Symposium ST01—Mechanical Behavior at Micro/Nano-Scale

We present the results of atomic-force-microscopy-based friction measurements on Re-doped molybdenum disulfide (MoS₂). In stark contrast to the seemingly universal observation of decreasing friction with increasing number of layers on two-dimensional (2D) materials, friction on Re-doped MoS₂ exhibits ananomalous, i.e. inverse dependency on the number of layers. Raman spectroscopy measurements reveal signatures of Re intercalation, leading to a decoupling between neighboring MoS₂ layers and enhanced electron-phonon interactions, thus resulting in increasing friction with increasing number of layers – a new paradigm in the mechanics of 2D materials.

Light-powered and chemically fueled artificial molecular applications have long been recognized as the key building blocks of the functional materials of the future. In this work we employ an azobenzene based molecules A3 that undergoes reversible structural change between trans and cis forms upon irradiations of UV and visible light respectively. Using this, we develop highly sensitive phototransistors with tunable and ultra-high field-effect carrier mobility based on tungsten diselenide (WSe₂) monolayer. By stretching and contracting the underlaying A3 molecules using UV and visible light (from 9 Å to 5.5 Å), the hole mobility of the hybrid FET device can be tuned from 100 cm²/Vs to as high as 1025 cm²/Vs, which is over an order of magnitude higher than typical TMDs based transistors. The observed mobility is comparable to that of conventional semiconductors such as silicon and germanium. We investigated the enhancement mechanisms by electrically characterizing the hybrid device before and after UV exposure. Consequently, we conclude strain to be the primary mechanism for channel current enhancement.
Furthermore, we monitored the UV photoresponse of the hybrid transistor and found that the UV photocurrent extinction ratio to be 75% higher than that of the bare WSe₂ device, making it a promising candidate for UV detection application. Such hybrid platform can potentially be used for designing next generation devices as well as ultrasensitive UV photodetectors.

Strong and tough materials are desired for light-weight applications such as electric cars and aerospace applications. Recently, heterostructures are found to produce unprecedented strength and ductility that are considered impossible from our textbook knowledge and materials history. Heterostructured materials consist of heterogeneous zones with dramatic (>100%) variations in mechanical and/or physical properties. The interaction in these hetero-zones produces a synergistic effect where the integrated property exceeds the prediction by the rule-of-mixtures. Importantly, HS materials can be produced by current industrial facilities at large scale and low cost. There are many scientific issues with such materials that challenge the communities of experimental materials science and computational material mechanics. Heterostructured materials is quickly becoming a hot research field in the post-nanomaterials era. In this talk I’ll present the current advances as well as future challenges and issues in this emerging field.

For safety of nuclear fuel, pure metallic uranium disqualifies due to its anisotropic thermal expansion and cycling behavior of its equilibrium alpha phase under operation conditions. In contrast, the body-centered cubic gamma-phase provides the desired irradiation stability, but as this phase is not stable at room temperature, it is necessary to alloy it with elements such as Nb, Ti or Mo. Still, the alloy needs to be quenched rapidly from the high temperature gamma-phase regime down to room temperature in order to preserve the gamma phase. Uranium-molybdenum alloys, with 4.5 wt.% to 15 wt.% Mo, have been favored, however, the best compromise between fuel performance and uranium density must be made. In order to provide a safe usage, two conditions must be ensured during the entire operation cycle: On the one hand, the stability of phases must be given throughout the operation cycle and on the other hand, the interaction with other materials, especially with the cladding, must be minor or at least predictable.

Exposed to elevated temperatures, the meta-stable gamma-phase of uranium-molybdenum alloys decomposes into other thermal equilibrium phases, i.e. U₂Mo and alpha-uranium following isothermal phase transformation kinetics. Until now, the transformation enthalpy, which is the energy required to transform the decomposed phases back into pure gamma, has not been studied by accounting for the relative fraction of the phases during this re-transformation. The enthalpy change of the phase transformation can be studied by differential scanning calorimetry (DSC).

In the present study, an uranium-molybdenum alloy with 7.8 wt.% has been used, which was deposited onto a copper substrate using physical vapor deposition (PVD). In order to compare the phase composition, X-ray diffraction (XRD) measurements were conducted on fresh and tempered samples, respectively. After heat treatment, the phase proportion between alpha and gamma phase exists according to the corresponding time temperature transformation (TTT) diagram. During the measurement, the respective fraction of the volume undergoes a transformation back from alpha- to gamma-uranium, which can be measured using DSC. However, the phase transition enthalpy cannot directly be normalized by the weight or volume of the sample. Hence, an additional correction of pre-measured fractional phase composition must be carried out. Analysis of DSC data is presented.

Nanoporous metals with bi-continuous porosity attracts considerable interest due to its high surface area which opens their potential for use in sensing [1], catalysis [2], and energy storage [3]. However, processing-structure (PS) relations for nanoporous gold (NPG) have been largely not understood. Recently, McCue et al. constructed and analyzed a large dataset of porous gold SEM images from various publications to establish correlations between structural parameters (i.e. ligament diameter, aspect ratio, solid area fraction (SAF)), and processing conditions (such as dealloying times and temperatures) [5]. However, the large scatter of this dataset also suggests issues with the reproducibility of NPG synthesis even with the reported fixed dealloying protocol (e.g. precursor alloy composition, etching solution, concentration, etc.). In this study, we experimentally examine the reproducibility of the synthesis of NPG thin films fabricated at constant conditions (i.e. 60 at.% Ag in precursor alloy, 7.85 M HNO₃, 65 °C) and extract structural data to find PS relations. Moreover, we test the influence of processing conditions previously not reported such as the temperature equilibration time of the etchant solution, and sample aging effects such as stress release, grain growth and surface oxidation, all of which have been found to affect the uncertainty of NPG synthesis. This ground work may shed more light at future understanding of the kinetics of morphology evolution in chemical dealloying at a more quantitative level.

[1]
B.-K. Chao, H.-C. Ho, P. Yiu, Y.-C. Lai, C.-H. Shek and C.-H. Hseuh, "Gold-rich ligament nanostructure by dealloying Au-based metallic glass ribbon for surface enhanced Raman scattering," Scientific Reports, vol. 7, no. 7485, 2017.

[2]
V. Vij, S. Sultan, A. Harzandi, A. Meena, J. Tiwari, W. Lee, T. Yoon and K. Kim, "Nickel-based electrocatalysts for energy-related applications: oxygen reduction, oxygen evolution, and hydrogen evolution reactions," ACS Catalysis, vol. 7, no. 10, pp. 7196-7225, 2017.

[3]
J. Li, J. Pu, Z. Liu, J. Wang, W. Wu, H. Zhang and H. Ma, "Porous-nickel-scaffolded Tin-Antimony anodes with enhances electrochemical properties for Li/Na-ion batteries," Applied Materials and Interfaces, vol. 9, pp. 25250-25256, 2017.

[4]
A. Sharstniou, S. Niauzorau, P. M. Ferreira and B. P. Azeredo, "Electrochemical nanoimprinting of silicon," PNAS, vol. 116, no. 21, pp. 10264-10269, 2019.

[5]
I. McCue, J. Stuckner, M. Murayama and M. J. Demkowicz, "Gaining new insights into nanoporous gold by mining and analysis of published images," Scientific Reports, vol. 8, no. 6761, 2018.

There is a growing interest in developing strategies to increase the high temperature performance of metallic alloys used in extreme conditions, such as those found in many energy generations and propulsion systems, among others. Microstructure refinement has long been considered a potential route for improvement in this regard; however, nanocrystalline (NC) metals and alloys (with grain size less than 100 nm) exhibit significant microstructure instability at high temperatures and even at room temperatures under, e.g., cyclic loads. Thermodynamic (solute segregation to reduce grain boundary energy) and/or kinetic approaches (solute drag, Zener pinning, etc., to pin grain boundaries) can be used to suppress grain growth through the addition of alloying elements and second phases. Some NC alloys such as Cu-10%Ta fabricated using far from equilibrium processing techniques have displayed stability of their microstructures mostly through kinetic mechanisms at various thermo-mechanical loading conditions, leading to an enhanced creep behavior compared to its coarse-grained (CG) counterparts. However, the low melting point of Cu makes Cu alloys ill-suited for advanced applications compared to Ni and Ti. The fundamental creep mechanisms in these alloys also need to be studied in more detail.

This work seeks to study the microstructure stability as well as the fundamental creep mechanisms in hierarchical and NC Ni-Y alloys, where the hierarchical alloys were synthesized using the conventional arc-melting technique in an inert atmosphere and NC alloys by consolidating powders at high temperatures through equal channel angular extrusion (ECAE) processing to retain a fully dense NC microstructure. For the Ni-Y alloys, using a Ni-rich matrix can impart high ductility and formability while leading to high stiffness and a relatively high melting point, thereby delaying the onset of diffusional creep mechanisms. Small additions of Y to Ni would introduce second phase particles from Ni-Y (Ni₁₇Y₂), thus possibly improving the kinetic stability (Zener pinning of grain boundaries).

Current research is divided into three major tasks: a) evaluating microstructure and mechanical properties of hierarchical Ni-Y alloys over an appropriate range of compositions to establish a baseline to assess binary alloy behavior, b) high temperature uniaxial compression and creep-testing of both hierarchical and NC alloys, and c) advanced characterization techniques to understand the creep behavior of the alloys. The microstructure of hierarchical alloys studied using characterization techniques like optical microscopy, BSE imaging, EDX, and WDX analysis displayed three major length scales. The ultrafine eutectic length scale of ~300 nm led to the microhardness and yield strength of the alloys comparable to the existing Ni-superalloys. Furthermore, the matrix of the alloys defined by dendritic arm spacing of ~10 µm controlled the microstructural stability of alloys, ascertained by heat treatments and high temperature compression tests. Additionally, grain size (200 µm) in hierarchical alloys is expected to play a significant role in improving creep properties based on the observation of the creep rate possessed by the ultrafine-grained Ni-Y alloys being comparable to that of Ni-based single crystal superalloys. Characterization has also been performed on as-deformed hierarchical Ni-Y samples using EBSD techniques before and after deformation to see a combination of RT and temperature deformation in the matrix and eutectic phases, understand the deformation behavior, e.g., slip behavior, and correlate crystallography with slip behavior. EBSD combined with higher resolution imaging has also been used to see the effect on the eutectic phase with deformation. Furthermore, SAXS measurements are also performed on the deformed samples to understand the average particle size, particle size distribution, particle surface area, etc. in hierarchal and NC alloys.

Spark plasma sintering (SPS) is a unique powder metallurgy technique capable of rapid densification due to a combination of joule heating and pressure. The rapid heating rates in SPS and the tooling-sample contact resistances cause thermal gradients, which in turn lead to microstructural gradients. To elucidate the nature of the interaction between starting powder morphology, electric current pathways, and thermal gradients in SPS, the present work investigated the microstructure and mechanical properties of nominally austenitic stainless-steel samples fabricated via SPS of gas atomized and ball milled powders. Microhardness testing and nanoindentation were applied to investigate local mechanical properties at both a micro- and a nanoscopic level. Nanoscratch testing was incorporated to understand the effect of different particle-particle interfaces including the deformation behavior. It was found that a grain size gradient arising from the thermal gradients in SPS lead to changes in local microhardness (ΔH=100MPa~150MPa). Nanoindentation results revealed radial porosity gradients in the samples lead to a radial gradient in Young’s modulus (ΔE=30GPa~65GPa).

The remarkable strength of nanotwinned metals and metallic alloys has been attributed to a number of mechanisms, all involving frequent dislocation-twin interactions. Despite the high density of nanotwins that have been achieved through a variety of processing routes, the relative distribution of twins remain stochastic. For simple deformation modes, such a distribution of dislocation barriers can be accommodated by adapting traditional strengthening models. However, under cyclic loading conditions, the configuration and spacing between obstacles play a much more nuanced role. Modeling approaches have begun to tackle this problem, but without experimental validation in a controlled microstructure, the impact remains limited. We will emphasize the processing science necessary to develop well defined nanotwin microstructures, complemented by characterization of select twin-dislocation interactions enabled by these configurations. We will demonstrate the impact of deposition conditions and intrinsic properties on the nature and density of nanotwins using Ni-rich Ni-Ti alloys. Specifically, we have employed alloy chemistry, stacking fault energy and substrate constraint to engineer nanotwins that are inclined to the plane of the film. In such a configuration, simple tensile loading can activate complex dislocation-twin interactions, while still allowing for a systematic characterization of these interactions. When paired with computational tools and developing analytical model, these observations provide fundamental insight on the interfacial reactions relevant to fatigue damage accumulation.

Polycrystalline Ni-based superalloys are vital materials for disks in the hot section of aerospace and land-based turbine engines due to their exceptional microstructural stability and strength at high temperatures. In the drive to increase operating temperatures and hold times in these engines, hence increasing engine efficiency and reduction of carbon emissions, creep properties of these alloys becomes increasingly important. At these higher temperatures, new deformation modes become active. Several alloy compositions and microstructure variants of commercial disk alloys are being explored, including g’ strengthened alloys as well as compositions promoting g’ and g’’ co-precipitation. Microtwinning and stacking fault shearing are important operative mechanisms in the critical 600-800°C temperature range. Advanced characterization techniques using scanning transmission electron microscopy with diffraction contrast imaging, high resolution imaging, and energy dispersive spectroscopy have been used to gain new insights into these mechanisms and the rate-limiting processes during high temperature deformation. Atomic-scale chemical and structural changes associated with stacking fault and microtwin interfaces within g’ precipitates have been identified and indicate that local phase transformations (LPT) occur commonly during creep of superalloys. Three distinct LPT scenarios have been identified. In the first, the stacking faults and twin interfaces adopt a composition similar to that of the solid solution g matrix and is associated with alloys having lower creep strength. In the second scenario, the superlattice extrinsic stacking fault structure transforms to the DO₂₄ η phase and is associated with alloys having improved creep resistance. In the third scenario, superlattice intrinsic stacking faults transform to a local X (DO₁₉) phase, and also appears to provide creep strengthening. Thus, these important deformation modes can be modulated by LPT formation. For instance, for alloys in which η phase formation is favorable, microtwinning is inhibited as a deformation mechanism. The thermodynamic driving force for LPT formation is being explored using DFT modeling, and the predicted segregation profiles are compared with experiment. Phase field dislocation dynamics modeling is being used to explore the interaction of dislocations with g’ microstructures under the cooperative shearing and local compositional changes associated with the LPT mechanism. Finally, these time-dependent processes are being incorporated into a microstructure-based crystal plasticity model for creep.

Calcite, a polymorph of calcium carbonate with a trigonal crystal structure, has been predicted to exhibit auxetic behavior – materials with negative Poisson’s ratios – but this has not been experimentally determined. Work by Aouni et al.,¹ for example, calculated the predicted crystallographic directions of maximum auxeticity in calcite. In this work, we report the first experimental evidence of auxeticity in calcite by ion implanting (1010) oriented single crystalline calcite substrates with Ar ions. Calcite substrates were implanted at room temperature using an ion energy of 400 keV and a dose of 1.0 × 10¹⁴ cm^-2, resulting in a projected range of ~400 nm. Lattice compression normal to the substrate surface was observed, which is an atypical result when ion implanting materials. Usually lattice expansion normal to the substrate surface is expected when implanting substrates.^2–4 This is due to the ions inducing lattice expansion during the implantation process. The bulk of the substrate beneath the ion projected range applies compressive biaxial stress to the expanding ion implanted region. Hence, materials with positive Poisson’s ratios would expand normal to the surface, which can be measured as tensile strain with diffraction measurements. Triple-axis X-ray diffraction measurements were employed to examine the strain state of the implanted calcite substrates. Reciprocal space maps for the symmetric 3030 and asymmetric 1450 reflections were measured and revealed that the implanted region was fully strained (pseudomorphic) to the bulk of the substrate beyond the projected range of the implant. A symmetric ω:2θ line scan of the (3030) reflection was also measured and X-ray dynamical diffraction simulations were used to model the strain profile to extract the variation of compressive strain as a function of depth normal to the substrate surface. SRIM calculations were also performed to obtain a displacement-per-atom profile. It was found that the strain profile generally follows the displacement-per-atom profile. This study demonstrated the use of ion implantation and X-ray diffraction methods to probe mechanical properties of materials and to test predictions such as the auxceticity on the scale of nanometers to microns depending on the ion energy used.

References
1. N. Aouni, et al., Phys. Stat. Sol. (b), 245(11), 2454 (2008).
2. C. Miclaus, et al., J. Phys. D: Appl. Phys., 36, A177 (2003).
3. M.E. Liao, et al., ECS J. of Solid State Sci. and Technol., 8(11), P673 (2019).
4. Y. Wang, et al., Phys. Status Solidi B, 257, 1900705 (2020).

Metallic thin films typically show high strength but low strain hardening capacity, which leads premature failure. For example, Ni films with a mean grain size less than 100 nm shows ultimate tensile strength of around 1.5 GPa but fail at strains below 3%. Here, we report the mechanical behavior of bicrystalline (111) textured Ni films with incoherent twin boundaries that exhibit high strength and significant ductility. Ni films of thickness varying from 200 nm to 2.5 μm were deposited on Si (111) substrates with a 25 nm Ag buffer layer at room temperature using magnetron sputtering. The microstructure of the films was characterized by TEM, EBSD and XRD, which revealed the presence of two grain variants with (111) out-of-plane texture with an incoherent twin boundary at their interface. Freestanding, dog-bone shaped samples of the films were co-fabricated with MEMS tensile testing stages using standard microfabrication techniques and their uniaxial tensile stress – strain response was measured at quasi – static strain rates. The experiments revealed flow stresses approaching 1 GPa as well as significant strain hardening capacity, which resulted in uniform elongation >10% in the thickest films. Possible mechanisms for this unusually high strength and ductility will be discussed.

2D and layered materials are generating excitement due to their multitude of novel properties. Exciting applications include as high-performance solid lubricants due to their ability to achieve “superlubricity” (nearly zero friction), and as functional materials for flexible electronics due to their exceptional bendability, strength, and electronic behavior. Yet the limits of their durability, friction, and adhesion behavior and not understood.

To explore these limits, we study nanocontacts with 2D and layered materials including graphene, MoS₂, and other transition metal dichalcogenides (TMDs) with atomic force microscopy (AFM). We previously found that friction is intrinsically low for these materials, but depends strongly on the number of layers underneath the AFM tip. A model attributing this to adhesion-induced “puckering” of the ultrathin 2D layers around the tip [1] was then enhanced by molecular dynamics (MD) simulations showing a strong role of the “quality” of the contact. In particular, energy barriers to sliding are affected by interfacial pinning and commensurability due to subtle deformations of the 2D material [2].

We then discovered another contact quality effect when probing nanoscale sliding against graphite while varying the relative humidity. Water acts as a lubricant only above a threshold humidity; below that, adsorbed water increases friction six-fold relative to dry sliding. Such a non-monotonic dependence of friction has been previously attributed to a humidity-dependent water meniscus. We reveal a new possibility, where the number and location of water molecules at the interface controls friction [3].

Finally, we have discovered another factor that strongly affects static friction, namely, the lattice constant. We compared the friction behavior for a nanoscale single asperity sliding on MoS₂, MoSe₂, and MoTe₂ in both bulk and monolayer form through a combination of AFM experiments and molecular dynamics (MD) simulations [4]. Under otherwise identical conditions, MoS₂ has the highest friction and MoTe₂ the lowest. Simulations and further analysis revealed that the friction contrast is attributable to their lattice constants. While the energy barriers to sliding are similar for all three materials, the larger lattice constants associated with larger chalcogen atoms permit the tip to slide more easily across correspondingly wider saddle points in the potential energy landscape. This emphasizes the critical role of the lattice constant, which can be the determining factor for nanoscale friction behavior.

[1] C. Lee et al. Frictional Characteristics of Atomically-Thin Sheets. Science, 328, 76 (2010).

[2] S. Li et al. The Evolving Quality of Frictional Contact with Graphene. Nature 539, 541 (2016).

[3] Hasz, K. et al. Experiments and Simulations of the Humidity Dependence of Friction between Nanoasperities and Graphite: The Role of Interfacial Contact Quality. Phys. Rev. Mat. 2, 126001 (2018).

[4] Vazirisereshk, M.R. et al., Nanoscale Friction Behavior of Transition-Metal Dichalcogenides: Role of the Chalcogenide, ACS Nano (2020).

The nanomechanical properties of materials critically depend on the nature of atomic bonds. In the conventional epitaxy, large lattice mismatches inevitably result in the formation of defects which are usually detrimental for the device performances. The weak bonds obtained at quasi van der Waals interfaces on 2D layers can relax this constraint [1], but the detailed mechanical relaxation processes are not fully understood yet, since charge transfers and defect creation might occur, up to the buckling-driven delamination of the 2D films. The strain engineering is critical for devices applications because it can tune the optical and electronic properties [2],[3],[4] at the interface. Hence, a precise insight into the mechanical properties at the atomic scale is needed to probe the links between the grown structures and their physical properties.

In this work, we investigate the strain relaxation mechanisms at quasi van der Waals interfaces using aberration-corrected HRSTEM-HAADF imaging [5] with a spatial resolution of 50 pm. High resolution maps of mechanical strains and bond rotations are derived from cross-analysis of atomistic modeling and microscopy image simulations. In particular, the influence of the possible crystallographic phases on the simulated HRSTEM images is provided to guide our analysis. We report the case of AlN layers grown on multi-layered MoS₂. Our analysis shows that the lattice mismatch is mostly accommodated by interfacial shear strains and heterogeneous localized distortions. Plastic deformations are prevalently located in MoS₂. These results are compared to recent theoretical predictions about the possible reordering phenomena in 2D layers due to in-plane uniaxial stress [6]. A global overview of the elastic strain fields around the quasi van der Waals interface will be presented. These results provide a basis for future strain and interface engineering of similar systems.

[1] D. Liang et al., Nano Energy 69, 104463 (2020)
[2] J. He, et al. Phys. Rev. B 89, 1–11 (2014)
[3] F. Li et al., Phys. Chem. Chem. Phys., 20, 29131 (2018)
[4] P. Gant et al., Materials Today, Vol. 27 (2019)
[5] O.L. Krivanek et al., Nature 464, 571–574 (2010)
[6] A. B. Alencar et al., J. Phys.: Condens. Matter 33, 125401 (2021)

Acknowledgments: Nicolas Bernier is acknowledged for measuring the HRSTEM images, Audrey Jannaud for preparing the samples at the nano-characterization platform of Minatec, and François Martin for his valuable expertise.

It is understood that anisotropy in material behavior plays a critical role in deformation, but it is important to also understand how anisotropy is related to the very cores of materials, atoms and their structures, and how a material’s genome can be altered at the atomic level to increase strength and toughness. In this study, we apply some of the basic principles of quantum mechanics to explore how strength and toughness change in diamond cubic solids, by taking SiC as an example material. We attempt to understand how electron density changes due to stress and strain under different loading conditions. Two distinct conditions were investigated: the first condition involved hydrostatic deformation which preserves the lattice symmetry, and the second condition involved symmetry-breaking uniaxial deformation along the three principal high-symmetry directions, [100], [110], and [111]. Under hydrostatic deformation, it was found that the nearest-neighbor bonds in 3C-SiC deform equally, regardless of the deformation state of the lattice; thus, there is no relaxation within the lattice, and there is symmetric distortion in the electron density. In contrast, under uniaxial stress deformation, it was found that the elastic energy was highest for loading along the [100] direction, while it was the least for loading along the [111] direction. This is due to the nearest-neighbor bonds during [100] directional loading deforming uniformly, so all energy is equally distributed throughout the lattice, while for the [110] and [111] loading directions, bond lengths change non-uniformly, so parts of the lattice relax while the others do not, leading to quicker failure. After evaluation of the electronic structure basis of the maximum stress the lattice can sustain under uniaxial stress deformation, it was found that for the Carbon atoms, electron redistribution in the s and px orbitals was highest for loading along the [100] direction, with the [111] loading direction inciting the least change in electron occupation. For the Silicon atoms, the opposite results were observed, with the [111] loading direction experiencing the highest change in electron density for those orbitals. Further discussion of the methodology and conclusions from this study will occur during the presentation. In this presentation, it will be shown just how significant this study is; with the connection between solid mechanics and quantum mechanics understood, the future of tough, strong, and electrically conductive materials is brighter, and will allow applications of energy-efficient materials to grow in abundance.

Lightweight materials with tailorable properties have attracted attention for decades, yet stiff materials that can resiliently tolerate extreme forces and deformation, while being manufactured at large scales, have remained a rare find. Bio-inspired designs such as hierarchical foams and atomic-lattice-mimicking trusses have achieved optimal combinations of mechanical parameters but suffer from limited mechanical tunability, limited long-term stability, and low throughput volumes. Most mechanical metamaterials have relied on symmetry, periodicity, and lack of defects to achieve the desired mechanical response, resulting in sub-optimal mechanical response under the presence of inevitable defects. Shell-type designs, which can mitigate stress concentrations and flaws, have come with high densities and limited recoverability.

We exploit non-periodic nanoscale ceramic shell architectures of ultralow density (reaching 4 mg/cm³) whose engineered curvature distribution achieves close-to-optimal stiffness scaling and, moreover, whose careful combination of topology, geometry and base material results in mechanical resilience superior to previous mechanical metamaterials. We present a method for scalable fabrication of these materials, guided by natural spinodal decomposition, which results in shell-based ceramic architectures with thicknesses down to ∼10 nm and overall sample volumes of up to a few cubic centimeters. We show the capability of these architectures to maintain more than 50% of their original stiffness and strength after ten cycles of compression up to 30% while exhibiting no visible permanent deformation. Guided by simulations, we show how controlling the material morphology leads to the (an)isotropic material response whose directional stiffness distribution, impressively, remains constant over a wide range of shell thicknesses, as demonstrated experimentally. Our approach highlights pathways to harness self-assembly methods in the design and scalable fabrication of novel micro/nanoscale metamaterials with both tunable high stiffness and unsurpassed recoverability, simultaneously unachievable by previously reported mechanical metamaterials.

In recent years, graphene has captured wide attention among engineers and researchers for its unique electronic, thermal, chemical, and mechanical properties, which could revolutionize a number of application areas including electronics, nanotechnology, drug delivery, sensors, and catalysis. Since discovery, extensive research has been conducted theoretically, experimentally, and computationally to investigate the behavior of pristine graphene under no strain conditions. With the goal of applications in mind, it is critical to develop a fundamental understanding of the properties of defective graphene under deformed or strained conditions. While line-defects (such as grain boundary) within the graphene lattice are known to degrade its mechanical properties, they can serve as an important source for making the graphene layer function as a chemically reactive 2D material which may be unattainable otherwise. Under defect-free conditions, an ideal and infinite graphene lattice can be deemed non-polar, due to its symmetry preservation across the sheet, which can prevent any measurable interaction with external molecules that may come in contact with. In this work, using density functional theory simulations, we investigate the energetics of an isolated H₂O molecule at a grain-boundary formed between armchair and zigzag graphene sheets. By placing the molecule at different heights from the graphene lattice, we calculate the total groundstate energy of the system. Our findings suggest that if the initial placement of the molecule is parallel to the lattice within the van der Waals interaction distance, the molecule undergoes a reorientation, and the O atom faces the graphene lattice. On the other hand, if the initial position of the molecule is parallel but closer to the graphene lattice, the H atoms become more reactive and they face toward the graphene lattice. We considered six different sites on the hexagon and found similar behavior. An electronic structure analysis suggests that the electron redistribution at the grain boundary affects the behavior of the molecule. Also, due to a separation-distance dependent distinct chemical affinities between the C-O atom vs. C-H atom pairs, the groundstate interaction is strongly affected by the initial position of the molecule. The results are expected to have important implications to our understanding of strain-chemistry of defective graphene. This talk will present the analysis of the results from an electronic structure perspective.

Vacancy defects are unavoidable in 2D materials due to the challenges involved in fabricating defect-free lattice from experimental methods. While defects are known to degrade the mechanical properties of a lattice, they offer unique opportunities to tailor the electronic and thermal properties by controlling the mean free path of electrons and phonons. It is therefore important to develop a fundamental understanding of the role of defects in modulating the local electronic character of a lattice. Nevertheless, the understanding of defect-induced alteration of electronic properties (the electronic states for example) remains less understood, particularly under finite deformation. Although researchers have investigated the mechanical and electronic properties of pristine graphene under stress, there is little research on the chirality effects on electron distribution at defective sites in 2D materials under applied stress. In this work, we use the density functional theory simulations and the Mulliken charge population analysis to investigate the electronic behavior of an isolated defect in graphene under the uniaxial loading condition for five different chiralities of the lattice. We analyze how chirality and intensity of loading affect the mechanical properties, bond length and bond angle change, charge distributions, the density of states, and the partial density of states surrounding the vacancy. Our results show that there is a uniform pattern for bond length redistribution as well as electron redistribution at all orientation angles and that such bond and electron redistributions are directly correlated. During all simulations, we observe a bond reconstruction at the defective site at higher deformation. In terms of the orientation angle, we demonstrate that increased orientation angle yields changes in the strength, toughness, stiffness, bond distributions, and electronics surrounding a mono vacancy. We find that differing orientation angles result in different failure processes and speeds. Additionally, the redistribution at the defective site is chirality-dependent, and there is a significant localization of electronic states at the defective site. The pattern of density of states is deformation-dependent. This talk will present a comprehensive view on the specific effects of electron redistribution on mechanical and electronic characteristics of the defective lattice.

Two-dimensional (2D) materials, such as Graphene, hBN and MoS₂, are promising candidates in a number of advanced functional and structural applications, owing to their exceptional electrical, thermal and mechanical properties. Understanding mechanical properties of 2D materials is critically important for their reliable integration into future electronic, composite and energy storage applications. However, it has been a significant challenge to quantitatively measure mechanical responses of 2D materials, due to technical difficulties in the nanomechanical testing of atomically thin membranes. In this talk, we will report our recent effort to determine the engineering relevant fracture toughness of graphene with pre-existing defects, rather than the intrinsic strength that governs the uniform breaking of atomic bonds in perfect graphene. Our combined experiment and modeling verify the applicability of the classic Griffith theory of brittle fracture to graphene. Strategies on how to improve the fracture resistance in graphene, and the implications of the effects of defects on mechanical properties of other 2D atomic layers will be discussed. More interestingly, stable crack propagation in monolayer 2D h-BN is observed and the corresponding crack resistance curve is obtained for the first time in 2D crystals. Inspired by the asymmetric lattice structure of h-BN, an intrinsic toughening mechanism without loss of high strength is validated based on theoretical efforts. The crack deflection and branching occur repeatedly due to asymmetric edge elastic properties at the crack tip and edge swapping during crack propagation, which toughens h-BN tremendously and enables stable crack propagation not seen in graphene.

Isolated linear carbon chains (LCCs) encapsulated by multiwalled carbon nanotubes are studied under hydrostatic pressure (P) via resonance Raman scattering. The LCCs’ spectroscopic signature C band around 1850 cm−1 softens linearly with increasing P. A simple anharmonic force-constant model not only describes such softening but also shows that the LCCs’ Young’s modulus (E), Grüneisen parameter (γ), and strain (ε) follow universal P−1 and P2 laws, respectively. In particular, γ also presents a unified behavior for all LCCs. To the best of our knowledge, these are the first results reported on such isolated systems and the first work to explore universal P-dependent responses for LCCs’ E, ε, and γ.

This talk will present our recent study on the mechanical properties of free-standing wire-frame 3D DNA tetrahedra in air. The structure was coated with a nanometer thick layer of uranyl acetate. The free-standinig DNA tetrahedron structure, 93 ± 2 nm high in air, withstands 42 ± 22 nN of loading force. The effective hardness (9.1 ± 5.1 MPa) and Young’s modulus (77 ± 48 MPa) of this low-density (70.7 kg/m³) DNA nanostructure are comparable to other reported low-density high-strength materials. Our result suggests a new approach to fabricate low density materials.

Flexible electrodes suffer unexpected, complete electrical disconnection after the onset of inevitable mechanical fracture across metal thin-films during their uses, severely reducing the functional lifespan of flexible/wearable electronics. Sudden, unperturbed straight in-plane mechanical fracture of metal thin-film was revealed to cause a catastrophic electrical failure of complete disconnection. Here, we report two-dimensional (2D)-interlayer approach that modulates in-plane fracture modes of metals and results in augmented strain-resilient electrical performance of metal thin-film electrodes under a high degree of multimodal deformation. Atomically thin 2D-interlayers, such as semi-metallic graphene, semiconducting MoS₂ and/or insulating hBN, induce continuous in-plane crack deflection (vs. straight) of metal thin-film electrodes and enable unique electrical characteristics which we term ‘electrical ductility’, where electrical resistance gradually (vs. abruptly) increases with strain allowing extended regions of stable resistance. 2D-interlayer electrodes maintain several orders-of-magnitude (>10⁴~10⁵) lower electrical resistance with resistance locking beyond a strain where conventional metal electrodes would be completely disconnected. Our 2D-interlayer approach is not limited to a certain combination of metals and 2D materials. The electrical ductility enabled by 2D-interlayer offers strategies for early damage diagnosis for the next-generation flexible/wearable electronics preventing abrupt, complete device functional failures.

Metallic FCC-BCC nanolayers, such as Cu/Nb, have received wide attention due to their extraordinary mechanical properties as well as the unique self-healing capacities due to the interface characteristics. Most recently, the materials have also been shown to exhibit significant and tunable interfacial sliding mechanisms (based on defect structures in the interface). The significant interfacial sliding is all along while maintaining full contact between the layers, and thus one could expect negligible resistance increase upon straining, which would be attractive for stretchable metallic conductor technology. The interfacial sliding has been modeled with some combination of diffusional and displacive mechanisms – the extreme extents of which are afforded by the nanoscale layering in the materials. The exact mechanisms continue to be fully investigated with some of the most cutting-edge materials experimental/characterization techniques as reported in the literature in the past 2-3 years. Nanoindentation has been used widely to investigate mechanical properties of small scale materials such as elastic modulus, hardness and work hardening. In this work, we report a study using nanoindentation with particular emphasis on depth profile (of the indented surface) and the pile-up height upon Berkovich nanoindentation of Cu/Nb nanolayers fabricated by Accumulated Rolling Bonding (ARB) method. The Cu/Nb ARB nanolayers have been reported to have strong anisotropy in mechanical properties in general, but specifically in this study, we show there is strong correlation between pile-up height and the extent of interfacial sliding. Cu/Nb ARB nanolayers (with 16nm individual layer thickness) has been used used as a test material due to its crystallographic anisotropy owing to the presence of contrasting interfaces along rolling (RD) and transverse direction (TD). The Cu/Nb ARB nanolayers were first characterized with X-ray Diffraction with 4-axis diffractometer, resulting in complete 3D crystal orientation/texture of the nanolayers. Nanoindentation was then performed along TD as well as RD of ARB Cu/Nb nanolaminate and subsequently Scanning Probe Microscopy (SPM) data was collected to measure the pile-up along RD and TD in the Cu/Nb nanolaminate. The 16 nm Cu/Nb ARB nanolaminate along RD was found to exhibit significantly higher surface pile-up than TD which is attributed to the variation in the Cu/Nb interface-mediated plasticity (sliding, rotation, etc.) along RD and TD, as well as the crystallography and texture of the nanolayers which determine the availability of slip systems in both Cu and Nb crystals. Further, 2D axisymmetric FEA analysis was performed to compare and validate experimental indentation data. The simulated data was found to compare well with the experimentally generated load-displacement curves whereas qualitative agreement was obtained with the experimentally obtained pileup data. The authors believe that the characterization of surface pile-up is of significant importance for enabling the Cu/Nb ARB nanolayers as the novel stretchable metallic conductor technology (for emerging applications such as electronic skins, soft robotics, etc.).

Intermetallics are generally brittle by nature and exhibit little or no plasticity at ambient conditions. However, lamellar eutectics containing an intermetallic phase, such as CuAl₂-Al, may exhibit room-temperature plasticity due to confinement effects. In this study, we apply the same idea of plasticity confinement to a CuAl system by designing a CuAl₂ matrix containing a nano-scale CuAl-phase via the diffusion-couple approach. This nano-composite is found to undergo room temperature shear deformation during nano-indentation. The indentation curves exhibit a continuously increasing pop-in size with increasing load, a feature typically not known for crystalline nano-indentation plasticity. High-Angle-Annular-Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) observation of deformed regions underneath the indent locations provide evidence for shear-banding in the otherwise brittle CuAl₂ phase. TEM observations of the microstructure underneath the indent revealed furthermore the existence of a phase with three-fold symmetry, of which we discuss the origin and structure.

Yttria stabilized zirconia (YSZ) has important applications in thermal barrier coatings, dental implants and ceramic blades. Fracture toughness and wear resistance are two critical mechanical properties that determine the performance of YSZ. Al₂O₃-YSZ composites have the potential to combine the high hardness of Al₂O₃ and the enhanced toughness of the tetragonal phase of zirconia. The present work aimed to investigate the effect of adding Al₂O₃, specifically, the particle size and distribution of Al₂O₃, on the mechanical behavior of YSZ at both micro- and nanoscale, via the combination of micro-indentation, nanoindentation and nanoscratch. Spark plasma sintering was utilized to fabricate the Al₂O₃-YSZ ceramic composites for its rapid densification rate to minimize grain growth. The results show that the as-sintered Al₂O₃-YSZ composites retain the tetragonal zirconia phase and the grain size of the YSZ matrix decreases with increasing amount of Al₂O₃ addition. Enhanced fracture toughness, as high as 11.32 MPa×m^0.5, is achieved by tailoring the distribution of the Al₂O₃. Homogeneous distribution of Al₂O₃ with reduced particle size alters the nanoscratch-induced plastic deformation mechanism and increases the wear resistance of the Al₂O₃-YSZ composites.

Even though numerous engineering applications of ultrathin multilayers have already been demonstrated, their mechanical behavior under dynamic loading remains poorly understood. Recent advances in experimental techniques have enabled the dynamic testing of nanoscale films. For example, microprojectile impact tests have revealed that ultrathin polymer films absorb approximately ten times more energy per unit mass during the penetration process than that of traditional bulk armor materials. At the same time, it has been well established that metallic nanostructures possess extraordinary mechanical properties when their characteristic dimension is few nanometers. Therefore, multilayer arrangements of nanoscale polymer and metal films could possess superior ballistic performance, and more importantly, their ballistic performance could even be engineered by altering the relative domain size and the spatial arrangement. We tested this hypothesis by conducting a comprehensive molecular dynamics (MD) study on the ballistic impact response of multilayer aluminum-polyurea nanostructures.
Macroscopic experiments have shown that placing a polymer layer on the back face of a metallic target is more effective in terms of impact energy absorption. In contrast, our MD simulations revealed that placing a nanoscopic polymer layer on the strike face is more effective when the individual layer thickness is in the order of a few nanometers. Furthermore, our dislocation analysis revealed that the polymer layer has a remarkable influence on the dislocation processes in the metallic layer. Moreover, the MD simulations demonstrated that the ballistic limit velocity (V₅₀) and the specific penetration energy of the nanoscale multilayers are significantly higher than the experimentally measured values for any nanomaterial. In order to gain further insights into the nanoscale energy dissipation mechanisms, we computed the V₅₀ of the multilayers using two existing membrane models. The results of this study demonstrate a potential bottom-up design pathway for developing new structural barrier materials with superior dynamic properties.

Acknowledgement: This work was supported by the Natural Sciences and Engineering Research Council of Canada.

Nanolayered composites composed of metal-ceramic layers have been studied to have high strength by effectively constraining the dislocation motion at the interfaces, but unfortunately, unstable deformation occurs due to the brittleness of the ceramic layer. In this study, In order to investigate the effects of the brittle-to ductile transition of the ceramic layer, we explore the deformation behavior of metal-ceramic nanolayered composites at varying ceramic layer thicknesses. Ag and Al-doped ZnO was chosen for metal-ceramic material system. The thickness of Al-doped ZnO was first fixed at 9 nm, which is theoretically expected to be ductile, and the metal thickness varies from 85 nm to 240 nm. For further comparison with thicker Al-doped ZnO that is expected to be brittle, 150 nm Ag/120 nm Al-doped ZnO was studied. In-situ SEM pillar compression showed stable deformation for 9 nm thick Al-doped ZnO samples, and TEM analysis of the deformed specimen was used to confirm the co-deformation of metal and ceramic. For the same 9 nm thick Al-doped ZnO samples, systematic increase in strength was observed with reduction in Ag thickness from 240 nm to 85 nm. For the 150 nm Ag/120 nm Al-doped ZnO, A brittle fracture of the Al-doped ZnO layer was observed to lead to a reduction in flow stress or strain softening after 30% deformation. Finite element analysis showed that the stress of the Ag and the Al-doped ZnO layer reached the stress required for the co-deformation of the two phases.

Cellulose Nanocrystal(CNC) is a naturally abundant environmentally benign material. The large aspect ratio and many hydroxyl groups attached on the surface of CNCs greatly contribute to the orientation of polymer chains when produced as a polymer nanocomposite. In addition, CNC causes a strong interaction with polyacrylonitrile (PAN), a semi-crystalline polymer having nitrile groups used as a carbon fiber precursor, leading to an increase in physical and mechanical properties of polymer nanocomposite.
In the current study, PAN/CNC polymer fibers were prepared through in-situ polymerization by combining CNC to the PAN synthesis. We have conducted the thorough characterizations of CNC dispersion and polymers (molecular weight, molecular weight distribution, rheological properties, and the structure-property of the resulting precursor fibers). The orientation factor of the post-drawn PAN/CNC fiber increased about 2 times compared to the as-spun PAN/CNC fiber, and PAN/CNC fibers by in-situ polymerization had the highest mechanical properties at total draw ratio 18 than mechanical stirred PAN/CNC and control PAN fibers. . In other words, it was confirmed that the fiber with CNC can have high mechanical properties because crystallinity is clearly improved even if it is slightly stretched.
Also, in heat treatment for manufacturing carbon fiber, it is observed that the crystallinity improvement effect by CNC and the influence of low molecular weight molecules with a wide polydispersity index (PDI). These low molecular weight molecules play a role in increasing the intensity ratio of the D and G band (IG/ID) in Raman spectroscopy analysis in the heat treatment. And it confirmed that after carbonization in-situ polymerized PAN/CNC fibers acted as an element capable of having a tensile property value of 1.6 times or more compared to the carbonized control PAN fiber.
This experiment could suggest that the PAN/CNC nanocomposite fiber may be used as a precursor for high-performance carbon fiber.

Metal/graphene nanolayered composites are known to have ultra high strength due to the effectiveness of graphene in constraining the dislocation propagation during plastic deformation. Earlier study on metal/graphene nanolayered composites involved repeated physical vapor deposition of metal and wet-transfer of graphene and the fabrication method, therefore, has limitations in terms of scalability. Powder metallurgy techniques through mixing graphene flakes into metallic powders has also been reported, but the achieved strengthening effect is not as pronounced as in nanolayered composite produced by alternated thin film deposition and graphene transfer layer-by-layer. In this study, we report a scalable fabrication method for Al/ reduced graphene oxide (rGO) nanocomposite with interlayer spacing of ~200 nm in the bulk form using Accumulative Roll Bonding (ARB). rGO flakes are first sprayed on the surface of an aluminum sheet of thickness using air-spray gun, and the resulting rGO on Al sheet was used in ARB where the Al sheet was stacked, rolled, sectioned and stacked for subsequent rolling. A bulk Al/rGO nanolayered composite with ~200 nm interlayer spacing was successfully fabricated and the mechanical properties of the developed bulk nanocomposite was analyzed for mechanical properties using nanopillar compression testing to show the strengthening effect in comparison to the specimen prepared using layer-by-layer deposition. In order to study the effectiveness of the Al/rGO interfaces in hindering dislocation motion, deformed nanopillars were analyzed using TEM.

The strength of nanoscale materials is one of key properties in development of advanced devises such as semiconductors, small sensors, and micro- and nano-electromechanical-systems (MEMS and NEMS). It is inevitable to understand their fracture behavior in terms of the mechanics for keeping high reliability in manufacturing as well as in service. Especially, focus must be put on the point that the mechanics and mechanism for materials in nanoscale might be significantly different from that in macroscale counterpart. The internal nanostructures including dislocations, grain boundaries, and various defects strongly affect the mechanical properties coupling with the external structures (geometric factor), shape and size of components. Although there are many excellent papers published on the deformation behavior on the bases of nanoscale experiments, few research works have been conducted on the fracture characteristics. It is mainly due to the difficulty of fracture experiments in nanoscale. For describing the fracture laws of materials in macroscale, it is essential to understand the fracture characteristics under diverse fracture mechanism including toughness, fatigue, creep and so on. We have conducted various fracture experiments with in situ SEM/TEM observation. In this lecture, we present two topics on the experimental study in terms of the fracture nanomechanics.
Smallest applicable limit of the conventional fracture mechanics [T.Sumigawa et al, ACS Nano, 11, p. 6271, 2017]
Fracture of bulk brittle materials such as silicon is characterized by cracking and the mechanics is well explained by the singular stress field formed near the crack tip according to the fracture mechanics (Griffith) theory. The applicability of these continuum-based theories is, however, uncertain and questionable in a nanoscale system due to its extremely small singular stress field of only a few nanometers. We directly conduct in situ fracture toughness testing for silicon specimens with a nanocrack using a transmission electron microscope and demonstrate that the Griffith theory can be applied to even 4 nm stress singularity despite their continuum-based concept. We show that the fracture toughness of clean silicon is independent of the dimension of materials and therefore inherent. Quantum mechanics/atomistic modeling explains and provides insight into these experimental results. It also shows that the limit is on around 2-3nm. This work therefore provides a fundamental understanding of fracture criterion and fracture properties in brittle nanomaterials.
Fatigue process of small metal [T.Sumigawa et al, Acta Materialia, 153, p.270, 2018]
We develop an in situ scanning electron microscope observation technique of micron-scale metal specimens under fully-reversed tension-compression cyclic loading and investigate the unique cracking process in low-cycle fatigue of single crystal copper in micron scale under a constant displacement amplitude. While crystallographic slips spread over the entire test section during the tensile first half-cycle, a locally concentrated slip band appears during the first reverse loading (compression). Crystallographic slip takes place only near the localized band in consecutive cycles, and strain localization due to slip bands leads to nanoscale extrusion/intrusion at the surface. This indicates that, unlike during the fatigue of a typical bulk pure metal, no characteristic dislocation substructure is formed during the extrusion/intrusion process. Moreover, the process required a much higher stress amplitude than in the case of bulk copper specimen. Thus, the fatigue cracking process for copper micro-scale specimen differs significantly from that for bulk copper specimen.

Ceramic composites have various applications ranging from medical to high-temperature aerospace structures. These multi-material composites usually have two brittle materials sharing a bonded interface. For example, ceramic composites with SiC/SiO₂ interface find applications as microelectronics in semiconductors. However, like other bonded components, performance challenges are inevitable due to the high density of defects existing at SiC/SiO₂ composite interface. A crack or defect can be initiated at the interface during fabrication or as a result of residual mechanical/thermal stresses between the two components. We developed a multi-scale model that simulates the mechanics of interfacial fracture at a strong interface by combining reactive molecular dynamics (ReaxFF) and continuum fracture theory.
Cracks existing in strong interfaces tend to propagate out of interface under mixed-mode (i.e., combined opening/sliding) loading conditions. As temperature varies, the material properties change, thereby affecting the interfacial crack propagation behavior. The mechanical response of both the constitutive materials, namely silicon carbide and amorphous silica, was simulated within a temperature range of 300–1200 K. Amorphous silica (SiO₂) shows a continuous decrease in modulus and strength with increasing temperatures. The modulus of silicon carbide (SiC) plateau between 700 and 900 K while its strength drops continuously as temperature increases. Next, using a multi-scale modeling approach, these temperature-dependent properties are used as inputs to a continuum-based model to investigate fracture behavior as a function of temperature; by employing maximum tangential stress (MTS) criterion.
We found that altering the ambient temperature influences the interfacial fracture toughness in all crack tip deformation modes (pure mode I, mixed-mode I/II, and pure mode II). Depending on the imperfection level, the limits depicted in the mixed-mode fracture toughness curves should vary. It is worth mentioning that the current research was a preliminary analysis, and other significant parameters such as specimen geometry require additional investigations. Depending on the specimen geometry and applied loading configuration, the coefficients of the higher-order terms in the crack tip stress field may also be important in the fracture criteria as well as the singular terms. Future investigations will look into combined environmental and geometric effects for various materials, not only for ceramic composites but also for advanced composites.

The mechanical properties of disordered nanoparticle (NP) packings can be significantly improved through the infiltration of polymers to enhance the interactions between the NPs. Previous work has shown that capillary rise infiltration (CaRI) is a simple and highly effective route to stiffen, harden, and toughen NP packings. While results from previous nanoindentation-based fracture measurements suggest that polymer infiltration can improve the toughness of NP films, it is challenging to fully understand how the relative size of polymer and nanoparticles and the extent of confinement affect the toughness using nanoindentation-based measurements of fracture toughness. In the present work, a thin-film fracture testing method based on the double cantilever beam (DCB) specimen is developed and used to investigate the fracture properties of polymer-infiltrated nanoparticle films. In the DCB specimen, a crack is propagated in NP films over distances of tens of millimeters, allowing for highly accurate measurements of toughness in a mode I (tensile opening) configuration. The fracture toughness of the polymer-infiltrated NP films is found to be strongly dependent on polymer molecular weight (MW) and NP size. Low MW, unentangled polymers, effectively toughen small NP packings; whereas high MW, entangled polymers, show enhanced toughening in large NP packings. Possible toughening mechanisms, including confinement-induced polymer bridging and polymer entanglement, will be discussed.

Most of the brittle materials are usually very vulnerable to the existence of the crack because a lack of the intrinsic toughness mechanism, such as tip blunting by the plastic deformation, renders it to propagate unimpededly once a critical condition is reached. Therefore, most of the efforts to mitigate the sudden failure of brittle ceramics have been focused on developing the extrinsic toughening mechanisms that hinder crack propagation behind the tip, such as the fiber bridging. In this work, we experimentally demonstrate that the intrinsic toughening arises even in the brittle monolithic ceramic material like diamond-like carbon (DLC) when its external dimension reduces down to sub-micron scales. This unique phenomenon owes its origin to the decrease of the crack driving force in the small samples, which in turn enables them to bear high enough stresses to activate the local atomic plasticity. Through nanomechanical tensile and bending experiments, electron energy loss spectroscopy analysis and finite element method for stress distribution calculation, we confirmed that the local atomic plasticity associated with sp³ to sp²rehybridization is responsible for the intrinsic toughening.

Todays' materials design space reaches from electronic structure to microstructure and metamaterials structured on a mesoscopic scale. Nevertheless, the aim is typically to implement novel properties or tune them to a specific task. Instead, Programmable Materials aim to design materials' behavior and let them interact and adapt to the environment. In this talk, an outline will be given how this new design space could be approached and structured. The approach is based on translating physical and mechanical mechanisms into classes similar to modern programming languages. Materials’ specific molecular and mechanical mechanisms will be discussed which enable Programmable Materials to process information or specific user interactions. For a concerted behavior, the interaction between compartmentalized unit cells needs to be finely tuned. Based on simulations and optimization routines, local programming of mechanical metamaterials can be used to implement complex shape morphing into 3D-printed parts. Furthermore, possible routes for mass manufacturing Programmable Materials will be presented.

Resistive strain sensors (RSS) with ultrasensitivity has been investigated for widespread applications. However, due to physical/mechanical limits, the development of mechanosensors satisfying the prerequisite of stability, durability and controllability remains a challenge. Also, in-depth toxicology in in vivo system is imperative because of in direct contact with skin, even applied to organs, yet has not been reported for RSS. Here, we demonstrate that vertical graphene (VG) RSS can serve in vivo biocompatible sensors, and show remarkable sensitivity (gauge factor of 5,000) with revivable status even after broken current path. Three-dimensional tufted VG network structure allowed charge transport depending on crack geometry. We further showed that the signals from rat heartbeat depend on the direction of VG sensor directly attached to the heart, enabling to distinguish the cardiac contractility through its ventricle and atrium. Our finding provides new insight for controllable and permanent mechanosensing system, making VG promising option for in vivo biocompatible platform.

Structural Health Monitoring (SHM) of engineering structures involves damage detection and monitoring during its life-cycle. One of the phenomena used to detect changes within the structure is Acoustic Emission (AE). Structure undergoing the damage mechanism releases the energy in the form of elastic waves. These elastic waves propagating in a structure when captured could be used to characterize and localize the damage. In this work, we demonstrate the measurement of AE waves using the capabilities of two-dimensional nanomaterials - Titanium Carbide MXene (Ti₃C₂-MXene) films. Ti₃C₂-MXenes, discovered in 2011, have good electromechanical properties, film-forming ability, piezoresistivity, and relatively good dynamic response behavior to impact loads. Utilizing these properties of Ti₃C₂-MXene, we subject the films to the Hsu-Nielsen source. The test is performed with the pencil lead in contact with the Ti₃C₂-MXene film sample and the breakage of the pencil lead (which generates the acoustic wave) is detected by the Ti₃C₂-MXene film. Sensors based on Ti₃C₂-MXene can be deposited on the surface of the structure as a coating or potentially as a thin film layer. Ti₃C₂-MXene with its dynamic response properties provide a wide range of opportunities for SHM applications and these are discussed with the experimental results.

Ultra-portable, imperceptible^[1], and shapeable^[2] devices are expected to be widespread due to the emergence of flexible electronics as an industrial technology^[3,4]. Printing is an affordable and high throughput method to process electronics in soft substrates that is still to be optimized to deliver electrically and mechanically reliable electronic devices^[5].

In particular, printable magnetoresistive pastes have been developed as an alternative single-step fabrication method to obtain magnetic field sensors^[6]. These pastes usually consist of composites of magnetic particles embedded in a non-magnetic matrix^[7,8]. Particle-based pastes can achieve large magnetoresistance ratios at the expense of high resistivity and noise levels^[7-9]. Previously, Karnaushenko et al. reported magnetoresistive pastes based on giant magnetoresistive (GMR) microflakes as an alternative to overcome the problems presented in particle-based pastes^[10,11]. Magnetoresistive flakes were produced after the delamination of thin-film stacks from a deposited sacrificial layer. With this technology, it was shown that flakes-based Co/Cu printed sensors exhibit low resistance and 37% GMR response at moderate magnetic fields (500 mT)^[11].

Despite the advances in printable magnetic sensors, there are no reports of systems that show good sensitivity at low magnetic fields relevant for safe integration into wearable electronics. Electronics with magnetic components have to perform below the WHO limit of continuous exposure to magnetic fields (<40mT) to comply with this health standard^[12]. Furthermore, for on-skin electronics that experience considerable strain, there are not examples of magnetic printed sensors that deliver steady sensing behaviour upon mechanical stretching.

Here, we will present low-noise printable magnetic field sensors sensitive down to sub-mT, which are mechanically stretchable after printing. We demonstrate the fabrication of printable sensors in ultrathin foils (3-μm-thick Mylar) based on magnetoresistive pastes that can undergo 100 % strain and 16 μm bending radius maintaining stable sensing and mechanical performance. The pastes are composites of poly(styrene-butadiene-styrene) copolymer (SBS) with embedded magnetoresistive microflakes. Using [Py/Cu]₃₀ and [Ta/Py] flakes, we obtained printed GMR and anisotropic magnetoresistive (AMR) sensors, respectively. We address the key role of SBS to enable an enhancement of two orders of magnitude improvement in bendability and sensitivity at low magnetic fields.

Due to the good performance at low fields, reduced noise levels and high compliance, we will show the direct lamination of the printed sensors as an on-skin interactive device for scrolling through documents or digital maps. We envision that the proposed magnetic sensors will enable printing on-demand utilities for physical activity tracking systems or human-machine interfaces that can improve and even expand our sensing capabilities.

[1] M. Melzer et al., Nat. Commun. 6, 6080 (2015)
[2] D. Makarov et al., Appl. Phys. Rev. 3, 011101 (2016)
[3] S. Huang et al., Adv. Funct. Mater. 29, 1805924 (2019)
[4] W. Wu, Sci. Technol. Adv. Mater. 20, 187 (2019)
[5] Q. Huang et al., Adv. Mater. Technol. 4, 1800546 (2019)
[6] D. Makarov et al., ChemPhysChem 14, 1771 (2013)
[7] J. Meyer et al., Smart Mater. Struct. 22, 025032 (2013)
[8] J. L. Mietta et al., Langmuir 28, 6985 (2012)
[9] L. Ding et al. ACS Appl. Mater. Interfaces 12, 20955 (2020)
[10] D. Karnaushenko et al., Adv. Mater. 24, 4518 (2012)
[11] D. Karnaushenko et al., Adv. Mater. 27, 880 (2015)
[12] World Health Organization, Static fields (2006)

Sensors are a key issue in the present technological development and Surface Acoustic Wave (SAW) sensing is just one of the development directions in fluids detection with application from security to industry and medicine. Depending of the fluid composition there are various sensor types and used materials but all of them are basing their functionality on the mechanical and structural properties changes of an ‘active layer’ material in the presence of the detected fluid. Nanostructure or nanostructured materials present a series of enhanced properties compared with the bulk materials, due to the high surface-to-volume ration but their specific parameters are sometimes rather challenging to control for optimizing sensor performances.
For SAW sensor fabrication we use ZnO material as a versatile bio-compatible wide bang-gap semiconductor, with a good absorption of various gases including hydrogen isotopes. Through the ‘Bottom-UP’ approach, we use Pulsed Laser Deposition (PLD) - Vapour-Liquid-Solid (VLS) techniques to grow ZnO single crystal [0001] nanowires on sensor patterned active surface area. By controlling several experimental parameters and respectively VLS grow elementary processes we control nanowires morpho-structural properties. Some correlations between these parameters and sensor experimental response are presented. Results interpretation is presented together with few wave propagation theoretical modeling through the sensor active area and respectively simulations on ZnO nanowire absorption. Presented results are aimed for SAW applications in gas detection and pressure measurement and are exemplified for hydrogen isotopes detection.

Natural materials exhibit high specific toughness, strength, stiffness and impact resistance¹. The excellent properties of many such natural materials are attributed to hierarchical arrangement of their structure and spatial material gradients at different length scales². Nacre- an iridescent material, exhibits enhanced strength and toughness though it majorly (>95vol.%) comprises an extremely brittle phase (aragonite)³. Nacreous structures show extraordinary toughness due to sliding of the bricks (macroscopic work hardening) over neighboring bricks together with other energy dissipation processes such as crack deflection, micro cracking and crack bridging⁵. Failure in such nacreous materials usually initiates in structurally weak mortar near the corners of the bricks where steep stress gradients exist. In this study, we introduce compliant grading motif (CGM) observed in many natural materials such as spider fang, byssal thread and squid beak, in the mortar of nacreous structure utilizing multimaterial additive manufacturing (AM). We postulate that the CGMs in mortar could play a role in toughening and strengthening the nacreous system by diffusing the peak stresses and moving the stress concentration away from the potential failure zones. Mechanical performance assessment of the AM realized nacreous structures with CGMs in mortar show ∼60% improvement in strength, ∼ 70% in toughness, and ∼30% in strain-to-break, while retaining the macroscopic stiffness. The nature inspired CGMs introduced here is easily realizable in a wide range of 3D printing techniques for developing high performance architected materials with unprecedented properties.
References
(1) Gu, G. X.; Takaffoli, M.; Hsieh, A. J.; Buehler, M. J. Biomimetic Additive Manufactured Polymer Composites for Improved Impact Resistance. Extreme Mechanics Letters 2016, 9, 317-323.
(2) Studart, A. R. Biological and Bioinspired Composites with Spatially Tunable Heterogeneous Architectures. Adv. Funct. Mater. 2013, 23 (36), 4423-4436.
(3) Munch, E.; Launey, M. E.; Alsem, D. H.; Saiz, E.; Tomsia, A. P.; Ritchie, R. O. Tough, Bio-Inspired Hybrid Materials. Science 2008, 322 (5907), 1516-1520.
(4) Rabiei, R.; Bekah, S.; Barthelat, F. Acta Biomaterialia Failure Mode Transition in Nacre and Bone-Like Materials. Acta Biomaterialia 2010, 6, 4081-4089.
(5) Nalla, R.; Kinney, J.; Ritchie, R. Effect of Orientation on the in Vitro Fracture Toughness of Dentin: The Role of Toughening Mechanisms. Biomaterials 2003, 24 (22), 3955-3968.

We have developed hollow carbon fiber-aluminum sandwich plates with high strength-to-weight ratios and microscale thicknesses that can serve as structural components in microflyers and miniature robots. These panels consist of perforated aluminum sheet cores fixed between face plates of carbon fiber-reinforced polymer using epoxy. This combination of aluminum and carbon fiber exhibits both high impact resistance and high specific stiffness and strength^1,2 with areal densities of only 100-200 mg/cm² and normalized flexural rigidity values of up to ~7 N-m. Our work represents a simple and inexpensive platform for creating structural components of microflyers and small robots.

Emerging micro-aerial vehicles and small-scale robots require low-weight materials with sufficient structural rigidity to survive substantial collisions^3,4. This is difficult to achieve because thin light-weight elements suffer from low bending stiffness, which scales cubically with plate thickness. Because of this, sandwich composite plates with lightweight cores and rigid face sheets are widely used to achieve enhanced resistance to bending while maintaining low weight. Although there have been many efforts in creating sandwich plates with microscale thickness^5,6, many require costly and complicated microfabrication. For quick prototyping of mesoscale devices, such as microflyers and small robots, more straightforward methods of producing sandwich plate materials are required.

To fill this gap, we have developed sandwich plates whose faces are carbon-fiber reinforced polymer veneers and cores are aluminum sheets. This combination results in high strength with resistance to the brittle fracture failures that characterize single carbon fiber veneers. We fabricate these plates by using time-curing two-part epoxy to bond 300-μm thick carbon fiber weaves to both sides of a 300-μm thick aluminum sheet. To reduce the weight of our plates, we use waterjet cutting to remove material from the aluminum sheet, yielding a low-density honeycomb-patterned core spacer.

To test our specimens, we subject them to three-point bending experiments in which we monitor their mechanical properties as a function of the plates’ micro- and macro-scale geometry and composition, and subsequently we compare our results with finite element simulations. Our results reveal that the flexural rigidity values of our sandwich plates are enhanced by 1 to 2 orders of magnitude relative to those theoretically expected from carbon fiber veneers alone. We are currently exploring more aggressive mass-reducing methods, such as decreasing the fill factor of the aluminum honeycomb core and reducing the volume fraction of epoxy in the composite. Although we are currently investigating aluminum cores, our method can be easily adapted for other lightweight cores, such as a laser-micromachined polyurethane layers.

1) Aslan Pam. Univ. J. Eng. Sci. 24(2018)1062
2) Tamilarasan Mat. Res. 18(2015)1029
3) Sareh Sci. Robot. 3(2018)EAAH5228
4) Mulgaonkar IDETC-CIE (2015)47864
5) Arias J. Mat. Res. 16(2011)597
6) Kolodziejska APL Mat. 3(2015)050701


This material is based upon work supported by the Defense Advanced Research Projects Agency (DARPA) under Contract No. HR0011-19-C-0052. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the DARPA. Approved for Public Release, Distribution Unlimited.

By using kirigami-inspired structures, highly deformable microscale shape memory polymer structures can be realized. These have a variety of applications in the wearable electronics, biomedical, and aerospace industries. Since many types of structures can be made, a complex assortment of mechanical responses can be observed. Since it’s desirable to measure and verify performance, a sensitive mechanical testing instrument is needed. To that end, we fabricated various three-dimensional shape memory polymer (SMP) structures composed of epoxy monomers and studied their mechanical behavior using a nanoindentation instrument. Several important mechanical responses, such as load bearing capacity, hysteresis, and the effect of pre-existing cracks were investigated in detail. Full load-displacement curves were determined for the various structures, both at room temperature and an elevated temperature of 72 °C using a heating stage. The experimental results were coupling with modeling, which was performed to examine the internal stress distributions of the structure under compression conditions, both at room temperature and elevated temperature. This study demonstrates with detail how the mechanical performance of kirigami structures is significantly altered by its design.

There is a growing motivation to impart electrical properties into polymer fibers and textile structures, in order to enable multi-functional composites¹. A common approach to manufacturing electrically-conductive fiber-reinforced polymer-matrix composites (PMCs) is to modify the matrix polymer with conductive additives. However, these particle additives can affect the bulk mechanical properties of the final product, which may not be desired. An alternative method to impart electrical conductivity without significantly affecting the bulk mechanical properties is to modify the surface of the fibers by adding a thin, conductive coating. Among the coating techniques that can be utilized, atomic layer deposition (ALD) is an attractive technology that provides unparalleled conformality and sub-nanometer resolution in film thickness and composition.

In this work we demonstrate the fabrication of electrically-conductive Kevlar-reinforced PMCs without measurably affecting the bulk material properties, by pre-coating Kevlar 49 fabrics with aluminum-doped zinc oxide (AZO) via ALD. The core-shell fabric morphology and structure were characterized by SEM, XPS, and XRD. Four-point probe cyclic voltammetry measurements were performed to measure the electrical resistance of the AZO-coated Kevlar. Measurements were taken at both the single-fiber and fabric scales, after varying the thickness of AZO from 40-120 nm. At 40 nm the Kevlar fabric exhibits a low electrical conductivity, which is attributed to the film morphology. As the film thickness increases, the sheet resistance decreases, reaching a value 55.1 Ω/sq at 120 nm. The thickness-dependence of conductivity is well described by an analytical model, which allows for predictive design. This is sufficiently conductive to serves as an electrode material in textile-based devices ranging from thin-film electronics to energy storage devices.

Additionally, we studied the electrical conductivity and strain-sensing capabilities of AZO-coated Kevlar fabrics in PMCs. We demonstrate the potential for these conductive composites in strain-sensing and damage-monitoring applications. The electrical resistance of the sample was continually monitored in-situ while loading the sample to failure in a tensile load frame. The average ultimate tensile strength and Young’s modulus of the Kevlar-reinforced PMC was not affected by AZO interface coatings, illustrating the advantage of this approach over bulk conductive additives. Furthermore, significant changes in the electrical resistance (R-R₀)/R₀ as a function of the strain were observed. These results demonstrate the potential of AZO-coated Kevlar to monitor the strain and provide electrical feedback during the linear elastic regime of the composite, which could be used for structural health monitoring prior to failure.

References

1. Sabetzadeh, N.; Najar, S. S.; Bahrami, S. H. Electrical Conductivity of Vapor-Grown Carbon Nanofiber/Polyester Textile-Based Composites. J. Appl. Polym. Sci. 2013, 130 (4), 3009-30017. https://doi.org/10.1002/app.39447.

We report the synthesis of surface-attached nanofibers by chemical vapor polymerization of 4-hydroxymethyl-[2,2]paracyclophane into films of nematic liquid crystals (LC) (4-(trans-4-pentylcyclohexyl) benzonitrile and 4-(trans-4-propylcyclohexyl) benzonitrile) supported on indium tin oxide (ITO)-coated glass. Dense arrays of end-attached polymeric nanofibers are observed to form with shapes programmed by the strain engineered into the confined LC film. Upon application of an electric field across the LC-nanofiber film, we observe reorganization of the LC to be coupled to the deformation of the soft nanofibers, generating an electrooptical response that reflects coupling of elastic strain in both the LC and nanofiber. We construct a microscopic electromechanical model that captures the transmission of torques between the LC and nanofibers. We also observe patterned orientations of LCs to lead to patterned arrays of nanofibers (e.g., defining mesoscale vortices) during synthesis, which generate complex electro-optical responses that arise from elastic energy stored in both the LC and nanofiber arrays. Overall, these results reveal that LC-templated nanofiber arrays offer a promising route to optical metamaterials with emergent properties generated by the sharing of strain between soft nanofibers and LCs.

In the quest for improved materials, length scale effects are an option for tuning properties. Reduction in grain size can increase strength and manipulation of strut size and distribution in lattice structures can alter myriad properties. A key challenge in using length scale as a tuning parameter is understanding stresses and strains across scales to inform the design process. This talk reviews recent work that leverages a suite of spatially resolved, electron imaging, diffraction, and spectroscopy techniques to correlate global and local strains and stresses. Specifically, linking grain to subgrain scales is addressed. The work presented describes a platform for reliable, multiscale tuning in systems such as additively manufactured materials, high entropy alloys, and fine-grained metals.

Carbon-carbon (C-C) composites are often used as friction and brake materials due to their high strength, low density, excellent frictional properties, high thermal conductivity and high heat capacity. When worn, C-C composites produce particulate wear debris. The fine wear debris particles then form a layer on the wear surface, known as a friction film layer, which has the potential to change the overall frictional performance of the system. The size, shape and chemistry of these wear debris particles and friction layers can give information about the mechanisms of wear in the composite and the effects of temperature, humidity, oxidation and other chemical modifications that can occur depending on the wear environment. However, this analysis is rather difficult to perform due to the wide range of particle size, from small nanoscale primary particles to large microscale aggregates. The chemical and material similarities of the carbon fiber and matrix further complicates decoupling the effects of the different components of wear debris on performance. This research explores particle separation and characterization techniques to obtain information about the C-C wear process. The principle technique for particle separation is density gradient ultra-centrifugation (DGU) in which fibrous and matrix-rich particles are separated based on their density. Orthogonal analysis with Raman and UV-vis spectroscopy, optical and electron microscopy, and X-ray Photoelectron spectroscopy can then be made on separated aliquots. Understanding the properties of the wear particles and friction film gives information about the wear process and can help inform future design and material considerations.

Nanowires (NWs) are frequently employed as critical components in innovative devices including gate-all-around transistors and next-generation mass spectrometers. Despite all their promising aspects, the level of commercialization of such new technologies remains surprisingly low. There has been a vast effort towards the characterization of materials behavior at the nanoscale while a lot of improvement remains to be accomplished depending on the optimization or development of NW fabrication technologies, physical characterization tests, and integration techniques. Mechanical characterization of NWs considered as building blocks of nanoelectromechanical systems (NEMS) is mainly carried out through tensile or bending tests. The interpretation of test data obtained through such experimental measurements necessitates a comprehensive modeling effort to address the current challenge of understanding the size-dependent behavior of NWs. For instance, modeling of NW deformation at the atomistic level predicts size-dependent elastic properties for NWs only with cross-sectional dimensions less than ~10 nm. We undertake such a study by investigating the elastic modulus of Si NWs with a critical dimension of 28 nm via three-point bending test. Testing effort is accompanied by a comprehensive non-linear model of NW bending, including surface and intrinsic stress effects. The model considers all relevant effects including large deformations, intrinsic stresses, and surface effect with native oxide. The presence of intrinsic stresses is studied through micro-Raman spectroscopy with a precise evaluation of strain profile in the structure. Raman mapping was performed for analysis of the spectra as a line scan along the NW where residual stresses were quantified. In addition, we examine the surface effect by implementing the surface stress and surface elastic through atomistic simulations coupled with the continuum model by introducing the presence of native oxide layer covering the silicon NW. The native oxide structure is characterized through high-resolution transmission electron microscopy where inclusion of oxide layer effect into calculations verifies that the size effect is postulated to be primarily a result of differences in the NW surface and core elastic moduli. An in-depth understanding of the surface effect in NW mechanical properties is a critical step which requires an extensive mechanical analysis including all effective parameters in the deformation behavior of NWs. Overall, the study presents a comprehensive model for completely considering effects from axial extension, surface effect, and intrinsic stress to tackle the bending response of Si NWs.

Metallic glasses are alloys that exhibit a disordered atomic structure. The lack of dislocations makes these materials stronger and harder than their crystalline counterparts. Furthermore, the lack of grain boundaries provides a desirable microstructure for corrosion resistance. Therefore, metallic glasses provide a promising design space for the development of wear-resistant and corrosion-resistant coatings.

Magnetron sputtering of thin films provides binary metallic glasses over wide compositional ranges due to the process's ultra-fast cooling rates. As a result, the sputtering approach enables systematic studies exploring the effect of composition on metallic glass thin films' mechanical behavior. Zr-Ta is one of the promising systems for coating applications, whose constituent elements possess desirable properties of high strength, biocompatibility, and corrosion resistance. However, there has been no study to date focusing on this binary system and its potential as a metallic glass coating. In this work, we explored the structure-property relationships of magnetron sputtered ZrTa metallic glasses.

A magnetron sputterer deposited 1 μm-thick films of ZrTa on oxidized silicon substrates. We employed combinatorial sputtering, which provided a wide range of compositions at a single deposition step. EDS measurements verified the composition, and X-ray diffraction characterized the microstructure. Transmission electron microscopy analyzed selected compositions in further detail. Nanoindentation with Berkovich and cube corner tips determined the elastic modulus, hardness, and fracture toughness of the specimens.

EDS measurements show that the Zr content varies between 21 and 70 at.%. For Zr contents below 30 at.%, the microstructure includes nanocrystalline and amorphous domains. For Zr in the range of 30 to 70 at.%, the microstructure becomes fully XRD-amorphous. Nanoindentation measurements exhibit a monotonic decrease of elastic modulus and hardness with increasing Zr content. For the amorphous films, hardness varies in the range of 5.5 – 9 GPa, and elastic modulus varies in the range of 105 – 130 GPa. Energy-based analysis of nanoindentation data provided the fracture toughness of the coatings. Fracture toughness decreases with increasing Zr content.

The second part of the study investigated nanolayered films composed of alternating layers of Zr₃₅Ta₆₅ and Zr₇₀Ta₃₀, for layer thicknesses in the range of 10 – 100 nm. The results show that the nanolayered films provide a desirable combination of high strength and fracture toughness. We attribute these results to the strong modulation of strength and elastic modulus at a length scale comparable to the size of the shear bands. The abrupt variations in the mechanical properties distort the stress state in the structure and hinder the rapid propagation of the shear bands.

In summary, we obtained ZrTa metallic glass thin films over a wide compositional range and determined the effect of composition on the mechanical behavior. The nanolayered form of the metallic glass thin film provided promising results that combine high hardness and fracture toughness. The findings can guide future studies towards the development of wear-resistant coatings that exhibit corrosion resistance and biocompatibility.

Nanocontact plasticity of metal surfaces involves sudden imprint formation processes along with the inception and evolution of entangled defect networks in the subsurface. Our investigation provides a fundamental comprehension to nanoindentation experiments through massive molecular dynamics simulations of face-centered cubic Al, Cu and Ni; body-centered cubic Ta and Fe, and hexagonal closed-packed Mg performed across temperatures, tip penetration velocities, and nanoindenter tip sizes. We first investigate the critical contact resistance p_c rendering defect inception as a function of the incipient defect nucleation mechanisms and effective elastic modulus E of the crystal. Systematic analyses are performed on the subsequent pop-in events attained with increasing tip penetration, which lead to the onset of a steady-state nanoindentation regime characterized by constant hardness p_p. An insight is then gained into the specific nanoimprint formation mechanisms in cubic and hexagonal metals which comprise specific, collective dislocation emission and glide processes, individual nanotwin inception, and the development of unique nanostructured subgrain arrangements. A discussion is given on the role of these mechanisms upon the onset of higher values for the p_p/E ratio in BCC Ta and Fe and for the smaller p_p/E attained in FCC Cu, FCC Ni and HCP Mg. Finally, we investigate the correlation between the nanoimprint formation at the steady-state hardness p_p and the dislocation mobilization events triggered within the incepted defect networks at the uniaxial yield strength σ_ys. It is found that the p_p/σ_ys ratio approaches ≈7 and ≈10 in FCC and BCC metals, respectively.

Here we present temperature-dependent micro-compression data of Nb single crystals. We focus on the changes of the discrete plastic flow response between 370 and 170 K. Whilst the flow stress as a function of temperature is in excellent agreement with thermally-activated bulk plasticity models for bcc metals, a temperature insensitive component is revealed when tracing the change of displacement discontinuities with decreasing temperature. It I found that the scale of the displacement jumps changes from a scale-free like behavior at high temperatures to a scale-dependent behavior at the lowest temperature, where the length scale converges towards the scale of the lattice parameter. 3D discrete dislocation dynamics simulations are done to shed light onto the underlying dislocation behavior, revealing that the athermal plasticity component is governed by screw-dislocation activity. The temperature insensitivity of this screw-dominated plasticity can be shown to depend on high local stresses that significantly exceed the Peierls stress in the studied Nb microcrystals. This work is based on earlier efforts that aim at understanding stress-strain fluctuations in microcrystals (Phys. Rev. Mat. 2 (2018) 120601; Phys. Rev. Mat. 3 (2019) 080601).

Nano-cardboard structures are shell structures that consist of two face plates held together by hollow tubes that serve as ligaments [1]. These structures are fabricated entirely using thin films with thicknesses on the order of 100 nm or less. As a result of their geometry, nano-cardboard structures have a very low density and are ten times stiffer than aerogels or nanolattices of the same densities (5-20 kg/m³). Additionally, they exhibit recoverable deformation similar to nanolattices and have a low thermal conductivity similar to aerogels [2, 3]. Nano-cardboard structures are light enough to be levitated by photophoretic forces[4, 5], they can serve as substrates for sensors, and may be of interest in geoengineering applications [5]. Here, we describe a low-cost and fast process for fabricating scalable nano-cardboard structures. The fabrication process relies on inexpensive silicon wafers that are patterned by means of deep reactive ion etching to form sacrificial scaffolds. The sacrificial scaffolds are then uniformly coated with a thin ceramic layer using atomic layer deposition. In a final step, the silicon is etched away through small openings in the top face using XeF₂ gas, leaving a ceramic nano-cardboard with dimensions comparable to those of the original silicon wafer. Using this approach, nano-cardboard can be fabricated from many different ceramics and metals, with no limitation on the size of the nano-cardboard other than the wafer dimensions. We have analyzed the mechanical behavior of nano-cardboard with various periodic ligament patterns including a hexagonal and basket-weave patterns using finite elements. The nano-cardboard stiffness, buckling load, and post-buckling behavior depend sensitively on both the ligament pattern and dimensions, allowing optimization of the structure for maximum specific stiffness and strength. Because of their macroscopic dimensions, nano-cardboard structures can be tested relatively easily using a range of different mechanical techniques. Here, we present results of bending experiments on nano-cardboard structures with various patterns performed using nanoindentation.

[1] C Lin et al., Nanocardboard as a nanoscale analog of hollow sandwich plates, Nat. Commun., 2018
[2] L Meza et al., Strong, lightweight, and recoverable three-dimensional ceramic nanolattices, Science, 2014
[3] Zhang et al., Lightweight, flaw-tolerant, and ultrastrong nanoarchitected carbon, PNAS, 2019
[4] J Cortes et al., Photophoretic Levitation of Macroscopic Nanocardboard Plates, Adv. Mater., 2020
[5] D Keith, Photophoretic levitation of engineered aerosols for geoengineering, PNAS, 2010

The exceptional properties that have been reported for nanostructured metals have made them appealing in both applications at decreasing dimensions and increasingly complex environments. The incorporation of nanoscale metals into thin-film solar cells, electrical sensors, and electronic textiles means that these materials will experience the day-to-day fatigue from thermal cycling through bodily motion. Fatigue is one of the most common causes of failure in metals, but to date is poorly explored in nanostructured metals. This presentation will highlight recent in-situ transmission electron microscope (TEM) investigations into the tension–tension fatigue behavior of nanocrystalline copper and platinum at frequencies from 1 Hz to 100s of Hz, enabling accumulations of up to 10⁶ cycles within 1 h. Fatigue loading combined with the high spatial resolution of the TEM permits observations at crack growth rates of ∼10^–12 m/cycle. As a result of this exceptional resolution, new insights into grain growth, twin elongation, and crack healing mechanisms that occur in the crack initiation regime of pure nanocrystalline metals can now be directly observed. When in-situ TEM fatigue experiments are coupled with automated crystal orientation mapping (ACOM), the interplay among grain orientation, grain boundary character, and fatigue crack propagation becomes apparent. This addition of ACOM also permits direct coupling to atomistic and mesoscale modeling, which can add or remove microstructural elements not possible experimentally to fully elucidate the active mechanisms. This new capability to directly observe with nanometer resolution grain growth, twin elongation, crack healing and potentially other active mechanisms has the potential to drastically change our understanding of the crack initiation portion of fatigue failure. Ongoing efforts to expand this in-situ TEM to approach and other materials systems and extreme environments will also be highlighted if time permits.

This work is supported partially by the Division of Materials Science and Engineering, Office of Basic Energy Sciences, U.S. Department of Energy. This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. DOE’s National Nuclear Security Administration under contract DE-NA-0003525. The views expressed in the article do not necessarily represent the views of the U.S. DOE or the United States Government.

Nanofabricated systems, from electronics, to spintronics, to quantum devices, rely on increasingly 3D stacks of ultrathin films. However, as feature sizes push below 10nm, there are both new and long-standing challenges to improving device performance. For example, an established problem in nanoelectronics is that doping interlayer dielectric films with hydrogen (hydrogenation) to improve electrical performance can degrade mechanical performance to the point of device failure [1]. However, a potential new challenge is the dominant influence of surfaces and interfaces for ultrathin nanoscale films, which can change the film’s properties compared to bulk materials. Depending on the composition of the film, the influence of surfaces has been shown to either increase stiffness or increase compliance in ultrathin films [2,3]. Moreover, for sub-25 nm films at the leading edge of technology, traditional techniques struggle to measure the mechanical properties of films without overwhelming influence from the substrate. In this work, we use our ultrafast acoustic technique based on coherent extreme ultraviolet (EUV) light [4] to fully and nondestructively characterize the mechanical properties of films as thin as 5nm, even for an individual film in a bilayer. We study SiOC:H films and SiC:H bilayers to illuminate how surfaces affect the mechanical properties of these low-k (k < 4.2) dielectric materials being developed for next-generation nanoelectronics. Comparing the two materials, we distinguish between the stiffness degradation induced by dopants and that induced by the surface of the film. In particular, in very thin (5-nm) SiC:H films with low hydrogen doping, we find that surface effects induce a substantial reduction in stiffness—by almost an order of magnitude—compared with the same doping in thicker (46-nm) films [5]. However, for SiOC:H films with high hydrogenation, we observe no change in mechanical properties down to 11 nm. We attribute this difference in behaviors to the large loss of stiffness already induced by the hydrogen doping in SiOC:H, such that the surface has less of a relative effect. These findings have important implications for informed design of ultrathin films in a host of nano- and quantum technologies and most directly for improving the switching speed and efficiency of advanced nanoelectronics.

[1] D. Shamiryan, et al., Low-k dielectric materials, Mater. Today 7, 34–39 (2004).
[2] C. Q. Sun, Size dependence of nanostructures: Impact of bond order deficiency, Prog. Solid State Chem. 35, 1–159 (2007).
[3] R. J. Wang, et al., Effective geometric size and bond-loss effect in nanoelasticity of GaN nanowires, Int. J. Mech. Sci. 130, 267–273 (2017).
[4] A. Rundquist, et al., Phase-Matched Generation of Coherent Soft X-rays, Science 280, 1412–1415 (1998).
[5] T. D. Frazer, et al., Full characterization of ultrathin 5-nm low-k dielectric bilayers: Influence of dopants and surfaces on the mechanical properties, Phys. Rev. Mater. 4, 073603 (2020).

Three-dimensional (3D) microstructures of thin film materials are emerging technologies with a growing range of applications. The formation of residual compressive stresses is a common method to transform in-plane microstructures to 3D architectures. Here, we present a 3D strain scheme for compressive buckling in free-standing semiconductor thin films spanning between thin gold gridlines. The residual stresses are determined while transferring and bonding the films with the gridlines on their surfaces onto a substrate by cold pressure welding. Computational models are implemented using finite element analysis to predict the residual stresses and post-buckling morphologies in the free-standing films. Experimental results demonstrate a variety of scalable 3D architectures from nanometers to tens of microns, depending on the gridline spacings and bonding conditions. Additionally, the buckling microstructures elastically respond to external forces. We show the thickness dependent buckling mode transitions in the free-standing films. The film buckling phenomena offer an opportunity for achieving a diversity of 3D shapes such as nearly flat surface, upward buckling, telephone-cord buckling, and geometrical circle segmentation. Indeed, strain engineering of 3D thin metamaterial microstructures on inelastic platforms can lead to unprecedented material functions useful for energy harvesting and deployable microstructures.

Hierarchical elastomers, such as polyureas or polyurethanes, have gained interest for their high energy absorption capabilities and impact protection performance, where deformation-induced glass transition has been suggested to play a determining role. Here, we investigate the dynamic response of polyurea under temperature-controlled micro-impact experiments. We impact micron-scale silica projectiles at subsonic velocities on a polyurea target using the laser-induced microparticle impact test platform. Using high-speed imaging, we measure impact and rebound velocities and determine the absorbed impact energy as a function of target temperature from 20°C to 160°C. We observe a local elevated energy absorption around 115°C, which we attribute to the dynamic glass-to-rubber transition. The soft-segmental α₂-relaxation is extracted and fit with a Havriliak–Negami function from dielectric spectroscopy measurements. We find that the α₂-relaxation at 115°C correlates well with the characteristic strain rate of the impact experiments. The results confirm that the deformation-induced glass transition leads to enhanced energy dissipation. This work further supports the importance of the dynamical T_g as an important factor in the design of impact-resistant materials.

Ultrahigh surface-to-volume ratio in nanoscale materials, could dramatically facilitate mass transport, leading to surface-mediated diffusion similar to Coble-type creep in polycrystalline materials. Unfortunately, the Coble creep is just a conceptual model, and the associated physical mechanisms of mass transport have never been revealed at atomic scale. Akin to the ambiguities in Coble creep, surface atomic diffusion in nanoscale crystals remains largely unclear, especially when mediating the process of yielding and plastic flow. Here, by using in situ nanomechanical testing under high-resolution transmission electron microscope, we found that the diffusion-assisted dislocation nucleation induced the transition from a normal to an inverse Hall-Petch relation of the strength-size dependence, and the surface diffusional creep led to an abnormal softening in flow stress with the reduction in the size of silver nanocrystals, contrary to the classical “alternating dislocation starvation” behavior in nanoscale platinum. This work provides novel insights into the atomic-scale mechanisms of diffusion-mediated deformation in nanoscale materials, and impact on the design for ultrasmall-sized nanomechanical devices.

In metallic microstructures, hierarchical architectures of multiple phases with varying sizes and morphologies shown to be effective in simultaneously enhancing yield strength, due to the strong glide dislocation interactions with their nanoscale features, and plastic deformability, due to increased strain hardening from microstructure hierarchy. In some hierarchical microstructures, impedance of crack growth via deflection along interfaces that have relatively low shear strength and bridging via relatively softer micro-scale composite domains can also lead to enhanced crack growth resistance. Examples will be presented from a variety of microstructures produced by magnetron co-sputtering (Cu-Mo, Cu-Ta), laser rapid solidification (Al-Si) and laser direct metal deposition (Cu-Fe). The challenges and opportunities in elucidating nanomechanical response of a microstructure comprised of multiple length scales, morphologies and chemical heterogeneities will be discussed.

Classical continuum theory is widely used as a tool to predict structural behavior. However, the behavior of micro/nanostructures cannot be appropriately predicted based on classical continuum mechanics. The structural stiffness predicted by classical continuum theory is much more flexible than that of the real structure. In the past decades, various higher-order deformation theories have been developed to simulate size effect within a continuum scale. In higher-order deformation theories, additional material properties are introduced besides the Lamé constants. Since the additional material properties have a unit of length, they are called length scale parameters. In the couple stress theory, which has only one length scale parameter, couple stress is introduced as additional resistance to deformation.
In this study, structural behavior at micro/nano-scale is analyzed based on the couple stress theory by using the finite element method. It is observed that the bending rigidity increases as the thickness of microcantilever beams decreases. When a tensile load is applied to a flat plate with a hole, the stress concentration factor decreases as the size decreases. The buckling load of the thin structure increases as the thickness decreases.
In order to predict the structural behavior of micro/nanostructures based on the higher-order deformation theory, measurement of the length scale parameter must be preceded. We introduce two experiments to measure the length scale parameter. The first experiment is bending test. Microcantilevers can be fabricated via MEMS fabrication process and a line load is applied using a nanoindenter equipped with a wedge tip. The second experiment is buckling of membranes. After fabricating free-standing thin membranes, the temperature is gradually increased to measure the temperature at which buckling occurs. The material length scale parameter can be calculated inversely from the magnitude of the buckling loads. The experimental results are predicted from the finite element analysis based on the couple stress theory.

This research focuses on observations of polyacrylonitrile (PAN)/singe wall carbon nanotube (SWNT) composite films processed through pyrolization cycles for the production of ordered graphitic and activated carbon structures. The precursor composite films were processed using a solvent-based phase separation approach to promote the formation of extended-chain PAN interphase development at the SWNT surface as well as a hybrid through-thickness morphology. Previously reported work has outlined the advantages of this structural formation of crystalline PAN interphase regions within the precursor for the formation of ordered carbon at low temperatures between 900 °C to 1100 °C. In this work, PAN/SWNT precursor films underwent identical stabilization and carbonization cycles in air and argon gas, respectively. After low temperature turbostratic carbon formation, annealing cycles were then performed for various time and temperature combinations (up to 10 hours at 1700 °C) before activating the samples in carbon dioxide. These studies aim to correlate the stability of the low-temperature carbon forms to heal during annealing and subsequently survive activation toward forming graded porosity structures in these hybrid films. Dynamic mechanical analysis was used at each intermediary step of pyrolysis to evaluate the development of tensile strength (σ) and modulus (i.e., Storage (E') and Loss (E'') moduli). Overall material morphological composition was also evaluated using X-ray diffraction/scattering and electron microscopy both before and after activation. The results to be presented demonstrate the development and thermo-mechanical characterization of unique graded activated carbon architectures with potential applications such as filtration, electrochemical, and shielding.

Metal thin films are widely used in various small-scale devices such as integrated circuits, gas sensors, micro-actuators, and micro-heaters. In most of the devices, it is hard to find freestanding thin films; instead, a number of layers of different materials are stacked in a complex way. In other words, metal films are generally surrounded by other materials. Dielectric materials commonly cover metal thin films in order to insulate electricity and heat conducted through the metal layer, and to prevent oxidation of the metal surface.
In this presentation, we will introduce two in-situ experimental techniques utilizing Si-based micromachining process to characterize the mechanical behavior of passivated metal thin films. Experiments were carried out on submicron thick aluminum thin films passivated with a few tens of nm thick SiNx.
The first technique is performed through use of a custom-built in-situ scanning electron microscope (SEM) mechanical tester for measurement of the mechanical properties of Al thin films with and without a passivation layer. The apparatus has a stroke of 250μm with a displacement resolution of 10nm and a load resolution of 9.7μN. It utilizes pre-calibrated leaf spring, capacitive displacement gauge, and piezo actuator with a built-in displacement sensor. The second technique utilizes an in-situ SEM nanoindentation system equipped with a diamond wedge tip. Micro-scale bridge-shaped membranes and cantilever samples are fabricated via standard microfabrication process, and deflected by applying a line load. Stress–strain curves can be determined from load-displacement curves. Metallic thin films with a passivation layer exhibit enhanced strain hardening behavior because the passivation layer forms a strong interface, which acts as an obstacle to dislocation motion. Future plans for measurements at elevated temperatures will be introduced.

Complex multi-principle element alloys, also referred to as high-entropy alloys (HEAs), have complex chemical and topological fluctuations at the atomic scale. This lead to a pronounced locality of the dislocation behavior and therefore the plastic response of such alloys. Here we focus on single-phase solid solution HEAs and trace the microplastic response of collective dislocation rearrangements (avalanches). A detailed analysis of the time-resolved response of crystallographic slip suggests that internal length scales much larger than the interatomic distance control the dislocation motion. To shed further light onto this observation, we use spatially resolved property mapping at the nano-scale with the aim of linking the length-scales derived from time-resolved slip with correlation lengths determined from nano-mechanical fluctuations.

Carbon fibers have been of great interest in many industrial applications due to their high specific mechanical properties as compared to steel or glass fibers. The polyacrylonitrile (PAN)-based commercial carbon fiber has as high as 7 GPa of tensile strength, and the highest tensile modulus is about 600 GPa although their theoretical properties are known to be 150 GPa and 1060 GPa, respectively. The property discrepancy, especially in tensile strength, is attributed to the defective structures such as bulk defects (voids, cracks, and cavities), surface defects (notches and punctures), and structural defects (disordered carbon and radial heterogeneity). Therefore, many researches regarding the correlation between microstructure and corresponding mechanical properties of carbon fibers upon carbonization have been conducted. Irrespective of carbon fiber’s intrinsic properties, one has to keep in mind that it is mainly used in the form of carbon fiber-reinforced plastics (CFRP) or carbon-carbon composites rather than being used by itself. The carbon fibers are oftentimes subjected to very high temperature not only in the application but also during processing. In the due course, the structure and properties of carbon fibers are supposed to be varied. It would be beneficial that one has the information how their structure and the corresponding mechanical properties are changed upon application of such an extreme condition.
In the current study, the structure evolution and respective mechanical behavior of commercial PAN-based carbon fibers, H2550 and H3055 (Hyosung Co.), were analyzed upon post heat-treatment from 1600 to 2400 ^oC. With increasing heat-treatment temperature, the tensile modulus continuously increased due to the improvement of crystal size and orientation factor. However, the tensile strength exhibited abnormal behavior, which decreased in the temperature range of 1600-1800 ^oC, increased in the range of 2000-2200 ^oC, and decreased again at higher temperature than 2200 ^oC . This is correlated with the microstructure by considering above-mentioned various defects. The fibers heat-treated in the range of 2000-2200 ^oC showed the highest tensile strength value among them because of a high fraction of amorphous and disordered carbon, leading to effective energy dissipation. On the other hand, at 2400 ^oC, the strength was drastically diminished due to the graphitization and the generation of a rough surface. Radial heterogeneity and elongated void development were also observed at high temperatures, but it appeared that they don’t have a significant effect on strength. It is noticeable that the Weibull modulus, which represented the fiber’s uniformity and reliability in fracture, also showed unusual behavior depending on the temperature, and was highest at 2400 ^oC for both fibers (8.8 of H2550 and 6.4 for H3055). This suggests that post heat-treatment can also tailor the structural uniformity of fibers. The current study will provide the fundamental information of carbon fibers’ structure-property relationship and can be used to predict the property variations of fibers in composites when used in high-temperature applications.

Poisson's ratio (PR) of a material plays an important role in governing the lateral response of a solid subjected to symmetry-breaking deformation. Traditionally, PR is represented as a single material constant which, by construction, refers to the linear part of the stress-strain curve. The understanding of a possible coupling between crystallographic direction and PR remains more elusive. Here, we report the observation of anisotropic PR in defect-free 3C-SiC lattice, using quantum simulations based on the Density Functional Theory. We analyze the lateral response of 3C-SiC to an applied strain along the three high-symmetry crystallographic directions, [100], [110], and [111]. From our ab initio calculations, we use the linear and nonlinear regimes of mechanical deformation to determine Poisson's ratio as a function of applied strain for a given axial and transverse direction. Asymmetric deformation in the lattice of the [110] direction results in two Poisson functions dependent on the lateral orientation. One of these indicates a positive value, while the other a negative value (regardless of the strain state of the lattice). This suggests simultaneous existence of both positive and negative PRs in the material. Conversely, uniaxial loading along either of the other two high-symmetry directions, the [100] and [111] directions, yields identical lateral behavior, allowing for a single Poisson function. From the four Poisson's functions, the maximum and minimum Poisson's ratios are reached in the two different lateral directions of the [110] direction. The first function reaches a maximum Poisson's ratio of 0.5, indicating incompressibility and maximum contraction of the lateral direction for applied tensile loads. The second function is auxetic, exhibiting purely negative Poisson's ratios throughout the true strain range. This translates to lateral expansion in tension and lateral contraction in compression. In both the [100] and [111] directions, all values of the Poisson's functions exist within the range of 0 to 0.3. Anisotropic behavior is similarly witnessed in the strength and toughness of 3C-SiC for the three high-symmetry directions. Here it can be noted that loading along the [100] direction exhibits optimal strength and toughness while loading in the [111] direction displays maximum stiffness. Analysis at the atomic scale reveals relationships between crystallographic structure and anisotropy of Poisson's ratio. Linear and angular bond deformation influences the lateral strain behavior based on lattice symmetry with respect to the axial direction. Additionally, further investigation of the electron redistribution properties of both silicon and carbon atoms was investigated to determine relationships between Poisson's ratio and electron density in the various orbitals. Coupled with improvements in grain manufacturing technologies, these observations can be utilized toward the optimization of mechanical properties of 3C-SiC and other diamond cubic structures in practical applications.

Shape memory alloys (SMA) are a class of alloys which show unusual deformation depending on temperature or loading condition. Nickel-Titanium (NiTi) alloys is one of the most attractive SMAs due to its remarkable work output per unit volume, significantly higher strain recovery, and superior thermo-mechanical stability. However, majority of research on NiTi alloys have focused on the bulk-scale mechanical properties, and it is still challenging to apply NiTi SMAs in micron-scale applications such as electric and/or biomedical sensor. Few studies have reported that small-scale NiTi shows different shape memory behavior compared to its bulk counterpart, but our understanding on size effect is yet premature and a comprehensive study is required.
In this study, we conducted in-situ tensile test of NiTi alloys, utilizing two experimental techniques for investigation of the small-scale mechanical behavior. In the first technique, tensile specimens are prepared by sputter deposition and Micro-electro-mechanical-system (MEMS) fabrication process. Tensile tests are conducted by using a customized in-situ scanning electron microscope (SEM) tensile tester. In the second experimental technique, micro-scale tensile bars are fabricated within a bulk-scale NiTi sheet by utilizing femtosecond laser and focused ion beam (FIB) milling. Femtosecond laser machining is used for fast, but rough, specimen mass production, and FIB milling is performed for fine cleaning. Micro-scale mechanical testing is conducted utilizing a nanoindenter equipped with a diamond gripper. By comparing the mechanical response between sputter deposited NiTi thin films and micro-scale tensile bars carved out from a bulk crystal, we aim to gain better understanding of microstructural effect on the superelastic behavior of micro-scale NiTi SMA alloys.

It is well known that the mechanical behavior of polycrystalline material changes largely with their mean grain diameter. In this work, we explore the grain size-dependent phenomenon in heterogeneous structure material using a continuum dislocation theory coupled with ViscoPlastic Fast Fourier Transform (VP-FFT) method. A dislocation-based strain gradient plasticity model was developed to predict the grain size effect over a wide range of length scales. The effect of the GNDs density was incorporated into the model for the mean free path of dislocations.

Superalloys are structural materials for high-temperature applications such as gas turbines and space vehicles. Co-based superalloys have been spotlighted as they can be utilized at a higher temperature than traditional Ni-based alloys. Investigations of composition-microstructure-property relationships are required to develop next generation superalloys. However, due to the enormous number of the possible combinations, it is challenging to acquire the property data that can illustrate the relationships. In this study, we carried out the combinatorial synthesis and high-throughput experiments to investigate the composition and microstructure dependent properties of Co-Cr-Ti alloy thin films library, which are known to produce the γ/γ’ microstructures in bulk scale. Combination synthesis and high throughput experiments provide an effective pathway for obtaining large amounts of experimental property data. We used magnetron co-sputtering to produce hundreds of different Co-Cr-Ti thin-film alloys on a single wafer. We then characterized composition-dependent phase formation as well as electrical and mechanical properties by performing X-ray diffraction, 4-probe resistance mapping, and nanoindentation experiments. The correlation among the measured properties and their implication on bulk properties will be discussed.

In this study, Fe₃O₄ and Fe₂O₃ nanoparticles were used to improve the tribological performance of the Epoxy resins. The composites were prepared by adding the nanoparticles to the epoxy resin and techniques like sonfication and thinky mixing were used to get good dispersion of the nanoparticles. The effects of Fe₃O₄ and Fe₂O₃ nanoparticles on the wear rate, friction coefficient and hardness of the composites were investigated using Bruker’s UMT Tribolab. The results showed that the composite exhibited improved wear resistance, lower friction coefficient and better hardness values compared to neat epoxy resin. With the incorporation of 3% Fe₂O₃, the hardness values of the composites enhanced to 9.3 HV (increased by 165%), 16.9 HV (increased by 160%) and 26.2 HV (increased by 159%) for 20 N, 40 N and 60 N, respectively. Similar increasing trend can be seen with incorporation of 3% Fe₃O_4, with hardness values increased to 9.1 HV (increased by 160%), 17.1 HV (increased by 163%) and 27 HV (increased by 167%) for 20 N, 40 N and 60 N, respectively. From the scratch test results, we can see a decreasing trend in the COF values for the samples, with 5% samples showing the lowest COF values. The improved tribological performance of the composites can be ascribed to the improved mechanical and lubricating properties of the nanoparticles.

Tungsten (W) is one of the promising candidate materials for plasma-facing materials (PFMs) in fusion reactors due to its high melting point, high thermal conductivity, and low sputtering rate. PFMs are known to be irradiated by the by-products of fusion reaction, such as helium ions & neutron, which can lead to structural and mechanical degradation. It has been reported that the addition of rhenium (Re) and tantalum (Ta) can mitigate irradiated damages and hiders helium bubble formation. In this work, we performed helium irradiation experiments on combinatorially sputter deposited W-based alloy thin films to investigate the microstructure- and composition-dependent irradiation damages. XRD was used to analyze the structural damages, and nanoindentation and 4-point measurement were used to measure the mechanical and electrical property changes, respectively. The correlation between the microstructural changes and the materials’ properties are discussed.

Biocomposites can be specifically tailored to improve their multifunctional properties for a wide variety of applications by the addition of carbon nanoparticles. In this study, thin films of Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) nanocomposites were prepared by adding 0.1- 0.5 % Multi-Walled Carbon Nanotubes (MWCNTs). In the construction of the films, chloroform was used as the solvent for the PHBV nanocomposites and the MWCNTs. Sonication was used to enhance the dispersion of the MWCNT throughout the solvent, and a magnetic stirrer was used to help in mixing the solution with the polymer. When adding MWCNT into PHBV, there was an improvement during the following tests: tensile test, thermogravimetric analyses, and differential scanning calorimetry. In addition, the tested samples were analyzed using scanning electron microscopy and transmission electron microscopy to see how the effect of MWCNT's addition on the surface of PHBV.

Additive manufacturing is an emerging technology due to its superiority in complex and customized design. These advantages lead to applications such as turbofan engines, power plant turbines, and spacecraft engine parts. Inconel 718, which is widely applied to the above applications, has now become one of the main additive manufacturing research targets. However, it has the possibility of containing inclusions such as titanium nitride (TiN) due to being exposed to air during deposition despite the injection of shielding gas. The effect of such inclusions on the high-temperature mechanical properties of additively manufactured Inconel 718 is not yet well understood.
In this study, multi-scale (bulk-scale and meso-scale) uniaxial tensile tests are performed to investigate the effect of TiN inclusions on the mechanical properties of additively manufactured Inconel 718 at elevated temperatures. Inconel 718 is deposited on the substrate using the direct laser deposition (DLD) method. Wire electrical discharge machining (EDM) is applied to fabricate dog-bone shaped bulk-scale specimens with gauge lengths of 10mm, gauge widths of 2mm, and thickness of 0.3mm. Tensile tests are carried out at room temperature, 400°C, 600°C, 650°C, 700°C, 750°C, and 800°C at the strain rate of 10^-4 /s. According to the experimental results, additively manufactured Inconel 718 maintains a yield strength of 550MPa while increasing the temperature up to 650°C, and exhibits the highest value of 670MPa at 700°C. In addition, as temperature increases, uniform elongation decreases from 16.5% at room temperature to 1% at 700°C. Scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) analysis are conducted to examine the presence of TiN inclusions and the failure mechanisms of additively manufactured Inconel 718. EDS and XRD results show that TiN inclusions are contained numerously, and SEM images indicate that TiN inclusions show a trace of brittle fracture, while the matrix of the material is ductile.
We will also introduce ongoing efforts trying to understand the effects of a single TiN inclusion on the high-temperature tensile properties via small scale experiments. Femtosecond laser machining is utilized to fabricate dog-bone shaped meso-scale specimens with gauge lengths of 2mm, gauge widths of 0.5mm, and thickness of 30. A meso-scale tensile tester, capable of testing at temperatures up to 1000°C has been custom-built. Future plans of meso-scale experiments at elevated temperatures will be discussed.

The effect of cross-linking on the modulus (E) and hardness (H) of an epoxy polymer was studied using nanoindentation. Experiments were carried out on SU-8, which is an epoxy-based thermoset polymer extensively used for fabricating micro-electrical mechanical system (MEMS) components. The extent of cross-linking was varied by modifying the curing steps (post-exposure baking, hard baking) of standard photolithography process. The amount of cross-linked segments was measured via in-situ Fourier-transform infrared spectroscopy. Nanoindentation experiments were carried out under constant strain rate conditions. Using the conventional Oliver-Pharr method yielded high E and H values for less cross-linked sample and less values for high cross-linked samples. A careful analysis showed that the main reasons for these inverse values are adhesion between the tip surface and sample surface, viscoelastic effect on unloading and errors in calculation of contact area. We propose a new methodology to correct for these potential sources of error. Thereafter, E values of 4.61 ± 0.13 GPa and 5.02 ± 0.18 GPa, and H values of 256.97 ± 1.42 MPa and 285.48 ± 1.17 MPa were obtained for less (~ 82 %) and high (~ 95 %) cross-linked samples, respectively.
Keywords: SU-8; Photolithography process; FTIR; Nanoindentation; SPM

Aluminum is a widely used metal in transportation, aviation, and energy due to its advantages of low density. Aluminum and its alloys are also being used in MEMS (Micro-electromechanical system) devices due to properties such as low resistivity, high conductance, high reflectance, and low residual stress. However, as aluminum possesses low strength, it is crucial to increase strength by tailoring the microstructure via altering deposition conditions, heat treatment, and/or alloying. In previous works, nickel, molybdenum, and copper were alloyed with Al to increase the strength. One possible way to enhance the mechanical properties of aluminum is to reinforce it with strong materials (e.g., carbon nanotubes, graphene, etc.), producing aluminum-based composite. However, it is still challenging to uniformly disperse nano-carbon material in the aluminum matrix without forming carbides. Recently, a new way to increase strength of aluminum using super-saturated Al-C phases has been proposed. Carbon atoms from individually dispersed C60-fullerenes, are intercalated into the interstitials of aluminum matrix, producing supersaturated Al-C phases with artificially moderated lattice structures. However, the underlying strengthening mechanism of atomically adding carbon atoms to aluminum matrix has not been deeply understood.
In this presentation, we discuss the mechanical properties and microstructure of sputter deposited aluminum (Al)-carbon (C) alloy thin films. Aluminum-carbon alloy thin films with various compositions were produced by co-sputtering aluminum and carbon without substrate rotation. Through this method, Al-C coatings with compositional spread can be fabricated, and the effect of carbon on the deformation mechanism can be analyzed. Thin film specimens having micro-scale dimensions were fabricated utilizing MEMS processing techniques. The tensile characteristics of thin film specimens were evaluated utilizing a custom-built micro tensile tester. The sputter deposited Al-C alloy thin films exhibit higher yield stress (~ 420 MPa) compared to that of pure aluminum thin films without loss of ductility (~ 10%). The deformation mechanism of the aluminum-carbon alloy thin films was analyzed based on mechanical and microstructural analysis.

During deformation of bulk polycrystalline materials, boundaries impose constraints due to requirements of compatibility across boundaries. Pure bulk Copper and Copper-aluminium samples were subjected to interrupted tensile tests coupled with electron backscatter diffraction (EBSD) analysis, to study the evolution of misorientation at specific boundaries. Our results show that misorientation development (in terms of Kernel Average Misorientation, KAM) near boundaries is a function of orientation of grains on each side, but it is also influenced by the presence of other boundaries nearby, and constraints such as triple points. We have noticed that KAM values near boundaries does not behave monotonically for both Cu and Cu8Al. For Cu, in one case it increases as well as decreases in values, while in other case it remains constant. Similarly, for Cu8Al it reduces in values for most of the cases and increases or remains constant with strain in few cases. So while developing models for deformation of polycrystals, detailed information regarding grain orientation and neighbouring grains should also be incorporated. We have analysed our results in terms of Schmidt factors of the grains on either side of the boundaries. Results for copper are compared with Cu-Al alloy, which has significantly finer twins in the microstructure. Hence there are greater constraints.

Keywords: EBSD; Misorientation; Orientation gradient; Grain boundary; Schmid factor; KAM

Surface micro/nano patterning is essential for diverse applications including next generation-solar cells, photonics and bio-interface devices. Nanoimprint lithography offers such patterning with high resolution and high throughput. However, since such technology usually requires sophisticated instruments and clean room facilities, it is cost-effective only if performed in high-volume mass production that limits its role in most research and development needs and various niche applications. To overcome such limitations of nanoimprinting process, we utilize a thermoplastic polymer with a thorough knowledge of the imprinting conditions at micron to nano scale such as filling behavior, pattern fidelity, and surface properties.

In the present study, we describe an optimized process for imprinting sub-micron to nano-structures to poly(methyl methacrylate) (PMMA), a thermoplastic polymer, using simple soft PDMS (Polydimethylsiloxane) templates. Initially, a range of imprinting depth of 200nm PMMA thin layer was explored with micron to nano scale features of a template for a low imprinting pressure (~3 bar). Sub-micron patterned features have complete depth of trenches as template compared to micron size patterns; in both cases, the obtained patterns depend on applied pressure and imprinting temperature. The imprinting at temperatures higher than that for PMMA glass transition leads to increased pattern trench depths; however the imprinted PMMA layer shows poor surface quality due to the high temperatures employed. We addressed this problem by using PMMA solvent mixed with acetone to improve the surface quality of PMMA layer. Therefore, relatively low surface roughness and good anti-sticking properties were obtained. The thermal behavior and etching rate of the polymer prepared with mixed solvent were studied. By tuning nanoscale mechanical and surface properties, we successfully imprinted features at submicron and nano scales and demonstrate pattern transfer to substrates after metallization and lift off process.

The method to measure hardness and elastic modulus of small volumes of material by instrumented indentation presented in the seminal works of Oliver and Pharr in 1992 and 2004, has revolutionized the field of small scale nanomechanical testing. Several recent advances in measurement electronics have enabled testing over a wider range of test conditions (speeds) using methodologies that were developed earlier, which requires a critical assessment. In the backdrop of the latest developments in instrumentation and test methodologies, an overview of the various factors affecting the precision and accuracy of the nanoindentation test results at different test conditions with specific focus on Continuous Stiffness Measurement (CSM) technique will be presented.

Hardness is a complex physical property that includes both intrinsic and extrinsic effects. In the past, to design intrinsically hard materials, we have explored the use of late transition metal borides that possess intrinsically high incompressibility due to high valence electron density, and added covalency through the formation of metal borides as a way to increase resistance to shear. At the opposite end of the spectrum, we find extrinsic effects, including grain boundary processes, such as grain boundary sliding, diffusion and grain rotation. In nanocrystalline material, these extrinsic effects compete with intrinsic effects (grain interior dislocations) and lead to unique plastic deformation behavior. While the size effects have been explored in the ductile metal system, little is known about whether these same phenomena dominate in nanocrystals of much harder materials, such as transition metal boride nanocrystals, mostly due to a lack of materials with size tunable nanosized domains.

Here, we demonstrate the size dependency of plastic deformation mechanism in the nanosized superhard ReB₂ system. In order to understand the lattice specific mechanical failure that is related to bonding motifs, in-situ high-pressure radial X-ray diffraction was conducted. A large magnitude of uniaxial stress state was generated in the diamond-anvil cell (DAC), while X-ray diffraction is simultaneously collected, allowing us to probe the changes in bonds under compression and obtain the differential strain (t/G), which describes the material’s ability to resist shear stress. In addition, texture analysis on the radial diffraction data provides quantitative information on slip planes within a material, revealing the extrinsic effects interacting with the grain interior dislocations. Different sizes of nano-ReB₂ (20 nm and 50 nm) were synthesized by molten-salt method and bulk-ReB₂ was synthesized by arc-melting method. In-situ radial X-ray diffraction was performed under high pressure up to ~60 GPa. The lattice specific differential strain was experimentally measured and the diffraction patterns were analyzed by Rietveld refinement. In the nano-ReB₂, extrinsic effects compete with grain interior dislocations and lead to plastic deformation behavior that is very different from bulk-ReB₂. A Hall-Petch size dependence was observed, as the grain size gets smaller, the increasing the grain boundary content impedes plastic deformation and results in enhanced resistance to dislocation glide. Texture analysis was performed on the nano-ReB₂ system, because the grains in the nano-samples are fine enough that the peak intensity variation with azimuth angle correlates to the slip systems and grain orientation. From the inverse pole figures, the texture has its maximum intensity along the (0001) direction in both nano-samples, suggesting that the deformation is primarily controlled by the basal slip system. Furthermore, the pole density is more concentrated at (0001) pole as the grain size gets smaller, suggesting closing of slip system as the grain boundary concentration increases.

Materials at small scale behave differently from their bulk counterparts. This deviation originates from the abundance of interfaces at small scale. Quantifying the properties and revealing the underlying mechanisms requires experiments with small samples in situ in analytical chambers. However, small size poses the challenge of sample handling, but offers the opportunity of in situ inspection of mechanism during testing in analytical chambers. In order to overcome the challenge and take advantage of the opportunity, we developed a MEMS based micro scale testing stage where the sample and the stage are co-fabricated. The stage suppresses any misalignment error in loading by five orders of magnitude. The stage allows in situ inspection of samples during testing in SEM and TEM. We employed the stage to explore the effect of size on brittle to ductile transition (BDT) temperature (540C) in single crystal silicon. Here, the sample is a micro scale single crystal silicon beam subjected to bending which limits the high stress region to a small volume of the sample, and minimizes the probability of premature failure from random flaws. We found that silicon indeed deforms plastically at small scale at temperatures much lower than 540C. Ductility is achieved through a competition between fracture stress and the stress needed to nucleate dislocations from the surface. Our combined SEM, TEM and AFM analysis reveals that as a threshold stress is approached, multiple dislocation nucleation sites appear simultaneously from the high stressed surface of the beam with a uniform spacing of about 200 nm between them. Dislocations then emanate from these sites with time, lowering the stress while bending the beam plastically. This process continues until the effective shear stress drops and dislocation activities stop. A simple mechanistic model is presented to relate dislocation nucleation with plasticity in silicon.

Earthquakes are frictional instabilities that initiate on faults in Earth's crust at slow, quasi-static slip rates of nm/s to μm/s. Unstable fault slip results from interactions between the mechanical properties of the rocks which load the fault and the frictional behavior of the fault itself. Like several engineering materials, the quasi-static frictional behavior of rocks is described by rate-variable and state-variable friction (RSF) “laws”, which are actually empirical relations based on data from laboratory experiments. These descriptions of friction agree with many (but not all) data from lab experiments, and reproduce a rich variety of earthquake behavior when employed in full-cycle earthquake rupture models. However, the physical and/or chemical mechanisms that yield the observed rock friction behavior are unknown, making extrapolations of RSF laws beyond the limited range of conditions of the experiments that form their basis fraught with uncertainty.

The most fundamental observation from experimental studies of rock friction is that friction increases with the log of the time of stationary (or nearly stationary) contact. This is typically studied in slide-hold-slide experiments, wherein restarting sliding after a “hold” period without sliding results in an increase in peak static friction. This increase in state of the frictional interface during the hold may occur with the time of stationary contact, or with the slow slip of shear-loaded contacts. Time- or slip-dependent aging manifests as a velocity dependence of friction, the sign of which determines whether unstable frictional sliding (an earthquake) is possible.

Because the frictional behavior of faults emerges from the collective behavior of myriad nanometers- to microns-sized contacting asperities, fundamental rock friction mechanisms can be studied at the single-asperity level by atomic force microscopy (AFM) and nanoindentation. The canonical view is that aging results from increases in contact area due to plasticity and creep of asperities in contact. Results of our recent AFM and nanoindentation experiments, coupled with first-principles simulations, however, point to chemical bonding across frictional interfaces as an important mechanism of friction. Extensive AFM studies with oxidized silicon tips and samples, and more recently with quartz tips on quartz samples, and coordinated simulations demonstrate that log-time aging results from chemical bonding across asperity contacts, in the absence of plastic deformation. Moreover, our AFM experiments reveal many of the phenomena observed in friction experiments on rocks - notably a critical slip (or memory) distance D_c , variations in the dependences of friction on sliding velocity, increases of friction with slow as well as no slip during holds, and stick-slip behavior. Our nanoindentation experiments show that the yield stress and creep behavior of quartz at room temperature are independent of relative humidity (RH) in the range ~0 to 50%, whereas macroscale friction tests on quartz reveal no aging at RH <5% due to a supposed lack of asperity yielding and creep. These observations thus preclude plastic deformation as the primary mechanism of frictional aging. Our studies collectively point to chemical bonding as an important mechanism of rock friction, and motivate our current models of rough rock surfaces in contact that incorporate both chemical bonding and asperity yielding and creep.

Investigating mechanical properties at small scales is a challenging endeavor. It requires sophisticated micro-/nano-scale experimental methods combined with laborious/time-intensive finite element computations. In this talk, a new framework for materials characterization at small scales using the latest developments in machine learning will be presented. This framework involves multi-fidelity deep learning and active learning methods limiting the need for finite element simulations. Its application for predicting the fracture toughness of microscale pentagonal cross-sectional ceramic cantilevers will be showcased, demonstrating that it can significantly accelerate fracture toughness characterizations at small scales.

Impacts at very high velocities have been widely perceived to promote particle flattening and bonding during additive manufacturing processes such as cold spray. Using the laser-induced particle impact tester, we perform site-specific studies with well-known particle sizes and velocities, and show that particle impact at some very high velocities can counter-intuitively deteriorate coating quality. We identify a particle rim peeling mode that hinders extensive lateral particle flattening and also appear to cause debonding at the particle-substrate interface close to the particle periphery, even prior to the undesirable hydrodynamic particle penetration regime. To the extent that high velocities are required for permanent particle adhesion, this work points to the possibility of a more narrow velocity range within which high quality particle bonding may be achieved.