Z. Sekkat, and S. Kawata, J. Phys. Chem. B 112, 3586-3589 (2008). [2] S. Nakanishi, S. Shoji, S. Kawata, and H.-B. Sun, Appl. Phys. Lett. 91, 063112 (2007). [3]H. Ishitobi, S. Shoji, T. Hiramatsu, H.-B. Sun, Z. Sekkat, and S. Kawata, Opt. Express 16, 14106-14114 (2008). [4]S. Kawata, H.-B. Sun, T. Tanaka, and K. Takada, Nature, 412, 697 (2001).
11:00 AM - FF6/DD6:MEMS
BREAK
11:30 AM - **FF6.6/DD6.6
Nano- and Micron-Scale Fracture and Fatigue: From MEMS to OLED Structures.
Winston Soboyejo 1 2
1 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 2 Princeton Institute of Science and Technology of Materials, Princeton University, Princeton, New Jersey, United States
Show AbstractThis paper presents the results of experimental and theoretical studies of fracture and fatigue in nanoscale thin films thin films that are relevant to micro-electro-mechanical systems (MEMS) and organic light emitting devices (OLEDs). The underlying mechanisms of fatigue are elucidated using a combination of focused ion beam (FIB)and scanning electron microscopy. In the case of the nickel MEMS thin films, the observed mechanisms of slip band formation are modeled using dislocation mechanics and fracture mechanics concepts. The formation of spiral buckles in OLEDs is also modeled using a combination of heat transport and interfacial fracture mechanics concepts. The implications of the results are discussed for the design of robust nanostructured thin film structures.
12:00 PM - FF6.7/DD6.7
Performance and Reliability of Ru-coated Microrelays.
Maarten de Boer 1 , David Czaplewski 1 3 , Michael Baker 1 , Steven Wolfley 1 , Gary Patrizi 1 , David Tallant 2 , James Ohlhausen 2
1 Microsystems Science, Technology and Components, Sandia National Labs, Albuquerque, New Mexico, United States, 3 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois, United States, 2 Materials Science and Engineering, Sandia National Labs, Albuquerque, New Mexico, United States
Show AbstractMicroelectromechanical relays are of great interest in radio-frequency and power switching applications, and also in filtering circuits. DC relays must maintain low electrical resistance over millions to billions of cycles. Soft metals such as Au tend to cold-weld, causing failure due to adhesion. Harder metals such as Pt tend to accumulate hydrocarbons, which form graphitic species, leading to an increase in contact resistance as devices are cycled. We are exploring conducting metal oxides, which are hard yet have low affinity for hydrocarbons. The switch is an in-plane design employing polysilicon thermal actuators attached to contact bars that make electrical contact on vertical sidewalls. A self-shadowing design has been implemented, which protects the thermal actuators, isolates traces, and coats the sidewalls. To maximize cleanliness of the surfaces, devices are vacuum-baked at 200 degrees Celsius. The chamber is then cooled to room temperature to attain ultra-high vacuum levels, and backfilled to atmospheric pressure with ultra-high purity gas. Next, in-situ testing is conducted. Applying a film stack consisting of a thermally oxidized sputtered Ru coating on sputtered Al to the structural polysilicon, we obtain intrinsic contact resistance (5 ohms) with entirely stable resistance over more than 20 million cycles. This is a significant improvement over Pt-coated microrelays, in which resistance increases by a factor of five after only 100,000 cycles. In this presentation, we will detail the switch design, fabrication, resulting Ru-film residual stress, device packaging, test procedure, electrical results, and surface analysis (SEM, Raman and Auger) for this high-performance material system. Sandia ia a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000.
12:15 PM - FF6.8/DD6.8
Investigating the Dynamics of Adhesion and Separation of Microscale Silicon Contacts.
David Grierson 1 , John Nguyen 2 , Kevin Turner 1
1 , UW-Madison, Madison, Wisconsin, United States, 2 , Sandia National Laboratories, Albuquerque, New Mexico, United States
Show AbstractControlling adhesion is a primary challenge for improving the performance and reliability of micro-electromechanical systems (MEMS). Microscale components are highly susceptible to the influence of surface forces, and systematic micromechanical investigations into the nature of adhesion between MEMS materials need to be undertaken in order to fully characterize and ultimately surmount the constraints that adhesive forces impose upon successful device creation. Interfacial behavior is typically described by a single quantity such as the work of adhesion, but the adhesion and separation behavior of an interface is more complex, and a more complete description of the mechanical interaction between contacting surfaces is needed. To investigate this and other adhesion-related issues, we have constructed a novel micromachined test structure to characterize room-temperature adhesion of MEMS-material surfaces with nanoscale roughness. The measurement approach shares some similarities with on-chip micromachined beam arrays that have been employed previously, but is unique in that the microscale beams are not fabricated directly on the test substrate. Instead, a chip with micromachined silicon cantilever beams is controllably contacted with a separate test surface to form the interface to be studied. The separation gap between the beams and the substrate is controlled with a piezoelectric nanopositioner, and the deformed profiles of the adhered beams are measured with an interferometer. The use of a separate beam and test surface allows for the investigation of a broader range of materials and facilitates the inspection and analysis of the contacting surfaces before and after testing. The test method allows for dynamic, time-dependent characterization of both the adhesion and separation of microscale contacts. Here, we describe the technique, report theoretical and finite element analysis of the test structure, and present measurements of adhesion between hydrophilic, single-crystal silicon surfaces with nanoscale roughness (r.m.s. ~0.3 nm) as a function of relative humidity (RH). We find that the work of separation is three to seven times higher than the work of adhesion, which is considerably different than the conventional assumption that the work of adhesion and fracture are the same for room temperature contacts, and we find that the adhesion energy and hysteretic behavior are influenced by the RH. The underlying mechanisms that lead to adhesion and hysteresis in these contacts will be discussed. This work was partially supported by AFOSR MURI Contract #FA9550-08-1-0337.
12:30 PM - FF6.9/DD6.9
Mapping Nanoscale Wear Field by Combined Atomic Force Microscopy and Digital Image Correlation Techniques.
Zhi-Hui Xu 1 , Michael Sutton 1 , Xiaodong Li 1
1 Department of Mechanical Engineering, University of South Carolina, Columbia, South Carolina, United States
Show AbstractSurface wear of coatings occurring at extremely low loads and in nanocontacts is of great importance for the development and the reliability of structural/functional nanocomponents in micro/nanoelectromechanical systems. To date, appropriate tools for mapping the nanoscale wear of thin coatings are still lacking. In this study, a new method combining atomic force microscopy (AFM) and digital image correlation (DIC) techniques has been developed and applied for the determination and visualization of the nanoscale wear of a gold coating. It has been shown that the initiation and development of nanowear, which is usually difficult to detect directly from AFM topographical images, can be efficiently revealed by monitoring the correlation coefficient change in DIC analysis. A linear relation between the correlation coefficient and the wear depth is found and may be used to quantify the nanowear. The nanowear of gold coating is dominated by material removal without any plastic deformation.
FF7: Novel Nanomechanical Characterization Methods
Session Chairs
Chris Xiaodong Li
Cynthia Volkert
Wednesday PM, December 02, 2009
Room 304 (Hynes)
2:30 PM - **FF7.1
Deformation Mechanisms of Nanostructured Metals Studied by In-situ Electron Microscopy.
Gerhard Dehm 1 2
1 Materials Physics, Montanuniv. Leoben, Leoben Austria, 2 Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Leoben Austria
Show AbstractRecent advances in fabricating micro- and nanostructured materials have stimulated research on mechanical properties at small length scales. Especially the internal interfaces in nanocrystalline materials and thin films start to play a significant role. E.g., hard coatings consisting of nanocrystalline grains embedded by a few monolayers of amorphous matrix can reach theoretical strength. Metal films like Al and Cu of less than 100 nm thickness reveal flow stresses exceeding 1 GPa, while for the corresponding bulk single crystals plastic deformation sets in below 50 MPa. Novel miniaturized compression and tension tests as well as in-situ electron microscopy experiments have been recently implemented to shed light on the deformation behavior at small length scales. In this talk the current understanding of deformation mechanisms is discussed for metals and thin films with critical dimensions in the sub-micrometer regime.
3:00 PM - FF7.2
In situ Characterization of the Electrical and Mechanical Properties of Oxide Nanowires using STM-STEM.
Hye Jung Chang 1 , A. Borisevich 1 , S. Kalinin 2 , E. Strelcov 3 , A. Kolmakov 3 , B. Mirman 4 , S. Pennycook 1
1 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Department of Physics, Southern Illinois University Carbondale, Carbondale, Illinois, United States, 4 Department of Mathematics and Computer Science, Suffolk University, Boston, Massachusetts, United States
Show AbstractOxide nanowires are promising materials for chemical and biological sensors, photonics and photovoltaics. The macroscopic properties have been widely studied and efforts to characterize the individual wires’ properties and their surface also have been made recently using scanning electron microscopy (SEM) and scanning probe microscopy (SPM) [1]. However, the mechanisms of lattice-transport coupling and interplay between chemical changes, mechanical deformation, and transport still remain largely unexplored. In this study, combined STM-STEM technique was used to enable in situ structural/mechanical/electrical analysis. Single-crystal nanowires (SnO2, ZnO, VO2) were vapor grown in a tube furnace by thermal evaporation under Ar atmosphere. In-situ measurement of electrical properties and observation at the nanometer level was carried out by using a Nanofactory Instruments Scanning Tunneling Microscope (STM) holder in the Scanning Transmission Electron Microscope (STEM) (FEI Titan S 80-300) operating at 300 kV. A W tip for STM probe was fixed on the 3 dimensionally moveable end of a piezotube in the holder and was used as both a current probe and a mechanical manipulator. The nanowires were attached to other W rod acting as a counter electrode. Individual nanowire’s I-V characteristics were used to characterize transport properties. The nanowires were (elastically) bent to a varying degree, and then changes in transport properties were concomitantly recorded. While the changes in transport properties caused by mechanical deformation were interpreted as a signature of a strain-induced insulator-metal transition in a previous study [2], we find that in the experimental setup of STM-STEM there are many factors contributing to the observed changes. Even in the case of SnO2, where no phase transitions or piezogalvanic effects are expected, the apparent conductivity increases as the nanowire is bent. We attribute this change to the increase in tip-wire contact area driven by increasing contact strain. Detailed data analysis and estimates of the magnitudes of different contributions for VO2 and ZnO nanowires will also be presented. Research supported by ORNL’s SHaRE Use Facility (AYB) and Center for Nanophase Materials Sciences (SVK), sponsored by the Scientific User Facilities Division; by the Division of Materials Science and Engineering (SJP), Office of Basic Energy Sciences, U.S. Department of Energy, and by the ORNL LDRD program via a postdoctoral appointment administered jointly by ORNL and ORISE (HJC). The research at SIUC was supported through ACS PRF-G grant.[1] A. Kolmakov, U. Lanke, R. Karam, et al., Nanotechnology 17, 4014 (2006).[2] X. Bai, D. Golberg, Y. Bando, C. Zhi, C. Tang, M. Mitome, K. Kurashima, Nano Lett. 7 (2007) 632.
3:15 PM - FF7.3
In situ Nanoindentation Studies of Deformation Mechanisms in Nanoscale Al/Nb Multilayers.
Nan Li 1 , Jian Wang 2 , Jianyu Huang 3 , Amit Misra 2 , Haiyan Wang 1 , Xinghang Zhang 1
1 , Texas A&M University, College Station, Texas, United States, 2 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 3 , Sandia National Laboratory, Albuquerque, New Mexico, United States
Show AbstractIn situ nanoindentation inside a transmission electron microscope has been used to explore the deformation mechanisms in Al/Nb multilayers. These multilayers are sputter-deposited with individual layer thickness ranging from 1 to 100 nm and exhibit Kurdjumov-Sachs orientation relationship: {111}fcc // {110}bcc; <110>fcc // <111>bcc. In situ nanoindentation shows significant deformability even at layer thicknesses as small as 5 nm via dislocation mechanisms. Although significant dislocation activity is observed during deformation, no dislocation tangles, networks or cell structures form within the nanoscale layers. Analysis of high-resolution images indicates that glide dislocations are trapped at the interfaces and dislocations of opposite sign are annihilated. The role of interfaces in slip transmission, work hardening and recovery is discussed. The mechanical properties of Al/Nb multilayers are compared with Cu/Nb, a system with similar crystallography but different heat of mixing and lattice mismatch at interfaces. Overall, the hardness of Cu/Nb is significantly higher than Al/Nb and differences are discussed in terms of the interface barrier strength to slip transmission.
3:30 PM - FF7.4
Strain Gradients and Interface Strengths in NiAl-Mo Composites from 3D Spatially Resolved X-ray Microdiffraction.
R. Barabash 1 2 , H. Bei 1 , Y. Gao 2 3 , G. Ice 1 , E. George 1 2
1 Materials Science and Technology Div., Oak Ridge National Laboratory, Oak Ridge TN, Tennessee, United States, 2 2.Materials Science and Engineering Department, The University of Tennessee, Knoxville, Tennessee, United States, 3 3.Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractStrain gradients near the buried interfaces in NiAl-Mo eutectic composites containing about 500nm fibers are probed with 3D depth-resolved x-ray microdiffraction. Depth-dependent strain gradients beneath the free surface are determined for both the Mo fibers and NiAl matrix. It is found that the Mo fibers have a (100) d-spacing variation of about 1% along the fiber direction near the surface, while in the matrix the opposite sign strain variation is ~ 3 times smaller. This can be explained by the release of residual strain resulting from the CTE mismatch between NiAl and Mo. The CTE mismatch causes embedded Mo fibers to be in compression, but this compressive residual stress is relaxed at the surface, or when the matrix is etched away. From a detailed micromechanics analysis, the strength of the Mo-NiAl interface is estimated.
3:45 PM - FF7.5
Atomic Force Microscopy Measurements of High Resonance Frequency of Isolated Nanostructures.
Tom Parker 1 , Gwo-Ching Wang 1 , Toh-Ming Lu 1
1 Physics Department and Center for Integrated Electronics, RPI, Troy, New York, United States
Show AbstractOne of the most important physical properties in the performance of MEMS (microelectromechanical systems) and NEMS (nanoelectromechanical systems) is the mechanical frequency response. More specifically the resonance frequency of functionalized micro-cantilevers has been used to detect individual viruses and absorbed chemical species in the biological/chemical research. As devices/structures shrink to the nanometer size scale there is a corresponding increase in the resonance frequency that can range from the MHz into the GHz frequencies. Not only that, when the device components are in the nanoscale, the conventional techniques, including the traditional optical techniques such as doppler interferometry, become impractical. In this presentation, we demonstrated the use of a non-contact atomic force microscope (NCAFM) to measure the high frequency resonance of a PZT (piezoelectric transducer) and AFM cantilevers up to the tens of MHz range [1]. We also demonstrated that the effect of the coupling between the AFM probe and the sample such as a sample cantilever is insignificant, i.e., a shift of <0.3%. In addition one can also measure the mode shape of the characteristic first vibrational mode of the sample cantilever along its length. This shape agrees well with the theoretical shape of a single end-clamped vibrating cantilever. Both the measured small shift in resonance frequency and mode shape support a negligible coupling of the probe cantilever and the sample. Applying this NCAFM technique we measured high resonance frequency of individual nanostructures including nanorods and nanosprings. These nanostructures were fabricated using the oblique angle deposition technique [2]. We showed that individual nanostructures such as slanted amorphous Si (a-Si) nanorods and a-Si 4-turn nanosprings with resonance frequency ranging from kHz to hundreds of MHz can be measured directly using this NCAFM technique. Acknowledgement: Work partially supported by the NSF 0506738 and NSF 0333314. [1] T.C. Parker, F. Tang, G.-C. Wang, T.-M. Lu, Non-contact atomic force microscopy characterization of micro-cantilevers and piezo electric transducers with frequencies up to the tens of MHz, Sensors and Actuators A: Physical A 148, 306 (2008). [2] D.-L. Liu, D.-X. Ye, F. Khan, F. Tang, B.K. Lim, R.C. Picu, G.-C. Wang, T.-M. Lu, Mechanics of patterned helical Si springs on Si substrate, Journal of Nanoscience and Nanotechnology 3(6), 492 (2003).
4:30 PM - **FF7.6
Critical Issues in Making Small-Depth Mechanical Property Measurements by Nanoindentation with Dynamic Stiffness Measurement.
George Pharr 1 2 , Jeremy Strader 1 , Warren Oliver 3
1 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 , Agilent Technologies, Oak Ridge, Tennessee, United States
Show AbstractDynamic stiffness measurement (DSM), often referred to as continuous stiffness measurement (CSM), is a technique used commonly in nanoindentation to measure hardness and elastic modulus continuously as a function of depth as the indenter is loaded into the specimen. To apply the technique, a small, sinusoidally-varying oscillation is applied to the primary loading signal, and the amplitude of the resulting displacement oscillation at the same frequency is measured by means of a lock-in amplifier. For elastic contact, the ratio of the load to displacement oscillation amplitudes provides a measure of the contact stiffness, which can then be used to determine the hardness and elastic modulus by standard methods of analysis. However, we have recently realized that for materials with a high modulus-to-hardness ratio, e.g., soft metals, CSM techniques can produce significant errors in the measured properties at depths of indenter penetration of 100 nm or even greater. In this presentation, the origin of these effects is documented by means of experiments conducted in a soft copper single crystal, and a model is developed that allows the effects to be quantified. By correcting the data in accordance with model and performing measurements at smaller displacement oscillation amplitudes, the errors can be significantly reduced. The errors are particularly important in characterizing the indentation size effect in metals.*** Research sponsored by the National Science Foundation under contract CMMI-0800168 (GMP & JHS) and by the Division of Materials Sciences and Engineering, US Department of Energy (GMP).
5:00 PM - FF7.7
An Analytical Solution for the Stress Field around an Elastoplastic Spherical Indentation/Contact.
Gang Feng 1 , Timothy Montalbano 1
1 Mechnical Engineering, Villanova University, Villanova, Pennsylvania, United States
Show AbstractContact Mechanics becomes a scientific subject since Hertz published his seminal paper “On the Contact of Elastic Solids” in 1882. The elastic spherical contact is commonly called Hertzian contact, and the associate closed-form stress field solution is called Hertzian field and widely used in analyzing many contact-induced phenomena, such as fracture, delamination, and dislocation activities. However, due to the strong local stress concentration, the materials under the contact may deform plastically, inducing a stress field significantly deviated from Hertzian field, and the rigorous closed-form analytical solution is not available for elastoplastic spherical contact. In our previous paper [Feng et al., Acta Materialia, 2007, p2929], it was found that, to estimate the stress field outside the plastic zone around a conical indentation, the effect of the plastic zone is equivalent to an embedded center of dilatation (ECD). In this study, the same equivalence, namely plastic zone = ECD, is found to be satisfied for the elastoplastic spherical indentation. Consequently, at the fully loaded state, the indentation stress field around an elastoplastic spherical indentation can be estimated by the superposition of a Hertzian field and an ECD field, while the residual stress can be estimated by the ECD field alone. The analytical solution based on the ECD model matches the finite element analysis (FEA) very nicely. For an elastic-perfectly-plastic material of yield strength equal to Y, the strength and depth of the ECD are found to be functions of a dimensionless parameter: a*Er/(Y*R), where a is the contact radius, R is the radius of the spherical indenter, and Er is the indentation reduced modulus. Due to the simple closed-form expression, the model can be used to analyze the elastoplastic-spherical-contact-induced phenomena and provide valuable physical insights for spherical nanoindentations.
5:15 PM - FF7.8
Lattice Misorientation Patterns and Strain Gradient Effects in Single Crystals under Spherical Indentation.
Yanfei Gao 1 2 , Bennett Larson 3 , George Pharr 1 3
1 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 2 Computer Science and Mathematics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States
Show AbstractThe ability to quantitatively predict dislocation microstructure and its evolution on mesoscopic length scales is a critical step towards developing a mechanistic understanding of small scale crystal plasticity. Despite the fact that many scale-dependent plasticity theories are based on some measure of the dislocation microstructure, efforts to compare predictions of these theories with experiments have generally been limited to qualitative features such as stress-strain curves and hardness measurements. A unique opportunity that can resolve some of these difficulties is provided by the capability to measure the near-surface dislocation microstructure by the 3D x-ray structural microscopy. Experimentally, the three-dimensional measurements of lattice rotation and elastic strain fields with sub-micron resolution can be used to compute the lattice curvature and the dislocation density tensor. The modeling effort adopts a strain-gradient crystal plasticity theory in which the extra hardening to the slip strength arises from the geometrically necessary dislocations. Experimental/modeling comparisons suggest that when the ratio a/R (with contact radius a and indenter radius R) is less than about 0.2, the lattice rotation pattern is determined primarily by the crystallographic orientations, slip systems, and indenter shape, but is insensitive to the gradient effects. The magnitude of lattice misorientation angle can be estimated from the ratio a/R for spherical indenters or the pyramidal indenter angle.
5:30 PM - FF7.9
Microcompression as a Quantitative Technique: A Case Study on Fused Silica.
Erica Lilleodden 1 , Alfred Cornec 1
1 Institute of Materials Research, GKSS Research Center, Geesthacht Germany
Show AbstractMicrocompression testing has quickly become a widely used technique for studying small-volume deformation. However, there remains a need for better assessment of how quantitative it is as a mechanical characterization method. The relatively low yield stress, inherently inhomogeneous nature of crystal plasticity and the common observation that the absolute dimensions of the sample influence the behavior of metals complicates the validation of the testing method on metal samples. Fused silica, on the other hand, serves as a model material to assess the accuracy of microcompression testing due to its high hardness, largely elastic response, and isotropic properties. Through an iterative comparison of experiments and finite element simulations we have assessed the accuracy of the microcompression test method in the measurement of elastic modulus, and in turn have identified an appropriate microcompression testing protocol.
5:45 PM - FF7.10
Measuring the Elastic Modulus of Anisotropic Materials using Micro-compression Testings.
Gan Yixiang 1 , Daniel Kaufmann 1 2 , Ruth Schwaiger 2 , In-suk Choi 1
1 IMF II, Forschungszentrum Karlsruhe, Eggenstein-Leopoldshafen Germany, 2 IZBS, University of Karlsruhe, Karlsruhe Germany
Show AbstractMicro-compression testings have been used extensively in the last few years as an effective methodology to investigate size effects in the mechanical properties of materials. While a number of studies have focused on measuring flow stress, extracting elastic modulus was largely ignored because the uncertainty of the column and substrate geometries during micro-compression testing results in significant errors. However, evaluating elastic properties using micro-compression testings is of interest since the Young’s modulus can be a good validation of the actual micro-compression testing set-up. Furthermore, the methodology may be extended to determine the elastic modulus for a particular crystallographic orientation of anisotropic materials at small scales, which is not possible using conventional nanoindentation. In this study, we performed both computational and experimental studies of micro-compression testings on the micro-columns of various materials with different aspect ratios. Using anisotropic FE simulation, the micro-columns were elastically deformed on the substrate of the same material. The parametric simulation study shows that the Young’s modulus can be underestimated by up to 40% depending on the materials tested and on the aspect ratio of the micro-columns. Applying the Sneddon’s equation, which accounts for the compliance of the substrate, still significantly underestimates the elastic response. Hence, the proper adjustment of the substrate compliance is essential to measure the Young’s modulus using micro-compression testings. Micro-compression experiments were also performed on a number of micro-columns made of various single crystalline materials with different aspect ratios using a nanoindenter. The results are consistent with the FE simulation
Symposium Organizers
Jun Lou Rice University
Brad Boyce Sandia National Laboratories
Erica Lilleodden GKSS Forschungszentrum
Lei Lu Chinese Academy of Sciences
FF8: Mechanical Behavior of Nanocrystalline, Nanoporous & Nanotwinned Materials I
Session Chairs
Thursday AM, December 03, 2009
Room 304 (Hynes)
9:30 AM - FF8.1
Microstructure, Mechanical Behavior and Deformation Mechanisms of Nanocrystalline Ni-50wt%Fe.
Steven Van Petegem 1 , Julien Zimmermann 1 , Stefan Brandstetter 2 1 , Xavier Sauvage 3 , Marc Legros 2 , Bernd Schmitt 4 , Helena Van Swygenhoven 1
1 NUM/ASQ, Paul Scherrer Institut, Villigen Switzerland, 2 , CEMES-CNRS, Toulouse France, 3 Faculte des Sciences, University of Rouen, Saint-Etienne du Rouvray France, 4 Swiss Light Source, Paul Scherrer Institut, Villigen Switzerland
Show AbstractIt is well known that the strength of metals increases with decreasing grain size, a behaviour that is well described by the phenomenological Hall-Petch relation down to grain sizes of 100 nm and even lower. Deformation mechanisms in coarse grained metals are based on a dislocation mechanism where dislocations are created during deformation with their propagation and multiplication being an essential mechanism for the resulting ductility and strength. However with decreasing grain size a transition towards grain boundary dominated plasticity mechanisms is expected.Molecular dynamics simulations suggest that in nanocrystalline materials slip is generated at the grain boundaries (GB); dislocations nucleated at GBs propagate through the grains and are absorbed at the opposing GB, leaving no dislocation debris in the grain interiors. In order to understand the elastic and plastic deformation properties of nanocrystalline metals we have developed an in-situ synchrotron x-ray-diffraction technique which allows the simultaneous measurement of many diffraction peaks continuously during mechanical testing, providing a direct link between the evolving microstructure and macroscopic mechanical data (Rev. Sci. Instr. 77 (2006) 013902). Here we present recent results obtained for electrodeposited Ni-Fe with a nominal iron content of 50%. The microstructure of this alloy is characterized by a narrow grain size distribution with a mean grain size of 10nm. Atom probe tomography and electron energy loss spectroscopy indicate that the NiFe matrix is nearly randomly ordered without significant segregration at the grain boundaries. Geometrical phase analyses show that local strain gradients are present in most of the grains, especially near the grain boundaries. X-ray diffraction reveals the presence of high internal strain variations and a significant <100> texture.Uni-axial mechanical tensile tests indicate that Ni-Fe exhibits a transient regime after reloading or strain rate jump tests. This regime is characterized by an upper or lower yield point, which is not observed during continuous loading. Furthermore the strain rate sensitivity is relatively low. In-situ x-ray diffraction experiments reveal strong relaxation effects during initial loading in the microplastic regime. In contrast to nanocrystalline Ni (Adv. Mater. 18 (2006) 1545) no increase of the elastic inhomogeneous strain variations after loading in the macroplastic regime was observed.
9:45 AM - FF8.2
Grain Boundary Shear-migration Coupling: From Experiments to Modelling.
Frederic Mompiou 1 , Marc Legros 1 , Daniel Caillard 1
1 , cemes/cnrs, toulouse France
Show AbstractGrain boundary (GB) migration has been considered recently as an alternative deformation mode to dislocation mechanisms in nanocrystalline and ultra-fine grain metals. However, because of the difficulty of exploring plasticity mechanisms in very small crystallites, this mechanism has never received a clear confirmation. To investigate this phenomenon, we have performed a series of dynamical observations in a TEM on fine grained Al polycrystals subjected to an applied stress. They show the fast migration of general high angle GB in response to the stress. A shear strain, smaller than expected by existing models relying on peculiar GB, has been measured thanks to markers trapped in the specimen and image correlation analysis. We propose a general model able to describe the shear-migration coupling for a given GB. The Shear MIgration Geometrical (SMIG) model consists in finding the multiple ways two adjacent lattices are related by a combination of a rotation and a shear. For a given GB plane, misorientation angle, and shear strain, the migration distance can then be calculated. This model, compatible with small values of the shear strain as experimentally observed, gives a suitable description of GB migration in polycrystals.
10:00 AM - **FF8.3
Interface Effects on the Mechanical Properties of Nanocrystalline Nanolaminates.
Alan Jankowski 1 , H.S. Tanvir Ahmed 1
1 Mechanical Engineering, Texas Tech University, Lubbock, Texas, United States
Show AbstractNanocrystalline nanolaminates are widely used in the study of physical properties in order to engineer materials for a variety of industrial applications. Often, novel and interesting mechanical behaviours that are found in nanolaminate materials can be linked with two characteristic features of structure. These are the layer pair spacing (hlp) and the grain size (hg). For the case of nanolaminates synthesized by physical vapor deposition processes, the layer spacing corresponds with the composition wavelength to the repeating sequence of layer pairs and the grain size to the width of the columnar structure along the growth direction. The mechanical properties of strength and hardness are known to functionally vary with the separation between dislocations in crystalline materials. Hence, both structural features contribute to the number of interfaces, the interfacial area, and the characteristic separation of interfaces that mitigate dislocation motion. In this investigation, the contribution of layer pair spacing and grain size to the total interfacial structure are each quantified in an assessment of strength and hardness. A model is proposed that quantifies the interfacial area (Ai) of the material volume under plastic deformation. This interfacial area (Ai) accounts for the contribution from the layer pairs (Alp) as well as the grain size (Ag). It is found that each structural feature may dominate the plastic deformation of the nanolaminate as dependent upon the specific layer pair spacing, the grain size, and the volume of material under deformation. Sample cases will be evaluated for different metallic nanolaminate systems and the implications on the assessment of hardness and strength. This work was supported through the J.W. Wright Endowment for Mechanical Engineering at Texas Tech University.
10:30 AM - FF8.4
Deformation Crossover: From Nano to Meso Scales.
Xun-Li Wang 1 , Sheng Cheng 2 , Alexandru Stoica 1 , Joe Horton 3 , Chain Liu 3 4 , Yang Ren 5 , Jon Almer 5 , Donald Brown 6 , Bjorn Clausen 6 , Peter Liaw 2 , Liang Zuo 7
1 Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States, 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 4 Mechanical Engineering Department, Hong Kong Polytechnic University, Hong Kong China, 5 X-ray Science Division, Argonne National Laboratory, Argonne, Illinois, United States, 6 Los Alamos Neutron Science Center, Los Alamos National Laboratory, Oak Ridge, New Mexico, United States, 7 Key Laboratory for Anisotropy and Texture of Materials, Northeast University, Shenyang China
Show AbstractRecent studies of the mechanical behavior of nanocrystalline metals have generated considerable debate over the deformation mechanisms at small length scales. Using in-situ synchrotron and neutron diffraction, we carried out a systematic study of tensile deformation in Ni over a broad range of grain sizes. The experimental data show that unlike in coarse-grained metals, where the deformation is dominated by dislocation slip, plastic deformation in nanocrystalline Ni is mediated by grain-boundary activities, as evidenced by the lack of intergranular strain and texture development. For ultrafine-grained Ni, although dislocation slip is an active deformation mechanism, deformation twinning also plays an important role, whose propensity increases with the grain size.
10:45 AM - FF8.5
De-twinning Mechanisms of Growth Twins in Face Centered Cubic Metals.
Jian Wang 1 , Nan Li 2 , Osman Anderoglu 2 , Xinghang Zhang 2 , Amit Misra 1 , John Hirth 1
1 , LANL, Los Alamos, New Mexico, United States, 2 , Texas A&M, College Station, Texas, United States
Show AbstractIn nanotwinned (nt) fcc metals, the presence of high density coherent twin boundaries can strengthen materials, without losing ductility, by acting as strong barriers for slip transfer. Using in-situ transmission electron microscopy, we observed rapid migrations of coherent and incoherent twin boundaries in epitaxial nt Cu, and found that very thin nanotwins are unstable and can be reversed through de-twinning process, which is accomplished via a collective glide of multiple twinning dislocations (TDs). Molecular dynamics simulations show that de-twinning becomes the dominant deformation mechanism when the thickness of twins is on the order of a few nanometers, and results in the annihilation of twin boundaries in rolled highly textured nt Cu films, and softening in ultrafine-grained nt Cu.
11:30 AM - FF8.6
New Mechanisms for Ultrahigh Strength and Ductility in Nano-Twinned Metals.
Zhaoxuan Wu 1 , Yong-Wei Zhang 2 3 , David Srolovitz 3 4
1 NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, Singapore Singapore, 2 Materials Science and Engineering, National University of Singapore, Singapore Singapore, 3 , Institute of High Performance Computing, Singapore Singapore, 4 Department of Physics, Yeshiva University, New York, New York, United States
Show AbstractUltra-fine polycrystalline metals with growth nano-twins exhibit simultaneous ultrahigh strength and ductility. We study the plastic deformation of such materials through molecular dynamics simulations. Based upon these simulations, we trace the sequence of dislocation events associated with the initiation of plastic deformation, dislocation interaction with twin boundaries, dislocation multiplication and deformation debris formation. We report two new dislocations mechanisms that explain the observation of both ultrahigh strength and ductility found in this class of microstructures. First, we observe the interaction of a 60o dislocation with a twin boundary that leads to the formation of {100}<110> Lomer dislocation which, in turn, dissociates into Shockley, stair-rod and Frank partial dislocations. Second, the interaction of a 30o Shockley partial dislocation with a twin boundary generates three new Shockley partials during twin-mediated slip transfer. The generation of a high-density of Shockley partial dislocations on several different slip systems contribute to the observed ultrahigh ductility while the formation of sessile stair-rod and Frank partial dislocations (together with the presence of the twin boundaries themselves) explain observations of ultra-high strength. Our simulation highlights the importance of interplay between the carriers of and barriers to plastic deformation in achieving simultaneous ultrahigh strength and ductility.
11:45 AM - FF8.7
Nanotwins and Partial Dislocations in Ultrafine-grained Gold Wires.
Effie. Y. H. Chew 1 5 , Hui Kim Hui 2 , C. Ferraris 3 , Yong Hao Zhao 4 , E. Lavernia 4 , Johnny Yeung 5 , Chee Cheong Wong 1
1 5School of Materials Science and Engineering, Nanyang Technological University, Singapore Singapore, 5 , Heraeus Materials Singapore Pte Ltd, Singapore Singapore, 2 , Institute of Materials Research and Engineering, Singapore Singapore, 3 , Laboratoire de Minéralogie, Paris France, 4 , University of California Davis, California, California, United States
Show AbstractSimultaneous high ductility and strength is a much sought-after goal by researchers studying mechanical properties of nanocrystalline (NC) and ultrafine-grained (UFG) materials. As predicted from both computational and experimental studies, dislocation activity and twinning are known to behave differently in NC and UFG materials vis-à-vis conventional coarse-grained (CG) materials.Firstly, in materials having grain size < ~ 50 nm, partial dislocations are believed to become activated in face-centered cubic (FCC) metals, that grains may be sheared by partial dislocations, which are absorbed in the opposite grain boundaries (GB), leaving an intrinsic stacking fault (SF) behind. This is so-called the partial dislocation mediated process (PDMP). In the next regime (grain size between ~50 nm and ~1000 nm), deformation is believed to take place via conventional unit dislocations, i.e. the trailing partial can be nucleated before the entire grain is sheared by the leading partial. The major difference of this regime with traditional CG metals is that the lattice dislocations are nucleated from the GB, while both boundary and intragranular sources are important in traditional metals. Besides deformation mechanisms, twinning effect also change as a function of grain size. Twinning is harder to occur in metals with finer grain size, as the twinning stress gets higher with reduced grain size. However, molecular dynamics simulation has suggested a transition of twin nucleation mechanism, from pole mechanism to partial dislocation initiated, as grain size gets into the very fine size regime. Also, twinning was reported to give rise to simultaneous high ductility and strength, and is therefore receiving much attention.In this work, large amount of localized partial dislocations and twins, generating from the GB, are observed on UFG Gold (Au) wire. This shows that PDMP has taken place. Our grain size range of 100-500 nm is much higher than the suggested boundary for PDMP, which is between 10 to 50 nm. Here, the smallest possible nanotwins are observed, consisting of only two atomic layers (~ 0.12 nm) at each side of the twin plane. Detailed characterization of the types of partial dislocations revealed that the partial are of both Shockley and Frank type. It is seen that twinning density in the UFG Au wires gets higher with more addition of calcium inside the Au wires. It is known that in CG metals, dopants that segregate to GB can create ledges which are conducive to dislocation generation. Similar scenarios can happen in UFG or NC materials, but probably partial dislocations instead of full dislocations shall be emitted, as dictated by the grain size, eventually giving rise to twinning. It is postulated that dopant that can preferentially segregate to the GB shall lead to the proliferation of SF and twin.
12:00 PM - FF8.8
Yielding and Plasticity of Periodically-Twinned Nanowires in FCC Metals from Molecular Dynamics Simulations.
Chuang Deng 1 , Frederic Sansoz 1
1 School of Engineering and Materials Science Program, The University of Vermont, Burlington, Vermont, United States
Show AbstractMetal nanowires with a diameter of tens of nanometers have attracted considerable attention in recent years because of their ultra-high strength in comparison to conventional bulk metals. This work poses a fundamental question: can microstructural design, both internal and external, give rise to ideal strength in realistic metal nanowires? Here, the underlying mechanisms for yielding and plastic deformation in various FCC metal nanowires, such as Au, Cu, Ag, Al, Ni and Pb, containing a periodic arrangement of (111) coherent twin boundaries along the axis are investigated in detail by molecular dynamics simulations. High plastic flow and strong size effects from the sample diameter and twin boundary spacing are discovered in twinned Au nanowires. It is demonstrated, however, that the stacking fault energy significantly influences the plastic behavior of FCC metal nanowires by changing the stress required for dislocation nucleation and that for penetration of dislocations through twin boundaries. These findings may reconcile some conflicting observations made in the past regarding the mechanical behavior of metal nanowires with nanoscale twins. Furthermore, it is shown that surface faceting in Au nanowires results in a novel yielding mechanism associated with the nucleation and propagation of full dislocations along {100}<011> slip systems, instead of the common{111}<112> slip observed in FCC metals, which enables increased strength.
12:15 PM - FF8.9
Maximum Strength of Copper with Nano-scale Twins.
Lei Lu 1
1 SYNL, Institute of Metal Research, Shenyang China
Show AbstractBelow a certain critical size (in the nanometre scale), a softening phenomenon is expected by atomistic simulations, which suggested the grain boundary (GB) related mechanisms will be the dominating deformation mechanism. Nevertheless, such a critical grain size of maximum strength has never been identified experimentally so far. Coherent twin boundaries (TBs), a special kind of low-energy boundary, are known to be as effective as conventional GBs in strengthening materials. Here we provide a discovery of a critical twin thickness in Cu: the highest strength in the nano-twinned Cu was observed at 15 nm, which is followed by a rapid softening at smallertwin thickness and a significant enhancement in both the strain-hardening and tensile ductility. The critical twin thickness relates to a transition in the yielding initiated from a mechanism by the slip transfer across TBs to a mechanism by the motion of pre-existing mobile dislocations. The finding reveals the scale-dependent nature of plastic deformation of materials in the nanometre scale.
12:30 PM - FF8.10
Temperature-Induced Amorphization in Periodically-Twinned III-V Zinc Blende Nanowires During Tensile Loading.
Rassin Grantab 1 , Vivek Shenoy 1
1 Mechanics of Solids, Brown University, Providence, Rhode Island, United States
Show AbstractUsing molecular dynamics simulations, we have studied the tensile behavior of periodically-twinned GaAs zinc blende nanowires at 300K and 1200K, and have observed two completely different modes of deformation and failure. The concave edges of the wire where two {111} surface-facets intersect present a stress singularity of power 0.35; during deformation at 300K, these singular regions serve as nucleation sites for cracks, which propagate across the wire in a brittle fashion, causing complete failure at roughly 12% strain. Similar brittle fracture behavior is observed at 600K and 900K, however, at 1200K, crack nucleation and propagation is no longer observed. Instead, amorphization occurs at the singular regions and spreads across the nanowire through coalescence and growth of smaller amorphous regions. As these regions spread along the nanowire, the {111} surface-facets disappear and the wire begins to lose any semblance of crystal structure, as evidenced by the radial distribution function. The amorphous wires fail at strains exceeding 120%, in a manner resembling the failure of a viscous material. We have concluded that the amorphization of GaAs nanowires requires both high temperature and strain, and have not observed this behavior in the absence of one of those elements. It is believed that this behavior occurs at elevated temperatures because the combination of increased thermal energy and strain energy exceeds the bulk diffusion barrier in GaAs, thereby allowing atoms to migrate from their crystallographic sites. Previous studies have shown that amorphization in metallic nanowires are strain-rate-induced, but this was not observed for GaAs; on the contrary, reducing the strain-rate increased the degree of amorphization in GaAs due to the increase in the amount of time available for atomic diffusion.
12:45 PM - FF8.11
On the Role of Nanoscale Twins in the Deformation of fcc Metals: A Competition of Hardening or Softening Mechanisms.
Karsten Albe 1 , Alexander Stukowski 1 , Diana Farkas 2
1 Materials Modeling Division, Institute of Materials Science, Darmstadt Germany, 2 Materials Science and Engineering, VirigniaTech, Blacksburg, Virginia, United States
Show AbstractThe strengthening effect of twins in nanocrystalline metals has been reported both in experimental and simulation work and is explained by the fact that twins are effective barriers to slip transfer of dislocations. However, it is also possible that the twins provide nucleation sites for dislocations or migrate during the deformation process, contributing to plasticity. The interplay among these various effects of the twin boundaries may result in the fact that the twins do not always harden fcc metals.Here we present a comparative atomistic study of the effect of twins on the deformation behavior of nanocrystalline Cu, Al and Pd. Identical samples of 20 nm grain size were tested with and without twins grown in at various twin spacings. While Cu shows hardening from the twins, Pd and Al show the opposite effect. The results provide evidence that slip transfer across the twin boundary is much easier in Pd than in Cu, due to the smaller dissociation distances for dislocations in Pd. However, in case of Pd and Al, the twin planes provide additional dislocation sources for twinning dislocations, which is the main reason for softening of these samples as compared to identical nanocrystals without twins.
FF9: Mechanical Behavior of Nanocrystalline, Nanoporous & Nanotwinned Materials II
Session Chairs
Thursday PM, December 03, 2009
Room 304 (Hynes)
2:30 PM - **FF9.1
Mechanical Behaviors of Bulk Nanostructured Metals Synthesized by Means of Dynamic Plastic Peformation (DPD).
Ke Lu 1 , Nairong Tao 1
1 SYNL, Institute of Metal Research, Shenyang, Liaoning, China
Show AbstractBy means of plastic deformation at high strain rates and at cryogenic temperatures, referring to as dynamic plastic deformation (DPD), bulk nanostructured samples have been synthesized in a number of metals and alloys, consisting of nano-sized grains (with an average size below 100 nm) and nano-scale twin bundles. The nanostructured samples exhibit superior mechanical behaviors such as high strength and high fracture toughness. Mechanical behaviors of the nanostructured samples will be introduced with emphasis on effects of volume fraction of nano-twins, twin thickness, and grain sizes on the mechanical properties. Optimization of strength-ductility combination of the bulk nanostructured materials will be addressed by tailoring the microstructure via proper thermal and mechanical treatments.
3:00 PM - FF9.2
High-strain Rate Behavior of Nanocrystalline Tantalum Processed by High Pressure Torsion.
Zhiliang Pan 1 , Xiaolei Wu 2 , Brian Schuster 3 , Laszlo Kecskes 3 , Qiuming Wei 1
1 Mechanical Engineering, University of North Carolinat at Charlotte, Charlotte, North Carolina, United States, 2 , Institute of Mechanics, CAS, Beijing China, 3 , US Army Research Lab, Aberdeen Proving Ground, Maryland, United States
Show AbstractNanocrystalline (NC grain size < 100 nm) metals exhibit some fascinating properties, particularly very high strengths under quasi-static loading (strain rates below 10-1 s-1 ) due to the presence of large population of grain boundaries which greatly impede dislocation activities, as well as the very small grain size which disallows the multiplication of dislocations through the conventionally well known Frank-Read source. In the past two decades, a great amount of work has been undertaken to understand the mechanical properties and their relation to the microstructure of NC metals, with tremendous progress. However, not much has been known about the dynamic high-strain rate mechanical behavior of such materials. Recent experimental results have shown that many nanocrystalline and ultrafine grained body centered cubic metals exhibit adiabatic shear localization under impact loading. This is explained by the vanishing strain hardening and reduced strain rate hardening, combined with the much elevated strength. However, the smallest grain size achieved thus far has been at best in the upper bound of the nanocrystalline regime. No work has been available on the dynamic mechanical behavior of fully-dense, truely nanocrystalline metals. In this work, we have examined the dynamic response (i.e., strain rate >103 s-1) of nanocrystalline tantalum (Ta) (grain size ~40 nm), processed by high-pressure torsion. To test the very small dynamic samples (~1.0 mm thick) at high strain rates, we used a miniaturized Kolsky bar (or Split-Hopkinson Pressure Bar) system, for which the input and output bars are made of high-strength maraging steel of 5 mm in diameter. Combined with the quasi-static testing results, we have found that the NC Ta has increased strain rate sensitivity compared to its coarse-grain counterpart. Unlike other typical body-centered cubic NC metals such as Fe, V, or W which exhibit strong tendency to adiabatic shear localization under dynamic compression, NC Ta dynamic specimens deform in a uniform manner. This is explained by the persistent strain hardening and the increased strain rate sensitivity of the NC Ta.
3:15 PM - FF9.3
Effect of Annealing on the Residual Stress in Electrodeposited Nanocrystalline Ni-W Alloys.
Tiffany Ziebell 1 , Christopher Schuh 1
1 Dept. of Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractCharacterizing the residual stress of nanocrystalline electrodeposits poses several unique challenges due to the inherent fine grain structure, non-uniform deposit, and matte surface. This talk will describe a profilometry-based approach that addresses each of these complicating factors and enables accurate quantitative analysis of residual stresses. The specific emphasis of this work is nanocrystalline Ni-W electrodeposits, in which residual stresses arise from both the deposition process and post-deposition heat treatments. These stresses are associated with warping, delamination, and reduced fatigue strength of a coated component. The present measurements offer quantitative insight into the mechanisms of stress development and evolution in these materials. Additionally, the technique developed here can be extended to evaluate more complicated residual stress states in multi-layer and functionally graded nanocrystalline electrodeposits of commercial relevance.
3:30 PM - FF9.4
Atomic Scale Studies of Micro-Mechanisms related to Dynamic Failure in Ultra-Fine Grain Sized Nanocrystalline Cu.
Avinash Dongare 1 2 , Arunachalam Rajendran 3 , Bruce LaMattina 4 , Mohammed Zikry 2 , Donald Brenner 1
1 Materials Science and Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina, United States, 3 Mechanical Engineering, University of Mississippi, Uinversity, Mississippi, United States, 4 , U. S. Army Research Office, Research Triangle Park, North Carolina, United States
Show AbstractNanocrystalline metals have been extensively studied for their high strengths and mechanical properties as compared to their polycrystalline counterparts comprised of larger grains. The plastic deformation mechanisms of nanocrystalline materials depend on the interplay between dislocation and grain boundary processes. Dislocation based processes dominate at larger grain sizes; whereas grain boundary (GB) based processes (sliding) dominates at smaller grain sizes. Shock loading of these ultra-fine nanocrystalline metals with ultra-fine grain sizes (< 20 nm) at speeds that are greater than the speed of sound, limits the grain-boundaries sliding mechanism, and results in ultra-high strength values for the nanocrystalline metal. The resistance of nanocrystalline metals to dynamic failure can be understood based on the material response to plastic deformation and failure. While there has been significant progress in the understanding the deformation mechanisms of nanocrystalline metals, the understanding of failure mechanisms at high strain rates is still at a stage of the initial exploration. To understand the dynamic failure of nanocrystalline metals (spallation), Large-scale MD simulations are also carried out to study the micromechanisms related to nucleation, growth, and coalescence, of voids for conditions of deformation that lead to the onset of spallation during shock loading. The effect of shock pressure, strain rates, and grain size on the spall strength and microscopic failure mechanisms as obtained from MD simulations will be discussed.
4:30 PM - **FF9.6
The Mechanical Behavior on Nanoporous Au: Tailoring Properties by Changing Length Scale.
Eike Epler 1 , Burkhard Roos 1 , Cynthia Volkert 1
1 Institite of Material Physics, University of Göttingen, Göttingen Germany
Show AbstractNanostructured materials often exhibit unusual stability, strength, and toughness due to their size. In the case of nanoporous Au with ligament and pores sizes between 10 and 100 nm, higher strengths and lower toughnesses are measured than expected from the classical scaling laws for foams. At a given porosity, the strength and toughness can be changed by annealing the nanoporous Au to coarsen its structure. In this presentation, the increased strength is described in terms of constrained dislocation activity in small volumes and is compared with in-situ TEM observations of dislocations during deformation of single crystal Au nanowires. The low toughness is explained by the fact that the plastically deforming volume at the crack tip is limited by the ligament diameters. The effect on toughness and strength of infiltrating the nanoporous Au with polymer is also discussed.
5:00 PM - FF9.7
The Micromechanisms of Deformation in Nanoporous Gold.
Rui Dou 1 , Brian Derby 1
1 School of Materials, University of Manchester, MANCHESTER United Kingdom
Show AbstractWe have fabricated arrays of nanoporous gold nanowires by the electrodeposition of Au-Ag alloys into anodized aluminum oxide templates followed by dealloying. Individual nanowires have diameter in the range 30 - 70 nm, with a relative density of 20% and ligament diameter of 5 - 10 nm. They are sufficiently electron transparent to allow observation in the transmission electron microscope (TEM) without further thinning. On loading in compression the ligament yield stress is approximately 3 GPa and this is shown to be consistent with prior studies in the literature. We have carried out a TEM investigation of the micromechanisms of deformation in these nanoporous gold specimens after compression testing. We find that the nanoporous specimens show deformation localised to the nodes between the ligaments of the foamed structure, with very high densities of microtwins and Shockley partial dislocations in these regions. These deformation structures are very different from those seen after solid nanowires are tested in compression, which show very low dislocation densities and a few sparsely distributed twins. However, similar dislocation structures to those found in the nanoporous specimens are observed in the larger nanowires when they are deformed in bending. The currently accepted model for the deformation of nanoporous gold, implicitly assumes that the deformation of these structures is by bending near the nodes where ligaments intersect. We hypothesis that the much higher dislocation densities seen in both the nanoporous gold and the nanowires deformed in bending are evidence for the presence of geometrically necessary dislocations in these deformed structures.
5:15 PM - FF9.8
Mechanical Behavior of Gold Nanofoams using Atomistic Simulations.
Kedarnath Kolluri 1 , Michael Demkowicz 1
1 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractWe present atomistic simulations of a model Au nanofoam (nf-Au) using an EAM potential. A model nf-Au structure forms spontaneously upon relaxing a random distribution of atoms with a density of 20% of perfect crystalline FCC Au. The relaxed microstructure is polycrystalline and contains numerous defects in both the nanofoam ligaments and nodes. Annealing using Molecular Dynamics (MD) at 300 K for 0.812 ns causes the nanofoam to coarsen. During volume conserving uniaxial compression of the model nanofoam, ligaments are in a combined state of tension, shear, bending, and torsion. Peaks and valleys of the stress-strain curves after initial elastic loading correlate with the necking and pinch-off, respectively, of nanofoam ligaments. Similar mechanisms are found to operate during nanofoam coarsening.
5:30 PM - FF9.9
Anelastic Behavior of Nanocrystalline Nickel and Nanoporous Metals.
Nicolas Briot 1 , Jochen Lohmiller 1 2 , Christoph Eberl 2 , Oliver Kraft 2 3 , T. John Balk 1
1 Chemical & Materials Engineering, University of Kentucky, Lexington, Kentucky, United States, 2 Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, Karlsruhe Germany, 3 Institut für Materialforschung II, Forschungszentrum Karlsruhe, Karlsruhe Germany
Show AbstractAnelasticity, the time-dependent elastic behavior of materials, may arise from mechanisms including the interaction of dislocations with point defects, nucleation of kink/anti-kink pairs, or grain boundary diffusion. A home-built system equipped with laser vibrometer for sample tracking and vacuum chamber for minimizing air damping was used to study the anelastic behavior of free-standing cantilever samples during low amplitude loading. Internal friction peaks were identified near room temperature for nanocrystalline Ni samples, which appear to result from interactions between dislocations and point defects. Damping behavior was also studied after the samples had been annealed to achieve micron-scale grains. Additionally, nanoporous metals obtained by dealloying were investigated. The nanoscale, porous structure with high surface-to-volume ratio is expected to affect the internal relaxation mechanisms, as it does the nucleation and motion of dislocations. Results of these investigations will be presented and mechanisms for anelasticity in nanoscale metals will be discussed.
5:45 PM - FF9.10
Porosity Dependence of Elastic Constants of Nanoporous Silicon.
Gazi Aliev 1 , Bernhard Goller 1 , Paul Snow 1
1 Department of Physics, University of Bath, Bath United Kingdom
Show AbstractPorous silicon is produced by the electrochemical etching of crystalline silicon wafers, with the pore morphology depending on the doping of the silicon wafer. Using heavily boron-doped silicon wafers produces samples that have pore sizes of ~10 nm and a volume fraction of voids (porosity) between 25 and 85%. The porosity is determined by the etching current used and is continuously variable across this range. The functional dependence of the mechanical properties on porosity is important for device design but has not been extensively measured or adequately modelled. Indeed, porous silicon has frequently been approximated as an isotropic material. Hence, we have obtained the porosity dependence of the elastic constants of porous silicon by analysis of sound velocity measured in different crystallographic directions. The velocity of longitudinal acoustic waves in the (100) and (110) directions has been measured for a range of porosities, by microechography using 1 GHz transducers. This is a non-destructive method performed on layers with thickness of ~50 μm. The relations between the elastic constants and the acoustic velocity in different crystallographic directions are well known. The c_{11} elastic constant is directly obtained from the velocity measurement in the (100) direction. In porous silicon, the atomic configuration of the silicon skeleton retains the symmetry of bulk crystalline silicon. The mechanical properties of the silicon skeleton must depend on the innate silicon bonding modified by the nanoscale morphology. Using the (110) results and Keating’s relation between the elastic constants for cubic diamond-like anisotropic crystals, the elastic constants c_{44} and c_{12} have also been obtained as a function of porosity. We also note that our samples demonstrate a porosity dependence of the Young’s modulus that is very close to that obtained theoretically for the open cell foam-like porous materials.
FF10: Poster Session: Mechanical Behavior of Nanomaterials---Experiments and Modeling
Session Chairs
Brad Boyce
Erica Lilleodden
Jun Lou
Lei Lu
Friday AM, December 04, 2009
Exhibit Hall D (Hynes)
9:00 PM - FF10.10
Mechanical Behaviors of Nanoporous Carbon.
Mi Xi 1 , Yunfeng Shi 1
1 Materials Science and Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States
Show AbstractNanoporous carbon is widely used in industry for its attractive properties on transport, adsorption and reaction. However, the mechanical behavior of nanoporous carbon has received much less attention. We will present a systematic study on the porous structure-mechanical property relation in nanoporous carbon using large-scale molecular dynamics simulations. We have previously demonstrated a virtual synthesis route in which curved and defected graphene sheets are grown from monatomic carbon atoms at high temperatures [1]. Our nanoporous carbon models have excellent agreement with saccharose-based activated carbon in terms of density, chemical composition and pair-correlation function. This virtual synthesis process enables independent control over density and pore size distribution, and produces mechanically stable structures. Thus, nanoporous samples with prescribed porous structures can be prepared. For each nanoporous sample, the bulk modulus, Young’s modulus and Poisson’s ratio will be calculated. The yield strength will also be calculated through simulated nanoindentation tests. Both the elastic and plastic properties will be correlated with the porous microstructure. We will investigate whether a single porosity is sufficient to characterize its mechanical properties in Gibson-Ashby type power laws, or a higher order characteristic such as the pore size distribution has to be included.[1] Y. F. Shi, "A mimetic porous carbon model by quench molecular dynamics simulation", Journal of Chemical Physics, 128, 234707 (2008).
9:00 PM - FF10.11
Localized Characterization of Carbon Nanotubes and Carbon Nanotube Reinforced Nanocomposites using Novel Micromechanical Devices.
Yogeeswaran Ganesan 1 , Yang Lu 1 , Cheng Peng 1 , Hao Lu 1 , Roberto Ballarini 2 , Boris Yakobson 1 , Jun Lou 1
1 Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States, 2 Civil Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe knowledge of carbon nanotube (CNT) strength and fundamental mechanisms that govern mechanical behavior at the nanotube-matrix interface are critical for CNT reinforced nanocomposite development i.e. in order to realize the theoretically and computationally predicted potential of CNTs as reinforcements for high performance composites. We have recently developed a simple micro-fabricated device that could be used within a scanning electron microscope (SEM) chamber in order to perform in situ tensile tests of individual CNTs treated with different functional groups and nanoscale CNT pullout experiments using different matrices quantitatively. In this work, we report on the usage of such device that works in conjunction with a quantitative inSEM nanoindenter, for the in situ quantitative tensile testing of an individual multi-wall carbon nanotube (MWNT) and for an individual MWNT pullout experiment from an epoxy matrix. Load applied on the sample and sample deformation/displacement were ascertained using the indenter load vs. displacement curve. The values for the Young’s modulus and the tensile strength of an individual MWNT and the average interfacial shear strength (IFSS) of a MWNT-epoxy composite have been presented. The insights gained from this research could potentially help engineer superior CNT reinforced nanocomposites by enabling the development of powerful predictive models.
9:00 PM - FF10.12
Size Effects in Nanomechanical Behavior of Protein Materials.
Sinan Keten 1 , Markus Buehler 1
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractElasticity and strength of individual protein domains govern key biological functions and the mechanical properties of biopolymers including spider silk, amyloids and muscle fibers. The ultrastructure of protein materials consists primarily of regular structures such as alpha-helices and beta-sheets, stabilized by hierarchical assemblies of H-bonds (Ackbarow, Chen, Keten and Buehler, PNAS, 2007). Despite the weak nature of H-bond interactions, these materials combine exceptional strength, robustness, and resilience (Buehler, Keten, Ackbarow, Prog. in Mat. Sci., 2008). We show that the rupture strength of H-bond assemblies in beta-sheets is governed by geometric confinement effects, suggesting that clusters of at most 3-4 H-bonds break concurrently, even under uniform shear loading of a much larger number of H-bonds. This universally valid result leads to an intrinsic strength limitation that suggests that shorter strands with less H-bonds achieve the highest shear strength at a critical length scale. The hypothesis is confirmed by direct large-scale full-atomistic MD simulation studies of beta-sheet structures in explicit solvent (Keten and Buehler, Nano Letters 2008) as well as experimental evidence (Keten and Buehler, PRL, 2008; Keten and Buehler, PRE, 2009). Our finding explains how the intrinsic strength limitation of H-bonds can be overcome by the formation of a nanocomposite structure of H-bond clusters, thereby enabling the formation of larger and much stronger beta-structures as found in silks and muscle fibers. Our results agree well with experimental proteomics data, suggesting a correlation between the shear strength and the prevalence of beta-strand lengths in biology as well as typical H-bond cluster sizes in other structures such as alpha-helices and beta-solenoids.
9:00 PM - FF10.15
Computational Simulations of Blast Response of Biological Cell.
Guoxin Cao 1 , Namas Chandra 1
1 , university of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractTraumatic brain injury (TBI) caused by the blast wave resulting from primary explosions to head is one of the main casualties of military personnel. TBI can cause the short or long term effect on daily life and work of victims, such as debilitating cognitive deficits, severe headaches, depression, memory deficits and personality changes. The mechanism of TBI induced by blast wave is still not clear. One of the most possible reasons is the severe stretch of brain cells (neuron or glial cell) caused by blast wave. Therefore, it is very important to study the blast response at cellular level.In the present work, the blast response of cell is studied using numerical simulations. Cell is modeled as a two-phase composite system: Phase I comprising microfilaments and intermediate filaments (main components of cytoskeleton); Phase II is the cytoplasm. Since these filaments behave more like strings, Phase I can only resist tensile deformation. Phase I is modeled as a group of linear viscoelastic fibers with the same properties using standard linear solid (SLS) model. Phase II behaves like liquid and can resist only compressive deformation, and is simulated using a Maxwell model. The effects of the volume fraction and the diameter of viscoelastic fibers on blast response are also examined. The blast loading applied on cell surface is modeled by a pressure history: a compressive pressure pulse with less than 1ms duration and sharply reduce to zero. For the sake of reference, the blast response of homogeneous and shell-core cell models are also computed. The blast response of cells can be described by the following parameters: cell pressure, volumetric tension, Von Mises stress. The qusistatic behavior of cell is also shown based on AFM indentation with a flat tip and a similar load as the applied blast pressure. The results in this work show the blast response at cellular level and the effects of geometric cytoskeleton components on the dynamic behavior of biological cells, which can provide a useful guideline to understand the mechanism of blast induced TBI.
9:00 PM - FF10.16
Correlating Nanoparticle Dispersion to Surface Mechanical Properties of TiO2/Polymer Composites.
Yongyan Pang 1 , Stephanie Watson 1 , Aaron Foster 1 , Chang Kwon Moon 1 , Li-Piin Sung 1
1 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractThe main objectives of this study are to characterize the nanoparticle dispersion in polymeric matrices with different mixing chemistry conditions; and to correlate dispersion to surface mechanical properties of the nanopaticle-polymer system. Two types of TiO2 nanoparticles (with and without surface treatment) were chosen to mix in two polymeric matrices of different chemistries: water-borne butyl-acrylic styrene latex coating (Latex) and solvent-borne acrylic urethane (AU). Nanoparticle dispersion (cluster size and spatial distribution) was characterized using laser scanning confocal microscopy (LSCM). Overall, Particle A (without surface treatment) dispersed better than Particle B (with organic dispersant) in both systems and the Particle A/AU system exhibited the best dispersion state. Surface mechanical properties, such as hardness and Young’s modulus at micron and sub-micron length scales were determined from depth sensing indentation. The surface mechanical properties were strongly affected by the dispersion of nanoparticle clusters, and a good correlation was found between locations of the clusters near surface and the modulus-depth mapping.
9:00 PM - FF10.17
Temperature Variation in Energy Absorption System Functionalized by Nanomaterials.
Aijie Han 1 , Yu Qiao 2 , Weiyi Lu 2 , Venkata K. Punyamurtula 2 , Zhongyuan Sun 1
1 Chemistry, University of Texas-Pan American, Edinburg, Texas, United States, 2 Structural Engineering, University of California-San Diego, San Diego, California, United States
Show AbstractA high performance energy absorption system (EAS) is developed by using functionalized nanoporous material. The energy density associated with the volume variation can be higher than that of conventional EAS by 2-5 orders of magnitude. The temperature variation of the system during the energy absorption process of a MCM-41 (the nanopore size is nearly 1.9 nm) is analyzed in considerable detail. It is observed that in the nanopores the energy exchange between solid and liquid phases is dependent on the direction of liquid motion: liquid infiltration is exothermic and liquid defiltration is endothermic. The temperature increases more profoundly in infiltration than it decreases in defiltration, leading to a net temperature change around 2.2 oC per loading-unloading cycle, fitting well with the overall energy dissipation measurement. This phenomenon is fundamentally different from the nanofluidic properties in nanopores larger than 5 nm, where no temperature variation can be detected as the confined liquid moves.
9:00 PM - FF10.18
Mechanics of Silica-PDMS Nanocomposites.
Adrian Camenzind 1 , Thomas Schweizer 2 , Michael Sztucki 3 , Sotiris E. Pratsinis 1
1 Particle Technology Laboratory, Mechanical & Process Engineering, ETH Zurich, Zurich Switzerland, 2 Institute of Polymers, ETH Zurich, Zurich Switzerland, 3 , European Synchrotron Radiation Facility (ESRF), Grenoble France
Show AbstractSilica nanoparticles of 50 to 300 m2/g specific surface area (SSA) were admixed into vinyl-terminated dimethylsiloxy monomer with a planetary mixer. The evolution of mechanical properties of uncured particle/monomer suspensions with increasing mixing duration was investigated with a rheometer (dynamic mode) focusing on the Payne Effect. Thin sections of cured nanocomposites obtained from a cryostate-microtome were analysed by TEM. Small and ultra-small angle X-ray scattering (SAXS/USAXS) was used for monitoring composite structure such as particle morphology, aggregate (chemically or sinter-bonded particles) and agglomerate (physically-bonded particles) size evolution as a function of mixing duration. The composite strength was analyzed with tensile test (Young’s modulus and elongation at break) and composite swelling experiments. The reinforcing ability of SiO2 particles was correlated to the “bound rubber” theory.
9:00 PM - FF10.19
Multiscale Computer Simulation of Tensile and Compressive Strain in Polymer-Coated Silica Aerogels.
Brian Good 1
1 Materials Division, Glenn Research Center, Cleveland, Ohio, United States
Show AbstractWhile the low thermal conductivities of silica aerogels have made them of interest to the aerospace community as lightweight thermal insulation, the application of conformal polymer coatings to these gels increases their strength significantly, making them potentially useful as structural materials as well. In this work we perform multiscale computer simulations to investigate the tensile and compressive strain behavior of silica and polymer-coated silica aerogels. Aerogels are made up of clusters of interconnected particles of amorphous silica of less than bulk density. We simulate gel nanostructure using a Diffusion Limited Cluster Aggregation (DLCA) procedure, which produces aggregates that exhibit fractal dimensions similar to those observed in real aerogels. We have previously found that model gels obtained via DLCA exhibited stress-strain curves characteristic of the observed brittle failure. However, the strain energetics near the expected point of failure were not consistent with such failure. This shortcoming may be due to the fact that the DLCA process produces model gels that are lacking in closed-loop substructures, compared with real gels. Our model gels therefore contain an excess of dangling strands, which tend to unravel under tensile strain, producing non-brittle failure. To address this problem, we have incorporated a modification to the DLCA algorithm that specifically produces closed loops in the model gels.We obtain the strain energetics of interparticle connections via atomistic molecular statics, and abstract the collective energy of the atomic bonds into a Morse potential scaled to describe gel particle interactions. Polymer coatings are similarly described.We apply repeated small uniaxial strains to DLCA clusters, and allow relaxation of the center eighty percent of the cluster between strains. We present energetics and stress-strain curves for looped and nonlooped clusters, for a variety of densities and interaction parameters.
9:00 PM - FF10.20
Investigation of the Mechanical Properties of High Nanoparticle Loading Fraction Polymer Nano-Composites.
Theodore Kramer 1 , Jeffrey Kysar 2 , Irving Herman 1
1 Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York, United States, 2 Department of Mechanical Engineering, Columbia University, New York, New York, United States
Show AbstractWe have made dense nanoparticle-polymer films and investigated their mechanical properties via nano-indentation and other methods. Electrophoretically deposited (EPD) films of cadmium selenide nanocrystals were infiltrated with various monomers that are subsequently polymerized. This hybrid material exhibits the desirable photoluminescence properties of the constituent CdSe nanocrystals but does not fracture, as do thick electrophoretically grown nanoparticle films. This may be the result of reducing strain in the films via void filling or the result of enhanced film toughness. The mechanical properties of these films differ from those of EPD films without the introduction of polymer, and provide a model system for the study of nano-composites at high nanoparticle loading fractions. The wide range of film thickness (0.1 - 2 micron) attainable could make this a useful class of material for optical and energy applications.
9:00 PM - FF10.21
Novel Nanostructured Metals by High Rate Severe Plastic Deformation.
Shashank Shekhar 1 , Jiazhao Cai 1 , Ravi Shankar 1
1 Department of Industrial Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Show AbstractHighly refined nanostructured materials created by imposing Severe Plastic Deformation (SPD) at low strain-rates in Equal Channel Angular Pressing, High Pressure Torsion etc. offer substantial improvements in weight specific strengths in an array of metal alloys. However, the resulting materials are usually bedeviled by limited ductility and thermal stability. Recently, we explored consequences of SPD, but over a much larger range of strain rates than conventional SPD, spanning several orders of magnitude ranging from 10^0 – 10^3/s using a machining-based approach. The ability to control strain-rate by modifying the deformation rate, in addition to the usual control of SPD strains, can engender intriguing nanostructural characteristics in two prototypical systems - low stacking fault energy (SFE) brass and medium SFE Cu. In a particular subset of these studies that involved High Rate Severe Plastic Deformation (HRSPD) characterized by strains >2 at rates > 10^3/s, we will demonstrate control not only over distributions of the size of the nanostructures, but also the distribution of the character of their interfaces. Manipulating these two attributes help design nano-scale microstructures that may help overcome the usual problems of ductility and stability. Our characterizations will also reveal how the interplay of strain and strain-rates in HRSPD intrinsically offers control over the in situ temperature rise in the deformation zone. This obviates the need for an explicit modification of deformation temperature by say, external thermal agitation that characterizes conventional deformation processing paradigms. These observations will also be shown to be interwoven with an overarching influence of HRSPD on the stored energy in the fine-grained system.In brass, the utility of HRSPD at modifying the density of nanotwins that strengthen and possibly stabilize the microstructure will be presented. In Cu, HRSPD can manipulate the grain boundary misorientation distribution as well as nanostructure size distribution. The control over the distribution of the size of the fine grains also involved the generation of unimodal and multimodal grain sizes, as and when needed, to control the resulting levels of ductility and toughness. Manipulation of the stored energies and the interface structure in nanostructured metals from HRSPD will be shown to offer clues for enhancing their stability.
9:00 PM - FF10.22
High-Throughput Optimization of Adhesion in Multilayers by Superlayer Gradient.
Sergey Grachev 1 , Coraly Cuminatto 1 , Alexander Mehlich 1 , Jan-Dirk Kamminga 1 , Elin Sondergard 1 , Etienne Barthel 1
1 , Saint-Gobain Recherche, Aubervilliers France
Show AbstractThin films under compressive stress exert a load on interfaces and might cause buckling driven delamination. This phenomenon has been studied in recent years in the view of application as an adhesion measurement technique (superlayer method). The load exerted onto an interface scales with the elastic energy stored in the compressed film, which is, in turn, proportional to the film thickness. We show that in this context thickness gradients are especially useful for high throughput adhesion measurements. In addition, the effect of subtle modifications of interface chemistry on adhesion can be assessed in a single test. Namely, we used this methodology for optimisation of the Ti interlayer thickness for adhesion at the SiO2/Ag interface. In the listed above experiments, the shape of the blisters observed was telephone-cord-like. The sizes of the telephone-cords depend on a number of parameters including the elastic properties of and the stress in the film before buckling. The connections between these values and the blister sizes are well established for simple case of a straight blister often formed when a film is loaded uniaxially. The telephone-cord blisters form at large biaxial compressions and known to relax the strain energy more efficiently than straight blisters. Using the gradient technique we were able to measure the blister sizes in a range of film thickness and quantify the relation between the size of the telephone-cords and the residual compressive stress, which was measured in-situ by the curvature method.
9:00 PM - FF10.23
Electrospun Polymer/MWCNTs Nanofiber Reinforced Composites.
Elif Ozden 1 , Yusuf Menceloglu 1 , Melih Papila 1
1 Material Science and Engineering, Sabanci University, Istanbul Turkey
Show AbstractThe focus of this study is to investigate electrospun nanofibers in reinforcing polymer to enhance mechanical behavior. Polystyrene-co-glycidyl methacrylate (PSt-co-GMA) and PSt-co-GMA with multiwalled carbon nanotubes (MWCNTs) composite nanofibers first were produced by electrospinning. The process optimization for electrospun Polystyrene-co-Glycidyl Methacrylate /MWCNTs was also investigated. An emprical relationship between polymer and MWCNTs concentration parameters and average fiber diameter was sought by response surface methodology (RSM). The nanofibers, were then embedded into epoxy matrix to form polymer composites. The experimental procedure was designed in order to see the effects of GMA composition in structure and the effect of additional crosslinker agent by spraying method. The effect of PSt-co-GMA and PSt-co-GMA/MWCNTs nanofibers in the composites was reported in comparison to neat epoxy. These three effects and the mechanical response were investigated by Dynamic Mechanical Analyzer (DMA) instrument. The dynamic-mechanical reponses from the composite specimens were remarkable compared to the neat epoxy specimen which had the lowest storage modulus. Epoxy reinforced with 2 wt% mass fraction of electrospun PSt-co-GMA/MWCNTs nanofibers has the highest storage modulus E’ (eight to ten times higher than unreinforced epoxy). Nanocomposite of PSt-co-GMA electrospun mat, has also shown substantial increase in storage modulus by a factor of five compared with the neat epoxy at room temperature. It was observed that spraying of crosslinker agent has impact on mechanical properties of the reinforced composites. Moreover, GMA composition in copolymer structure also affects the performance. Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) characterization methods will be further utilized to observe the morphology of nanofibers and the distribution of MWCNTs in electrospun mats.
9:00 PM - FF10.24
Improving the Crack Resistance of Indium-Tin-Oxide (ITO) Films on Polymeric Substrates using Ductile Metal Interlayers.
Eunhye Kim 1 , Chan-woo Yang 1 , Kyeong-hwi Min 1 , Jin-woo Park 1
1 , Yonsei University , Seoul Korea (the Republic of)
Show AbstractIn this study, we investigate the effects of interlayers on improving the mechanical reliability of conductive oxide films on polymeric substrates. ITO has been most frequently used transparent conductive oxides (TCO) as electrodes in flexible electronics due to the superior optical and electrical properties to other TCO materials. However, the inherent brittle nature limits the flexibility of the substrates where ITO is deposited. The internal stress induced in ITO during deposition has been known to be one of the most important factors reducing the resistance to cracking of ITO under tension. In addition, our recent studies revealed that the resistance is affected significantly by the film microstructures such as crystallinity and surface roughness. In this study, we insert ductile metal interlayers such as Ag and Ti and investigate the effects of interlayers on releasing internal stresses and changing microstructures. We found that both Ag and Ti reduced internal stresses, but affected the microstructures differently due to the different chemical reactivities. We sputter-deposited the interlayers and ITO with different thicknesses on PET at fixed deposition conditions. The crack resistance of the samples was evaluated by measuring the crack initiation strains using uniaxial tension test. By finite element method (FEM), the tension test was simulated and the results are compared with the tension test results. The effects of different interlayers and film thicknesses on the degree of crystallinity and surface roughness were analyzed by transmission electron microscope (TEM) and atom probe microscope (AFM). The deleterious effects of the microstructures compensating the internal stress release on improving the crack resistance were discussed in detail.
9:00 PM - FF10.25
Thermo-Mechanical Analysis of the Polymer Materials with High Spatial Resolution.
Maxim Nikiforov 1 , S. Gah 2 , L. Germinario 3 , Stephen Jesse 1 , R. Composto 2 , Sergei Kalinin 1
1 CNMS, ORNL, Oak Ridge, Tennessee, United States, 2 , University of Pennsylvania, Philadelphia, Pennsylvania, United States, 3 , Appalachian State University, Boone, North Carolina, United States
Show AbstractBehavior of the materials at the nanoscale determines the macroscopic properties of the materials. The origin of many failures lies at the nanoscale, for example, crack nucleates at the mechanical defects in the material, electrical failure happens at the electrical defects etc. In polymeric materials mechanical behavior of the components determines the mechanical behavior of the mixture as well. Phase separation commonly causes failure in polymeric materials, explaining the amount of attention attracted to study local thermomechanical properties of mixed-phase polymeric materials and composites in recent decade.We developed a quantitative method for local thermomechanical analysis of polymers. Commercial atomic force microscope was used as a platform for our method. Our measurements of the mechanical properties are based on the determination of tip – surface contact resonance frequency as a function of the temperature. Measurements of the dynamic property, such as resonance frequency, provide us with the increased sensitivity to the changes in materials properties. To date, local thermal analysis techniques utilize either the displacement of the tip due to penetration into the sample or the change in thermal impedance as detection mechanisms for the onset of melting transition. Due to the large noise inherent in static cantilever deflection detection systems, a measurable signal cannot be determined before a large-scale (>100nm) cavity is formed, thus limiting the spatial resolution. Our approach allows to measure mechanical properties with 50 nm spatial resolution. The experimental protocols that provide reproducible measurement of a polymer's thermomechanical properties were validated on several polymer materials (PET, acrylic polymers and PETG). The theory for the mechanical response of the material to the localized heating was built in order to convert measured parameters (contact resonance frequency and quality factor of the oscillations) to Young’s and loss moduli of the material.Glass transition temperatures of PMMA-rich and SAN-rich domains obtained during the phase separation in PMMA:SAN system was measured with 50 nm spatial resolution. The effect of the annealing time on spatial distribution of the glass transition temperatures is discussed.This research at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy.
9:00 PM - FF10.27
Characterization of Mechanical Properties of Polymer Nanocomposites by Nanoindentation and Influence of Nanoparticulate Filler on the Viscolastic Response.
Jiri Nohava 2 , Nicholas Randall 1
2 , CSM Instruments, Peseux Switzerland, 1 , CSM Instruments, Needham, Massachusetts, United States
Show AbstractNanostructured polymers have recently gained high importance due to the possibility of tailoring their mechanical properties by the amount and type of nanofillers. This is mainly due to the fact that the nanofillers have large surface to volume ratio and therefore can significantly affect the overall mechanical properties, even in small amounts, including the creep characteristics of the polymer. At early stages of development these materials are not always available in sufficient volume for standard tensile testing and therefore the nanoindentation procedure has become one of the only methods available to test the mechanical properties. Although the nanoindentation method is now established for measurement of hardness and elastic modulus, determination of creep and viscoelastic properties in general is still quite a challenge mainly due to the existence of thermal drift. This paper presents the results of ongoing research on several types of polymeric materials with various ratios of nanoparticulate fillers. The characterization was done by instrumented indentation with a Berkovich indenter at loads of several tens and hundreds of micronewtons. The creep properties were determined during a pause at maximum force in duration of 60 s to 300 s. The thermal drift of the instrument was verified by indentation on fused silica at the same load and pause duration and it was found that the nanoindentation system has thermal drift of less than 0.5 nm/min while the creep of the tested polymers was in the range of hundreds and tens of nanometers. The level of creep of all samples was measured and its amount was related to the nanofiller content and loading rate. It was found that irrespective of the content of the nanofiller, the amount of creep was higher when the loading rate was higher. At the same time, oscillation indentation measurements were performed to determine the storage and loss moduli of the tested materials. Results show that even very small amounts of nanofiller have a significant influence on the viscoelastic properties of the polymer.
9:00 PM - FF10.28
Training Effect in Artificial Muscle using Polyaniline.
Hikaru Hashimoto 1 , Kazuo Tominaga 1 , Wataru Takashima 2 , Keiichi Kaneto 1
1 Biological Functions and Engineering, Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu, Wakamatsu-ku Hibikino 2-4, Japan, 2 , Research Center for Advanced Eco-fitting Technology, Kyushu Institute of technology, Kitakyushu Japan
Show AbstractConducting polymers show electrochemomechanical deformation (ECMD), resulting from the insertion of bulky ions and the change of polymer conformation due to the delocalization of π-electron by oxidation. Polyaniline (PANi) has been known to exhibit the stable ECMD [1] and higher mechanical strength compared with those of polypyrrole [2]. Characteristics of ECMD in PANi film have been studied to elucidate the mechanism of deformation under high uniaxial tensile stresses. PANi films were prepared by common chemical polymerization of aniline in ammonium peroxodisulfate. The emeraldine base powder was dissolved in NMP, followed by casting on a glass plate. A piece of PANi film was electrochemically cycle under tensile stresses up to 3MPa in an aqueous 1 M HCl. The PANi film showed elongation and contraction (cyclic ECMD) by approximately 6% upon electrochemical oxidation and reduction, respectively. When tensile stresses up to 3MPa were applied to the film, the film showed creeping by about 20%, showing superimposition of cyclic ECMD. However, it was found that by release of the high tensile stress the creeping was recovered and the film turned back to the almost original length. It is interesting to note that the magnitude of cyclic ECMD increased by more than 10% compared with the magnitude of cyclic ECMD before applying the tensile stress of 3MPa. The enhanced cyclic ECMD by the experience of large tensile stress is called as the training effect of conducting polymer artificial muscles. The creeping is commonly resulted from the conformation change, realignment, slipping and/or breaking of polymer chains. The present result of full recovery of creeping and the training effect suggests that the creeping is due to conformation change, since the other mechanisms should result in permanent deformation. The training effect is discussed by taking the anisotropic deformation of the film under high tensile stress into consideration.
9:00 PM - FF10.30
A Comprehensive Study of Cuticle Organisation at Different Hierarchical Levels in Functionally Differentiated Parts of the Exoskeleton of the Edible Crab Cancer pagurus .
Keerthika Balasundaram 1 , Helge Fabritius 1 , Dierk Raabe 1
1 , MPIE, Düsseldorf Germany
Show AbstractThe Arthropod exoskeleton is a structural entity formed by the cuticle, a hierarchically organized chitin-protein based nanocomposite. On the molecular level, it consists of chitin associated with proteins forming fibers, which are organized in the form of twisted plywood. On the higher levels, the twisted plywood organization is modified and forms structural units with elaborate functions. In crustaceans, parts of the cuticle also contain biominerals, mostly calcium carbonate. During evolution, all parts of the exoskeleton were optimized to fulfill different functions according to the different ecophysiological strains faced by the animals. The cuticle has to perform such diverse functions like resistance to mechanical loads and friction in joints, elasticity in joint membranes and transparency in eyes. This is mainly achieved by modifications in microstructure and chemical composition. In order to understand the relationship between structure, composition and resulting properties and function we structurally, mechanically, and chemically characterized selected body parts of the edible crab Cancer pagurus . We performed tensile and compression experiments to investigate mechanical properties on the macroscopic scale, nanoindentation tests were performed to study local mechanical properties. Characterization of the microstructure included scanning electron microscopy (SEM), Energy dispersive X-ray spectroscopy (EDX) and thermogravimetric analysis (TGA). The comparison to the well investigated lobster Homarus americanus shows that although both species share the same basic construction principle of their cuticles there are pronounced differences in the way they achieve similar functions in corresponding body parts, especially in the way they mineralize their cuticle.
9:00 PM - FF10.31
A Mechanically Actuated Plasmonic Nanoparticle Fluid.
Rama Bhattacharjee 2 , Ruipeng Li 1 2 , Emmanuel Giannelis 2 , Aram Amassian 1
2 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 1 Materials Science and Engineering, King Abdullah University of Science and Technology, Jeddah Saudi Arabia
Show AbstractNanoparticle ionic materials (NIMs) are a new class of hybrid organic-inorganic fluidic materials consisting of a hard inorganic nanoparticle corona and a soft organic canopy. NIMS possess high nanoparticle content, zero vapor pressure, as well as inexpensive and green mass production capabilities. Their design provides multiple avenues for tuning properties of nanomaterials. In this work we show that step-wise surface modification of gold nanorods (GNRs) by simple aqueous solution-based assembly and acid-base chemistry can result in the formation of a solventless, GNR-based plasmonic and ionic fluid. These fluidic materials show dynamic and reversible color changes upon mechanical shearing, which are visible to the naked eye in ambient lighting. The color changes and their time scale can be tuned by changing the aspect ratio of the GNRs as well as via plasmon coupling by controlling interparticle interactions and arrangements in the fluid matrix. NIMS-based complex fluids appear to be a new way class of plasmonic materials with fascinating physical and mechanical properties.
9:00 PM - FF10.32
Deformation Behavior of Nanocrystalline Co-Cu Alloys.
Motohiro Yuasa 1 , Hiromi Nakano 2 , Kota Kajikawa 1 , Takumi Nakazawa 1 , Mamoru Mabuchi 1
1 Department of Energy Science and Technology, Graduate School of Energy Science, Kyoto University, Kyoto Japan, 2 Cooperative Reserach Facility Center, Toyohashi University of Technology, Toyohashi Japan
Show AbstractThe present paper describes deformation behavior of two kinds of nanocrystalline Co-Cu alloys; one is the nanocrystalline Co-Cu alloy containing high-density fine nanoscale lamellar structure with a narrow spacing of 3 nm and another is the supersaturated nanocrystalline Co-Cu alloy with a high Cu concentration of about 20 %. These nanocrystalline Co-Cu alloys showed the high hardness of 4 - 7 GPa and the low activation volume of about 3 b3 from Berkovich hardness testing. The rate controlling process of deformation for the nanocrystalline Co-Cu alloys is the dislocation emission at grain boundaries, and therefore, the high strengths and low activation volume are attributed to the characteristic grain boundary structures caused by the grain boundary segregation and the nanoscale lamellar structure. In the present paper, molecular dynamic (MD) simulations are performed in the nanocrystalline Co-Cu alloys to investigate effects of the characteristic grain boundary structures on the deformation mechanism. The MD simulations revealed that the dislocation emission at grain boundaries is strongly affected by the grain boundary structures.
9:00 PM - FF10.33
Adhesion Influence of Diamond Coatings on WC-Co Turning Inserts for High Performance Machining Applications.
Humberto Gomez 1 4 , Feng Qin 2 , Ashok Kumar 1 , Kevin Chou 2 , Bob Johnson 3
1 Mechanical Engineering, University of South Florida, Tampa, Florida, United States, 4 Ingenieria Mecanica, Universidad del Norte, Barranquilla Colombia, 2 Mechanical Engineering, The University of Alabama, Tuscaloosa, Alabama, United States, 3 , Seki Technotron USA, Santa Clara, California, United States
Show AbstractThe importance and interest of diamond films for machining applications is based on some of the unique diamond properties such as the highest thermal conductivity, extreme hardness and low friction coefficient, making it very attractive to improve the wear resistance of cutting tools materials. Diamond coatings have been proposed in order to enhance the dry machining performance for magnesium and aluminum alloys in the automobile and aerospace industries. However, machining performance of the diamond coated tools is subjected to technical barriers in terms of the resulting adhesion between the substrate tool material and the deposited film. The effect of diamond films on WC-Co cutting inserts (SPG422) is compared for microcrystalline and nanocrystalline diamond films synthesized by hot filament and microwave plasma-assisted CVD techniques, respectively. The quality of the film is analyzed using Raman spectroscopy, EDS, SEM, AFM, and hardness measurements. In addition, the resulting adhesion of the film is measured in terms of the film delamination morphology by the effect of different surface pretreatments including heat treatment in H2 and CH4 atmospheres, Cr/CrN/Cr interlayer pre-deposition and chemical etchings. Correlations for thermal and mechanical stresses are discussed as result of experimental techniques and finite element modeling used to simulate and evaluate the corresponding deposition stresses. Finally, machining performance tests were conducted with the aim of establishes the wear failure mode of the tool, and tool life in terms of the delamination occurrence.
9:00 PM - FF10.34
Effect of Nitrogen Doping on the Mechanical Properties of Sol-gel Prepared TiO2 Thin Films: A Nanoindentation Study.
Murat Kurtoglu 1 2 , Travis Longenbach 1 2 , Yury Gogotsi 1 2
1 Drexel Nanotechnology Institute, Drexel University, Philadelphia, Pennsylvania, United States, 2 Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractPhotocatalytic TiO2 films have enjoyed a great deal of interest among the scientific community in the last decade because of their exceptionally high catalytic activity in the UV range, corrosion and wear resistance, and mechanical integrity. Being an inexpensive, readily available, non-toxic and biocompatible material makes them perfect candidates for use in the large scale production of UV and visible light activated self-cleaning glassware, capable of removing pollutants from air and water, as well as killing various forms of viruses and bacteria. In order to take full advantage of the photoinduced catalytic properties of titania films, they must be engineered to work within the visible light spectrum. By doping titania films with nitrogen, it is possible to see activity within this range. However, the effects of nitrogen doping on the mechanical properties of the films have not been investigated. In this study, we found by nanoindentation that while it was possible to create titania based films capable of catalyzing under visible light by nitrogen doping, there was a significant amount of decrease in the hardness values of the films. We discuss the ways to optimize the mechanical properties of titania films, while maintaining their high photocatalytic activity. Further research will include the usage of various other elements within the films to increase the hardness and modulus while retaining the visible light activity of the films.
9:00 PM - FF10.35
Microstructural and Mechanical Properties of Boron Carbide Ceramics by Methanol Washed Powder.
Kyoung Hun Kim 1 , Jong Pil Ahn 1 2 , Joo Seok Park 1 , Jae Hong Chae 1 , Hyung Sun Kim 2 , Sung Min So 1 2
1 , KICET, Seoul Korea (the Republic of), 2 , Inha university, Incheon Korea (the Republic of)
Show AbstractBoron carbide is currently used in lightweight armors and high temperature materials, because it has high meting point, good hardness, low specific gravity and good mechanical properties. The sintering of boron carbide, however, is restricted by its high covalent bonding and B2O3 coatings on B4C particles surface which can cause a microstructural coarsening during sintering. Therefore, it is necessary to remove B2O3 film of B4C particles surface to restrict microstructural coarsening and densification of B4C. B4C ceramics were fabricated by a hot-press sintering and its sintering behavior, microstructure and mechanical properties were evaluated. The relative density of B4C ceramics were obtained by a hot-press sintering reached as high as 99% without any sintering additives. The mechanical properties of B4C ceramics was improved by a methanol washing which can remove B2O3 phase from a B4C powder surface. This improvement is resulted from the formation of homogeneous microstructure because the grain coarsening was suppressed by the elimination of B2O3 phase. Particularly, the mechanical properties of the sintered samples using a methanol washed powder improved compared with the samples using an as-received commercial powder.
9:00 PM - FF10.36
Dispersion Rheology of Carbon Nanotubes in a Polymer Matrix.
Yan Yan Huang 1 , Samit Ahir 1 , Eugene Terentjev 1
1 Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractWe report on rheological properties of a dispersion of multiwalled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t* needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing t smaller than t*, we find a number of nonequilibrium features characteristic of colloidal glass and jamming of clusters. A thoroughly dispersed nanocomposite, at t larger than t*, has several universal rheological features; at nanotube concentration above a characteristic value n~2–3 wt.% the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and reaggregation.
9:00 PM - FF10.37
Variable Elastic-Plastic Properties of the Grain Boundaries and Their Effect on the Macroscopic Flow Stress of Nano-Crystalline Metals.
Malgorzata Lewandowska 1 , Romuald Dobosz 1 , Krzysztof Kurzydlowski 1
1 Faculty of Materials Science and Engineering, Warsaw University of Technology, Warsaw Poland
Show AbstractThe effect of the grain boundaries on flow stress of metals, which is especially prominent in the case of nano-sized ones, has been a subject of numerous experimental and theoretical studies in the past. Recently, remarkable progress has been made in the understanding of this effect due to the new experimental results obtained on metals subjected to nano-grain refinement via Severe Plastic Deformation and as a result of advances in modelling of plastic behaviours of nano-polycrystalline aggregates. In particular, so called composite approach to the properties of the nano-metals proved to be successful in explaining such unique properties of nano-metals as reverse Hall-Petch relationship. However, some key questions with regard to the role of grain boundaries remain unanswered and quantification of their effect on the macroscopic properties is still a challenge.The present paper describes new experimental results obtained on nano-metals produced via hydrostatic extrusion, HE. Nano- and sub-micron structures have been produced by HE in a series of aluminium alloys. Size of grains and mis-orientation of the grain boundaries in the sample subjected to HE have been quantified using high resolution microscopic techniques. It has been found that the obtained nano-structures differed in the average size of grains, normalized width of grain size distribution and grain boundary mis-orientation distribution functions, in the way which seem to be typical of nano-metals obtained by SPD. As HE processed billets are relatively large in dimensions, it was possible to characterize the mechanical properties of the processed materials in tensile tests with relatively good statistics. The results of the tensile tests showed that the flow stress significantly depends on the average grain, grain size distribution and the distribution function of mis-orientation angles. In order to explain the observed difference in the properties of nano- and micro-sized structures, a Finite Element Method models have been developed, which assumes that both grain boundaries and grain interiors may accommodated elastic and non-linear plastic deformation. These models assumed true geometry of grains (which differed in size and shape). Also, variable mechanical properties of grain boundaries have been taken into account (elastic modulus, yield strength and work hardening rate). The results of modelling explain in a semi-quantitative way macroscopic deformation of nano-crystalline aggregates. In particular, they illustrate the importance of the interplay between properties of grain boundaries and grain interiors in elastic and plastic regime. These results provide also insight in such phenomena as nano-scale strain localization. Finally they can be used to optimize the structure of nano-metals in terms of their mechanical properties.
9:00 PM - FF10.39
The Measurement of Mechanical Properties on Chars with Nanoclays Using Nanoindentation.
Seongchan Park 1 , Miriam Rafailovich 1
1 Materials Science and Engineering, Stony Brook University, Stony Brook, New York, United States
Show AbstractDuring combustion char formation becomes a crucial factor to reduce flammability of polymeric materials. The char acts as a barrier against combustible gases, conducts heat away from the interior of the sample and reflects the heat from the approaching heat front. We have previously observed that large aspect ratio nanoparticles can act synergistically with flame retardant formulation, rendering a large group of polymers flame retardant. Cone calorimety data has shown that this phenomenon is also related to the formation of chars, even in systems which normally burn without char formation. In order to understand the mechanism through which the nanoparticles increase the flame resistance, we must also study the properties of the chars that are formed. Here we describe a study on Polystyrene (PS) and polymethacrylate (PMMA) polymer blends where neither of the polymers forms a char upon burning and where they can only be rendered flame retardant with the addition of clay and tube nanoparticles. SEM images of the chars indicate that they consist of carbonateous remains mixed with clay platelets or carbon nanotubes. High magnification images reveal that the clay particles are folded, forming long tube-like structures. These structures are only observed in blends and result from differential combustion of the two components where the pressure collapses the clays adsorbed at the polymer interface regions. The mechanical properties of the chars were measured with a micro-indentor since the chars are too brittle to be measured using standard mechanical testing apparatus. We found that the mechanical response of chars from compounds with different compositions varied greatly in their mechanical properties. In particular, the PMMA/C30B char was the hardness, with a modulus of G= 0.2GPa, Similarly PS/C15A and PS/PMMA/C20A had modulii of 0.06GPa and 0.035GPa, respectively. Examination of the, the morphology of the PMMA/C30B residue showed many more small cracks when compared to that of the PS/PMMA/C20A compound. Video images obtained during the cone calorimetry combustion studies revealed that the chars swelled by more than a factor of 4 and then collapsed abruptly. The elasticity of the chars can then be directly related to their degree of swelling and the lack of cracks when the char is deflated, after heat treatments. The compound which had the most elastic chars, also had the best flame retardant performance, passing the UL-94 V0 criterion.
9:00 PM - FF10.4
Micromechanical Modeling of Layer-by-Layer Coated Electrospun Nanofiber Mats for Proton Exchange Membrane Fuel Cells.
Meredith Silberstein 1 , Mary Boyce 1 , J. Ashcraft 2 , Chia-Ling Pai 2
1 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Chemical Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractProton exchange membrane fuel cells are a promising alternative energy conversion method for low to moderate temperature applications. The polymer electrolyte membrane (PEM) is responsible for rapid conduction of protons from anode to cathode. The PEM is subject to hygro-thermal cycling within tight physical constraints during cell operation. This can lead to plastic deformation and thinning of the membrane contributing to gas crossover and accelerating fuel cell failure. The multifunctional requirements of a PEM (high conductivity, low gas crossover, resistance to chemical corrosion, and mechanical durability) indicate exploring a nanocomposite solution for an alternative membrane design. Novel processing techniques are used to produce a nanocomposite PEM. Layer by layer (LbL) processing has recently been found to successfully produce a highly proton conductive PEM (Argun et. al. Adv. Mat. 2008). Mechanical integrity is provided to the membrane by using an electrospun nanofiber mat as the substrate. Preliminary results show that the nanocomposite design provides damage tolerance and increased failure strain while maintaining conductivity comparable to the LbL film alone.Here the mechanics of the nanofibrous mats and the LbL/mat nanocomposite are studied through experiments and micromechanically-based constitutive modeling. The behavior of the mats is studied from the fiber level to the mat level. Single fibers are extracted from the mats and evaluated to obtain the elastic-plastic stress-strain behavior of the constituent fibers. The electrospun mats are characterized and modeled in both randomly oriented and aligned configurations. The mats exhibit linear elastic-plastic behavior similar to the single fibers but with a much lower elastic modulus and yield stress due to the high porosity and fiber orientation. The elastic-plastic stress-strain behaviors and failure of the LbL films and the nanocomposite are found to be strongly dependent on hydration, rate, and temperature. A constitutive model has been developed treating the random mat as a superposition of triangulated networks of the single fiber material. This model is able to predict the elastic-plastic behavior of the network as well as the increased stiffness and yield that arise from initial orientation of the mats. This model is then combined with a constitutive model of the LbL to obtain a model of the nanocomposite. By building the continuum model from the microstructure up, the rate, temperature and hydration dependence of the composite structure is naturally incorporated via these dependencies of the constituent materials. The model is shown to successfully capture the mechanical behavior of this particular PEM. Further, it establishes a method for predicting the mechanical behavior of composites synthesized in the same manner with different fiber dimensions or from different base polymers. This can then be used to parametrically screen prospective PEMs for mechanical suitability.
9:00 PM - FF10.40
A Comparison of Dry Sliding Wear of Conventional and Nanocrystalline Eutectic Al-Si.
Ian Baker 1 , Michael Gwaze 1 , Ye Sun 1 , Adam Dohner 1 , Annika Grosse 1 , Francis Kennedy 1 , Paul Munroe 2
1 Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire, United States, 2 Electron Microscope Unit, University of New South Wales, Sydney, New South Wales, Australia
Show AbstractAl-Si alloys are of increasing interest due to their applications in many wear conditions in automobile and other industries. In this work, a pin-on-disk tester was used to study the wear behaviour of as-cast eutectic Al-Si against a Y-stabilized ZrO2 counterface under both Ar and air atmospheres and the results were compared with those of Al-Si alloys of similar composition made from both commercial powders and nanocrystalline powders produced by mechanical milling followed by equal channel angular extrusion. Scanning electron microscopy and transmission electron microscopy, including energy dispersive X-ray spectroscopy, were used to examine the worn pin surfaces. X-ray diffraction was used to characterize the wear debris, and pits produced by focused ion beam milling were used to study subsurface cracking on the pins.This work was supported by U.S. National Science Foundation Grant CMMI 0651642.
9:00 PM - FF10.41
Independent Characterization of Second Phase Particle in a Dual Phase Material using Nanoindentation.
Akbar Aftabi Gilvan 1 , Vis Madhavan 1 , Mahdi Saket Kashani 1
1 Industrial and Manufacturing, Wichita State University, Wichita, Kansas, United States
Show AbstractThe influence of particle size (second phase) embedded in a matrix material (main phase) on the results of nanoindentation is investigated using finite element simulations. For a given depth of penetration, it is attempted to determine the minimum acceptable size of the second phase for measuring the hardness and modulus of elasticity of the particle independent of the matrix. The particle is modeled in cylindrical shape and its radius and height are varied in a series of axisymmetric FEA simulations. The analysis of a model entirely made of the second phase was performed separately and the obtained hardness and modulus were set as references. The criterion of 1% deviation from the reference values was assumed to determine the acceptable sizes of the particle for measuring accurate hardness and modulus. The simulations suggest that the determined sizes are only valid for the specific mechanical properties that are assumed for each phase; however, the conclusions are discussed to set a general frame work for any given dual phase material. Some case studies for different materials are presented to verify the conclusions. The results of this study would help to determine the upper limit for the depth of penetration for a given dual phase material.
9:00 PM - FF10.42
Structural Carbon Aerogels for High Temperature Thermal Insulation.
Wendell Rhine 1 , Shannon White 1 , Nicholas Zafiropoulos 1 , Redouane Begag 1 , Wenting Dong 1 , Irene Melnikova 1
1 , Aspen Aerogels, Northborough, Massachusetts, United States
Show AbstractAspen Aerogels has developed a high-performance carbon aerogel insulation material that is lightweight and thermally and structurally efficient. Aerogels have fine pores of nanometer dimensions, extremely high porosities (generally between 90 and 99%), and very unique lattice structures. Because of their nanoporous, low-solid structures, there is a complex interrelationship between gas conduction, solid conduction, and radiation components of thermal conduction within aerogel structures making them excellent insulating materials. Carbon aerogels have been prepared with variable final densities. Thermal and mechanical performances were evaluated across the density range studied. Extensive compression analysis on these samples reveals their superior strength over other aerogel systems. Results of these tests will be discussed along with BET porosimetry, TGA/DSC, infrared spectroscopy, SEM, and hot-plate thermal conductivity data. In this paper, we will address the high temperature thermal performance capability of carbon aerogels.
9:00 PM - FF10.43
Theoretical Modeling of Isostructural Decomposition in Cubic M(1-x)AlxN (M=Ti, Cr, Sc, Hf).
Igor Abrikosov 1 , Bjoern Alling 1
1 Department of Physics and Measurement Technology (IFM), Linkoping University, Linkoping Sweden
Show AbstractTo increase the hardness and prolong the lifetime of cutting tools, coating with nanocomposite thin films of multicomponent transition metal nitrides has become standard procedure within the industry. Today, complex multicomponent nitrides have, to a large extent, replaced the binary nitrides such as TiN. For example, it has been found that the alloying of TiN with AlN increases the cutting performance of the coating. This has been attributed to increased oxidation resistance, but more recently also to an age-hardening mechanism. A related coating material, Cr(1−x)AlxN, has also been very successful in cutting tools applications. We have used first-principles calculations to investigate the mixing enthalpies, lattice parameters and electronic density of states of the ternary nitride systems Ti(1−x)AlxN, Cr(1−x)AlxN, Sc(1−x)AlxN and Hf(1−x)AlxN in the cubic B1 structure, where the transition metals and aluminium form a solid solution on the metal sublattice. The inclusion of Sc(1−x)AlxN and Hf(1−x)AlxN systems in a common comparison adds an extra dimension to the study of the mixing thermodynamics since ScN and HfN, in opposite to TiN and particularly CrN, show a large lattice mismatch with c-AlN. Thus we are able to analyze the contribution of both the band structure effect and volume mismatch to the atomic level driving force for isostructural decomposition in M(1−x)AlxN-systems [1,2]. It was found that in all four systems, the non-bonding transition metal d-state developed into a narrow almost atomic like state as the AlN content increased. However this development effects the mixing enthalpy only in the cases of Ti(1−x)AlxN and Hf(1−x)AlxN leading to a high and asymmetric mixing enthalpy and an accelerating driving force for isostructural decomposition at higher AlN concentrations. A large lattice mismatch adds a symmetric contribution to the mixing enthalpy in the Sc(1−x)AlxN and Hf(1−x)AlxN systems. A large size mismatch however, although adding to the driving force for decomposition, might hinder coherent decomposition and instead promote an incoherent decomposition through nucleation and growth of hexagonal AlN, a process believed to be unfavorable for coating performance. It is also shown that the magnetism is the key to understand the difference between the Cr-containing system and other systems considered in this study [3]. Our results indicate that the use of the disordered local moments approximation rather than nonmagnetic simulations is important to model the Cr(1−x)AlxN system at elevated temperatures.[1] B. Alling, A. V. Ruban, A. Karimi, O. E. Peil, L. Hultman, and I. A. Abrikosov, Phys. Rev. B 75, 045123 (2007).[2] B. Alling, A. Karimi, and I. A. Abrikosov, Surface & Coatings Technology 203, 883 (2008).[3] B. Alling, T. Marten, A. Karimi, and I. A. Abrikosov, J. Appl. Phys. 102, 044314 (2007).
9:00 PM - FF10.45
Thermomechanical Behavior of Aligned Carbon Nanotube Composites with Ideal Morphology.
Hulya Cebeci 1 , Roberto Guzman de Villoria 1 , Brian Wardle 1
1 , MIT, Cambridge, Massachusetts, United States
Show AbstractSimilar to advanced composites, nanocomposites with controlled morphology can be tailored at the nanoscale to achieve property enhancement, particularly anisotropy. Many fundamental questions related to carbon nanotube (CNT) inclusion in polymer nanocomposites (PNCs) are studied using novel processing to achieve a unique aligned-CNT composites up to high volume fractions (exceeding 20%) of CNTs. This allows assessment of properties without the typical issues and uncertaintly associated with the inhomogeneous distribution, agglomeration and dispersion challenges, and lack of orientation that arise due to uncontrolled morphology in the literature. In this study, aligned, as opposed to randomly-oriented, aligned-CNT polymer nanocomposites (A-PNCs) are fabricated, characterized, and tested to assess thermo-mechanical property effects of the CNTs on the resulting composite. Particularly important is the continuous nature of the CNTs across the sample, reaching lengths of ~2 mm. Such ideal morphology CNTs are important to characterize as they form a constituent in larger-scale nano-engineered composites containing advanced fibers such as carbon.A-PNCs are fabricated using capillarity-driven wetting [1], avoiding dispersion issues and then mechanically densified [2] to obtain desired volume fractions so that the CNTs can dominate composite properties. Cured samples are machined to mm scale and several techniques used to characterize their morphology such as alignment, dispersion, voids, and the effect of the closely-packed CNTs on polymer curing. Thermal properties are assessed with differential scanning calorimetry and thermogravimetric analysis with good agreement in weight/volume fraction of CNTs with curing kinetics studies. Thermo-mechanical behavior was studied by dynamic mechanical analysis. Overall, the A-PNC properties show a volumetric effect from the CNTs, and CNT waviness is noted as a dominant morphological effect controlling modulus by comparing the experimental results to standard composites theories modified to account for fiber (CNT) waviness.1.Garcia, E.J., Hart, A. J., Wardle, B. L., Slocum, A. H., Fabrication of composite microstructures by capillarity-driven wetting of aligned carbon nanotubes with polymers. Nanotechnology, 2007, Vol. 18 (16), 165602 (11pp).2.Wardle, B.L., Saito, D.S., Garcia, E.J., Hart, A.J., deVilloria, R.G.,, Fabrication and Characterization of Ultra-High Volume Fraction Aligned Carbon-Nanotube-Polymer Composites, Advanced Materials, Vol. 20, 2008, pp. 2707-14.
9:00 PM - FF10.46
Finite Element Modeling of Nanoindentation of Nanowires on a Deformable Flat Substrate
Davood Askari 1 , Gang Feng 1
1 Mechanical Engineering, Villanova University, Wayne, Pennsylvania, United States
Show AbstractThe fascinating properties of nanostructured materials and their promising applications have been the main driving force for the development of more advanced material characterization tools and techniques. It should be mentioned that the accuracy of calculated results directly depends on the capability of employed techniques and the assumptions that have been made for the analysis of measured data. In general, due to the complexity of material’s behavior in small scales and difficulties in data reductions and analysis, many simplifying assumptions have been made for the calculation of material properties. Among the many mechanical properties measurement techniques, indentation/nanoindentation has been a very effective technique for the properties measurements of thin films and nanostructured materials. However, in majority of the published reports regarding the nanoindentation of nanowires/nanotubes, deformations and the compliance of substrate material (i.e., upon which nanowires/nanotubes are laid) are completely ignored for the material properties calculations and data analysis. Here in this work, we present a more sophisticated approach that accommodates the substrate deformations for the calculations of hardness and stiffness of nanowires/nanotubes using a depth-sensing nanoindentation technique. Furthermore, receding of the nanowires during the nanoindentation is allowed in our simulation and thoroughly investigated. A 3-dimensional nanoindentation finite element model of a GaN nanowire on a flat surface Si substrate is created as a double contact receding problem. An indentation load is applied over the nanowire by means of a rigid spherical nanoindenter tip, incrementally, and solved. The stiffness and the hardness results are calculated for both the contact interfaces as a function of indentation load and then discussed in details. It is concluded that the substrate deformations has a significant influence on material properties calculations. Finally, the results are compared to those obtained from experiments and analytical solutions reported in authors’ earlier works.
9:00 PM - FF10.5
Nanomechanical Characterization of Raspberry-like Core-shell Nanocomposite.
Xinnan Wang 1 , Li Tao 2 , Xiaodong Li 2 , Qian Wang 2
1 , North Dakota State University, Fargo, North Dakota, United States, 2 , University of South Carolina, Columbia, South Carolina, United States
Show AbstractBioconjugation of virus and virus-like particles with other polymers is a promising approach of producing good vehicles for the drug delivery and therapeutic purposes. This work investigated the nanomechanical behavior of turnip yellow mosaic virus (TYMV, radius: 14 nm) coated Poly(4-vinylpyrindine) (P4VP, radius: 95 nm) composites using atomic force microscope (AFM)-based nanoindentation technique. The elasticity coefficient was determined from the elastic region of indentation force vs. displacement curves. Structural and interfacial characteristics of the particles were studied by comparing the mechanical properties of the constituent materials. Results showed that the mechanial properties of P4VP particles can be altered with a mono layer of TYMV coating. The deformation behavior of the biocomposite is further discussed.
9:00 PM - FF10.6
Influences of Crystallization Behavior of Poly(L-Lactide) and Morphologies of Hydroxyapatite Particles on the Mechanical Properties of Co-Electrospun Scaffolds.
Fei Peng 1 , Montgomery Shaw 1 2 , James Olson 3 , Mei Wei 1
1 Department of Chemical, Materials, and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 Polymer Program, Institute of Materials Science, University of Connecticut, Storrs, Connecticut, United States, 3 , Teleflex Medical, Coventry, Connecticut, United States
Show AbstractHighly porous hydroxyapatite (HA)/biopolymer hybrid scaffolds have attracted much attention as prospective candidates for bone tissue engineering. However, in most cases, their poor mechanical properties compared to bone have greatly limited their use in many clinical applications. In this project, highly porous HA/poly(L-lactide) (PLLA) nanofibrous scaffolds with mechanical strength remarkably enhanced by excellent alignments of HA particles and of composite nanofibers were prepared using co-electrospinning technique. In these scaffolds, three different kinds of HA particles of different lengths ranging from 100 nm to 2 μm and of a round or needle-like shape were aligned within the PLLA fiber matrix along the fiber long axes. And the HA incorporation ratio could be precisely controlled with the initial feeding ratio. The fibers constructing the scaffold had a diameter adjustable between 100 nm to 1 µm and could also be aligned by using a rotating drum running at a high speed of 800 m/min as a collector. It was found that the highly aligned fibrous assembly increased the tensile modulus and strength of the PLLA scaffold by 498% and 306%, respectively. And 20wt% HA particles of each studied kind were found to remarkably enhance the mechanical strength of the composite scaffolds. Among them, the micro-size and needle-shaped MNHA particles demonstrated the best reinforcing effect by increasing the tensile modulus and strength of the composite scaffold to 897% and 320% of those of the PLLA scaffold. The reinforcing mechanism of the aligned fibrous assembly was studied considering the crystallization behaviors of PLLA molecular chains within the scaffolds of random (RD) or aligned (AL) fibrous assembly. And that of the HA particles of a different morphology were investigated using shear-lag model.
9:00 PM - FF10.8
Deformation of Nanocrystalline Mg by Large-Scale Molecular Dynamics Simulation.
Dong-Hyun Kim 1 , M. Manuel 1 , F. Ebrahimi 1 , J. Tulenko 2 , S. Phillpot 1
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Nuclear and Radiological Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractLarge scale molecular-dynamics simulations of deformation under tensile stress were performed for nanocrystalline Mg, a prototypical nanocrystalline h.c.p. metal. The atomistic processes of slip and twinning created by a tensile load were observed in a [11bar20]-textured columnar structure with grain size of 60nm. Considering individual defect mechanisms, Shockley partial dislocations, 1/3[bar1100] and 2/3 [bar1100] with basal slip plane were observed; the 1/6 [20bar23]Frank partial dislocation was also activated. Primary tensile{10bar12}<10bar11> twinning was also nucleated at grain boundaries. By comparison to primary twinning, secondary tensile twinning,{10bar11}<10bar12>, mainly seen in the grain interior, was created during the interaction between Shockley dislocations with opposite Burgers vectors. Competitive process of slip and twinning in plastic deformation of h.c.p. metals with increasing external stress were also observed. This work was supported by DOE NERI contract DE-FC07-07ID14833.
9:00 PM - FF10.9
Atomistic Simulations of the Mechanical Response of Copper/Polybutadiene Joints under Stress.
Fidel Valega Mackenzie 1 , Barend Thijsse 1
1 Materials Science and Engineering, Delft University of Technology, Delft Netherlands
Show AbstractMetal/polymer system joints are widely encountered nowadays in microscopic structures such as displays and microchips. In several critical cases they undergo thermal and mechanical loading, with contact failure due to fracture as a possible consequence. Due to their variety in nature and composition, metal/polymer joints have become major challenges for experimental, theoretical and numerical studies. Here we report on the first results of molecular dynamics simulations to study the mechanical response of a metal/polymer joint, in this case the Cu/polybutadiene model system. The behavior of Cu and crosslinked polybutadiene are modeled, respectively, by the Embedded Atom Method (EAM) and the Universal Force Field (UFF). Loading is applied under shear and by means of tensile stress. Studies are performed below the metal melting point and both below and above the polymer glass transition temperatures. Different potentials are used to describe the interactions in the metal/polymer interface which allows us to analyze, in a qualitative manner, possible mechanisms for failure occurring in these joints.
9:00 PM - FF10: PosterII
FF10.1 Transferred to FF12.11
Show Abstract9:00 PM - FF10: PosterII
FF10.29 Transferred to FF9.4
Show Abstract
Symposium Organizers
Jun Lou Rice University
Brad Boyce Sandia National Laboratories
Erica Lilleodden GKSS Forschungszentrum
Lei Lu Chinese Academy of Sciences
FF11: Electromechanical, Thermomechanical & Chemomechanical Behaviors of Nanomaterials
Session Chairs
Friday AM, December 04, 2009
Room 304 (Hynes)
9:30 AM - **FF11.1
Electrical, Mechanical, and Thermal Damage Modes during High Current Stressing of Damascene Copper Interconnects.
Robert Keller 1 , David Read 1 , Roy Geiss 1
1 Materials Reliability Division, NIST, Boulder, Colorado, United States
Show AbstractWe present observations of several different forms of damage to damascene copper interconnects subjected to various types of thermal and electrical stressing expected in processing and use. Two factors were varied in these studies: (i) AC versus DC stressing at high current densities (typically in the range ~ 10 MA/cm2 and higher), and (ii) full encapsulation versus partial encapsulation (top surface uncovered, specimens tested in vacuum). We measured lifetimes to open circuit as well as changes and damage to microstructure. Plots of lifetime as a function of applied temperature range due to joule heating during electrical testing revealed that AC-stressed specimens exhibited somewhat longer lifetimes (by a factor of approximately two to five) than DC-stressed specimens for a given temperature range and level of constraint. Both forms of stressing resulted in the formation of voids early in the testing of fully encapsulated specimens. However, their morphologies differed, with AC voids being rounder and DC voids being more faceted. AC stressing produced a density of voids higher than that induced by DC stressing, and those AC voids remained largely stationary during continued testing. Conversely, DC stressing produced voids that moved toward the anode, suggesting behavior typical of electromigration damage. DC stressing of samples containing voids initially produced by AC stressing resulted in the motion of only some of those voids. AC stressing also resulted in considerable grain growth, whereas this was negligible for DC-stressed specimens. One prominent effect of mechanical constraint due to full encapsulation by rigid dielectric was to increase AC lifetimes by a factor of up to approximately fifty over partially encapsulated specimens, for a given applied temperature range. Further, whereas a transition from low temperature range voiding to higher temperature range dislocation-induced damage was observed at around 250 °C during AC stressing of partially encapsulated lines, this did not take place in fully encapsulated specimens, which exhibited only void damage. We will discuss these observations in terms of the relative contributions and interactions of diffusive, thermal, and mechanical damage mechanisms for the different test conditions.
10:00 AM - FF11.2
Computational Studies of Effect of Electron Doping on the Hardness of Aluminum Magnesium Boride.
Scott Beckman 1
1 Material Science and Engineering, Iowa State University, Ames, Iowa, United States
Show AbstractAluminum magnesium boride, AlMgB14, is an ultrahard material, having a hardness greater than 30 GPa. It is an ordered nanostructure made of orthorhombically stacked boron icosahedra. The icosahedra are joined by two B atoms that rest in the plane of the icosahedra. Between the stacked sheets of B is an atomic layer of Al and Mg. It is experimentally observed that doping with Si increases the hardness by 5-10 GPa. Using ab initio methods the structure and properties of AlMgB14 is studied. The influence of substitutional doping is investigated by modifying the number of electrons in the calculation (a jellium background charge is used to maintain overall charge neutrality). It is observed that the charge in the vicinity of the Al atom is strongly distorted whereas the Mg atom's charge is not as effected. Presumably the charge from the Al atom is moved to the B nano-layer. It is observed that increasing the number of electrons in the calculation results in a decrease in the B—B bond length. This suggests that doping with an electron donor, such as substitutional Si on an Al site, will result in increasing the B—B bond strength. The hardness and bond lengths are reported as a function of electron doping and isostatic dilatation.
10:15 AM - FF11.3
Modeling the Sliding Wear of Single Asperity Copper/Aluminum Contacts Under Electromagnetic Stress with Atomistic+Continuum Multi-Scale Simulations.
John Crill 1 , Douglas Irving 1 , Don Brenner 1
1 , North Carolina State University, Raleigh, North Carolina, United States
Show AbstractThe integrity of metallic sliding surfaces under an applied electrical potential is highly influenced by the wear of individual contacting asperities. For systems comprised of copper and aluminum, deformation mechanisms include surface grooving, Al melting, and intermixing of Cu atoms into molten Al, all of which can be dependent on sub-surface dislocation emission and propagation. To investigate the relationship between voltage, deformation and micro-structural features, we have carried out simulations of the sliding of a single asperity contact using large-scale molecular dynamics simulations coupled to a continuum treatment of Joule heating and heat transfer. A 200 nm2 facetted asperity was compressed and slid against a flat 1700 nm2 substrate with and without voltage applied. The simulations show asperity wear and plastic deformation to be increased commensurate with the enhanced heating produced by electrical current. Extrication of the effects of frictional heating from Joule-heating, as well as ascertainment of the impact of micro-structural features such as crystal grains, substitutional atoms and precipitates will be discussed and compared to experimentally-observed results in metallic contacts. The work was supported by a Multi-University Research Initiative of the U.S. Office of Naval Research through a subcontract from the Georgia Institute of Technology and by ONR grant N00014-09-1-0330. Prof. C. Padgett is thanked for helpful discussions.
10:30 AM - FF11.4
Thermo-mechanical Behavior of Nano Scale Al Thin Films using a Resonance System.
Seungmin Hyun 1 , Jungmin Park 1 , Hak-Joo Lee 1 , Byung-Ik Choi 1 , Walter Brown 2
1 , Korea Institute of Machinery & Materials, Daejeon Korea (the Republic of), 2 , Lehigh Unviersity, Bethlehem, Pennsylvania, United States
Show AbstractThe mechanical behavior of nano scale thin films has been widely explored due to many applications for such films in MEMS and semiconductors. However the thermo-mechanical behavior of nano scale thin films has not been heavily investigated. In this study, we present the thickness dependent thermo-mechanical behavior of nano-scale aluminum thin films using a resonance system in the thickness range between 30 and 80 nm. Al thin films were sputter deposited onto silicon nitride membranes surrounded by a thick Si frame. The system electrostatically actuates and detects the resonant frequency of the membrane during thermal cycling. The resonant frequency of the composite Al and silicon nitride membrane is converted to stress. The stress of silicon nitride is separately evaluated to extract the stress of the Al thin film only. Al films were thermally cycled up to 300oC and the thermo-mechanical behavior of the films was examined for different film thicknesses. At the end of a thermal cycle, Al films do not show a thickness dependent residual stress at room temperature. However, 30nm thick Al films show large stress relaxation at high temperature. The temperature dependent stress and relaxation behavior in the films will be presented and discussed.
10:45 AM - FF11.5
Thermal Stability and Stress Relaxation of Nanostructured Cu–Nb-based Wires.
Jean-Baptiste Dubois 1 , Ludovic Thilly 1 , Pierre-Olivier Renault 1 , Florence Lecouturier 2 , Marco Di Michiel 3
1 PHYMAT, University of Poitiers, Futuroscope France, 2 , LNCMI, Toulouse France, 3 , ESRF, Grenoble France
Show AbstractThe development of novel nanostructured materials with a combination of functional properties such as high electrical conductivity and high mechanical strength is needed to develop magnets providing non-destructive high pulsed magnetic fields over 80 T. Currently, the best candidates are the Cu-based high-strength nanocomposite wires reinforced with Nb nanostructures (nanofilaments or nanotubes) that are produced by accumulative drawing and bundling (ADB: series of hot extrusion, cold drawing and bundling stages). This severe plastic deformation (SPD) process leads to a multiscale copper matrix containing up to 854 (52.2 106) continuous parallel Nb nanofilaments or nanotubes. These nanocomposite wires combine low electrical resistivity (0.6 μΩ cm at 77 K) and ultra-high strength (2 GPa at 77 K) very much higher than the rule of mixtures predictions.After SPD, high internal stresses appear in the nanocomposites. The niobium exhibits a residual axial tensile stress while the copper exhibits a residual axial compressive stress. In-situ heat treatments under synchrotron high energy x-rays have been performed to study the evolution of the nanostructure and the internal stresses with temperature. Experiments have been carried at the high energy beamline ID15B at the European Synchrotron Radiation Facility (ESRF). The diffracted patterns are continuously collected on a large 2D detector giving access to (i) many reflections and (ii) information about crystalline orientation in each phase. Furthermore, peak positions allow measuring elastic strain of the different crystallographic planes and peak width can be related to microstructure evolution (grain size, inhomogeneous strains).It was evidenced that internal stresses relaxation in niobium occurs at temperatures which are far below from its normal recrystallization temperature. Experiments reveal also a very strong microstructure size dependence of the stress relaxation temperature that decreases when the dimensions become finer. This phenomenon seems to be explained by a proximity effect where the recrystallization of copper in the vicinity of niobium may cause significant early changes in internal stresses. Dimensional dependence of this phenomenon may be related to the high surface-to-volume ratio of grains in nanocrystalline materials, allowing for increased relaxation of intragranular stresses thanks to discrete changes in the atomic structure of grain boundaries and interfaces.
11:30 AM - FF11.6
Tuning the Mechanical Properties of a Nanoporous Gold.
Hai-Jun Jin 1 , Lilia Kurmanaeva 1 , Joerg Weissmueller 1 2
1 Institute of nanotechnology, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 Technische Physik, Universität des Saarlandes, Saarbrücken Germany
Show AbstractThe conventional approach towards tailoring the mechanical properties of materials is to manipulate their atomic structure or microstructure, for example by changing the grain size of a polycrystalline material or by controlling the diameter of a (nano-) wire or pillar. However, instead of these “permanent” changes one might also envisage to manipulate control parameters that allow a reversible tuning of the materials mechanical response on demand and during service. Here we introduce a new concept to develop a “smart” material which allows prompt and reversible variation of its mechanical properties. The idea is essentially to combine two well-documented facts: first, the crystal deformation is greatly influenced by the surface as marked by the well known size-effect, and second, the surface parameters (e.g., surface stress, surface energy) can be controlled by chemical or electrochemical means. Thus it is expected that the mechanical properties of a high specific area crystalline material will respond to changes in the chemical or electrochemical environment.Our experiments use bulk samples of nanoporous gold prepared by dealloying, which have a very small structure size and large surface area. More importantly, the material, if prepared carefully, can undergo compression with appreciable plastic strain which even allows strain rate sensitivity measurements [1]. We demonstrate in this material that both the yield stress and the brittleness can be “tuned” in an electrochemical environment. Compression tests were preformed in situ with samples immersed in the electrolyte (HClO4) and with potentiostatic control. Potential changes allow a reversible variation of the flow stress by as much as the factor two. Furthermore, the samples are obviously more brittle at positive potential. Flow stress changes were observed during cyclic OH adsorption/desorption as well as during capacitive charging in the double layer region. Effects of structure size and solid fraction were also documented. The underlying mechanisms will be discussed in terms of surface dislocation nucleation and dislocation motion concerning the roles of the surface stress, surface energy and even the surface diffusivity. [1] H.J. Jin, L. Kurmanaeva, J. Schmauch, H. Rösner, Y. Ivanisenko, J. Weissmüller, Acta Mater. 57 (2009) 2665-2672.
11:45 AM - FF11.7
Hydridation and De-hydridation Behavior of Nanoporous Palladium-Nickel Ultra-thin Films.
Wen-Chung Li 1 , T. John Balk 1
1 Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky, United States
Show AbstractNanoporous palladium-nickel (np-PdNi) films were prepared by dealloying of co-sputtered Pd and Ni alloy films on Si substrates. Microstructure and composition were characterized using electron microscopy and energy dispersive x-ray spectrometry. Hydrogen absorption and desorption behavior was investigated by measuring changes in film stress with wafer curvature. Np-PdNi films exhibit a crack-free and uniform porous structure, with pores and ligaments as small as 5 nm through the film thickness (25 nm). Depending on dealloying conditions, Ni content in the final np-PdNi structure can be either 10 at.% or 30 at.%. The effect of residual Ni on hydrogen cycling behavior of np-PdNi films is discussed in relation to the film microstructure. Overall, np-PdNi films exhibit short response times to reach saturation during hydriding. They have good mechanical stability and increased sensitivity to hydrogen cycling, due to their high amount of surface area, and show promise for hydrogen sensing applications.
12:00 PM - FF11.8
Multi-scale Simulation on H embrittlement and H-Pd interactions.
Hieu Pham 1 , Tahir Cagin 1 2
1 Chemical Engineering, Texas A&M University, College Station, Texas, United States, 2 Materials Science and Engineering, Texas A & M University, College Station, Texas, United States
Show AbstractThe hydrogen-charged palladium system has been the subject of tremendous experimental studies for the last two decades, as the reports on its anomalous behaviors draw such a big interest. However, due to the complication of their microscopic origins, the controversy over many phenomena still continues that needs novel approaches to elucidate the problem. In our work, various levels of theory, namely Density Functional Theory (DFT) and classical molecular dynamics (MD) simulations, will be used to study the problem of H-Pd interaction, H-loading and H-embrittlement in Palladium under different thermal and mechanical conditions. This study focuses on the behaviors of Pd defects, such as vacancy, interstitials and grain boundary (GB), under the influence of H segregations. The dynamic loading of H into Pd should be in a strong coupling with chemistry, microstructure and mechanics. In the first part, ab initio calculations will be carried out (using VASP simulation program) for moderate-size cells of Pd containing point and planar defects. The interaction of H with different kinds of crystal defects will be reported. The presence of grain boundary in material is of special interest as it is essentially related to the issue of impurity diffusion, segregation and embrittlement. We use the concept of binding energy to demonstrate the tendency that H atoms are bound to the Pd GB. Big binding energy values indicate the chemical nature of these bonds, instead of physisorption. Also, the investigation on behaviors of Pd atoms around GB under the concentration of H may reveal the mechanism of H-embrittlement in Pd and the efficiency of H-charging. To overcome the size limitation and short-time scale of first-principles quantum mechanical method, we employed the condensed phase molecular dynamics simulations, in which an embedded atom method (EAM) is used to express the total energy of metal atoms. Larger-scale simulations, therefore, are conducted (by LAMMPS simulation package) for supercell models containing up to 10,000 Pd atoms. In contrast to DFT, these simulations will be able to include the mechanical and thermal effects to the H-Pd interactions, ranging from ambient to extreme conditions (high temperature, pressure and H-loads). Again, the energetics, thermodynamics and microstructure of Pd GB under H-loading are investigated in order to understand the nanoscale nature of H-Pd interactions and H embrittlement.
12:15 PM - FF11.9
Nanocomposites with Stimuli–Responsive Mechanical Behavior.
Kadhiravan Shanmuganathan 1 , Jeffrey Capadona 2 3 , Dustin Tyler 2 3 , Stuart Rowan 1 2 4 , Christoph Weder 1 5
1 Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, Ohio, United States, 2 Dept of Biomedical Engineering, Case Western Reserve University, Cleveland , Ohio, United States, 3 Rehabilitation Research and Development, Louis Stokes Cleveland DVA Medical Center, Cleveland , Ohio, United States, 4 Department of Chemistry, Case Western Reserve University, Cleveland , Ohio, United States, 5 Adolphe Merkle Institute, University of Fribourg, Route de l'Ancienne Papeterie , Marly, Switzerland
Show AbstractPolymer nanocomposites that exhibit stimuli responsive changes in morphology and mechanical behavior are interesting materials for ‘smart’ protective devices or adaptive biomaterials. Here we report on a new class of biomimetic stimuli-responsive nanocomposites which change their mechanical properties upon exposure to physiological conditions. Cellulose nanofibers or “whiskers” derived from tunicates were used as reinforcement in poly(vinyl acetate) matrix. Below the glass-transition temperature (Tg ~40°C), a modest increase in elastic storage modulus from 1.8 GPa to 5 GPa with only 12-16% v/v whiskers was observed. The reinforcement was much more dramatic above Tg (i.e. neat = 0.9MPa, and 16.5% v/v whisker nanocomposite = 620 MPa). Upon exposure to physiological conditions with modest swelling (70 %) the 12-16%v/v whisker nanocomposites showed a drastic decrease in storage modulus (1000 fold) from ~5 to 12 MPa. This dramatic mechanical morphing as a result of changing nanoparticle interactions is described in the framework of two mechanical models: percolation model and the Halpin Kardos model based on a mean field approach. The high contrast in mechanical behavior, the temperature range (23°C to 37°C) and time (1hr) required for switching opens up broad range of applications for these nanocomposites as adaptive biomaterials.
12:30 PM - FF11.10
Anisotropic Strain and Training of Conducting Polymer Artificial Muscles under High Tensile Stresses.
Keiichi Kaneto 1 , Hikaru Hashimoto 1 , Kazuo Tominaga 1 , Tomokazu Sendai 1 , Wataru Takashima 1
1 Life Science and Systems Engineering, Kyushu Institute of Technology,, Kitakyushu Japan
Show AbstractElectrochemomechanical deformation (ECMD) in polypyrrole (PPy) and Polyaniline (PANi) films under tensile stresses up to 5 MPa was studied. PPy film was electro deposited in pyrrole/dodecylbenzene sulfonic (DBS) acid/methylbenzoate solution with the thickness of 20-30 μm. PANi was polymerized by the common chemical method, and the film was obtained by casting the NMP solution of emeraldine base. The PPy/DBS film was electrochemically cycled in LiCl, showing cation movement. The cyclic stroke of ECMD in PPy/DBS film was approximately 2% by insertion and extraction of cations. The stroke of ECMD depended linearly on the amount of oxidative charge. By the application of tensile stresses higher than 2 MPa, a creeping associated with conformation change, slipping and/or breaking of polymer chains was observed. The creeping was 25% at the tensile stress of 5MPa. A nonlinear ECMD against the injected charges was observed under tensile stresses above 2MPa. Upon removal of high tensile stresses, the creeping was recovered to some extent, and interestingly the cyclic stroke was slightly larger than that before the application of large tensile stresses. The increased cyclic ECMD is a training of the artificial muscle.The cyclic stroke of ECMD in PANi film was approximately 6%, which was obtained by the operation of rectangular voltage wave form in aqueous 1M HCl, showing anion movement. By the application of tensile stresses more than 1 MPa, a creeping of the PANi film was observed by 20% at 3MPa. After the removal of tensile stress, the creeping was recovered completely to the original length after15-20 electrochemical cycles. Furthermore, the magnitude of cyclic stroke of ECMD was increased by 15 % compared with that before the application of tensile stress. The larger training effect was observed in PANi film.We propose the mechanisms of creeping and training in ECMD of conducting polymers under high tensile stresses. In the electrochemical oxidation of conducting polymers the oxidation occurs by the insertion of anions into the polymer matrix near the polycations (polaron or soliton). The anion may interact with polycations nearby and bridges adjacent polymer chains (cross linkage). The cross linkage may be loosen by the reduction, and the cross linkage should be rearranged by oxidation under larger tensile stresses, resulting in the creeping or hysteresis. The training results from the relaxation of uniaxial anisotropic strain formed during the creeping by the thermal fluctuation and the elasticity of polymer chains.
12:45 PM - FF11.11
Electrochemically-Controlled Mechanical Properties of a Polymer Nanocomposite.
Daniel Schmidt 1 , Fevzi Cebeci 1 2 , Ilke Kalcioglu 2 , Samantha Wyman 1 , Christine Ortiz 2 , Krystyn Van Vliet 2 3 , Paula Hammond 1
1 Chemical Engineering, MIT, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States, 3 Biological Engineering, MIT, Cambridge, Massachusetts, United States
Show AbstractStimuli-responsive polymer nanocomposites hold promise for applications in a number of areas including control of cellular or protein adhesion on surfaces, drug delivery, separations, and biomimetics, among others. Electrochemical stimuli are particularly advantageous as they can be applied rapidly, reversibly, locally (i.e., at an electrode instead of throughout the bulk), and remotely, while maintaining a mild operating environment. Here we present the layer-by-layer assembly of an electroactive polymer nanocomposite thin film containing cationic linear poly(ethyleneimine) (LPEI) and 68 vol% of anionic Prussian Blue (PB) nanoparticles. Incorporation of these PB nanoparticles allows for electrochemical control over film thickness and mechanical properties. Electrochemical reduction of the PB doubles the negative charge on the particles, inducing an influx of water and ions from solution to maintain electroneutrality in the film; swelling and increased elastic compliance of the film result. Upon cycling of the applied potential between -0.2 V and +0.6 V (versus Ag/AgCl), reversible film swelling on the order of 2-10% was measured with spectroscopic ellipsometry and electrochemical atomic force microscopy. Reversible changes in the elastic modulus of the hydrated composite film on the order of 50% (from 3.40 GPa to 1.75 GPa) were measured with in situ nanoindentation of these composites in the fully immersed state. These results present a new framework for electrically modulating the stiffness of a composite. Further, this swellable nanocomposite system opens up the possibility of investigating disruptable percolative networks in which interactions between nanoparticles are turned on and off with an electrochemical trigger, resulting in more dramatic mechanical changes. This work serves as a starting point for further studies on mechanomutable coatings with potential future applications in micro- and nanoscale devices.
FF12: Mechanics of Nanocomposite Materials
Session Chairs
Bob Keller
Erica Lilleodden
Friday PM, December 04, 2009
Constitution B (Sheraton)
2:30 PM - **FF12.1
Multiscale Modeling of Nanoceramic Composites.
William Curtin 1 , E. Byrne 2 , M. McCarthy 2 , F. Pavia 1 , Z. Xia 3
1 , Brown University, Providence, Rhode Island, United States, 2 , University of Limerick, Limerick Ireland, 3 , University of Akron, Akron, Ohio, United States
Show AbstractThe impressive mechanical properties of carbon-nanotubes are driving research into the creation of new strong, tough nanocomposite systems. Due to the nanolength scale of the reinforcements, both traditional and new phenomena are relevant for designing these materials. Here we present an underlying concept for designing strong and tough ceramic nanocomposites based on multiwall carbon nanotubes with controlled interwall coupling via sp3 bonding (MWCNT-sp3). We first show via molecular modeling that such nanotubes can be stronger than either single-wall or multiwall nanotubes that do not have interwall bonding, which is attributable to the reinforcing effects of the interwall bonding and consistent with recent experiments1. Use of these properties in composite strength theory then shows that composites based on MWCNT-sp3 can perform comparable to the best possible composites that contain small-diamater SWCNTs [1]. We also show that load transfer from outer to inner walls in MWCNT-sp3 depends on the degree of sp3 interwall bonding and can be quantitatively understood using a shear-lag model. We then put MWCNT-sp3 in a surrounding diamond matrix without any interfacial bonding and show that they can have higher frictional sliding stresses than SWCNTs or MWCNTs, which should enhance composite strength further [2]. Adding interface bonding between fibers and matrix, via direct bonding and C interstitials, we quantify the extra energy dissipation and damage mechanisms that can occur during fiber pullout. Collectively, these results show that careful tailoring of the nanotube geometry (interwall coupling, diameter, wall thickness), interface properties, and residual stresses, may permit engineering of these materials for high hardness and damage tolerance at submicron scales, making them excellent candidates for wear-resistant coatings. 1. E. Byrne et al., Phy. Rev. Lett. (2009)2.L. Li et al., J. Am. Cer. Soc. (2009)
3:00 PM - FF12.2
Mechanical and Interfacial Properties of Individual Carbon Nanofibers in Epoxy Matrices.
Tanil Ozkan 1 , Qi Chen 2 , Ioannis Chasiotis 2
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Aerospace Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States
Show AbstractThree grades of vapor grown carbon nanofibers (pyrolytically stripped, heat-treated, heat-treated and oxidatively treated) with diameters 100-200 nm were tested for their tensile strength and interface adhesion properties to epoxies by a novel MEMS-based mechanical testing platform. The nominal tensile strengths of the pyrolytically stripped and the heat-treated nanofibers followed Weibull distributions with Weibull characteristic strength between 2.8-3.3 GPa. The true material strengths excluding the hollow fiber cores were twice the nominal strengths. The nanofiber fracture surface had all the elements of brittle fracture with concurrent slip of the oblique graphene layers comprising the nanofibers. SEM and TEM images of matching ruptured surfaces pointed to a failure geometry that agrees with the “dixie cup” structure of vapor grown carbon nanofibers, which is actually the limiting factor in the tensile strength of this class of nanofibers. Furthermore, pull-out experiments of individual vapor grown carbon nanofibers provided their average interfacial shear strength inpolymeric matrices. The average interfacial shear strength of as-fabricated (pyrolytically stripped) vapor grown carbon nanofibers was 50% hiugher than that of high temperature heat treated nanofibers, which points out to the need for surface functionalization after fiber heat treatments.
3:15 PM - FF12.3
Carbon Nanotube Pullout from a Diamond Matrix.
Fabio Pavia 1 , William Curtin 1
1 , Brown University, Providence, Rhode Island, United States
Show AbstractDiamond-CNT nanocomposites have a broad range of applications because of the exceptional mechanical properties of both matrix and fibers. The interface plays a key role in determining strength and toughness of ceramic composites, and so here it is studied using molecular dynamics. The deformation of the interface during quasistatic pullout of the carbon nanotube is investigated as a function of the degree of interfacial bonding between the matrix and the outer nanotube wall, as well as the degree of interwall coupling inside the nanotube. Preliminary results using the REBO potential for C-C bond formation and breaking indicate that bonding between matrix and carbon nanotube can be tailored to increase the toughness of the modeled nanocomposite. The original REBO potential can show inappropriate fracture mechanisms and overestimation of the stress for bond breaking. We have thus used a modified potential introduced by Pastewka et al. [1] that uses an environmental screening coefficient to better capture bond breaking and reforming. Results will be reported on energy dissipation and interfacial "sliding" as a function of material parameters, and related to macroscopic composite performance. [1] Lars Pastewka et al., "Describing bond-breaking processes by reactive potentials: importance of an environment-dependent interaction range", Phys. Rev. B 78, 161402(R)(2008)
3:30 PM - FF12.4
Direct Measurement of the Matrix-Filler Interactions in A Graphene-Polymer Nanocomposite and Their Relationship to the Composite's Macroscopic Mechanical Performance.
Minzhen Cai 1 , Sarah Cotts 2 , David Kranbuehl 2 , Hannes Schniepp 1
1 Department of Applied Science, The College of William and Mary, Williamsburg, Virginia, United States, 2 Department of Chemistry, The College of William and Mary, Williamsburg, Virginia, United States
Show AbstractWe produce single-layer functionalized graphene sheets via exfoliation of oxidized graphite in gram quantities. Tuning the number of hydroxyl and epoxide surface groups of these sheets allows us to adjust their interfacial properties and thus to achieve compatibility with a large range of solvents ranging from water, alcohols, tetrahydrofuran, to dimethylformamide. Employing this high solvent compatibility, we produce nanocomposites by dispersing the functionalized graphene sheets in many polymers, including polyetherimide, polymethylmethacrylate, and polyvinyl-alcohol via solution processing. Due to the outstanding mechanical properties of graphene, some of these nanocomposites exhibit significantly enhanced properties at nanofiller concentrations of as little as 1%—for example a several-fold increase of the Young's modulus. In order to develop nanocomposites with the best possible mechanical performance, we want to understand the importance of the matrix–filler interface. We therefore developed two independent methods to measure the polymer–graphene interactions at the level of individual sheets based on scanning probe techniques. We compare the results of these measurements at the nanoscale with stress-vs-strain curves determined via mechanical testing of macroscopic samples. Using these tools we can measure how tuning the surface functionality of the sheets influences their interaction with the polymer matrix, and we can determine for which degree of interaction the best mechanical performance of the composites is observed.
3:45 PM - FF12.5
Design of Co-Continuous Composite Materials for Stiffness, Strength and Energy Dissipation.
Lifeng Wang 1 , Mary Boyce 1
1 Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMechanical behavior is a critical function across of natural and engineered solids. The combination of hard and soft materials can enable an outstanding combination of mechanical performance properties including stiffness, strength, impact resistance, toughness, and energy dissipation as has been demonstrated in biological materials such as bone, nacre and exoskeleton. The mechanical properties of the constituent materials and the relative geometric arrangement of the constituents provide avenues for truly engineering and tailoring the resulting composite properties. In this work, we demonstrate that the mutual constraints each constituent phase imposes on the other constituent phase in co-continuous microstructures can lead to unique and superior mechanical behaviors. Triply periodic minimal surfaces have been of great interest to physical scientists, material scientists and biologists; these co-continuous structures have been observed in block copolymers, micellar materials, nanocomposites, cell membranes, and biological exoskeletons. Here we consider co-continuous structures with simple cubic (SC), body-centered-cubic (BCC), and face-centered cubic (FCC) Bravais lattices, which correspond to the triply periodic minimal surfaces known as the tubular P, I-WP, and F-RD respectively. Prior works have found: (1) these level set structures to exhibit better elastic properties than their rod-connected model counterparts; 2) these structures to provide multifunctional optimization, such as simultaneous optimization of transport of heat and electricity; (3) these structures can be scaled down and fabricated at submicron length scales that enables the coupling of mechanical deformation with photonic or phononic properties. Herein, we explore the linear and nonlinear mechanical behavior of these structures including their elastic stiffness, yield, post-yield, and dissipative behaviors. Finite element based micromechanical modeling of the co-continuous composites are constructed. The large deformation mechanical responses of these 3D periodic cocontinuous composites are investigated using a periodic representative volume element (RVE) of the microstructure together with the nonlinear constituent material stress-strain behaviors where the combinations of polymer/elastomer, polymer/metal, and polymer/polymer are considered for these two-phase composites. The properties obtained for these co-continuous structures are compared with conventional particle-reinforced composites, fiber-reinforced composites, and lamellar composites. We show that 3D periodic cocontinuous composites with interfaces that are the minimal surfaces can have great mechanical performance achieving a unique combination of stiffness, strength and energy absorption. These results provide guidelines for engineering and tailoring the nonlinear mechanical behavior and energy absorption of the composites for a wide range of applications.
4:30 PM - FF12.6
The Mechanical Response of Ag-Based Metallic Nanoinks.
Andrew Birnbaum 1 , Ray Auyeung 1 , Ji Wen Wang 1 , Kathy Wahl 2 , Maxim Zalalutdinov 3 , Alberto Pique 1
1 Materials Science and Technology Division, Naval Research Lab, Washington, District of Columbia, United States, 2 Chemistry Division, Naval Research Lab, Washington, District of Columbia, United States, 3 Acoustics Division, Naval Research Lab, Washington, District of Columbia, United States
Show AbstractNon-lithographic techniques for fabricating micro and nanostructures such as ink-jetting and laser forward transfer have received significantly increased interest recently due to their inherent flexibility. These processes have facilitated the fabrication of high resolution, three dimensional structures and devices on non-planar, flexible and low temperature substrates. However these techniques rely on the non-lithographic deposition of nanosuspensions which give rise to partially dense agglomerations of nanoparticles upon deposition and curing. In order to employ these techniques to fabricate devices for MEMS-like applications, the mechanical behavior of the cured nanosuspensions must be characterized and understood. In fact, due to their nanoporous nature, the mechanical response of the resulting structures tend to deviate significantly from their solid, fully dense counterparts, thus requiring that either the structure be modified or the material be altered in order to achieve a desired functionality (or behavior). This work utilizes nanoindentation to characterize the mechanical properties of nano-porous, silver (Ag) based metallic nanoinks. Cross sectional analysis of individual indents is also presented via the utilization of a focused ion beam (FIB). Finally, the dynamic response of laser transferred Ag nanoink beams are also characterized by laser micro-vibrometry, while the static responses of these micro-beams are also probed via nanoindentation.
4:45 PM - FF12.7
Heterogeneity in Epoxy Nanocomposites Initiates Crazing: Significant Improvements in Fatigue Resistance and Toughening.
Iti Srivastava 1 , Yue-Feng Zhu 2 , Catalin Picu 1 , Nikhil Koratkar 1
1 Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 2 Department of Mechanical Engineering, Tsinghua University, Beijing China
Show AbstractA major limitation of thermosetting epoxy polymers is their brittle failure, low toughness and poor resistance to fatigue. While toughness in such systems can be enhanced by adding rubber particles and plasticizers, these fillers decrease the stiffness of the composite and lower its glass transition temperature which severely limits their utility. In this paper we report that amido-amine (–NHCH2CH2NH2) functionalized multiwalled carbon nanotubes (A-MWNT) can be used to fundamentally alter the fracture behavior of thermosetting epoxies by causing them to Craze. This increases the ductility of the epoxy as manifested by a 10-fold lowering in fatigue crack growth rate coupled with significant enhancements in the strain-to-break, and fracture toughness. Importantly these enhancements were obtained without sacrificing the stiffness/strength of the composite. In fact the Young’s modulus was ~30% larger and the average hardness was ~45% larger for the nanocomposite epoxy compared to the pristine epoxy.Crazing is a failure mode of bulk polymers and occurs under predominant uniaxial tensile load when the bulk eventually forms denser ligaments (or fibrils) while preserving its continuity. The bridging of cracks by such fibrils is an important mechanism for energy dissipation and toughening in polymers. While craze phenomena are routinely seen in the fracture of thermoplastic polymers, crazing is typically not observed in thermosetting polymers such as epoxies due to the high cross linking density of the epoxy chains, which limits molecular mobility and inhibits craze fibril formation. We show that the reason for crazing in our system is heterogeneous epoxy crosslinking near the A-MWNT fillers which induces large localized fluctuations in material properties. Differential scanning calorimetry analysis indicated that a significant amount of unreacted epoxy is kinetically trapped in the crosslinked matrix structure that is formed at the A-MWNT/epoxy interface. Such local heterogeneity in the curing may be caused by a variety of factors such as, for example, the fact that the chemistry may be modified locally due to the presence of amido-amine groups. Epoxy chain alignment, which is known to influence the crosslinking density, may also be modified locally due to presence of these functional groups. Heterogeneous crosslinking results in localized pockets of enhanced molecular mobility; the correlated evolution of such mobile regions under mechanical load can initiate crazing as observed in our experiments. It should be noted that the heterogeneities that we report here are localized to the nanotube–matrix interfaces since we do not observe any reduction in the global (macroscale) stiffness, hardness, or strength of the epoxy.
5:00 PM - FF12.8
Electrochemically-Induced Percolative Mechanical Properties of Polyaniline Nanofiber-Polymer Composites.
Fevzi Cebeci 1 , Daniel Schmidt 1 , Christine Ortiz 2 , Paula Hammond 1
1 Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractStimulus-responsive percolative materials are of great interest due to expected dramatic changes in properties, in particular mechanical behavior, below and above the percolation threshold whereby the responsive units contact each other and form a continuous path through the entire system. Responsive constituents with high aspect ratios are expected to produce extremely low (as low as 1% volume fraction) percolation thresholds. In this work, we expect to create an easily disruptable percolative network that will yield impactful changes in composite modulus. Here, we describe the design of an electrochemically-responsive, percolative composite composed of stiff, high aspect ratio, conducting polymer nanofibers interdispersed with more compliant macromolecular spacer layers. Specifically, the responsive nanofibers are polyaniline interdispersed with hydrogen-bonded layer-by-layer (LbL) assemblies of poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA). Synthesis of polyaniline nanofibers was carried out similarly to previously published methods in the literature using an ammonium persulfate (oxidant)-hydrochloric acid (dopant) solution added to an acidic aniline and p-phenylenediamine solution. The average length and diameter of the nanofibers were calculated as approximately 2μm and 40nm by TEM, respectively, with an aspect ratio of 50. The mechanical properties of the nanocomposite films (10 layers of polyaniline embedded within 20 bilayers of PEO/PAA) were investigated upon application of an electric potential of -0.2V for reduction and 0.4V for oxidation vs. Ag/AgCl by aqueous atomic force microscopy-based nanoindentation. Incorporation of nanofibers into the PEO/PAA LbL films increased the indentation modulus of the nanocomposite film, and upon application of electrochemical potentials (at -0.2V and 0.4V vs. Ag/AgCl), the films were found to undergo several-fold changes in indentation modulus. The effects of multilayer assembly architecture (i.e., volume fraction of the nanofibers and composition of the spacer layers in the film) on the mechanical switching of the nanocomposite films were also explored. Such "mechanomutable" material systems hold great potential for a variety of applications such as responsive armor textiles and substrates for tunable cell interactions for tissue engineering.
5:15 PM - FF12.9
The Mechanical Properties of Brush-Coated Nanoparticles in Homopolymer Matrix.
Sascha Pihan 1 , Sebastian Emmerling 1 2 , Ruediger Berger 1 , Jochen Gutmann 1 2
1 , Max-Planck-Institute for Polymer Research, Mainz Germany, 2 Institute of Physical Chemistry, Johannes Gutenberg University, Mainz Germany
Show AbstractThe central problem in preparation of nanocomposite materials is the intrinsic incompatibility between the high energy inorganic filler surfaces (e.g. SiO2) and the lower energy polymer matrix (e.g. polystyrene). The approach to graft polymer chains to inorganic nanoparticles can drastically enhance the dispersion of particles in a homopolymer matrix. Furthermore the mixture of the brush-coated nanoparticles with different molecular weight homopolymer allows tailoring mechanical properties of the composite material [1].In our studies we prepared polysilsesquioxane nanoparticles having diameters of 15 to 30 nm [2]. Polysilsesquioxane-synthesis was chosen because it offers the possibility to create very small particles with a polydispersity down to 0.13. In a “grafting from” step we used atom transfer radical polymerization (ATRP) to grow poly-ethyl-methyl-acrylate (PEMA) brushes on these particles yielding well defined PEMA-grafted-microgels (PEMA-g-µgels). We expect that the PEMA brush interface determines the structure of homopolymer surrounding phase significantly. The above mentioned preparation route enabled us to separately control the diameter of the particles and the length of the polymer brushes (35 nm to 50 nm).The radii of individual nanoparticles were investigated by means of non-contact scanning probe microscopy (SPM) and dynamic light scattering (DLS). By analyzing the phase contrast images we were able to distinguish between the core particle and the brush. From the SPM analysis we could determine the size of the core and the surrounding, independently. We found that the calculated brush-length and the core diameter from DLS measurements are significantly smaller compared to our SPM investigations attributed to swelling effects of the core particle and the PEMA brush. Mechanical properties of the above mentioned PEMA-g-µgel system are expected to depend on the core material, grafting density, molecular weight of the grafted polymer and molecular weight of the homopolymer matrix. In order to avoid exhaustive synthesis of PEMA-g-µgels we developed a nanomechanical cantilever sensor method to analyze the mechanical properties of nanogram materials. We deposited materials by inkjet printing of toluene dissolved PEMA-g-µgels dispersed in PEMA homopolymer matrix. Dried structures were then analyzed by investigating the vibration spectra of the nanomechanical cantilever [3].The analysis of a thin, homogeneous polymer film made by plasma polymerization of norbornene has revealed that the applied nanomechanical method is suitable to determine viscoelastic properties. The Young’s modulus derived from cantilever resonance-shift is consistent with complementary measurements. The investigation of thin heterogeneous films made from PEMA-g-µgels and PEMA homopolymer is currently performed.1. McEwan, M. et al., Soft Matt., 2009, 1705-1716.2. Jakuczek, L. et al., Polymer, 2008, 843-856.3. Whiting, W. et al., Mat. Sci. Eng., 1994, 35-38
5:30 PM - FF12.10
Disclosing Mechanical Properties of Composite Magnetic Nanoparticles,
Antonio Rinaldi 1 2 , Veronica Salgueirino 3 , Miguel Correa-Duarte 3 , Silvia Licoccia 1 , Enrico Traversa 1 4 , Ana Davila-Ibanez 3 , Pedro Peralta 2 , Karl Sieradzki 2
1 NAST center and Department of Chemical Science and Technology, Univ Rome Tor Vergata, Rome Italy, 2 MAE, Arizona State University, Tempe, Arizona, United States, 3 Departamentos de Física Aplicada e Química-Física, Universidade de Vigo, Vigo Spain, 4 4International Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), Tsukuba Japan
Show AbstractMechanical response is actively pursued for NEMS applications. Central to these efforts is addressing the relationship between confined volumes (nanosized structures) and applied stress. The mechanical properties of single-phase nanocrystalline materials have been typically measured and interpreted but none of the many and more complex multi-phase nanostructures that can be produced nowadays seem to have been explored yet. Part of the reasons lays in the inherent and fastly-growing complexity of the mechanical behavior of materials other then simpler single-phase or isotropic cases. The present study aims to indicate a conceptual approach to tailor or adapt the basic tools (e.g. the standard Oliver-Pharr's method) for complex composite nanostructures. By taking into account core-shell structured silica-coated cobalt boride nanocomposites (with average outer diameter of 100nm and 10nm coating skin), we discuss several fundamental processes that can govern nanomechanics. The complex nature of these nanoparticles (simultaneously composite, non-crystalline, clustery, defective, and magnetic) sets these objects apart from nanoparticles examined previoulsy. Different mechanisms (of mechanical, chemical and magnetic types) contributes to a characteristic mechanical response made of three regimes associated with elastic and plastic (with low and high hardening rates) deformation. A combination of quantitative in-situ compression tests and contact theory, aided by finite-element analysis (FE) for estimating Young’s modulus and hardness, led us to show the direct correlation between mechanical data with nanostructural evolution. A hardening effect driven by the applied pressure was observed in analogy to what reported in literature for other nanoparticles or highly confined structures under compression. Beyond elasticity, the onset of plasticity can be traced back to the presence of dislocation-like radial defects (observed via HRTEM) and resolved shear stresses supposedly greater than 1 GPa (estimated via FE). The pre-existent distribution of such radial dislocations found in the cobalt boride core represents itself a signature feature of such nanocomposites and reflect the internal ordering resulting from the inner electrostatic and magnetic interactions. Also, reverse plasticity and strain hardening are highlighted experimentally and discussed in light of this distinctive nanostructure.
5:45 PM - FF12.11
Modeling of Interfacial Friction in Amorphous Carbon/Nanotube Nanocomposites.
Lili Li 1 3 , Zhenhai Xia 1 , W. Curtin 2 , Yanqing Yang 3
1 , The University of Akron, Akron, Ohio, United States, 3 School of Materials, Northwestern Polytechnic University, Xi'an China, 2 Division of Engineering, Brown University, Providence, Rhode Island, United States
Show AbstractAmorphous carbon matrix nanocomposites reinforced with nanoscale reinforcement such as carbon nanotubes (CNTs) and nano-particles are promising as next generation super-tough and wear resistant coatings. The strength and toughness properties of these composites are determined by the interface between reinforcement and amorphous-phases. Here, interfacial sliding and friction in CNT/amorphous carbon composites are analyzed using molecular dynamics simulations. We investigate pullout in a nanoscale composite using a unit cell composed of one carbon nanotube or a nanofiber and surrounding amorphous carbon matrix with a certain degree of sp2/sp3 interfacial bonding. The nanotubes full of amorphous carbon as nanofibers were compared with the tube configurations. The results show that the interfacial bonding break and formation play a key role in interfacial friction. In the case of no chemical bonding formation at the interface, the nanoscale roughness determines interfacial friction and it is found that the frictional behavior can be described by a molecular friction law.