Andrew Minor University of California-Berkeley
Conal Murray IBM T. J. Watson Research Center
Nobumichi Tamura Lawrence Berkeley National Laboratory
Lawrence Friedman The Pennsylvania State University
Air Force Office of Scientific Research
IBM T.J. Watson Research Center
Tuesday PM, April 14, 2009
Room 3011 (Moscone West)
9:30 AM - **II1.1
Recent Progress in Direct-Observation Contact Mechanics in the Transmission Electron Microscope.
Oden Warren 1 , Zhiwei Shan 1 , S.A. Syed Asif 1 Show Abstract
1 , Hysitron, Inc., Minneapolis, Minnesota, United States
Contact mechanics has advanced beyond continuum models to atomistic simulations; therefore, it would be attractive to have a real-time “atomistic” contact probe at one’s disposal. Combining a nanoindenter with a transmission electron microscope (TEM), to create an in-situ nanoindentation technique, is a major step in this direction. Direct observation of microstructure evolution, e.g. dislocation bursts or phase transformation, in response to quantitative contact probing (force vs. displacement measurement) would be possible by this pairing. Recently, we have developed several in-situ nanoindentation instruments that enable imaging the deformation with the TEM during load- or displacement-controlled nanoindentation, compression, or bend tests. Nanoindentation has been carried out on polycrystalline films and single-crystal materials, compression tests have been performed on nanopillars and nanospheres, and bend tests have been conducted on nanowires. This presentation describes the novel instrumentation and provides several remarkable scientific results about deformation mechanisms in small volumes. It is also our hope to present in-situ nanotribology data from our ongoing development of three-dimensional force sensors for the TEM. Our efforts, as well as those of others, have substantially removed the spatial resolution gap between contact experiments and simulations, although a very large temporal resolution gap still remains.
10:00 AM - II1.2
Dislocations Nucleation and Starvation in Metallic Nanowires.
Scott Mao 1 Show Abstract
1 Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania, United States
Nanowires (NWs) are versatile building blocks for fundamental studies in nanoscience, and for the assembly of functional devices and integrated systems that promise to impact diverse areas of technology. Owing to their small length scale and high surface area/volume ratio, NWs exhibit unique mechanical properties as compared with bulk materials. The issue we focus on is size-mediated plasticity. In NWs, the free surface will act as both dislocation nucleation and sink source. The dislocation activities are determined by the length-scale competition between the NW diameter d and the dislocation splitting distance. It’s been found the mechanical deformation behaviors of single-crystalline nickel nanowires are quite different from their large size with MD simulation. Correlation between the obtained stress-strain curves and the visualized defect evolution during deformation processes clearly demonstrates that a sequence of complex dislocation slip processes results in dislocation starvation, involving dislocation nucleation, propagation and finally escaping from the wire system, so that the wires deformed elastically until new dislocation generated. This alternating starvation of dislocations is unique in small-scale structures.
10:15 AM - II1.3
Examining Nanomechanical Properties of Mg alloys through Quantitative in situ TEM Compression Testing
Jia Ye 1 , Raj Mishra 2 , Andrew Minor 1 Show Abstract
1 , Lawrence Berkeley National Laboratory, Berkeley, California, United States, 2 , General Motors Research and Development Center, Warren, Michigan, United States
Mg alloys hold great promise for applications in the automotive industry due to their low density and high strength. However, because of their hexagonal close-packed (HCP) structure, they demonstrate severe plastic anisotropy, which lowers their practical formability. Understanding the fundamental deformation mechanisms, especially at the nanometer scale, has become a priority for Mg alloy development. In this study, we used in-situ TEM nanocompression testing to investigate the nanometer scale deformation mechanism of two Mg systems: (1) pure Mg and (2) Mg-0.2wt.% Ce alloy. The pure Mg demonstrated basal plane sliding and extensive twinning behavior. Importantly, our in situ technique measures the stress imposed at the point of twin nucleation and during the progression of the twinning. Our results show that for pure Mg, the critical stress for twinning is about two times higher than for basal plane sliding. In contrast, the two stresses are about the same for MgCe alloy. Moreover, twinning is suppressed in the MgCe alloy, while dislocation plasticity is noticeably more active. These observations reveal explanations as to the reduced texture and improved elongation of bulk MgCe alloys and shed light on future Mg alloy design principles.
10:30 AM - II1.4
Characterizing the Mechanical Behavior of Nanoporous FCC Noble Metals.
Wen-Chung Li 1 , Ye Sun 1 , Jia Ye 2 , Andrew Minor 2 3 , T. John Balk 1 Show Abstract
1 Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky, United States, 2 , National Center for Electron Microscopy, Berkeley, California, United States, 3 Materials Science and Engineering, University of California, Berkeley, California, United States
Nanoporous metals with nanoscale ligaments offer a unique opportunity to explore the deformation behavior of highly confined metallic volumes and understand the mechanisms that govern mechanical behavior at the nanometer length scale. The narrow ligaments, with widths of 5 to 15 nm, provide an extreme constraint on the motion of dislocations, which must traverse very small volumes. This presentation will discuss several face-centered cubic noble metals that were fabricated in nanoporous form and mechanically tested. Thin film and bulk samples of nanoporous Au, Pd and Ir exhibit high strength levels and mechanical properties much different from those of their fully dense counterparts. Nanoporous Au exhibits equivalent strength values (calculated with scaling equations) that may approach the theoretical strength level. While the measured stress in thin films corresponds to an equivalent stress of one-half the theoretical shear strength, the equivalent elastic modulus appears to be underestimated by scaling equations for porous materials. This suggests that the equivalent strength of nanoporous Au may be even higher than previously thought. Additionally, the time-dependent mechanical behavior of nanoporous Au and Pd is strongly affected by the high surface-to-volume ratio resulting from the porous structure. Stress relaxation in nanoporous Au thin films was observed at near-ambient temperatures. In nanoporous Pd films, hydrogen was absorbed much more quickly than in dense Pd films, leading to large compressive stresses and a gradual transformation from the alpha to the beta hydride phase. Comparison of these three nanoporous noble metals, which have significantly different melting temperatures, allows the mechanical behavior of FCC noble metals to be investigated over a broad homologous temperature range. These measurements will be interpreted in light of the nanoporous structure of each metal, as determined by scanning and transmission electron microscopy. In-situ nanoindentation in the TEM provides clear evidence of dislocation-mediated plasticity in nanoporous Au. These and other aspects will be discussed in light of additional observations of the microstructure and mechanical behavior of nanoporous noble metals.
11:30 AM - **II1.6
Evaluating Mechanical Properties at the Microscale in Nickel Superalloys using Microcompression Testing.
Paul Shade 2 , Robert Wheeler 3 , Michael Uchic 1 , Hamish Fraser 2 , Dennis Dimiduk 1 Show Abstract
2 , The Ohio State University, Columbus, Ohio, United States, 3 , UES, Inc., Dayton, Ohio, United States, 1 , Air Force Research Laboratory, Wright Patterson AFB, Ohio, United States
Mechanical testing of micron-size samples provides distinct advantages over macroscopic testing for quantifying selected fundamental processes governing plastic flow, such as the characterization of intrinsic size effects and direct, quantitative measures of strain heterogeneity and intermittency. We have utilized microcompression testing (both ex-situ and in-situ methods) to examine local variations in mechanical properties of a single-crystal nickel base superalloy, Rene N5, oriented for single slip and tested at room temperature. Testing has been performed both in the dendrite cores and in the interdendritic region, and the size-affected mechanical response differs for the two regions. Within the dendrite cores, typical FCC-like size-affected flow behavior is observed, which is manifested in a strong increase in the strain-hardening rate at small strains that has been previously attributed to the statistical availability of dislocation sources. Size dependent strengthening is observed to surprisingly large sample volumes, as the transition to bulk-like behavior is predicted to occur at sample sizes greater than 100 micrometers in diameter. By comparison, the interdendritic region displays a more complicated size-affected response, where for 5 and 10 micrometer diameter samples the samples are significantly weaker on average than those from the dendrite core, but this behavior is not observed outside this size range. These variations in the size-affected responses are related to changes in the local chemistry, microstructure, and initial dislocation substructure within the two regions.
12:00 PM - **II1.7
From Pop-in to Pillars: The Utility of Nanoindentation in the Study of Small-Scale Plasticity.
Sanghoon Shim 1 , Hongbin Bei 3 , Easo George 2 3 , George Pharr 2 3 Show Abstract
1 Research Institute of Industrial Science and Technology, Steel Structure Research Laboratory, Hwaseong, Gyounggi Korea (the Republic of), 3 Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee, United States, 2 Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, United States
Since its development in the mid-1980's, nanoindentation has proven to be a primary tool for discovering and characterizing a variety of unique deformation phenomena that have improved our understanding of the mechanisms of small-scale plasticity. Among these are the indentation size effect, in which hardness at small length scales increases due to plastic strain gradients; indentation pop-in, in which sudden displacement excursions are caused by homogenous nucleation of dislocations at stresses approaching the theoretical strength; and micro-pillar testing, in which the nanoindenter is used as a small-scale compression testing apparatus to explore deformation phenomena in samples small enough to probe single dislocation events. Many of these phenomena are interrelated in ways which are not at first obvious, and studying them by nanoindentation methods can be used to quantify some of the fundamental "unit events" that control dislocation plasticity. In this presentation, experimental observations are presented for a unique new class of micro-pillar specimens prepared by methods that don't suffer from damage imparted by focused ion beam milling. The observations are explained by means of pop-in studies in similar materials. ******** Research sponsored by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy (HB, EPG and GMP); and the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of FreedomCAR and Vehicle Technologies, as part of the High Temperature Materials Laboratory User Program (SS).
12:30 PM - II1.8
Yield Strength of Coherently Strained InGaAs Superlattice by in-situ Micropillar Compression.
B. Ehrler 1 2 , R. Ghisleni 2 , K. Png 1 , Andy Bushby 1 , D. Dunstan 1 , J. Michler 2 Show Abstract
1 Centre for Materials Research, Queen Mary, University of London, London United Kingdom, 2 Mechanics of Materials and Nanostructures Laboratory, EMPA, Thun Switzerland
In recent years micropillar compression tests have become commonplace for metallic materials. Such experiments have shown remarkable size dependent strengthening effects in many situations as the pillar diameter is reduced below a micron. However, there are few reports in the literature of micropillar compression tests for ceramic materials. Jayaweera et al. (Proc. Roy. Soc. A459, 2049, 2003) showed that the nanoindentation yield pressure could be controlled by the superlattice parameters such that the in-built coherency strain in the superlattice reduced the yield strength significantly at room temperature. This was an unexpected result, since coherency strain in metallic precipitation hardening alloys is normally associated with an increase in strength rather than a reduction. Here we conduct micropillar compression experiments, in-situ in the scanning electron microscope, using a displacement controlled compression device. We show that the yield strength of the superlattice materials does indeed decrease with increasing coherency strain at room temperature. This result proves that the effects observed by Jayaweera et al. (2003) were not an artefact of nanoindentation and allow a deeper understanding of size dependent yield to be developed.
12:45 PM - II1.9
Mechanical Properties and Microstructural Evolution in Nano-scale Materials via in-situ Uniaxial Compression and Tension Experiments.
Julia Greer 1 , Sarah Lansing 1 , Dongchan Jang 1 Show Abstract
1 Materials Science, California Institute of Technology, Pasadena, California, United States
Understanding mechanical behavior of materials at reduced dimensions is important for fabrication and reliability of devices at nanometer scales such as MEMS and NEMS, bio-cell sensors, and fuel cells. Yield strength of metals, a size-independent property for bulk, is found to strongly depend on sample size and initial microstructure as dimensions are reduced to nano-scale. To investigate material mechanical response under homogeneous deformation, we have developed an in-situ micro-deformation methodology, where nano-structures are mechanically deformed while capturing real-time video and data in a custom-built instrument, SEMentor. We report significant dependence of stress on sample size as shown by the results of uniaxial compression and tension of single-crystalline, nano-crystalline, and amorphous nano-pillars, as well as of carbon nanotubes (inidividual and bundles). We compare these mechanical strengths with one another and discuss their unique plasticity mechanisms governing deformation in these structures at nano-scale. We find that in structures where dislocations play key role in plasticity strength increases with smaller size, while when other plasticity mechanisms are engaged (grain boundary processes, shear transformation zones, buckling, etc.) smaller is generally softer. TEM analysis and deformation modeling are also presented.
Andrew Minor University of California-Berkeley
Conal Murray IBM T. J. Watson Research Center
Nobumichi Tamura Lawrence Berkeley National Laboratory
Lawrence Friedman The Pennsylvania State University
Thursday AM, April 16, 2009
Room 3011 (Moscone West)
9:30 AM - **II7.1
Plasticity at Small Scales.
Oliver Kraft 1 2 , Dan Gianola 1 , Reiner Moenig 1 , Cynthia Volkert 3 Show Abstract
1 IMF II, Forschungszentrum Karlsruhe, Karlsruhe Germany, 2 izbs, Universität Karlsruhe, Karlsruhe Germany, 3 Institut für Materialphysik , Georg-August-Universität Göttingen , Göttingen Germany
Since the 1950’s, size effects on strength and deformation of metallic materials have been studied. Prominent effects that have been reported range from Hall-Petch behavior to the critical thickness theory for thin films to the indentation size effect. Most recently, unexpected size effects in micro-compression tests on sub-micron single-crystalline metallic pillars have drawn quite some attention, and have led to a debate about the underlying deformation mechanisms. However, preparation artifacts and testing geometry may contribute to some of the observations in an undesired manner. We have made an effort in our work to conduct in situ tensile experiments on metallic nanowires. These tests are conducted in a dual-beam scanning electron microscope and focused ion beam (SEM/FIB), where specimen manipulation, transfer, and alignment are performed using a manipulator and the FIB. Gripping of specimens is achieved using electron-beam assisted Pt deposition. Results shown will include tests on single-crystalline nanowires having diameters between 30 and 300 nm. Typically, fracture of the nanowires occurs locally without homogeneous ductile deformation. Measured strengths are of the order of the theoretical strength. The observed behavior will be discussed in the context of discrete dislocation simulations as well as statistical analysis with respect to the specimen volume.
10:00 AM - II7.2
Separating Intrinsic Size Effects from Extrinsic Geometric Effects in Micro-pillar Compression of Metallic Glasses.
Brian Schuster 1 , Qiuming Wei 3 , Kaliat Ramesh 2 Show Abstract
1 Weapons and Materials Research Directorate, US Army Research Laboratory, Aberdeen Proving Ground, Maryland, United States, 3 , University of North Carolina at Charlotte, Charlotte, North Carolina, United States, 2 , Johns Hopkins University, Baltimore, Maryland, United States
In the literature, there are studies that employ both tapered and non-tapered microcompression specimens that are fabricated by focused ion beam machining. In some crystalline metals, one could reasonably assume that yielding is controlled by the maximum effective shear stress criterion and the maximum compressive strengths of these materials should therefore be relatively insensitive to specimen taper. An alternative yield criterion may be more applicable to metallic glasses. Packard and Schuh have proposed that the critical stress for deformation is the minimum shear stress on the shear band trajectory (rather than the local maximum compressive stress). As a result, the apparent strengths of metallic glasses are highly sensitive to specimen taper. We show the effect of specimen geometry on the observed compressive yield strengths of metallic glasses, with a particular focus on to micro-pillar compression tests. When metallic glasses are modeled as Mohr-Coulomb materials, finite element analysis calculations show that compressive yield strengths increase with specimen taper. However, analytical calculations applying the shear plane criterion indicate a stronger dependence of strength upon specimen taper. The later calculations appear to be confirmed by experimental evidence. In calculations and experiments, a specimen with a taper angle of ~5 degrees exhibits a maximum compressive strength that is ~50% larger than that found in non-tapered specimens.We will present measurements of the compressive strengths of a Pd-based metallic glass using the preferred method, namely by employing non-tapered specimens. We show a moderate dependence of strength upon specimen size; the compressive yield strengths of micrometer-sized specimens are roughly 9% larger than their millimeter-sized counterparts. This reinforces the notion that metallic glasses possess relatively size-independent mechanical properties over this range of sizes.
10:15 AM - II7.3
Universal Scaled Strength Behaviour for Nanowires and Pillars.
Brian Derby 1 , Rui Dou 1 Show Abstract
1 School of Materials, University of Manchester, Manchester United Kingdom
There has been considerable interest in the mechanical properties of small metallic pillars, with diameters in the range 10 – 1000 nm, tested in compression or in a few cases in bending. Although the majority of published work has focussed on the deformation of fcc metals, notably Au and Ni, a number of other metals have been examined and reported in the literature with data from bcc materials and the plastic flow of semiconductors included. These small metallic structures have been fabricated by a range of techniques including: nanofabrication from larger structures through focussed ion beam (FIB) machining, electrochemical deposition into templates, selective etching of fine columnar eutectic microstructures. In the majority of reports the authors have observed a distinct size effect with the structures displaying greater strength as their diameter decreases, although there is no universally accepted mechanism for this behaviour. Here we present a comparison of the reported plastic flow strength of a large range of materials with dimensions < 1000 nm. We show that if the yield strength, σ, is scaled with the materials shear modulus, μ, and the smallest dimension, d, (diameter or section width for non-circular section specimens) is scaled with the material’s Burgers’ vector, b, the mechanical behaviour of different materials shows a universal scaling behaviour within a given testing methodology. For metallic specimens fabricated by FIB machining or electrodeposition there is a remarkable correlation showing an empirical relation between specimen diameter and strength with σ/μ = A(d/b)-0.6, where A is a constant. Limited data for specimens tested in bending shows a similar universal law but with a different constant. However, specimens obtained from etched eutectics show no size effect and a higher yield strength than from the FIB machined and electrodeposited specimens. These results are discussed within the context of possible mechanisms for the high strength of metallic nanostructures.
10:30 AM - II7.4
Effect of Orientation and Loading Rate on Small-Scale Mechanical Behavior of BCC Molybdenum
Blythe Clark 1 , Andreas Schneider 2 , Carl Frick 3 , Patric Gruber 4 , Eduard Arzt 5 Show Abstract
1 , Sandia National Laboratories, Albuquerque, New Mexico, United States, 2 , Max Planck Institute for Metals Research, Stuttgart Germany, 3 Mechanical Engineering Department, University of Wyoming, Laramie, Wyoming, United States, 4 Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe, Karlsruhe Germany, 5 , INM - Leibniz Institute for New Materials and Saarland University, Saarbrücken Germany
Recently, much work has focused on the size effect in face centered cubic (fcc) structures, however few pillar studies have focused on body centered cubic (bcc) metals. This study investigates the small-scale deformation behavior of FIB-machined  and  Mo pillars. Compression results show a size effect in bcc Mo, smaller than that shown previously for fcc single crystals. Although it is not clear yet what dislocation mechanisms are responsible, results suggest source-controlled deformation. In addition, tests at various loading rates reveal that small-scale Mo pillars exhibit a strain rate sensitivity similar to bulk Mo. Additionally, calculated activation volumes are close to bulk values, suggesting that the same thermally activated kink pair nucleation required to propagate screw dislocations in bulk Mo may also be dominant at small size scales.
10:45 AM - II7.5
Size Dependence of the Elastic Modulus of Nanowires and Nanotubes Measured by Contact-resonance Atomic Force Microscopy.
Gheorghe Stan 1 , Robert Cook 1 Show Abstract
1 Ceramics, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Mechanical property measurements at the nanoscale are critical in optimizing and predicting the performance and reliability of emerging micro- and nano-electromechanical devices. The metrological challenges being overcome are poised to enable reliable characterization at this scale and, in this way, provide quantification and understanding of mechanical properties at the nanoscale: a link between atomistic simulations and micromechanical models. Endowed with nanoscale spatial resolution, contact-resonance atomic force microscopy (CR-AFM) provides distinctive localized elastic property measurements. Here we report on advancing the applicability of CR-AFM for quantitative measurements of the elastic modulus of 1D nanostructures, such as nanowires and nanotubes. By using CR-AFM we have systematically investigated the size-dependence of the elastic modulus of ZnO nanowires , Te nanowires , and AlN nanotubes . Precise measurements of the resonance frequencies of an AFM cantilever probe first in air and then in contact with a nanostructure, enable the contact stiffness for nanostructures of variable shape and dimensions to be determined. Appropriate contact mechanics considerations provide the quantitative conversion of the measured contact resonance frequencies into the elastic modulus of the nanostructure probed. In the case of ZnO and Te nanowires, increases in elastic moduli with reduction in nanowire diameter directly proved the stiffening enhancement that occurs in the outer layers of nanowires of reduced diameter. To determine the elastic modulus at different locations on prismatic AlN nanotubes, i.e., on edges, inner surfaces, and outer facets, we demonstrate a novel methodology of combining CR-AFM measurements with finite element analysis. This new approach extends the applicability of CR-AFM to elastic modulus measurements of 1D nanostructure with cross-sections of general shape. Our results conclusively demonstrate CR-AFM as a viable technique for elastic property measurements of simply substrate-supported 1D nanostructures and, in general, for local elastic modulus mapping at the nanometer-length scale.  G. Stan, C. V. Ciobanu, P. M. Parthangal, and R. F. Cook, Nano Lett. 7, 3691 (2007). G. Stan, S. Krylyuk, A. V. Davydov, M. Vaudin, L. A. Bendersky, and R. F. Cook, Appl. Phys. Lett. 92, 241908 (2008). G. Stan, C. V. Ciobanu, T. P. Thayer, G. T. Wang, R. J. Creighton, K. P. Purushotham, L. A. Bendersky, and R. F. Cook, Nanotechnology (in press) (2008).
11:30 AM - **II7.6
Making Silicon Mechanically Reliable.
Brad Boyce 1 , E. David Reedy 1 , James Foulk 2 Show Abstract
1 , Sandia National Labs, Albuquerque, New Mexico, United States, 2 , Sandia National Labs, Livermore, California, United States
Silicon microfabrication has seen many decades of development, yet the structural reliability of microelectromechanical systems (MEMS) is far from optimized or predictable. The fracture strength of Si MEMS is limited by a combination of poor toughness and nanoscale etch-induced defects. There is little evidence of processing pathways for extrinsic toughening of Si. However, careful control of defect distributions allows for enhancement of mean strength and a reduction in its stochastic variability. A MEMS-based microtensile technique has been used to characterize the fracture strength distributions of both standard and custom microfabrication processes. Recent improvements permit 1000’s of test replicates, revealing subtle but important deviations from the commonly assumed 2-parameter Weibull statistical model. Subsequent failure analysis through a combination of microscopy and numerical simulation reveals salient aspects of nanoscale flaw control. Grain boundaries, for example, suffer from preferential attack during etch-release thereby forming failure-critical grain-boundary grooves analogous to sharp cracks with Williams singularizes (r^-1/2). The crystallographic elastic anisotropy of polycrystals is found to be relatively unimportant, while the periodic spacing of grain-boundary grooves actually serve to improve strength by providing a shielding effect. Chemical effects such as dopant segregation and surface-passivation are shown to have non-trivial effects on strength. These and other factors will be discussed in an attempt to outline a pathway towards highly reliable micro- and nano-devices. 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.
12:00 PM - II7.7
In situ TEM study of Bauschinger Effect in Unpassivated Freestanding Metal Films
Jagannathan Rajagopalan 1 , Christian Rentenberger 2 , Hans-Peter Karnthaler 2 , Gerhard Dehm 3 , Taher Saif 1 Show Abstract
1 Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois, United States, 2 Physics of Nanostructured Materials, Faculty of Physics, University of Vienna, Vienna Austria, 3 Erich Schmid Institute for Materials Science, Austrian Academy of Sciences, and Dep. Materials Physics, University of Leoben, Leoben Austria
Recently, we reported that the stress-strain response of unpassivated freestanding metal films, contrary to expectation, deviates substantially from linear elastic behavior during unloading even at large overall tensile stresses (J. Rajagopalan et al, Scripta Materialia 59, 734-737 (2008)). We hypothesized that microstructural inhomogeneities, such as a distribution in grain sizes, and the nonuniform stress distribution they create were responsible for this observed Bauschinger effect. To explore the mechanism of the Bauschinger effect, we performed in situ TEM straining experiments on freestanding gold (mean grain size 60 nm) and aluminum films (mean grain size 150-200 nm) with concurrent macroscopic stress-strain measurement. During loading, dislocation activity is primarily confined to relatively larger grains while the smaller grains deform elastically, leading to a build up of internal stresses in the film. During unloading, after an initial phase of elastic deformation, noticeable dislocation activity occurs in the larger grains as the internal stresses induce reverse plastic deformation in these grains. This reverse plasticity in the larger grains leads to a macroscopic deviation from linear elastic behavior, resulting in Bauschinger effect. The experiments demonstrate the fundamental role of microstructural heterogeneity in determining the macroscopic behavior of ultra-fine grained and nanocrystalline metals
12:15 PM - II7.8
Single Dislocation Plasticity Probed by Force Microscopy.
Roland Bennewitz 1 2 , Philip Egberts 1 2 , Tobin Filleter 2 Show Abstract
1 , INM Leibniz Institute for New Materials, Saarbrücken Germany, 2 , McGill University, Montreal, Quebec, Canada
High-resolution force microscopy contributes to the study of plasticity on small scales through the combination of techniques. Using the force microscopy as an indenter, we can detect the emission of single dislocations as sudden excursions of the tip into the substrate over the length of one single Burgers vector. Using the force microscope for imaging, we can subsequently localize and analyze points of dislocation emergence at the surface with atomic resolution in ultrahigh vacuum. The emission of a mixture of screw and edge dislocations after indenting with a nanometer-scale tip will be shown for KBr single crystals. The critical shear stress deduced from these experiments is close to its theoretical value thanks to the almost perfect substrate and the controlled environment. Furthermore, electric charges at emerging dislocations can be detected by means of Kelvin probe force microscopy. Similar results with a more complex dislocation structure are found on Cu single crystals. Ongoing studies apply the method in ultrahigh vacuum to the surfaces of nano-crystalline materials.
12:30 PM - II7.9
Nanomechanical Resonance of Clamped Silicon Nanowires of Ultra Small Diameters Measured by Optical Interferometry.
Miro Belov 1 3 , Nathaniel Quitoriano 2 , Theodore Kamins 2 , Stephane Evoy 1 3 Show Abstract
1 , National Institute for Nanotechnology NRC, Edmonton, Alberta, Canada, 3 Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta, Canada, 2 Information and Quantum Systems Laboratory, Hewlett-Packard Laboratories, Palo Alto, California, United States
Resonant cantilevers have been proposed as highly-sensitive transducers for the detection and assaying of molecular systems. The mass sensitivity of resonant nanowires scales with decreasing masses of such resonators. The direct growth of cantilevered silicon nanowires (SiNW) by chemical vapor deposition (CVD) enables the production of high-quality NEMS resonators down to device dimensions in the 20 nm range. We report the synthesis and characterization of mechanically vibrating SiNWs grown by CVD. These highly-oriented and clamped silicon structures were laterally grown from the sides of etched silicon posts using a metal-catalyzed chemical vapor deposition process. The substrates were actuated at varying frequencies until the mechanical resonance of the suspended wires was observed by optical interferometry. Such data was acquired at room temperatures and at pressures 10-2 to 10-5 Torr range. The resonant frequencies of SiNWs with 20 nm diameter from f = 2-3 MHz and quality factors from Q = 600-1000 were observed. We will discuss aspects of our experimental approach, mechanical properties, and surface-related energy dissipation processes in these small nanostructures.
12:45 PM - II7.10
In-situ Deformation of Individual Polymer Nanofibers using AFM with SEM.
Fei Hang 1 , Asa Barber 1 Show Abstract
1 Department of Materials, Queen Mary, University of London, London United Kingdom
The mechanical properties of polymers are frequently improved by increasing the molecular alignment of the polymer chains within the direction of an applied load. Methods to improve molecular alignment are numerous but electrospinning is a relatively recent method which shows promise in manufacturing continuous polymer fibers typically with sub-micron diameters. Stress-strain curves are commonly used to evaluate the mechanical properties of materials but methods to evaluate electrospun polymer fibers is challenging due to their relatively small size. We describe in this work how a stress-strain curve for the tensile deformation of these electrospun fibers can be produced using an atomic force microscope (AFM) within the chamber of a scanning electron microscope (SEM). The AFM provides manipulation and applies deformation of the polymer fibers while the imaging of these events is achieved using the SEM. Our results show how the mechanical response of electrospun fibers is non-linear and provides a detailed analysis of polymer fiber deformation.
II9: Poster Session
Friday AM, April 17, 2009
Salon Level (Marriott)
9:00 PM - II9.1
Measuring Stress Corrosion Crack Growth Rates of Single Grain Boundaries in 304 Stainless Steel.
David Armstrong 1 , Francis Herbert 1 , Michael Rogers 1 , Steve Roberts 1 Show Abstract
1 Department of Materials, University of Oxford, Oxford United Kingdom
Stress corrosion cracking is a well documented phenomenon which occurs in wide range of materials of engineering importance. It is estimated to cost industry billions of dollars a year. The relationships between crack growth rate and load, grain boundary crystallography and chemistry are very poorly understood.Recent work has shown that some grain boundaries are more susceptible to intergranular stress corrosion cracking than others. However traditional methods of SCC testing on bulk specimens can only test multiple grain boundaries in polycrystalline materials. We describe here a novel technique, using micron-scale cantilever specimens, which allows the susceptibility to SCC of individual grain boundaries to be characterized.A 304 stainless was swaged and annealed to produce an elongated microstructure and then sensitized at 650 degrees Celsius to precipitate carbides at the grain boundary. Focused ion beam machining was used to make test specimens consisting of a cantilever (20μm by 3μm by 5μm) with a pentagonal cross section, each containing a single grain boundary. A custom nanoindenter test cell was developed which allows scanning and indentation to be carried out under a liquid. This has been used to image and then load the cantilever in the presence of acidified potassium tetrathionate solution. The cantilever was held at a constant force for 180 seconds and the rate of crack growth measured. Removal of the liquid while a microcantilever is under load stops the crack propagating, demonstrating that the crack growth is due to SCC.EBSD was used to calculate the misorientation of the grain boundary in each beam. This has been used to compare the resistance to SCC of low-Σ boundaries, to those of general orientation.
9:00 PM - II9.10
Nanoindentation Study of Friction and Wear of Silicon Nanolines.
Zhiquan Luo 1 3 , Huai Huang 1 , Scott Smith 1 , Bin Li 1 , Zhuojie Wu 1 3 , Qiu Zhao 1 3 , Rui Huang 2 , Paul Ho 1 Show Abstract
1 Microelectronics Research Center, University of Texas, Austin, Texas, United States, 3 Physics Department, University of Texas, Austin, Texas, United States, 2 Department of Aerospace Engineering and Engineering Mechanics, University of Texas, Austin, Texas, United States
The mechanical properties of silicon nanolines (SiNLs) at the nano-scale was investigated using the nanoindentation method based on atomic force microscopy (AFM). The SiNLs studied were high-quality single-crystals with line width ranging from 25nm to 90nm and height to width aspect ratio of 10 to 20. The loading-displacement indentation curves showed that the critical load to induce the buckling of the SiNLs can be correlated to the contact friction of the nano-indenter. In this report we present the linewidth dependence of the friction coefficients that were extracted from the indentation load-displacement curves and finite element method (FEM) simulations. In addition, the effect of the interface between the indenter and the nanoline was investigated by coating silicon dioxide or chromium on the SiNLs. The contact interface was found to have a significant effect on the contact friction and its behavior under cycling tests. Transmission electron microscopy was performed to investigate the deformation mechanism and the wear process at the nano-contact.
9:00 PM - II9.12
Observation of Giant Serrated Yielding during Controlled Strain Rate Flat Punch Nanoindentation of Thin Film Polymer Glasses.
Roseanne Reilly 1 , Graham L. W. Cross 1 Show Abstract
1 CRANN, Trinity College, Dublin Ireland
During plastic deformation of amorphous solids, strain softening can lead to shear localization phenomena such as the shear bands observed in compression of bulk metallic glasses and glassy polymers. Serrated flow behaviour can emerge as shear bands become active and inactive under changing strain conditions. Direct witness to this is made by sequences of small jumps or serrated yielding in load vs. displacement measurements of bulk metallic glass nanoindentation. Using a modified nanoindentaton setup we present results of low aspect ratio flat punch nanoindentation into thin polymer films. This technique generates large strain, thin film extrusion of precise geometries that approximate constant area squeeze flow rheometry performed on thin films of high polymers of various molecular weights. Low roughness, well aligned flat punch dies with large contact areas provide precise detection of surfaces with standard nanoindenter stiffness sensitivity. Punch radii varying from 6200nm to 400 nm have been realized in an eight-to-one aspect ratio testing of thin polymer films indented in the glassy state to a gap width of a few nanometers.We find that measured stress vs. strain curves into thin polymer films exhibit serrated yielding similar to conical nanoindentation into bulk metallic glass, but with a staircase of large jumps as opposed to many small jumps. The serrated yield phenomena only occurs under displacement control provided by the feedback loop in the nanoindenter control system, disappearing when the system is run under conventional load control. We also present scanned probe microscopy results that show the appearance of linear surface features at the periphery of the indentation. These generally correspond to the serration steps in a one-to-one ratio and we argue the observations may constitute the first direct measure of single shear band activity in glassy polymers.
9:00 PM - II9.13
Size Effects in Nanomechanical Behavior of Protein Materials.
Sinan Keten 1 , Markus Buehler 1 Show Abstract
1 Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Elasticity 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 - II9.2
Fluorescent Reporter Molecules with Mechanical Sensitivity.
Mariya Barch 1 5 , Matthew Lang 2 3 , Paul Matsudaira 3 4 5 Show Abstract
1 Chemistry, MIT, Cambridge, Massachusetts, United States, 5 , Whitehead Institute, Cambridge, Massachusetts, United States, 2 Mechanical Engineering, MIT, Cambridge, Massachusetts, United States, 3 Biological Engineering, MIT, Cambridge, Massachusetts, United States, 4 Biology, MIT, Cambridge, Massachusetts, United States
While mechanical measurements have greatly enriched the landscape of molecular information, they have also often required specialized instrumentation and expertise. Here, we present our progress towards an application of a novel class of molecules that are sensitive to applied loads of picoNewton-forces, report the applied force via fluorescent readout, and can be integrated into a system of interest by changing the terminal functional groups. The objective is to bring force measurement to fluorescence microscopy. Previously, we reported the force-to-fluorescence relationship for a sensor based on a modified DNA hairpin. To address the adaptability of this molecular sensor, we will apply it in the cell-adhesion mechanical model. For cell adhesion, both cellular level, and molecular level forces have been studied. The direct and non-invasive nature of this molecular approach to force measurement provides a distinct advantage: the ability to extend the measurement into three dimensional environments without additional mathematical complexity. We will report on our progress characterizing the bulk behavior of the sensor molecule and detecting cell-sensor interactions using fluorescence microscopy.
9:00 PM - II9.3
Mechanical Properties of Nanoparticle Superlattices by Nanoindentation.
Enrico Tam 2 , Paul Ashby 1 , Elena Shevchenko 3 , Marie-Paule Delplancke 2 , D Frank Ogletree 1 Show Abstract
2 Matter and Materials, Universite Libre de Bruxelles, Bruxelles Belgium, 1 Molecular Foundry, Lawrence Berkeley National Lab, Berkeley, California, United States, 3 Center for Nanophase Materials, Argonne National Laboratory, Argonne, Illinois, United States
Colloidal nanoparticles can self-assemble into macroscopic crystalline superlattices under appropriate growth conditions, creating a new class of nanostructed materials with novel properties. While investigations have started into the optical, electrical and thermal properties of these materials, relatively little is known about their mechanical properties. The inorganic nanoparticle cores are coated with organic ligands, and the superlattice mechanical properties depend primarily on the ligand-ligand interactions. Nanoindentation experiments have been performed on colloidal PbS superlattices. SEM images were used to identify well-ordered super-lattices 10-20 um in lateral size and ~ 2 um in thickness, then nanoindentation experiments were performed in ambient conditions using an Asylum Research MFP-3D force microscope with the low-force indentation accessory and a Berkovich diamond tip under load control. The indents were subsequently imaged by high-resolution AFM and by SEM. The elastic and plastic response and the residual stress of the PbS superlattices were determined from an Oliver-Pharr analysis of the load/unload curves and from post-indent images. Determination of the elastic modulus, yield strength and fracture toughness of this novel nanostructured material also provides insight into nanoparticle ligand-ligand interactions in well-controlled geometries.
9:00 PM - II9.4
Mechanical Properties of Nano-Thin Films by Use of Atomic Force Acoustic Microscopy.
Malgorzata Kopycinska-Mueller 2 1 , Andre Striegler 1 2 , Arnd Huerrich 3 , Bernd Koehler 1 , Norbert Meyendorf 1 , Klaus-Juergen Wolter 2 Show Abstract
2 Electronics Packaging Laboratory IAVT, Technical University Dresden, Dresden Germany, 1 NDE for Micro- and Nanostrauctures, Fraunhofer Institute for Non-Destructive Testing, IZFP-D, Dresden Germany, 3 , Fraunhofer Institute for Photonic Microsystems IPMS, Dresden Germany
We used the atomic force acoustic microscopy (AFAM) technique to characterize mechanical properties of nano-thin films of silicon oxide grown on a silicon substrate. The films thicknesses ranged from 7 nm to 28 nm, as measured by ellipsometry method. The results of AFAM measurements showed that it is possible to determine the elastic properties of the film if its thickness is known. In addition, the preliminary analysis of the AFAM results indicated that it may be possible to use the AFAM technique to determine the film thickness if the elastic properties of the components of the thin-film system are known. AFAM is a contact based method and as such provides information on the sample elastic properties from a certain volume that is compressed under an AFM tip. The information on the sample stiffness is obtained from the analysis of resonant vibrations of an AFM cantilever beam. The AFM cantilever is described as a rectangular micro-sized beam clamped on one side. The resonant frequency of an AFM beam that is not in a contact with a sample surface depends on the geometry and mechanical properties of the beam. When the cantilever is in contact with the sample surface, the forces acting between the sensor tip and the surface change the boundary conditions of the vibrating system. The previously free end is then spring-coupled and the resonant frequencies shift to higher values. In standard AFAM experiments static forces applied to the tip dominate the tip-sample interactions. Thus, the values of the contact resonance frequencies are used to determine the so-called tip-sample contact stiffness.The contact stiffness depends on the reduced Young’s modulus and the diameter of the contact area. For a system defined by a Hertz model for contact mechanics, where a smooth, elastically isotropic indenter is pressed against an elastically isotropic half plane, the reduced Young’s modulus is a constant quantity depending only on the Young’s modulus and the Poisson ratio of the indenter and of the indented body. However, a thin-film system is not elastically isotropic, and its effective Young’s modulus Eeff depends on the elastic properties of the film and the substrate, and the film thickness. In our experiments, we measured the contact resonance frequencies for the thin-film samples of silicon oxide as a function of the increasing static load. The static load was increasing from about 60 nN to 1200 nN in 20 steps. The values of the contact resonance frequencies obtained from the AFAM measurements were then used to calculate the values of the effective tip-sample contact stiffness and further to obtain the values of Eeff. We found a different dependence of the effective Young’s modulus on the static load for each of the thin-film samples. In addition, we were able to fit the experimental function and the values obtained for the elastic moduli of the silicon and the silicon oxide were in very good agreement with the literature values.
9:00 PM - II9.5
Mapping Young’s and Loss Moduli Maps in Polymer Materials from Curves to Numbers.
Maxim Nikiforov 1 , Stephen Jesse 1 , Louis Germinario 2 , Sangah Gam 3 , Russel Composto 3 , Sergei Kalinin 1 Show Abstract
1 , ORNL, Knoxville, Tennessee, United States, 2 , Eastman Chemical Co., Kingsport, Tennessee, United States, 3 Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Local thermomechanical properties of mixed-phase polymeric materials and composites have attracted much attention in the recent decade. We developed a quantitative method for local thermomechanical property analysis of polymers. The method is based on the combination of band excitation atomic force acoustic microscopy and localized heating with active thermal probes with integrated heaters. We developed and validated an experimental protocol that provides reproducible measurement of a polymer's thermomechanical properties. To date, LTA utilizes 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 sub-100 nm resolution.Spectroscopic version of the technique was used to measure the cross-linking kinetics of automotive refinish clearcoat formulations and to study the glass transition and de-polymerization reaction in polymer system based on polyethyleneterephthalateglycole (PETG). The kinetic of phase separation in PMMA:SAN system was studied using spatially resolved maps of mechanical properties of the polymers.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 - II9.6
Thermoelastic Damping in Silicon Micromechanical Resonators.
Bradford Pate 1 , Thomas Metcalf 1 , Douglas Photiadis 1 , Brian Houston 1 Show Abstract
1 , Naval Research Laboratory, Washington, District of Columbia, United States
The significance of thermoelastic damping in micro- and nanomechanical resonators has been an open question. We show here that the dominant energy loss mechanism in a plate mode of a 1.5 μm thick silicon micromechanical resonator is thermoelastic damping. In situ ultra-high vacuum annealing lowers the dissipation of two neighboring resonance modes (460 kHz and 510 kHz) at 120 K to Q−1 ≤ 5 × 10−7. From 120 K to 400 K, damping (Q−1) of these modes increase with temperature at different rates, in quantitative agreement with a modification (accounting for mode shape) of Zener’s theory of thermoelastic damping. The difference in the measured Q−1 between the two modes is evidence that the observed dissipation is the result of thermoelasticity, as opposed to other well-known internal friction mechanisms or to attachment losses.
9:00 PM - II9.9
Intrinsic Stress Fracture Energy Measurements for Low Dielectric Constant Thin Film Materials
Sean King 1 Show Abstract
1 Portland Technology Development, Intel Corporation, Hillsboro, Oregon, United States
The fracture energies of a series of tensile plasma enhanced chemical vapor deposited low dielectric constant (low-k) SiOxCy:H and SiOxNy:H thin films were measured by determining the critical thickness at which spontaneous cracking occurred. The fracture energies determined for the SiOxCy:H films were in the range of 2-3 J/m2 , whereas for the SiOxNy:H films, the calculated fracture energies were higher and ranged from 5-11 J/m2. For the SiOxNy:H films, the addition of nitrogen was not found to significantly increase the fracture energy of the SiON films relative to pure SiO2. The fracture toughness, however, was improved due to the increase in modulus from the addition of nitrogen. Overall, the fracture energies determined by this method were found to be consistent with those determined by other techniques.
Andrew Minor University of California-Berkeley
Conal Murray IBM T. J. Watson Research Center
Nobumichi Tamura Lawrence Berkeley National Laboratory
Lawrence Friedman The Pennsylvania State University
Friday AM, April 17, 2009
Room 3011 (Moscone West)
9:30 AM - II10.1
Thermal and Mechanical Size Effects in Electrospun Polymer Fibers.
Asa Barber 1 , Andrew Bushby 1 , Wei Wang 1 Show Abstract
1 Department of Materials, Queen Mary, University of London, London United Kingdom
Electrospinning is an adaptable method used to manufacture polymer fibers with a range of diameters. This provides an opportunity to understand how the physical properties of polymer fibers vary as their diameters reduce. In this paper we use scanning probe microscopy (SPM) to evaluate the mechanical properties of electrospun fibers from individual fiber indentation and bending. Our results indicate that the mechanical properties of the fibers are anisotropic and improve dramatically at fiber diameters below 300nm. Indentation of individual fibers with increasing temperature can be used to evaluate polymer fiber thermal transitions such as glass transition and melting temperatures. These thermal properties are shown to be strongly dependent on temperature. However, relatively small fiber diameters show poor mechanical behavior at elevated temperatures due to surface driven effects, in contrast to room temperature results.
9:45 AM - II10.2
Probing the Mechanical Responses of 1D Polymer Nanostructures
Jing Zhou 1 , Hyun Wook Ro 1 , Christopher Soles 1 Show Abstract
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Polymer nanostructures, such as 1D nanolines, 2D ultrathin films, and 3D scaffolds, have been utilized in electronics, photonics, medical devices, and tissue engineering. These wide applications are due to their ease of manufacture, variability of materials, and their tunable physical, chemical, and mechanical properties. It is therefore essential to develop novel techniques to accurately probe their physical, chemical, or mechanical responses at the nanoscale. Here, we report our efforts to quantify the mechanical properties of 1D polymer nanostructures. We adapt the surface wrinkling technique, widely used to quantify the mechanical modulus of thin films, for use with 1D polymer nanostructures. Linear polymer nanostructures are fabricated by nanoimprint lithography (NIL) and transferred to prestained polydimethylsiloxane (PDMS) surfaces. The polymer nanostructures buckle upon releasing the preapplied strain. A theoretical model is established to describe the relationship between the modulus of polymer nanolines and the geometry of buckling. Base on this model, we can measure the modulus of polymer nanolines with dimensions (line width/thickness) from 400 nm down to about 10 nm. For comparison, optical measurements based on Brillouin light scattering (BLS) are also applied to the same systems for comparison. In this presentation we discuss the dependence of measured mechanical modulus on the physical dimensions of the polymer lines in terms of nanoscale confinement effect.
10:00 AM - II10.3
Ultra-Shallow Nanoindentation of Soft Materials Using a Quartz-Crystal-Resonator Force Sensor.
Yen Peng Kong 1 , Ling Chen 1 , Michael Arnold 1 , Albert Yee 1 Show Abstract
1 Chemical Engineering & Materials Science, University of California, Irvine, Irvine, California, United States
The near-surface mechanical properties of materials are important for understanding adhesion, friction and wear, and environmental resistance. To date an instrument that can accurately measure the mechanical properties of surfaces of soft materials does not exist. Results from conventional AFM are unreliable because of the relatively high compliance of cantilevers typically used. We previously reported on the adaptation of a commercial AFM based on a quartz-crystal-resonator (QCR) as a force sensor. The mechanical compliance of a QCR can be orders of magnitude lower than that of a typical cantilever. The instrument we developed can be used to apply ~ nN loads to nanostructured surfaces, e.g., for compression of a nanopillar, while maintaining adequate force and spatial resolution. It can also be used as a nanoindenter that is capable of making extremely shallow nanoindents, down to 1 nm. It is therefore also useful for studying near-surface mechanical properties of soft metallic materials as well as the interphase in composites. We report on recent nanoindentation results on these material systems in this presentation.Shallow indentations, < 20 nm, on several glassy polymers using indenters with tip radii smaller than 100 nm produced results not previously reported, probably because polymers are known to have surface layers with significantly higher chain mobility than in bulk. Indentations larger than 20 nm produced loading curves consistent with post-yield plastic instability and unloading curves consistent with delayed yielding, a phenomenon unique to polymers. To ensure that these results are not due to artifacts in our instrument we conducted tests to validate the force measurements and compared the results against materials with known moduli.We made indentations ranging from 2 to 20 nm using a 28 nm radius indenter on lead with an oxidized surface. Indentations deeper than 6 nm produced apparent modulus in the 10 GPa range. Indentations at 2 and 4 nm produced much lower apparent moduli. These results are consistent with the existence of a thin oxide film on the Pb surface. These results give validity to our instrument. The paucity of modulus data on uncontaminated surfaces of materials for comparison will ultimately be the limiting factor in our ability to further validate our instrument. Further validation results on highly ordered pyrolitic graphite will be presented.We also performed nanoindentation experiments across the surface of sections in an epoxy-graphite fiber composite to characterize the spatial extent and relative moduli in the composite's interphase, a region that has been widely hypothesized to exist. Our results suggest that an interphase does exist. Further results will be presented.
10:15 AM - II10.4
Real-time Topography and Mechanical Property Mapping of Soft Materials by FIRAT Using Actively Controlled Transient Tap Forces
Zehra Parlak 1 , Mujdat Balantekin 1 , Levent Degertekin 1 Show Abstract
1 Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Although the lateral forces are largely reduced in Tapping mode AFM, gentle imaging of soft samples in air is still challenging as one needs to use significant oscillation amplitude to overcome the attractive forces, which in turn increases the repulsive forces on the tip. This deforms the sample surface leading to inaccurate topography measurement. An ideal AFM sensor for soft materials should act as a soft cantilever while the tip is in contact with the sample, and as a stiff cantilever for stable operation against attractive forces. This requires a fast, active sensor which can change its characteristics during each tap. Since regular AFM cantilevers are passive force sensors, they cannot provide these probe characteristics. In contrast, the force-sensing integrated readout and active tip (FIRAT) is ideally suited for this purpose since it comprises a broadband (fast) sensor tip with an integrated electrostatic actuator. Time resolved interaction forces (TRIF) can be measured without reconstruction while imaging topography to map the mechanical properties . In this study, we improve the FIRAT system by exploiting the integrated electrostatic actuator in a switched force feedback scheme to actively control the tap force at every cycle: Each tap on the sample has the same repulsive force maximum and this maximum value is limited by the electrostatic actuator at a user defined setpoint. To control the tip actively, an amplified version of PD signal in the repulsive regime is added to the bias of the FIRAT probe. Since the probe has a high cut-off frequency, the beam is pulled away from the surface during the repulsive part of the tap so that contact force is limited to the pre-defined threshold value. This approach effectively softens the probe during the contact decreasing the contact force while keeping it stiff against the attractive forces. Since the contact force is limited, the topography of soft samples can be obtained with higher accuracy without sacrificing any information related to the mechanical properties. As a demonstration, we imaged some soft polymers such as polymethyl methacrylate (E=3.2GPa) with peak repulsive tap force level of 630 nN and active tip controlled 250 nN. We observed that topography data changes up to 3 nm when the contact force maximum is decreased, while the mechanical properties can still be measured with the same resolution. Similar imaging experiments on stiff samples (aluminum on silicon) do not reveal any difference in measured topography. The results indicate that the FIRAT probe with active tip control may be an effective tool to characterize mechanical properties of soft samples such as biological structures with more accurate topography data. M. Balantekin, A. G. Onaran, and F. L. Degertekin, "Quantitative mechanical characterization of materials at the nanoscale through direct measurement of time-resolved tip-sample interaction forces," Nanotechnology, vol. 19, p. 085704, 2008.
11:00 AM - **II10.5
Reinforcement of Sol-gel Glasses at the Molecular Level and their Applications in the Microelectronics Industry.
Geraud Dubois 1 Show Abstract
1 Advanced Organic Materials, IBM, San Jose, California, United States
The intrinsic mechanical properties of a given material strongly depend upon its chemical nature: the organics tend to be soft, but tough, while the inorganic materials, on the other hand, are hard but brittle, and are prone to fracture. The later characteristic gets even worse for porous materials and is of major concern in the microelectronics industry as porous organosilicates (mainly inorganic) will constitute the next insulating layers in future electronic devices  . In this presentation, we demonstrate that significantly tougher organosilicates glass thin-films prepared by sol-gel process, can be obtained by introducing carbon bridging units between silicon atoms present in the organosilicate network [2-4] . Fracture energy values of 14-16 J/m2 were measured, surprisingly higher than those for dense silicon dioxide (10 J/m2), suggesting mechanical properties that lie somewhere in between those of conventional glasses and organic polymers. We also found that the Young’s modulus follows a linear decay when porosity is introduced, a unique property when compared to traditional organosilicates. As a result, crack resistant films were obtained at high level of porosity, opening potential applications in the field of low-k materials for future integrated circuits, membranes, sensors, waveguides, fuel cells and micro-fluidic channel.  G. Dubois, W. Volksen, R.D. Miller, in Dielectric Films for Advanced Microelectronics, (Eds: M. Baklanov, K. Maex,M. Green), Wiley, New-York, 2007, Chap 2. G. Dubois et al. US 7,229,934 B2. G. Dubois et al, Adv. Mater. 2007, 19, 3989-3994.  J. Rathore, L. Interrante, G. Dubois, Adv. Funct. Mat. 2008, in press.
11:30 AM - II10.6
Deformation, Strain, and Stress Mapping with Nano-scale Spatial Resolution using Diffraction, Spectroscopy, and Scanned Probe Microscopy.
Ryan Koseski 1 2 , Mark Vaudin 1 , Stephan Stranick 1 , Gheorghe Stan 1 , Robert Cook 1 Show Abstract
1 Ceramics, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Materials Science and Engineering, The Pennsylvania State University, State College, Pennsylvania, United States
Mechanical testing methodologies are currently being proposed and employed to characterize the mechanical properties of materials at the micro- and nano-scale. In addition to measuring experimental stress states during mechanical testing, it is desirable to measure strains and stresses in small microelectromechanical systems (MEMS) devices and layered electronic materials. In order to demonstrate measurement techniques to achieve these goals we have introduced static, known residual stress fields in a silicon surface via indentation, and mapped the resulting strains using electron backscatter diffraction (EBSD) and confocal Raman microscopy (CRM). In doing so we determined the resolution of these techniques; results showed that as the effective sampling depth of CRM (as determined by the incident light wavelength) was increasingly near surface, agreement in the mapped surface strain and stress field was achieved when compared to EBSD. Additionally, EBSD rotation values that have been interpreted as uplift at the near indentation contact areas have been corroborated via atomic force microscopy. Consequently, while EBSD exhibits a finer spatial resolution (10 nm for EBSD compared to 100 nm for CRM), CRM shows promise for depth profiling and crack detection (up to 1 µm normal to the sample surface). These two complementary approaches are being applied to an experimental MEMS-scale mechanical test fixture, the so-called “theta” specimen, to measure strains in situ as well as to verify stresses under applied load.
11:45 AM - II10.7
Probing Nanomechanics Using Band Excitation Scanning Probe Microscopy
Stephen Jesse 1 , Peter Maksymovych 1 , Maxim Nikiforov 1 , Sergei Kalinin 1 Show Abstract
1 Center for Nanophase Materials Science, Oak Ridge National Laboratory, Oak Ridge , Tennessee, United States
Probing elastic and loss moduli on the nanometer scale is one of the key challenges in nanomechanics. Here, we present the universal and quantitative approach for mapping local loss moduli on the nanometer scale based on dynamic scanning probe microscopy. Imaging and spectroscopy using force-based scanning probe microscopes universally relies on data processing that links the microsecond time scale of cantilever motion to the millisecond time scale of image acquisition and feedback. In most SPMs, the cantilever is excited to oscillate sinusoidally and the time-averaged amplitude and/or phase are used as imaging or control signals. The step of converting the rapid motion of the cantilever into an amplitude or phase is performed by phase sensitive homodyne or phase-locked loop detection. In this presentation, I discuss the fundamental limitation of lock-in detection as applied to probing energy dissipation and complex cantilever dynamics, and introduce the band excitation method (BE). The BE method is based on the excitation and detection of a signal having a finite amplitude over a selected region in the Fourier domain. The detected signal is Fourier transformed and fit by an appropriate model to extract multiple properties describing nanoscale mechanics simultaneously. This data acquisition scheme substitutes standard lock-in or PLL detection. This band excitation (BE) SPM allows very rapid acquisition of the full frequency response at each point in an image and in particular enables the direct measurement of energy dissipation through the determination of the Q-factor of the cantilever-sample system. We demonstrate this technique with electromechanical imaging, the investigation of dissipative defects in magnetic force microscopy, and in force-distance spectroscopy. We also investigate non-linear effects and thus probe details of contact mechanics at the nanoscale. The BE method thus represents a new paradigm in SPM, beyond traditional single-frequency excitation and is applicable as an extension to many existing SPM techniques.Research was sponsored by the Division of Materials Sciences and Engineering and the Center for Nanophase Materials Sciences, Office of Basic Energy Sciences, U.S. Department of Energy with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC.
12:00 PM - II10.8
Mechanical and Electrical Properties of Alkanethiol Self-assembled Monolayers on Gold Surfaces.
Frank DelRio 1 , Cherno Jaye 1 , Daniel Fischer 1 , Robert Cook 1 Show Abstract
1 Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
The mechanical and electrical properties of alkanethiol (CH3(CH2)n-1SH) self-assembled monolayers (SAMs) on gold surfaces were investigated using a conducting-probe atomic force microscope in an ultra-high vacuum chamber. Current-voltage measurements of the junction were performed as a function of applied load. Using a contact mechanics model and a parabolic tunneling model, we determined the contact area, barrier height, mean transmission probability, monolayer elastic modulus, and extent of plastic deformation. To assess the role of SAM chain length and packing density, these properties were extracted for different alkanethiol chain lengths (n = 5, 8, 12, and 18) and compared with contact angle, ellipsometry, Fourier transform infrared (FTIR) spectroscopy, and near edge x-ray absorption fine structure (NEXAFS) spectroscopy measurements.
12:15 PM - II10.9
On the Mechanical Properties of Tungsten Disulfide Nanotubes.
Ifat Kaplan-Ashiri 1 , Sidney Cohen 2 , Gartsman Konstantin 2 , Seifert Gotthard 3 , Hanoch Wagner 1 , Reshef Tenne 1 Show Abstract
1 Materials and Interfaces, Weizmann Institute of Science, Rehovot Israel, 2 Chemical Research Support, Weizmann Institute of Science, Rehovot Israel, 3 Institut für Physikalische Chemie , Technische Universität , Dresden Germany
WS2 forms multiwalled nanotubes which seem to be almost defect free and their structure can be precisely defined. Hence they can serve as good candidates for the study of nanomechanics. Various mechanical tests were applied on individual WS2 nanotubes to reveal their mechanical properties and behavior. First, nanotubes were axially compressed in atomic force microscope, and their Young's modulus was observed according to Euler's buckling point. An average value of 170GPa was obtained. A similar test which was conducted in the scanning electron microscope resulted in large elastic deformation of the nanotube. Here the Young's modulus was obtained from the post buckling equations, and found to be 150GPa. In a third experiment, the nanotubes were axially strained until fracture occurred. The Young's modulus was then observed according to Hooke's law and found to be 152GPa. These moduli values are in good agreement between themselves and also with density functional tight-binding (DFTB) calculations and the bulk material (150GPa).Tensile strengths and strain values as high as 16GPa and 14% were observed as well. These values reveal that WS2 nanotubes reached their theoretical strength, hence they are suspected to be defect free. The high strain value is unique to the tubular nanophase of WS2 and is also in good agreement with molecular dynamics simulation of MoS2 nanotubes. The nanotubes were deformed elastically until failure, in "sword in a sheath" mechanism and probably fractured in a brittle mode. Clamped nanotubes were bent and the shear (sliding) modulus was obtained according to Timoshenko's bending equation and found to be 2GPa. This value is in good agreement with DFTB calculations (4GPa) for sliding of two adjacent layers of MoS2. Furthermore, a unique nonlinear elastic deformation was observed both in post buckling and in bending tests. This mode of deformation is associated with the tubular structure.
Friday PM, April 17, 2009
Room 3011 (Moscone West)
2:30 PM - **II11.1
The Mechanisms and Implications of High Strength in Small Samples.
Cynthia Volkert 1 Show Abstract
1 Institute for Materials Physics, University of Göttingen, Göttingen Germany
Two important trends have recently been confirmed in small-scale samples with dimensions between 10 and 1000 nm: strength increases and dislocation storage becomes rarer as the length scale is decreased. The logical interpretation of these trends is that deformation in sub-micrometer samples is controlled by dislocation nucleation, and that dislocation nucleation requires larger stresses the smaller the crystal volume. Using examples from micro-compression and micro-tensile tests on single crystal copper, possible reasons for the sample size dependent nucleation stress will be discussed, including effects of surface roughness, statistics of flaw distributions, background stresses from flaws and surfaces, and interactions between incipient dislocation sources. The often contradictory observations of work hardening and the implications on the expected ductility of small scale samples will also be presented.
3:00 PM - II11.2
Development of Wafer- and Die-Scale Standards for Deformation, Strain, and Stress
Robert Cook 1 , Stephan Stranick 2 , Mark Vaudin 1 , David Owen 3 Show Abstract
1 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Chemical and Microanlaysis Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 , Ultratech, San Jose, California, United States
Many advanced industries require accurate and precise measurements of the state of deformation, or of strain or stress, in materials in order to improve or control device performance. For example, the microelectromechanical systems (MEMS) industry requires knowledge of stress in components in order to optimize the sensitivity of pressure sensors, the semiconductor microelectronics industry has implemented stress-engineered channel structures with increased field-effect transistor carrier mobility to improve device performance, and the optoelectronics industry takes advantage of substrate effects on photonic quantum-well structures to generate differences in strain states and thus changes in the output color of visible light-emitting diodes. All of these industries use many different methods to measure deformation and strain or stress, with little to no reconciliation between measurement methods, and no easy way of verifying the accuracy of measurements on components either on the manufacturing line or in development laboratories. This presentation will describe the development of a National Institute of Standards and Technology standard reference material (SRM) that will enable deformation, strain, and stress measurements to be calibrated on a range of commonly-used instruments. Specifically, the SRM will allow calibration of instruments that infer film stress from wafer deformation measurements, that determine stress from strain measurements using electron backscattered diffraction (EBSD) measurements in the scanning electron microscope or x-ray diffraction (XRD), or that infer stress from shifts or broadening of peaks in confocal Raman microscopy (CRM) measurements. The SRM will be based on the reaction stresses generated in a strain mismatched Si(1−x)Gex epitaxial layer deposited on a Si substrate. Two versions of the SRM will be developed: (i) A whole wafer form suitable for wafer curvature, XRD, and possibly CRM calibrations and (ii) a die form suitable for EBSD, XRD, CRM, and possibly curvature calibrations. Measurements and the extent of agreement between measurements using these techniques on prototype SRMs will be presented.
3:15 PM - II11.3
In-situ TEM Investigation of Deformation Behavior of Submicrometer-diameter Metallic Glass Pillars.
Changqiang Chen 1 2 , Yutao Pei 1 2 , Jeff De Hosson 1 2 Show Abstract
1 Dept Appl Phys, University of Groningen, Groningen Netherlands, 2 , Netherlands Materials Innovation Institute, Delft Netherlands
In this presentation we show some new striking results of in situ TEM quantitative investigations on the compression behaviors of amorphous micropillars fabricated by focused ion beam from two kinds of metallic glass ribbons, Cu-based and Zr-based respectively. More than 40 pillars with well defined gauge sections and tip diameter ranging from ~90 nm to ~650 nm are studied. Compression tests were performed by a recently developed Picoindenter TEM holder (Hysitron, Minneapolis), whose several unique features especially its rapid instrument response and data acquisition rates allow transient flow events well resolved and make the examination of individual shear band evolution feasible. It is found that the deformation of the MG pillars at the present size domain is still dominated by intermittent plastic flow, which is accommodated by discrete shear banding events. However, the frequency, amplitude and velocity of these shear banding events are strongly size dependent, leading to an apparently transition in deformation mode from inhomogeneous to homogeneous deformation with decreasing pillars diameter. Evolution of individual shear bands is followed in real time by in-situ TEM monitoring during compression. Yield strength as a function of size is also investigated.
3:30 PM - II11.4
Measurement of Critical Stresses during Pattern Collapse and Failure in High Resolution Lithography.
David Noga 1 , Richard Lawson 1 , Cheng-Tsung Lee 1 , Clifford Henderson 1 Show Abstract
1 School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
The development of lithographic technologies that allow for the patterning of high resolution polymeric photoresist relief images has been a critical and gating technology for semiconductor manufacturing for essentially the entire history of microelectronics. Currently, feature sizes in modern integrated circuit devices are on the order of 50 nm and the size of these features is expected to rapidly decrease to approximately 20 nm over less than the next decade. Producing such small relief structures in conventional polymeric resists is a significant challenge since a variety of requirements must be satisfied simultaneously. For example, the resist structure must be sufficiently thick to survive long enough to provide physical masking of the underlying substrate during an etch process. On the other hand, as the film thickness increases, the aspect ratio of the resulting resist structure increases for a given feature size. This increase in aspect ratio is extremely important since the resist relief structure is developed from an initially continuous polymeric resist film through an exposure and development process that involves immersion of the resist film in liquid media. As such, this liquid phase processing necessarily requires that the resist relief structures be dried at some stage before they are used in subsequent high vacuum processes. During this drying process, significant capillary force can be exerted on the resist structures that can result in either deformation or failure of the resist feature. These "pattern collapse" modes provide a limiting feature resolution that can be achieved in a particular resist process that may be larger than the intrinsic resolution that the photoresist. As feature sizes have continued to decrease, it is indeed the case that such resolution limits due to pattern collapse are now being encountered. It is also known that the physiochemical properties of polymer thin films can vary as a function of film thickness. However, there has been little direct measurement of the behavior of polymeric materials confined into nanostructured elements. In particular, the measurmenet of the mechanical modulus of resist polymer structures has to our knowledge gone largely unexplored. In this work, a method that utilizes analysis of pattern collapse in well defined resist nanostructures to determine the critical stress at pattern failure and the resist mechanical modulus will be presented and discussed. In addition, the use of this technique to analyze the impact of film thickness and resist feature dimensions on the resist modulus will be discussed. It is observed that there is indeed a critical resist feature length scale below which significant decreases in the mechanical modulus of the material are apparently observed for some reasist materials. These structure-property relationships will be discussed in the context of the different resist material types that may be used for next generation lithography.
3:45 PM - II11.5
Measuring Viscosity Variation in Thin Films and the High Viscosity of Interfacial Water
Nathan Moore 1 , Jack Houston 1 Show Abstract
1 Surface and Interface Sciences, Sandia National Laboratories, Albuquerque, New Mexico, United States
Various nanometric films possess unique lubrication behavior because their viscosity changes either spatially or under confinement. Useful analytical expressions are derived, which describe the frictional force between spherical or conical probes dragged across a thin film in the lubrication regime. We consider several models of how the film viscosity might change spatially within the film, and, separately, with the extent of local confinement. The solutions for the friction-vs.-penetration curves that would be measured differ in form only near the film edge for a given probe shape. These apparent coincidences are explained in terms of geometrical similarity, and hold relevance to lubrication between rough surfaces. We demonstrate the utility of these models by evaluating friction inside an interfacial water film ~ 2 nm thick, measured with Interfacial Force Microscopy. We show that the water film's high apparent viscosity cannot be explained by variation, which implies a quasi-uniform structure. Alternative strategies for discriminating viscosity variation inside thin films are discussed. Sandia is a multiprogram laboratory operated by Sandia Corporation, Lockheed Martin Company, for US DOE’s NNSA, Contract DE-AC04-94-AL85000.
4:30 PM - II11.6
Adhesion and Cohesion of Transparent Hard Coatings on Poly (methyl methacrylate).
Ani Kamer 1 , Stephanie Cai 1 , Liam Pingree 2 , Vasan Sundaram 2 , Reinhold Dauskardt 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 Boeing Phantom Works, The Boeing Company, Tukwila, Washington, United States
Acrylics and polycarbonates are tough and light and have replaced glass in many applications. However, the surface of these polymers is prone to scratching and water absorption, which is why hard transparent coatings with high adhesion are critical for reliable function. Sol-gel derived nanostructured hybrid coatings based on polysiloxanes have proven very versatile in terms of high hardness, ease of additive incorporation and optical properties, although their adhesion to plastics is not well characterized or understood. In our previous work, we reported quantitative thin film techniques to characterize the adhesion energy of hard transparent coatings on elastically soft poly (methyl methacrylate) substrates. In our present work, we report on the cohesive properties of these coatings using channel cracking methods. We also vary synthetic parameters such as the degree of hydrolization of the precursor chemicals, silica addition, and curing time and investigate their effect on the adhesive and cohesive properties of the coatings. The subcritical adhesive and cohesive crack growth rates are reported for a range of temperatures and moist and chemically active environments. Implications for the reliability and service life of plastics with hard coatings are discussed.
4:45 PM - II11.7
Transition in Plastic Flow Behavior of Tantalum (100)Studied by High Temperature Nanoindentation.
Andrea Hodge 1 , Koteswararao Rajulapati 1 , Monika Biener 2 , Juergen Biener 2 Show Abstract
1 Aerospace and Mechanical Engineering, University of Southern California, Los Angeles, California, United States, 2 Nanoscale Synthesis and Characterization Laboratory, Lawrence Livermore National Laboratory, Livermore, California, United States
The plastic flow behavior of Ta (100) as a function of temperature is studied by instrumented indentation. The observed characteristics are due to thermally activated deformation processes at the atomic scale. It was concluded that contributions from oxide layer and/or thermal drift arenegligible. An increase in temperature decreases the critical load required for the dislocation nucleation whereas the effect of loading rate seems to be minimal. It appears that at 200oC, the BCC lattice of Ta (100) is offering least resistance for the movement of screw dislocations. The resultant plastic flow pattern of BCC Ta (100) at 200oC resembles that of an FCC material at room temperature.
5:00 PM - II11.8
Investigating the Hall-Petch Efect at Grain Boundaries in Metals.
Siddhartha Pathak 1 2 , Roger Doherty 2 , Kilian Wasmer 1 , Johann Michler 1 , Surya Kalidindi 2 Show Abstract
1 Laboratory for Mechanics of Materials and Nanostructures, EMPA - Swiss Federal Laboratories for Materials Testing and Research, Thun Switzerland, 2 Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania, United States
The discovery of the Hall-Petch relationship (σY = σI + kY*d^-1/2) for the grain size strengthening of bcc ferritic steels has been greatly exploited by the steel industry, for example in the development of high strength low alloy steels. However, despite its huge technological success, doubts still linger as to the theoretical basis of the equation. For instance, the current models for the relationship propose a high density of dislocations being produced at or close to yield in the immediate vicinity (1-5µm) of the grain boundaries. Thus, as the grain size is reduced, a larger volume of the sample possesses this higher dislocation density giving the enhanced yield strength. In this work, we utilize the combined capabilities of spherical nanoindentation and orientation imaging microscopy (OIM) to analyze the Hall-Petch theory in the vicinity of grain boundaries experimentally.Annealed and 30% deformed polycrystalline samples of Fe-3%Si steel with large grains (of the order of few mm) were chosen for this study. Nanondentation measurements with 1 and 13.5 µm radii spherical indenters were carried out at various distances from the grain boundaries in these samples. The raw indentation load-displacement data were converted into much more meaningful indentation stress-strain curves using our novel data analysis techniques , which allow an accurate measure of the changes in indentation yield strengths across the grain boundaries in these samples. Combined with the orientation information from OIM, these results were used to explore the constitutive response of grain boundaries under contact loading.Our results demonstrate that the well known “Pop In” phenomenon in the undeformed metal is frequently absent when spherical indentations are made very close to most but not all grain boundaries. Pop-Ins are now believed to be due to the lack of mobile dislocations in the very small stress field under the indenter, especially when the smaller 1 µm radii spherical indenter is used. This result supports the idea that grain boundaries are usually effective sources of dislocations. Additional tests were also performed on 30% deformed Fe-3%Si steel samples, which show a lower propensity for the occurrence of pop-ins due to the additional degree of cold work. The yield point values measured from the indentation stress-strain curves on both the annealed and the deformed Fe-3%Si steel samples show an apparent lack of hardening when the indentations are carried out close to the grain boundary (sufficiently close to the boundary for the elastic and plastic strain fields to be felt in both grains). This observation seems to contradict the current models for the Hall Petch theory. Current ongoing in-situ indentation measurements inside the SEM are expected to shed more light on this problem.  S.R. Kalidindi, S. Pathak. Acta Mat. 56 14, 3523-32 2008.
5:15 PM - II11.9
Torsion of Thin Wires Shows a Size Effect at the Initiation of Plasticity in a Soft Metal.
B. Ehrler 1 , R. Bossis 1 , S. Joly 1 , K. Png 1 , Andy Bushby 1 , D. Dunstan 1 Show Abstract
1 Centre for Materials Research, Queen Mary, University of London, London United Kingdom
A size effect in the elastic limit has been observed in nanoindentation experiments in ceramics (Zhu et al. J. Mech. Phys. Sol. 56, 1170, 2008) and was suggested for soft metals in simulation of spherical nanoindentation experiments (Spary et al. Phil. Mag. 86, 5581, 2006). This size effect in the yield point has been attributed to the Matthews critical thickness effect but has not been observed before in fcc metals. Here we describe an experiment on the torsion of thin copper wires of 50 micrometre and 10 micrometre diameter, annealed to give a range of grain sizes in different wires. The experimental method employed here gives a sensitive in strain of 0.1 microstrain and the size effect in the elastic limit is clearly observed, as anticipated from geometrical critical thickness theory (Dunstan and Bushby, Proc. Roy. Soc. A460, 2781, 2004). The classical Hall-Petch behaviour with grain size is observed in the 50 micromtre diameter wires. The corollary of these results is that the strength of nano-scale structures is dependent on geometry so that there cannot be a constitutive law for plasticity size effects.
5:30 PM - II11.10
SEM In-situ Compression of Metallic Nanostructures.
William Mook 1 , Rudy Ghisleni 1 , Xavier Maeder 1 , Christoph Niederberger 1 , Zhao Wang 1 , Mikhael Bechelany 1 , Laetitia Philippe 1 , Johann Michler 1 Show Abstract
1 Mechanics of Materials and Nanostructures, EMPA - Swiss Federal Laboratories for Materials Testing and Research, Thun Switzerland
Characterizing the mechanical response of isolated nanostructures is vitally important to fields such as micro- and nanoelectromechanical systems where the behavior of nanoscale contacts can in large part determine system reliability and lifetime. To address this challenge directly, freestanding, single crystal metallic nanostructures have been compressed in-situ inside a high-resolution scanning electron microscope (SEM). The structures first load elastically where the stress distribution is modeled using a finite element (FE) simulation. This is followed by a distinct yield event after which the structures flow in a repeatable manner. Electron backscatter diffraction (EBSD) both before and after compression is used to quantify crystallographic differences within the structures due to the compressions. It will be shown that in order for a freestanding nanostructure to respond in a repeatable and reliable manner, it should have defects such that its deformation is determined by an internal and not an external length scale.