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. N