Determination of the mechanical response of polymeric materials with dimensions less than 100 nm is a continuing challenge. Here we describe a novel membrane (“nano-bubble”) inflation method we have developed for the purpose of making measurements of the creep response of ultrathin polymer films and show two major findings. The first is that the material dynamics as measured by the creep response of the membranes depends dramatically on film thickness. For example, in polystyrene films, the dynamics is accelerated so much that the glass transition temperatureT_g of a 11 nm thick film is reduced by approximately 50 K relative to the macroscopic T_g [1]. Furthermore, we have discovered that the nominal rubbery plateau in ultrathin films is stiffened by upwards of two orders of magnitude relative to the macroscopic state and the rate of stiffening (stiffening index S) correlates with the shape of the segmental relaxation in accordance with a recent model proposed by Ngai, Prevosto and Grassia[2]. We have elaborated this finding further and observe a strong correlation with the fragility index m that is related to glass formation according to the Angell categorization [3] of super cooled liquids. These results will be discussed in terms of current understanding of the impact of nanoconfinement on the glass transition behavior of polymers. In addition to being able to characterize the creep response of the ultrathin polymer films, we have also succeeded in adapting the bubble inflation method to make measurements on a graphene/polymer nano-sandwich structure and show that the method can be used to not only extract the stiffness of the graphene inner layer of the composite but that the method can be used to extract the interfacial shear strength of the polymer-graphene couple [4].
[1] P.A. O&’Connell, S.A. Hutcheson and G.B. McKenna, Journal of Polymer Science: Part B: Polymer Physics, 46, 1952-1965 (2008).
[2] K.L. Ngai, D. Prevosto and L. Grassia, Journal of Polymer Science: Part B: Polymer Physics, 51, 214-224 (2013).
[3] C.A. Angell, Journal of Non-Crystalline Solids, 73, 1-17 (1985).
[4] X. Li, J. Warzywoda and G.B. McKenna, Polymer, 55, 4976-4982 (2014).

The measurement of thin layer properties is important for both fundamental and technological reasons. Measurement of stress versus strain mechanics in one of the fundamental tenets upon which materials engineering is based. Direct measurements of elastic moduli and plasticity in nanometer thick polymeric glass layers is non-trivial, and one must often turn to indirect probes or small scale mechanical measurements that require complex interpretation. We introduce a novel strain-gradient free nanoscale compression test for thin soft layers on high stiffness substrates, which permits direct extraction of stress versus strain mechanics of the layer in elastic and plastic states from a single loading curve. Confined layer compression testing is built upon the compression of a precisely aligned flat probe into the layer forming a confined, stationary volume of highly compressed material in a well-defined, invariant geometry. Due to natural confinement from the surrounding material, we show that a state of uniaxial strain is created beneath the probe under small axial strains. Under a uniaxial strain stress state lateral strains within the film are zero throughout the entire volume under the probe. By this methodology we are able to directly probe via load-displacement measurement the stress-strain relationship in uniaxial flows under both reversible elastic and irreversible plastic conditions. Such nanoscale uniaxial strain testing is in some sense the complementary antithesis of nanopillar compression testing that has become so valuable for mechanical testing of size effects in hard materials. However, nanoscale uniaxial strain testing is suitable for in situ testing of both soft materials and continuous coatings on solid supporting substrates, in both elastic and plastic states. Confined compression measurements via instrumented nanoindentation apparatus or force microscopy apparatus provide new means to investigate mechanics and physics of thin films of structured or free volume materials such as glasses and biomaterials, including the plasticity of molecular materials, non-equilibrium states and behavior under the presence of high shear stress and hydrostatic pressure.

Secreted proteins experience multiple collisions with other biomolecules. To address how a single protein copes with collisions requires to measure its mechanical response in physiological-like conditions. Force microscopy methods have been applied to study the mechanical properties^1,2 of proteins. However, most of the studies involving folded proteins are performed on two dimensional arrays³, and characterizing the nanomechanical response of a single, isolated protein remains a challenge. Here, we develop a force microscopy method to measure the stress-strain curve of a single antibody pentamer by applying forces in the 20 to 300 pN range. Elastic recovery is shown for up to 0.4 compressive strains. We report the Young modulus and the yield strength of the protein&’s central region, respectively, 3.1 MPa and 2.1 pN/nm². The yield strength explains the capability of the protein to sustain multiple collisions without any loss of biological functionality. It also indicates the upper forces that could be used in force microscopy for the non-invasive imaging of β-sheet domains.
[1]Martinez-Martin , D., Herruzo, E. T., Dietz, C., Gomez-Herrero, J. & Garcia, R. Noninvasive protein structural flexibility mapping by bimodal dynamic AFM. Phys. Rev. Lett. 106, 198101 (2011).
[2]Dong, M.D. & Sahin, O. Determination of protein structural flexibility by microsecond force microscopy. Nature Nanotech.4, 514-517 (2009).
[3]Medalsy, I.D.; Muller, D.J. Nanomechanical properties of proteins and membranes depend on loading rate and electrostatic interactions. ACS Nano.2013,7, 2642-2650.

The Atomic Force Microscope (AFM) is an ideal tool for probing the nanometer-scale mechanical response of a free interface, with industrial applications in the design and quality control of nano-composite materials. Recent developments with dynamic AFM have enabled quantitative determination of tip-surface force with unprecedented resolution, speed and accuracy. The dominant paradigm for understanding these tip-surface forces builds upon contact mechanics, where force is considered to be a function of tip position, arising from elastic deformation in the contact volume. However, fundamental scaling arguments suggest that surface forces should dominate over volume forces at the nano-scale, especially with soft materials. Furthermore, many applications desire an understanding of the viscous, or energy-dissipating mechanical response of soft nano-fillers and their interphase to the surrounding matrix.
To understand both the viscous and elastic nature of surface forces we must go beyond contact mechanics with its assumption of quasi-static force equilibrium, and consider the tip-surface interaction as a dynamic two-body problem. We describe a multifrequency measurement technique and analysis method for AFM which, similar to Dynamic Mechanical Analysis (DMA) extracts the force that is in phase with, and quadrature to the harmonic tip motion [1]. We present measurements on several soft polymer blends that clearly show the significant effect of viscous forces arising from motion of the soft material surface. We introduce a new type of dynamic interaction model that takes in to account surface motion, allowing for a correct extraction of the elastic stiffness and viscous damping constants of the soft material interface.
[1] D. Platz et al. Nature Comm. 4, 1360 (2013).

Nanoindentation techniques have recently been adapted for the study of hydrated materials, including biological materials and hydrogels. There are unique challenges associated with testing hydrated materials in commercial instruments not designed for this application. Some key results from recent works using nanoindentation to evaluate hydrated materials including soft biological tissues, bulk hydrogels and thin hydrogel layers will be reviewed. Both natural and synthetic hydrogels have been characterized using indentation and nanoindentation across a wide range of experimental length-scales. The material response is shown to be greatly dependent on the chemical bonding within the hydrogel, i.e. whether the network is physically or chemically cross-linked. Hydrogels in particular are an attractive system for studying structure-properties relationships, as the water fraction can be systematically varied for a single polymer, and different polymers with the same water fraction can be compared. Based upon knowledge of the properties of each individual component, composite hydrogels can be created to mimic the overall response of complex biological materials to create multi-component tissue engineering scaffolds.

The application of polymeric materials in engineering is rapidly increasing, in both bulk and coating form, in technologies from automotive to plastic electronics to medical applications. Determining the mechanical properties is important for structural applications, high throughput screening of materials and understanding of polymer physics. Nanoindentation is a convenient means to determine mechanical properties but the response is often difficult to interpret. Unlike metal or ceramic materials, the mechanical response of polymers can depend on strain, strain rate, strain state and load history. The test method, contact geometry and load cycle can also strongly influence the measured response. In indentation, assumptions about the contact area derived from displacement measurements may not hold, and the induced strain gradients can lead to variable time-dependent responses within the strained volume, making determination of material parameters more problematic. It is often difficult to obtain values equivalent to tensile, flexure or dynamic test methods, with measured indentation modulus and hardness values often too high, leading to the supposition of size effects.
Here we present the results of an inter-comparison study of indentation methods on a range of engineering polymers. Different indentation geometries and test methods are compared to tensile and dynamic data from the same materials. The results show that the choice of indenter geometry, load cycle and analysis method have a large influence on the measured mechanical property values obtained. The results imply that confinement of the polymer within the high pressure beneath the indenter can significantly change the elastic and flow properties, creating apparent size effects. However, comparative values for time-dependent elastic properties can be obtained using appropriate geometries and analysis methods. The work reported here formed part of the European ‘MeProVisc&’ project, providing background data for the planned ISO 14577 part 5, instrumented indentation measurement of time dependent materials.

There is a rapidly growing interest in using plate-like nano-fillers such as graphene to reinforce polymers. It is also known that the spatial orientation of the reinforcing elements in polymer-based composites plays a vital role in controlling mechanical properties. There is, however, no generally-accepted way of quantifying the spatial orientation at the nanoscale of plate-like fillers in nanocomposites.
It was found in our previous study that the intensity of scattering of the Raman band is dependent on the axis of laser polarization when the laser beam is parallel to the surface of the graphene plane and it was demonstrated that a generalized spherical expanded harmonics orientation distribution function (ODF) could be used to quantify the spatial orientation of the graphene. Based on this approach, polarized Raman spectroscopy has been used to quantify, as an example, the level of spatial orientation of graphene oxide (GO) flakes in different nanocomposites using a variety of polymeric matrices. It is demonstrated further how it is possible to relate the spatial orientation of nanoplatelets to the mechanical properties of the in the nanocomposites and, from the stress-induced Raman band shifts, to stress transfer to the reinforcement. In particular, it has been possible to determine the well-known Krenchel orientation factor for these plate-like fillers directly from the experimental polarized Raman data.
Apart from the quantification method for the spatial orientation of nanoplatelets in the nanocomposites, another significant finding of this study is that the Krenchel factor for 3D randomly-oriented nanoplatelets is 8/15. This means that random orientation of fillers such as graphene should reduce the Young&’s modulus of the nanocomposites by less than a factor of 2 compared with the fully-aligned material. Compared to the reduction in the modulus of a factor of 5 going from aligned to 3D randomly-oriented fibres and nanotubes, it means that better levels of reinforcement should be achievable with misaligned nanoplatelets compared with, for example, nanotubes and there is less need to ensure accurate alignment of nanoplatelets in composites. Beyond just graphene and GO, this approach should be more widely applicable to the determination of the orientation of other nanoplatelet fillers for which well-defined Raman spectra can be obtained. Moreover, the effect of spatial orientation upon the mechanical properties of composites predicted in terms of the Krenchel orientation factor will be valid for all types of 2D platelet reinforcement.

An alternative approach to nanoscale electromechanical coupling is to exploit the flexoelectric effect, which is a linear coupling of strain gradient and the electric field. Flexoelectricity is functionally different from piezoelectricity, which in contrast is a linear coupling of strain to the electric field, in two significant ways. First, it lifts the restriction of the material structure being non-centrosymmetric, making it a universal effect whose magnitude depends upon the strength of the strain gradient and the geometrical arrangements of the molecular moieties. Second, the dependence on the strain gradient makes flexoelectricity a promising approach to nanoscale functionality and devices.
We report the results of a study on the dependence of the flexoelectric response on thickness in the ultrathin films of polar and non-polar polymers of vinylidene fluoride (VDF). The flexoelectric response was measured using a bent cantilever method and the results were corrected for the contribution from the interfacial electrode oxide layer. The results indicate enhancement of the response with decreasing film thickness in polar and non-polar polymers by up to a factor of 40 as compared to the bulk values, reaching such enhanced values in films only 10 nm thick. These results are consistent with a model accounting for interfacial contributions, and underline how large electromechanical coupling can be produced at the nanoscale.

Dentin is a naturally occurring nano-composite material (found in teeth) consisting of a collagen matrix, mineralized hydroxyapatite, and anisotropic tubule structures. This natural structure is of interest for biomimetic applications due it&’s remarkable mechanical properties, derived largely from the strength of the mineralized component combined with the unique fracture behavior dictated by the tubule features. An understanding of the damage evolution in such a naturally occurring structure can possibly aid in the design of man-made materials with similar properties. To that end, this work will present an in situ nanoindentation study of a sample of elephant dentin, performed in a commercial nanoscale 3D X-ray microscope. The X-ray microscope system provides spatial resolution on the tens of nanometer scale for samples ranging from tens to hundreds of microns in size, enabling new scientific investigations by serving a middle ground between the length scales of 3D in situ techniques possible with higher resolution TEM and somewhat coarser resolution classical micro-CT. The 3D data is collected by performing tomographic data acquisition by rotation and imaging of the sample through a range of angular orientations. A new load stage designed specifically for manipulation of samples in this system was implemented, allowing progressive indentation steps to induce fracture in the dentin sample under increasing load. In this study, 3D imaging was performed at multiple states of applied load to monitor the crack initiation and growth processes at the nanoscale, and gain insight into the connections between the novel microstructure, crack shielding mechanisms, and the material&’s fracture toughness. Visualization and quantitative analysis will be presented to help understand the process.

The damage evolution and structural failure in trees and other plants are mainly originated from the plastic yielding in the wood cell wall at the microstructural level, which consists of cellulose fibers embedded in a matrix of hemicellulose and either lignin or pectin¹. Understanding the mechanical behavior of wood cell wall at the plastic regime is critical to the investigation of the fracture characteristics of trees at macro-scale. In this research work, the wood cell wall, which consists of cellulose fibers, hemicellulose chains and lignin macromolecules, is modeled at the mesoscale to investigate the mechanical responses under tensile deformation. By examining the stress-strain relationship, the mechanical behaviors of the wood cell wall at the plastic yielding range will be obtained, which will be compared with experimental measurements and theoretical predictions to provide a bottom-up description of micromechanics of the wood cell wall, and to explain the damage evolution and structural failure occurred at the larger scales. The wood cell wall investigated here can be applied to the construction of wood hierarchical structure as a basic modeling unit and such application can be further extended to other natural materials.
1. L. Köhler and H.-C. Spatz, Planta 215 (1), 33-40 (2002).

Bulk GaN substrate has attracted much attention because of high quality and low dislocation density. We previously reported the dislocation formation and movement in surface dimples nano-indented on the high quality bulk GaN, and also proposed the mechanism of r-plane (-1012) slip initiating plastic deformation, so called pop-in event, which is supported by molecular dynamics simulation [1,2]. However, by TEM analyses, it was difficult to confirm the clear evidence of r-plane slip initiating the plastic deformation because of the generation of several kinds of dislocation multiplications and dislocation loops as secondary and tertiary slips accompanying the r-plane slip. In this paper we present evidence of r-plane (-1012) slip mechanism in indented GaN surface by using precise nano-indentation with smaller radius indenter (~100 nm) and lower pop-in load (~400 mu;N), comparing with our previous studies, and TEM observation right after the plastic deformation.
Nano-indentation was precisely performed on the surface of c-plane and m-plane bulk GaN. We chose a Berkovich diamond indenter with smaller radius of ~100 nm. The maximum load of transition from elastic to plastic deformation (pop-in load) was observed at 400 mu;N which is relatively smaller than that of our previous studies.
Cross-sectional TEM observation was performed right after the pop-in event beneath the indented surface of the c-plane bulk GaN. The pyramidal dislocation line of r-plane slip was clearly observed which is inclined by 43 degree from c-plane surface. And the basal c-plane slip and the prism m-plane slip occurred as a secondary or tertiary slip system were not observed, which is previously observed in larger radius indenter and higher pop-in load.
From these results including the nano-indentation for the m-plane bulk GaN, we confirm the mechanism for initiating plastic deformation as follows: i) indentation stress caused small-scale shuffles of atoms at the center of the diamond tip beneath the contact surface, ii) a metastable boundary arose locally along the r-plane, and iii) plastic deformation was initiated via the r-plane slip. The characterization by TEM analyses agrees well with the initial stage of plastic deformation with r-plane dislocation predicted by the molecular dynamics simulation.
[1] Masaki Fujikane, Toshiya Yokogawa, Shijo Nagao, and Roman Nowak, Jpn. J. Appl. Phys. 52, 08JJ01 (2013).
[2] Toshiya Yokogawa, Masaki Fujikane, Shijo Nagao, and Roman Nowak, 2015 MRS Spring Meeting & Exhibit, San Francisco, FF2.09, April 07, 2015

High Entropy alloys (HEAs) are solid solution alloys containing five or more principal elements in equal or near equal atomic percent (at %). In this study, the Al_0.7CoCrFeNi HEA was synthesized by vacuum arc melting and homogenized at 1250 ⁰C for 50 hours. The microstructure shows the presence of 2 phases. The BCC (A2+B2) and the FCC phase. Nano-pillars were fabricated from the single crystal Al_0.7CoCrFeNi HEA in the [001] of the BCC (A2+B2) phase and [324] of the FCC phase having diameters ranging from 400 nm to 2 mu;m using the focused ion beam and compressed using the Hysitron Nanoindenter. High yield strengths of about 2.2 GPa was observed in the BCC (A2+B2) phase and about 1.2 GPa for the FCC phase. Size effect occurs in both phases, the smaller pillars having the highest strength values. Comparing the BCC phase of the HEA with pure BCC metals shows a reduced size dependence with increased yield strength but the FCC phase in comparism with pure FCC metals shows similar size dependence with increased yield strength. This is attributed to the difference in lattice resistance of the two phases.

During the manufacturing process of electronic devices, semiconductor materials are subject to handling and contact with surfaces that could lead to plastic deformation or even fracture. Thus, an understanding of the mechanical deformation mechanisms of semiconductor materials is important. This can be achieved using nanoindentation techniques that allow the study of mechanical deformation mechanisms in nanoscale. Studies have been performed on the mechanical properties of gallium nitride (GaN) using nanoindentation with sperical tips with radius of curvature between 4 mu;m and 5 mu;m, but the early stages of plastic deformation and the dynamics of the deformation process are still unclear. In this work, we present a nanoindentation study using a conespherical tip with a radius of curvatore of 260 nm, which is approximatelly ~ 20 times smaller than the radius of curvature of the tips used in the literature. Therefore, our nanoindentations were able to produce highly-localized stress fields allowing a discussion on the incipient plasticity of GaN at the nanoscale. We report the deformation mechanism of GaN thin films with a [0001] orientation and the optical properties associated with the plastic deformation resulting from stress fields produced by nanoindentation. The residual indentation pits were studied using atomic force microscopy, transmission electron microscopy, and cathodoluminescence spectroscopy. The incipient plasticity was observed to be initiated by metastable atomic-scale slip events that occur as the crystal conforms to the shape of the nanoscale tip. Dislocation locking leads to pop-in events associated with the introduction of large volumetric material displacements at the indentation site. A large number of non-radiative defects were observed directly below the indentation. Regions under tensile strain extending from the nanoindentation were also observed. An insight of a plastic deformation process based on dislocation nucleation under highly-localized stress field is presented.

Micro-cantilever deflection testing has been used to determine fracture toughness at magnesium aluminate spinel bicrystal interfaces. Focused ion beam milling was used for micro beam fabrication and prenotch preparation. In the present study, the micro cantilever beams were fabricated along the bi-crystal grain boundary of magnesium aluminate spinel doped with either Ytterbium or Europium. The boundary location was confirmed using Electron Backscatter Diffraction after milling and a prenotch aligned with the interface was cut using a lower ion beam current. An in-situ nano mechanical test system, the Hysitron PI-85 Picoindenter, was used to perform the test and record the loading and displacement. A 3D finite element model of the micro cantilever beam was built in ANSYS APDL. A dedicated solver called Frac3D was utilized to calculate the finite specimen width correction term and estimate the fracture intensity factor. High resolution electron microscopy was used to identify the structure and chemistry of the boundaries. By combining the microscopy and mechanical testing results, a direct correlation between different complexion types and grain boundary mechanical behavior is to be established.

Deformation twinning is a critical component of HCP Mg&’s deformation mechanisms. Recently, anomalies were detected in an important {10-12}<10-1-1> twinning system[1]. This experimental result hinted that deformation twinning in Mg could be fundamentally different from traditional models for cubic metals. Theoretically, this topic has also been controversial and unsettled: shear-controlled or shuffling-controlled issue [2][3]. In this study, we computed atomistic pathways of deformation twinning with First-principles calculation, and quantified the pathways by two variables: strain which describes shape change of a periodic supercell, and shuffling which describes non-affine displacements of the internal degrees of freedom. The minimum energy path involves juxtaposition of both. If one can obtain the same saddle point by continuously varying the strain and relaxing the internal degrees of freedom by steepest descent, we call the path strain-controlled, and vice versa. With this way, we find the {10-12} <10-1-1> twinning of Mg is shuffling-controlled, that is, the reaction coordinate is dominated by non-affine displacement instead of strain. This has important consequences on the temperature and strain-rate dependences of {10-12} <10-1-1> deformation twinning due to the reduced importance of long-range elastic interactions. Thus, the room-temperature deformation twinning in magnesium is markedly and qualitatively different from traditional understanding.
[1] B. Yu, et al. Nature Comm. 5 (2014) 3297
[2] B. Li & E. Ma, Phys. Rev. Lett. 103 (2009) 035503.
[3]A. Serra, D. J. Bacon and R. C. Pond, Phys. Rev. Lett. 104 (2010) 029603.

Indenter angle effects on the cracking behavior of fused quartz were studied using nanoindentation experiments with different triangular pyramidal indenters with the centerline-to-face angles ranging from 35.3° to 85.0°. The indentation loads were varied from 10 to 500 mN to examine both the initial stages of cracking as well as the propagation of well-developed cracks with lengths much greater than size of the contact impression. Images of the indentations were obtained in a high-resolution scanning electron microscope (SEM) to observe the general features of the cracking and measure the indentation sizes and crack lengths for each indenter. The observations are used to examine the mechanisms of crack nucleation and crack growth in this classical brittle material.

Si pillars fabricated by focused ion beam (FIB) had been reported to have a critical size of 310-400 nm, below which their deformation behavior would experience a brittle-to-ductile transition at room temperature [1]. However, the potential effects from the high energy ion bombardment have been overlooked so far. Here, we demonstrated that the amorphous Si (a-Si) shell introduced during the FIB fabrication process was much softer and more ductile than the crystalline Si (c-Si), which can effectively inhibit the crack propagation in the c-Si core. Hence, the reported size-dependent brittle-to-ductile transition was actually stemmed from the imperfect sample fabrication method. Once the a-Si shell was crystallized by thermal treatment, Si pillars would behave brittle again with their modulus comparable to their bulk counterpart, showing that the crystalline Si is inherently brittle no matter in bulk or in submicron size. We also developed an effective analytical core-shell model to predict the modulus of as-FIBed Si pillars and derive the modulus of both crystalline Si core and a-Si shell, which fits well with the experiment results.
References
[1]Fredrik Östlund, Karolina Rzepiejewska-Malyska, Klaus Leifer, Lucas M. Hale, Yuye Tang, Roberto Ballarini, William W. Gerberich, and Johann Michler, Adv. Funct. Mater. 19, 2439 (2009).

There has been much research on highly nanotwinned Cu due to its potential for simultaneous high strength and ductility. Single element systems have limited engineering applications, though, and introducing alloying elements allows for a wide range of new materials with lower stacking fault energies which are more likely to produce fully twinned microstructures. In this study, the tensile properties of fully nanotwinned Cu-based binary alloys containing different alloying elements were determined with a custom-built small-scale tensile tester utilizing digital image correlation to generate in-situ strain maps. The effects of varying microstructure are examined for different alloy systems in order to understand their mechanical behavior. Yield strengths between 800 and 1500 MPa and strains to failure between 2 and 6% were achieved with varying twin thickness, grain size, and composition. Samples were synthesized by magnetron sputtering with stacking fault energies ranging from 6 to 45 mJ/m².

We report the crystallite-size-dependency of the compressibility of nanoceria under hydrostaticpressure for a wide variety of crystallite diameters and comment on the size-based trends indicating an extremum near 33 nm. Uniform nano-crystals of ceria were synthesized by basic precipitation from cerium (III) nitrate. Size-control was achieved by adjusting mixing time and, for larger particles, a subsequent annealing temperature. The nano-crystals were characterized bytransmission electron microscopy and standard ambient x-ray diffraction (XRD). Compressibility, or its reciprocal, bulk modulus, was measured with high-pressure XRD at LBL-ALS, using helium, neon, or argon as the pressure-transmitting medium for all samples. As crystallite size decreased below 100 nm, the bulk modulus first increased, and then decreased, achieving a maximum near a crystallite diameter of 33 nm. We review earlier work and examine several possible explanations for the peaking of bulk modulus at an intermediate crystallite size.

Flexible display is promising electronic devices which can act bending motion in anywhere. For some time past, it has been concerned from numerous manufacturer to apply flexible technology in e-paper, mobile phone and consumer electronics. Even if flexible display has many potentials in industry fields, it still has been issues to resolve yet. Traditionally, semiconductors and glass materials have been used as substrate. But these materials have trouble with manufacturing a flexible devices owing to material property such as brittleness, rigidity and weight.
Polymer (polycarbonate, PET, etc.) regarded as complementary materials instead of silicon and glass because device which is composed of the plastic material can be curved deeply as bendable display. Al-doped-ZnO (AZO) thin film was well known as low-cost transparent conductive oxide film instead of indium tin oxide (ITO). And Al-doped-ZnO is one of the most widely investigated electrode materials. Previous reports demonstrated that resistivity of their AZO film exhibited 2.2*10^-3 #8486;cm when 2 at% of Zn atoms were replaced by Al atoms.¹ However, not only electrical property but also mechanical property of AZO thin film as electrode is very significant in flexible devices.
Reports of mechanical property of AZO film are not enough, relatively. Especially, mechanical property such as strength, stiffness can be crucial factor to predict reliability, life time of device under environmental loading conditions. In this research, we fabricated AZO on polyimide film using atomic layer deposition (ALD) process. ALD is a thin film deposition technique allowing low process temperature, uniformity and controllability of thickness. Moreover, separated injection of the precursors in a reaction chamber allows use of highly reactive precursors and control of reaction time for each step. This facilitate to make thin film at low temperature and can be key of application of flexible display.
And electric-micro tensile test was performed to compare of the performance of the conductive oxide film deposited on polyimide substrate under process conditions such as deposition temperature, %Al doping ratio, O₂ plasma. As tensile force was loaded on specimen, electrical resistance & stress value was measured under %strain in DAQ system. Especially, it brought the result that elastic modulus of O₂ plasma treated-PI deposited with AZO was increased up to 20% then before. And in situ LFM (Lateral force microscopy) was performed to investigate interaction between thin film and substrate. An x-ray diffraction (XRD) was employed to reveal the structure of AZO using Cu Κα radiation. The surface morphology was analyzed by using the scanning electron microscope (SEM). To quantify the bonding strength, x-ray photoelectron spectroscopy (XPS) was used in this experiments.
Reference
J. W. Elam , D. Routkevitch , S. M. George , J. Electrochem. Soc. 2003,150 , G339

The use of nanoindentation to test through a range of strain rates is a growing field, with applications to both beginning, e.g. metal forming, and end of lifetime, e.g. impact deformation in many materials. The use of dynamic indentation methodology, superimposing an oscillating AC load over the DC quasistatic load, can be used to measure properties such as Hardness and Modulus as a function of time and depth. However, several assumptions need to be made to do so reliably. One such requirement is that the material response be such that the area function remains valid. For many materials, including the nanocrystalline copper tested here, that assumption is violated when the amount of pile-up varies with the strain rate. Theoretical strain rate pile-ups are compared to the experimental result, using both in and ex situ testing. A simple method for correcting the hardness as a function of the strain rate is presented. Additionally, a MEMS based transducer is used to test at strain rates approaching 10, to further extend the range of nanoindentation strain rate testing.

A good understanding of the true plastic stresses and strains are of vital importance in studying metal forming processes and in the fracture mechanics field. The information extracted from the tensile test has been used traditionally to derive the flow curve and to compute plastic properties of the metals like the strength coefficient and the strain hardening exponent. However, knowledge of the plastic behavior beyond necking is uncertain due to the triaxial state of the stresses generated in the sample, and therefore, the flow curve does not replicate the uniaxial condition sought during the tensile test. In this research, a correction method for the post-necking section of the flow curve is proposed by linking nanoindentation measurements, performed along the tensile axis, to the flow curve. Three bulk materials were studied, steel G101800, copper C11000 and stainless steel S30400. The true flow curve after the onset of necking was corrected utilizing the nanohardness data converted to true stress values through the constraint factor C proposed by Tabor. It was observed that this constraint factor is not constant for all materials as reported in various preceding works. The corrected section of the curves showed a strain hardening behavior different to that before the onset of necking since the converted nanohardness to flow stress revealed the uniaxial character of the deformation, in this manner, separating out the effects of triaxiality in the uniaxial true flow curve.

Multimillion-atom reactive molecular dynamics simulations are performed to study self-healing of cracks in Al₂O₃ by SiO₂ nanoparticles and by SiC/SiO₂ core shell nanoparticles embedded in the Al₂O₃ matrix. These simulations are carried out at 300 K and 1800 K for three different system sizes: 400Å*300Å*100Å, 600Å*300Å*150Å and 1600Å*800Å*400Å containing approximately 1.2 million to 60 million atoms. Brittle fracture is observed in alumina at 300K, while void nucleation and coalescence are observed to cause ductile fracture at 1800 K. Silica is found to be highly effective in healing cracks in the Al₂O₃ matrix at high temperatures. In addition, silica causes strain hardening and gives higher fracture toughness to alumina. Detailed analysis of atomistic mechanisms shows that crack healing is due to the flow SiO₂ into the crack, which stops the crack growth. Results will be presented for (1) stress distributions in alumina with and without SiO₂ and SiC/SiO₂ core shell nanoparticles, (2) size and shape of cavities in the damage zone, and (3) atomic diffusivities of silicon and oxygen during healing of cracks and damage cavities in alumina.
This work was supported by Department of Energy (DOE), Office of Science, Basic Energy Sciences, Materials Science and Engineering Division, Grant #DE-FG02-04ER-46130. Some of the simulations were performed on the 262144-core IBM Blue Gene/Q supercomputer at Argonne National Laboratory under the DOE INCITE program.

HCP materials like magnesium and magnesium alloys due to their high strength to weight ratio have been deemed as promising candidates for next generation armor materials. A fundamental understanding of the deformation and failure response of the material during high strain rate deformation and shock loading is required in order to develop the capability to withstand ballistic/blast impact. The mechanical response of these materials is primarily determined by the active deformation modes operating (slip, twinning, etc.) as well as the micromechanisms (voids, shear bands, etc.) related to failure. Of particular importance is the role of twinning in Mg due to the limited number of slip systems in HCP metals. The aim of this study is to identify the contribution and dependence of different atomic scale deformation mechanisms like basal, prismatic, pyramidal slip and twinning on the loading conditions (strain rates, shock pressures etc.), microstructure of the system (loading orientation, grain size) and temperature.
Molecular dynamics (MD) simulations allow the characterization and investigation of the evolution of defect structures at the atomic scales during deformation and failure. Large-scale MD simulations are therefore carried out to investigate the deformation and failure micromechanisms in nanocrystalline Mg with grain sizes up to 100 nm under uniaxial strain and shock loading conditions. The evolution of twinning during deformation, the mechanisms for nucleation, growth, and coalescence of voids, and the variation of the spall strength as a function of strain rate and grain size in comparison to that for single crystal Mg will be presented.

The protectiveness and performance of passive corrosion films as effective barrier layers depends greatly on their mechanical integrity under corrosion conditions. This mechanical stability is particularly important for two-dimensional layered surface films like mackinawite (Fe_1+xS), which are known to fail by delamination, leading to exposure of the bare metal and subsequent catastrophic localized corrosion [1,2].
In an effort to explain the mechanism behind this facile failure, we show, by analogy to three-dimensional passive films [3,4], how hydrogen point defects generated by cathodic proton reduction can weaken the interlayer van der Waals bonding in mackinawite leading to delamination. Specifically, we use density functional theory calculations to identify dominant hydrogen defects in mackinawite based on their formation energies and quantify their effect on important mechanical metrics like binding energies, elastic moduli and tensile and shear strengths by mapping the stress-strain behavior of mackinawite crystals containing hydrogen point defects.
We find that hydrogen is most detrimental to film passivity and protectiveness when it is present as molecular H₂ interstitials in the interlayer region. This configuration results in up to 80% degradation in the mechanical strength of mackinawite crystal and therefore increases the chances of passive film failure by delamination.
References
[1] D.W. Shoesmith, P. Taylor, M.G. Bailey, D.G. Owen, Journal of the Electrochemical Society 127 (1980) 1007.
[2] X.M. Dong, Q.C. Tian, Q.A. Zhang, Corrosion Engineering Science and Technology 45 (2010) 181.
[3] S.N. Rashkeev, K.W. Sohlberg, S. Zhuo, S.T. Pantelides, The Journal of Physical Chemistry C 111 (2007) 7175.
[4] M. Youssef, B. Yildiz, Physical Chemistry Chemical Physics 16 (2014) 1354.

Dislocations in fcc and hcp metals usually dissociate into a planar shape, and their slip is confined in the corresponding slip planes. Cross-slip usually requires transformation of the planar dislocation core into a linear, perfect dislocations, which requires some activation energy. Using an extensive DFT calculations, we have found a notable exception to this conventional view. The pyramidal screw dislocation in Mg consists of two partial dislocations connected by a stacking fault, and the stacking fault can migrate perpendicular to the plane by atom shuffling, enabling the dislocation to cross-slip without transforming into a perfect dislocation.

The mechanical properties of bulk metallic glasses, such as high strength and low ductility, are both of fundamental scientific interest and technological importance. It is now well recognized that the plastic deformation of metallic glasses below their glass transition temperature involves irreversible rearrangements of small clusters of atoms. In the present study, molecular dynamics simulations are performed to investigate the plastic response of a model glass to a local shear transformation in a quiescent system. The deformation of the material is induced by a spherical inclusion that is gradually strained into an ellipsoid of the same volume and then reverted back into the sphere. We show that the number of cage-breaking events increases with increasing strain amplitude of the shear transformation. The results of numerical simulations indicate that the density of cage jumps is larger in the cases of weak damping or slow shear transformation. Remarkably, we also found that, for a given strain amplitude, the peak value of the density profiles is a function of the ratio of the damping coefficient and the time scale of the shear transformation.

Metal-organic frameworks (MOFs) are a new class of porous materials with important applications ranging from gas sequestration to medicine. Understanding the processes of deformation of MOFs beyond the elastic limit is critical for several applications and processing where they are subjected to significant stresses. In this study, mechanisms of plastic deformation of metal-organic framework-5 (MOF-5) are investigated using large-scale molecular dynamics simulations. Atomistic-level details from the simulations of uniaxial compression along [001], [101], and [111] directions reveal that structural collapse of MOF-5 during compression occurs by slips along <100> directions on {001} planes due to the flexibility of the connection between Zn-O clusters and 1,4-benzenedicarboxylate (BDC) ligands. A dramatic decrease in stress of the system occurring during plastic deformation corresponds to the collapse of extensive free volume of the crystal structure. In addition to resolved shear stress, compressive stress normal to the active slip plane is essential for driving the plastic deformation depending on the orientation and this explains the anisotropic behaviors of this material. Plastic deformation of MOF-5 shows very interesting behaviors of strain localization that significantly affects the mechanical properties. Structural collapse propagates on the active slip planes along the directions perpendicular to slip directions and this is reminiscent of the glide of screw dislocation. The process exhibits interesting interaction between collapsing regions due to the effects of stress relieving by the volume shrinkage.

Nanoindentation testing has a “typical” protocol for execution of experiments and for data analysis, but there are many alternatives available. The choice of experimental parameters and technique for the analysis of data can depend on the stiffness of the material, its hydration state, the material constitutive response, and the presence or absence of pores in the material. Systematic experiments have been undertaken to examine a wide range of choices for nanoindentation experiments, particularly in the context of biological materials. Samples have been tested in dry and rehydrated conditions. Spherical and Berkovich indenter probes have been utilized, including spheres with radii across several orders of magnitude. Data analysis has been considered within elastic-plastic (Oliver-Pharr), viscoelastic and viscoelastic-plastic and poroelastic frameworks. Results suggest that the hydration state, probe geometry and the limitations and assumptions of each analysis method significantly influence the measured mechanical properties in materials such as bone. This systematic study has been carried out to illustrate that the discrepancies in the nanoindentation properties found in the literature should not be attributed only to the differences between the materials themselves, but also to the testing and analysis protocols.

Understanding the molecular network structure of polymer matrix composite and its relationship with mechanical performances can lead to a complete understanding of failure mechanisms. This work investigates the nanoscopic nature of PMC matrix as well as interphases in terms of topography, chemical mapping/bonding, thermal and mechanical properties to find a bridge between nanoscopic, microscopic and macroscopic mechanical properties. It is guided by ongoing simulations aimed at an understanding of bond scission, interphase structure and mechanical properties at the nanoscale level via a multiscale computational approach. DGEBA epoxy resins with known chemical structures are processed in-house with varying molecular weight, controlled stoichiometric ratios (resin and cross-linker) and degrees of cure, both in the presence and absence of carbon fiber. AFM-IR is used to investigate the chemical structure at ~ 100 nm resolution, which is calibrated by controlled experimental characterization at varying conditions using FTIR (mid-IR and near-IR) in bulk. The chemical mapping results show that there is a variation in the chemical network structure between the matrix and the interphase as well as within the matrix. Control experiments using spectroscopy and thermal analysis confirm that this variability in the three-dimensional network is due to differences in cross-linker density and degree of cure. To understand if these local chemical variations have any influence on the property, thermal AFM was used to investigate the glass transition temperature and modulus mapping was done by Peak-force tapping AFM on ultramicrotome surfaces. It is confirmed that these network structure variations resulted in changes in local thermal and mechanical properties.

Here we define a set of standard for providing data originating from atomic force microscopy AFM force measurements to be used to compare between sample properties, parameters or more generally, heterogeneity with a minimum number of points and conclusively. We exemplify our case by employing sapphire as a model system because it presents a homogeneous surface, that is, where the force of adhesion F_AD should be homogeneous even in the nanoscale. After setting the standards, we select a more challenging model system consisting of copolymer to show that approximately 100 suffice to conclusively establish the presence of nanoscale heterogeneity with a resolution of 10pN in F_AD. Employing the work of adhesion W_AD as an alternative parameter requires approximately ~200 data points to arrive at the same conclusion. . Our findings reinforce the relevance of multi-parametric models and analysis in the field of nanoscale force spectroscopy.
Keywords: nanoscale, adhesion, work, force

The thermally-induced nanostructure evolution in the thermotropic copolyester based on 1,3-hydroxybenzoic acid (B) and 2,4-hydroxynaphthoic acid (N) was investigated by small-angle X-ray scattering (SAXS). Heat treatments were carried out on extruded tapes at temperatures close to the solid-to-nematic transition (T_s→n) for up to five hours, under dry air conditions. Wide-angle X-ray scattering (WAXS) showed that the as-received tape exhibited fiber-like macromolecular orientation along the extrusion axis, along well defined crystalline reflections, denoting crystalline order. On the other hand, SAXS elucidated a highly anisotropic nanovoid morphology elongated along the extrusion axis over 17 nm and aspect ratios (axial/lateral dimension) in excess of 15. Upon heat treatment the crystalline reflections sharpened suggesting better molecular packing, and the degree of crystallinity increased. At the same time, SAXS showed that the lateral dimension of the nanovoids significantly decreased and the aspect ratio dramatically increased up to nearly 30, suggesting better molecular packing. There was also sharpening of azimuthal intensity around the 110 equatorial reflection, thus indicating an increase of molecular alignment along the extrusion axis. The quality of molecular alignment was quantified by the order parameter P₂. The thermally-induced molecular and nanometer scale changes correlated with a significant increase of mechanical Young&’s modulus, E, along the extrusion axis.

Complex nanostructures with semiconductor quantum dots (SQDs) and metallic nanoparticles (MNP) assembled in a close vicinity of each other provide a possibility for hybridization of the local intrinsic excitations, namely excitons in SQD and plasmons in MNP. Such hybridization gives rise to interparticle energy transfer, enhancement of local electromagnetic field and light-matter interaction with numerous practical applications if the host material, fabrication technology and target structures are compatible with those common for device production.
In this paper we show a possibility to self-organize InAs SQDs and SbAs MNPs in a close vicinity to each other in the GaAs matrix. For that we develop a combined process of the molecular beam epitaxy (MBE), in which self-organization of the InAs SQDs was achieved in the Stranski-Krastanow mode, whereas self-organization of the SbAs MNPs was realized by the low-temperature MBE with the subsequent anneal. Coupling of the InAs SQDs and SbAs MNPs was revealed by transmission electron microscopy. The distance between the SQD and MNP arrays was 10 nm. This phenomenon originates from local strain-stress fields surrounding both InAs SQDs and SbAs MNPs. We analyze the specific feature of the mechanical fields and their impact on the self-organization processes in our system.

Natural composites, like Nacre and wood, have attracted a lot of attention lately in the field of biomimetic materials given their unique combination of light-weight along with high strength and toughness. If the structure of natural composites could be successfully transferred to the conventionally man-made materials which are made of ceramics and polymer, it would benefit a wide area of fields, such as transportation and construction, biomedical implants and sensors. Inspired by unique microstructure of nacre, we present a new method to fabricate a material through layer-by-layer (LBL) deposition technique using an organic polymer poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] and an inorganic material (alumina platelets). The platelets are transferred to a silicon wafer substrate by dip-coating and a layer of organic polymer [P (VDF-TrFE)] is produced by spin-coating, resulting in strong hydrogen bonds formed between the two kinds of material. After repeating these two steps in succession several times, the 4-layer films are fabricated with a thickness of a few tens of microns. The nacre-inspired structure material composed by [P (VDF-TrFE)] and alumina platelets enhanced the mechanical properties overall when compared to the individual components in a very similar manner to nacre. The transfer of piezo-electrical properties of the copolymer P(VDF-TrFE) is another focus of this study. P(VDF-TrFE) shows the different behaviors after annealing under different temperature. Both the mechanical properties and piezo-electrical properties of the P(VDF-TrFE) are strongly dependent on the film&’s morphology and its fabrication process and that is the reason why we want the co-polymer [P(VDF-TrFE)] to be annealed. The Crystallization of [P(VDF-TrFE)]can be changed under different temperature , and that lead to morphology of the P(VDF-TrFE) film changed. This property was studied to see how it affected the nano-composite material and if there were any enhancements in annealing. Also the nacre-inspired structure material could be applied under different temperature based on this property. The resulting nacre-inspired material fabricated using our method is not only suitable for the small devices; it also satisfies the requirement for the large-scale applications. It is a simple time-saving and energy-efficient method to produce this type material.

Tunable gauge characteristics are important for wearable and conformal electronics. This report describes a gauge sensor device which is fabricated by assembling and printing molecularly linked thin films of gold nanoparticles on flexible microelectrodes with unusually high and anisotropic gauge factors. A sharp difference in gauge factors up to two to three orders of magnitude between bending perpendicular (B perp; ) and parallel (B || ) to the current flow directions is observed. The origin of the unusual high and anisotropic gauge factors is analyzed in terms of nanoparticle size, interparticle spacing, interparticle structure, and other parameters, and by considering the theoretical aspects of electron conduction mechanism and percolation pathway. A critical range of resistivity where a very small change in strain and the strain orientation is identified to impact the percolation pathway in a significant way, leading to the high and anisotropic gauge factors. The gauge anisotropy stems from molecular and nanoscale fi ne tuning of interparticle properties of molecularly linked nanoparticle assembly on flexible microelectrodes. The implication for the design of gauge sensors for highly sensitive detection of deformation in complex sensing environment or on complex curved surfaces such as wearable electronics and skin sensors will be discussed.

Sea sponge spicules possess an unusual combination of effective failure strain and optical light propagation properties due to their micro- and nanoscale hierarchical structure. We present measurements of the elasto-optic properties of Tethya aurantia and Euplectella aspergillum sea sponge spicules using Brillouin laser light scattering. Brillouin scattering is a powerful, non-invasive technique that can be used for determination of material elastic constants, sound velocities, and refractive indices. We non-invasively determine the full elastic stiffness tensor of these materials and show that spicules have optical propagation similar to that of man-made glass fibers. These measurements help us understand the interplay of flexibility, strength, and material microstructure for future functional biomimicry.

#8203;The unique properties of bamboo come from the natural composite structure of fibers that consists mainly of cellulose nanofibrils in a matrix of intertwined hemicellulose and lignin called lignin-carbohydrate complex (LCC). Here, we have used atomistic simulations to study the effect of water on the mechanical properties of and adhesive interactions between these materials in bamboo fibers. With this aim, the relationships between water content and elastic moduli and the adhesion energies between these materials and cellulose nanofibrils were investigated. The results imply that the densities of hemicellulose, lignin and LCC decrease with increasing water content. However, in low water content (around 1%) lignin and LCC maintain the density. The elastic moduli of lignin and LCC show an increase at low water content (around 1%) and decrease at higher water content, whereas the hemicellulose elastic modulus constantly decreases. This suggests that at the early stage of introducing water in these systems, water molecules enhance the structure of LCC and lignin and increase the modulus of elasticity while maintaining the density. For hemicellulose, however, the effect is always regressive. This behavior was found to be associated with the water molecules interference in the hydrogen bond energies of these materials.

This talk will be based on recent publication on In Situ Atomic-Scale Observation of Twinning Dominated Deformation in Nanoscale Body-Centred Cubic Tungsten, Nature Materials, doi:10.1038/nmat4228, March 2015 by Jiangwei Wang, Zhi Zeng, Christopher R. Weinberger, Ze Zhang, Ting Zhu and Scott X. Mao. Twinning is a fundamental deformation mode that competes against dislocation slip in crystalline solids. In metallic nanostructures, plastic deformation requires higher stresses than those needed in their bulk counterparts, resulting in the smaller is stronger&’ phenomenon. Such high stresses are thought to favour twinning over dislocation slip. Deformation twinning has been well documented in face-centred cubic (FCC) nanoscale crystals. However, it remains unexplored in bodycentred cubic (BCC) nanoscale crystals. Here, by using in situ high-resolution transmission electron microscopy and atomistic simulations, we show that twinning is the dominant deformation mechanism in nanoscale bi-crystals of BCC tungsten. Such deformation twinning is found to be pseudoelastic, manifested through reversible detwinning during unloading. We find that the competition between twinning and dislocation slip can be mediated by loading orientation, which is attributed to the competing nucleation mechanism of defects in nanoscale BCC bi-crystals. Our work provides direct observations of deformation twinning as well as new insights into the deformation mechanism in BCC nanostructures.

Polycrystalline Cu and CrN thin films were sputter deposited sequentially onto Si(001) substrates to form specimens of CrN/Cu bilayers on Si, in which Cu thin films were confined between elastic-brittle Si and CrN. Cylindrical micro-pillars were fabricated from CrN/Cu/Si(001) specimens through Ga⁺ focused ion beam (FIB) milling, in which the interfaces were either inclined at 45° to the pillar axial direction or normal to it. Load-displacement curves were measured through axial compression loading of CrN/Cu/Si micro-pillars, and together with post-compression morphological observations, demonstrated clearly the occurrence of extensive plasticity within the Cu interlayers confined between Si and CrN. Measured plastic flow stresses showed a marked dependence on the thickness of the Cu interlayer, exhibiting significant mechanical size effects. Detailed structural characterizations of the sputter deposited CrN and Cu thin films were carried out with techniques of X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM). A simple strain gradient plasticity model was applied to the present experimental situation, and comparision between experimental data and model output will be given. The present experimental protocol affects significant plasticity in metal thin films in simple deformation geometries, results from which offer new experimental test cases for non-local plasticity theories.
References:
1. K. Chen, Y. Mu, W.J. Meng, A new experimental approach for evaluating the mechanical integrity of interfaces between hard coatings and substrates, MRS Comm. 4, 19-23 (2014).
2. Y. Mu, K. Chen, W. J. Meng, Thickness dependence of flow stress of Cu thin films in confined shear plastic flow, MRS Comm. 4, 129-133 (2014).
3. Y. Mu, J. W. Hutchinson, W. J. Meng, Micro-pillar measurements of plasticity in confined Cu thin films, Extreme Mechanics Letters 1, 62-69 (2014).

Nanoporous (np) metals formed by dealloying are expected to have high strengths, because their solid component, the ligaments with diameter at nm-scale, may be as strong as similar-sized metal nanowires (with GPa scale strength). However, previous mechanical testing of bulk np Au samples (prepared by dealloying Au-Ag alloys) found that their strength are much lower than theoretical predictions, raising a question as to whether the np metals are intrinsically soft. Here we present that the theoretical high strength can be approached in np metals when the dealloying condition was optimized. By minor-alloying of Pt to suppress the coarsening of np structure, and by dealloying (AuPtAg parent alloy) at a lowest possible potential to prevent crack-formation, we prepared millimeter-sized np Au-Pt with strength of 345 MPa. This value is 700% larger than the strength of fully dense parent alloy and is much higher than that of any previous np metals. We will also show that a slight coarsening can severely degrade the strength of these np AuPt samples - it explains the low strength of previous np Au samples, which have more or less experienced coarsening. Our macroscopic np AuPt samples couple high strength, small structure size (~5 nm), and large surface-area-to-volume ratio (~10⁸ m^-1), which are promising for many applications, such as actuation.

Displacive deformation usually plays a dominant role in the plasticity of crystalline solids at room temperature. Here we report in situ quantitative transmission electron microscope deformation tests of single crystal Sn samples. We found that when the sample size was reduced from 450 nm down to 130 nm, diffusional deformation replaces displacive plasticity as the dominant deformation mechanism at room temperature. At the same time, the strength-size relationship changed from “smaller is stronger” to “smaller is much weaker”. The effective surface diffusivity calculated based on our experimental data matches well with that reported in literature for boundary diffusion. The observed change in the deformation mode arises from the sample size-dependent competition between the Hall-Petch-like strengthening of displacive processes and Coble diffusion softening processes. Our findings have important implications for the stability and reliability of nanoscale devices such as metallic nanogaps.

Most of our knowledge on the mechanical behavior of metals has been derived from conventional uniaxial testing. However production and application of metallic components involve often methods that include strain path changes. The effect of a sudden change in strain path on the operating deformation mechanisms, the evolving microstructures and their mechanical behavior is less well investigated. In-situ deformation studies of such strain path changes during X-Ray Diffraction (XRD) or during observation in a scanning electron microscopy (SEM) can give valuable insights on deformation mechanisms and microstructures.
While miniaturized uniaxial tensile and/or compression deformation set-ups that can be used in-situ during synchrotron radiation and/or in SEM are nowadays relatively common and known to provide valuable insights on deformation mechanisms and microstructures, miniaturized multi-axial deformation devices allowing to change the strain path during deformation are still rare.
Here we present a newly developed miniaturized biaxial tensile machine allowing operation of the two deformation axes independently on a sample that has been previously cut out from a bulk metallic material. The machine can be used in a synchrotron beamline (for transmission or reflection x-ray diffraction) or in a SEM environment for in-situ studies. The characteristics of the miniaturized device and the developed sample preparation technique using picosecond laser ablation will be discussed in detail. First in-situ studies will be presented including in-situ SEM and in-situ XRD analyses performed at the MS and the MicroXAS beamlines of the Swiss Light Source (SLS) on nanocrystalline and ultrafine grained metallic microstructures. The research is performed within the ERC advanced grant MULTIAX (339245)

Driven by the ongoing miniaturization, new exciting material behavior at small length scales was encountered. In the (sub-) micron range, pronounced size effects come into play and material properties are subject to change. This was a driving force for the development of miniaturized testing techniques that enable determination of local plastic properties of materials. While static testing techniques are well established, there were only few efforts regarding the determination of fracture properties in miniaturized systems. Moreover, when working with thin films, residual stresses are a permanent prime concern that needs to be addressed.
In this presentation, we studied the influence of residual stresses on the fracture properties of thin films. We will demonstrate the quantitative capability and precision of recent developments regarding miniaturized determination of residual stresses and stress gradients by a FIB based layer removal method, as well as consecutive miniaturized fracture testing of stressed Cu, W, and W-Cu-W multilayers, respectively, in situ in the SEM. Subsequently, for a detailed data analysis we introduce an accompanying finite element based modeling approach. The possibilities and benefits of these approaches will be demonstrated by examining the deformation and fracture properties of the above mentioned thin films. Moreover, possible limitations of commonly used data analysis approaches are addressed, and possible extensions are discussed

We report on the mechanical behavior of nanostructured W/Cu (3 nm/1 nm grain sizes) thin films deposited on Kapton® under controlled biaxial loadings thanks to a biaxial testing device developed on DiffAbs beamline at SOLEIL synchrotron (Saint-Aubin, France) [1]. In situ tensile tests were carried out combining 2D synchrotron x-ray diffraction (XRD) and digital-image correlation (DIC) techniques. In the elastic domain, the results show that the strain measurements (in the crystalline film by XRD and the substrate by DIC) match to within 10^-4. This result demonstrates the full transmission of strains in the elastic domain through the film-substrate interface and thus a good adhesion of the thin film to the substrate although no adhesion layer was used. Then, we have been able to extract the yield surface of W-Cu nanocomposites and demonstrate its brittle behavior [1] The elastic limit of the nanostructured W/Cu thin films was determined at the bifurcation point between strains obtained by XRD and DIC at ~0.50 % (depending on load ratio). After bifurcation the film elastic strain still increases linearly up to an applied load of ~100 N (i.e. a corresponding strain of ~0.67%, depending on load ratio) and then saturates. Deformation mechanisms such as strain localisation and film fragmentation are proposed and discussed in view of the behavior of monolithic Cu and W films obtained under similar conditions [2].
[1] S. Djaziri, D. Faurie, P.O. Renault, E. Le Bourhis, P. Goudeau, G. Geandier, D. Thiaudière, Acta Mater. (2013) 61, 5067.
[2] S. Djaziri, P.-O. Renault, E. Le Bourhis, Ph. Goudeau, D. Faurie, C. Mocuta, D. Thiaudière
J. Appl. Phys. (2014) 116, 093504

α-Fe single crystal nanopillars with different diameters were fabricated through focused ion beam (FIB) and then in situ tested using uniaxial microcompression methodology inside a SEM. Besides the well-established tenet of “smaller is stronger”, we demonstrated that the strain rate sensitivity of small-scaled single-crystal α-Fe decreased about 12 times as the pillar diameter decreased from 1000nm to 200nm. We propose that the observed phenomena are stemmed from the diminishing mobility difference between edge and screw dislocations which resulted from the increase of the external applied stress, i.e. the decrease of the sample size. This in turn renders the behavior of dislocations and the strain rate sensitivity of BCC metals similar to that of FCC metals.

The mechanical failure of amorphous materials is a complex and longstanding problem, and to this day the deformation mechanisms occurring in such materials remain poorly understood. With increased use in applications including microelectromechanical systems, and energy generation and storage devices, a better understanding of the mechanisms by which the materials deform and break is desired. Disordered nanoparticle packings are one useful class of amorphous materials; they have significant potential for usage in solar cells and other energy applications, where film integrity is important for electrical conductivity, and they also have applications in electrochemistry and optical coatings [1]. Importantly, they can also serve as a larger-scale model representation of disordered atomic systems like metallic glasses, whose structure and failure modes are extremely difficult to observe at the atomic level. In this study, quantitative nanoindentation techniques using atomic force microscopy are employed to investigate the mechanical behavior of thin film nanoparticle packings where the film thickness is varied. Mechanical properties such as the elastic modulus and hardness can be evaluated at the level of a single nanoparticle, allowing spatial inhomogeneities in the film structure to be mapped. The elastic modulus is measured to be 9.3 ± 3.6 GPa for a 100 nm thick film. The significant spread seen on any given sample is attributed to local variations in mechanical properties arising from the heterogeneous film structure. For loads below 500 nN on such films, outliers with very high moduli appear; these are never seen for higher loads or on thicker samples. The dependence of elastic properties on film thicknesses and load is attributed to mechanical communication between the nanoparticles being probed and the substrate. Topography scans before and after indentations allow for individual nanoparticle displacements to be resolved with nanometer accuracy. This resolution is significantly finer than the nanoparticle radius, which is unheard of for atomic displacements in metallic glasses. The hardness of the 100 nm film is measured to be 1.03 ± 0.51 GPa. Plastic indentations are accompanied by highly inhomogeneous nanoparticle displacements in the vicinity of the indent, in contrast to the displacement fields expected for crystals. The magnitudes of nanoparticle displacements after inelastic indentations are closely correlated with the measured mechanical properties. These relationships have implications for the spatial distribution of mechanical properties in other disordered packings including metallic glasses, for which so-called “soft spots” have been hypothesized but never directly observed. To illustrate this, the variance in the mechanical properties of soft spots as opposed to non-soft spots is estimated.
[1] Lee, D., Rubner, M. F., & Cohen, R. E. (2006). All-nanoparticle thin-film coatings. Nano Letters, 6(10), 2305-12. doi:10.1021/nl061776m

Nanoporous metals have been the subject of exciting studies in various fields such as catalysis, sensing, MEMS and biomedical applications, because of their large surface-area-to-volume ratio. The dimensions of the interconnected ligaments which create the nanoporous structure are typically below 100 nm. Despite the inherent ductility of metals, samples become brittle when the cell size is reduced to this scale, which hinders the development of direct applications.
Using a combination of free and electrochemical dealloying steps, bulk nanoporous gold (np-Au) samples of millimeter-scale dimensions were created from gold-silver alloys. This method led to crack-free nanoporous structures with negligible residual presence of the sacrificial element (silver), as verified by scanning electron microscopy and energy dispersive x-ray spectroscopy. The mechanical properties of np-Au were determined by tension and compression testing on a custom-built system, and by nanoindentation. Although np-Au specimens were extremely brittle at the macroscopic scale, observation of the fracture surface after testing revealed extensive plastic deformation and necking of the individual ligaments prior to rupture.
Classic scaling relations which link the mechanical properties of a porous metal to its dense counterpart do not take the cell size into consideration. Because the cells in nanoporous metals are of nanometer dimensions, the relations need to account for the change in mechanical properties of the individual ligaments. Based on the measurements realized on np-Au, a modified scaling relation will be presented. In addition, the deformation behavior of the cells under load will be discussed, based on some observations and 3D images of the deformed nanoporous structure, reconstructed from serial sectioning and imaging in a focused ion beam/scanning electron microscope.

A majority of refractory high-entropy alloys suffer from brittleness and limited formability at ambient temperature, but studies of their fracture properties are scarce. Here, we have conducted in-situ micro-cantilever tests to investigate the fracture behavior of a typical refractory high-entropy alloy, Nb₂₅Mo₂₅Ta₂₅W₂₅. The results show that the fracture toughness and strength of bi-crystal specimens are one order lower than those of single-crystal ones, suggesting that brittle intergranular fracture is a major fracture mode and strengthening grain boundaries is critical.

In situ nanomechanics is an emerging field that investigates the mechanical properties and deformation mechanisms of nanostructured materials. The study of in situ nanomechanics is often conducted by integrating the real-time mechanical testing inside an electron microscope and the mechanics modeling with atomic resolution. It provides a powerful approach to visualize the intrinsic nanomechanical behavior of materials - seeing is believing. In this talk, I will present recent in situ nanomechanics studies from my group, including the electrode degradation in nanoscale lithium-ion batteries (Nature Nanotechnology, 7, 749, 2012); deformation-induced stacking fault tetrahedra in FCC nanocrystals (Nature Communications, 4, 2340, 2013); fracture toughness of graphene (Nature Communications, 5, 3782, 2014); and twinning-dominated deformation in BCC nanowires (Nature Materials, 14, 594, 2015). The in situ nanomechanics studies provide new insights that cannot be offered by traditional mechanics studies. Ultimately, the in situ nanomechanics research aims to enable the design of nanostructured materials to realize their latent mechanical strength to the full.

It is now well-established that introducing nanoscale coherent twin boundaries (CTBs) is an efficient way to simultaneously increase strength and ductility in face-centered-cubic metals. Specific details on dislocation-CTB interactions have been studied extensively in the past using atomistic simulations, experiments and continuum theory, and are well understood when CTBs are considered to be perfect. However, recent nanodiffraction mapping experiments have revealed the existence of short segments of incoherent twin boundary (ITB) forming intrinsic kink-like twin boundary defects in the structure of nanotwinned copper, which contradicts the perfect CTB assumption and could play a major role in plastic deformation. This presentation will focus on a molecular dynamics simulation study of the effects of intrinsic kink-like twin boundary defects on plasticity and strengthening mechanisms in nanotwinned copper. Using simple bicrystal models, it is found that kink-like defects have a profound impact on screw dislocation - coherent twin boundary interactions, resulting in significant strengthening behavior. The dislocation products of a screw dislocation intersecting two different types of kink defects oriented at either 0 degree or 60 degrees with respect to the dislocation line, are studied at atomic scale to explain this phenomenon.

Extreme value theory is applied to the critical stress of the first plastic event which occurs in discrete intermittent plasticity. This could be the first pop-in during nano-indentation, or a displacement jump in a micro-deformation experiment. It is found that when the same deformation is repeated many times, the average of this first critical stress is related to the deforming crystal volume via an exponentially truncated power law. The present work demonstrates this trend and the expected Weibull fluctuations around it, for the experimental nano-indentation data of Morris et al Phys. Rev. Lett. 106, 165502 (2011), and for dislocation dynamics simulations suggesting a quite general phenomenon is at play. The identified truncated power law is found to uniquely define the underlying master distribution of critical stresses present in the deforming crystal, and also the density of discrete plastic events available to the system, thus providing a procedure via a deformation experiment to characterize a material's microstructure prior to loading.

The experiment results indicated that Al-TiN multilayers have unusual properties such as high strength, measurable plasticity and high strain hardening rate when both layers thicknesses are in the range of nanometers, and showed a pronounced size effect in these mechanical properties, not only depending on the layer thickness but also on the layer thickness ratio. Using atomistic simulations, Atomistically-informed Dislocation Dynamics (AIDD) simulator, and three-dimensional crystal elastic-plastic model (3DCEPM), we characterize interface structure and dislocation nucleation and study the effect of layer thickness and layers thickness ratio on mechanical properties. The high strain-hardening rate is ascribed to the accumulation of dislocations at interface and the load transfer related to the layer thickness ratio. The measurable plasticity implies the plastically deformable ceramic layer in which the dislocation activity is facilitated by the interaction force among the deposited dislocations within interface and in turn is strongly related to the ceramic layer thickness.

Metal nanowires are usually strong but not able to maintain a high plastic flow due to the lack of strain hardening in them. In this talk, I would like to report a new category of metal nanowires with hollow interior, metal nanotubes, to exhibit a combination of ultrahigh strength and extraordinary plastic flow. It was revealed based on tensile deformations through molecular dynamics simulations that by controlling the diameter, wall thickness and axial orientation, ultrahigh plastic flow stress of more than 2 GPa could be maintained for up to ~ 60% tensile strain in Au nanotubes, whereas the solid Au nanowires of similar size yielded at tensile st