Molecular Dynamics Simulations Of Nanoindentation Research Articles

Stress-assisted grain growth in nanocrystalline metals: Grain boundary mediated mechanisms and stabilization through alloying

The mechanisms of stress-assisted grain growth are explored using molecular dynamics simulations of nanoindentation in nanocrystalline Ni and Ni-1 at.% P as a function of grain size and deformation temperature. Grain coalescence is primarily confined to the high stress region beneath the simulated indentation zone in nanocrystalline Ni with a grain size of 3 nm. Grain orientation and atomic displacement vector mapping demonstrates that coalescence transpires through grain rotation and grain boundary migration, which are manifested in the grain interior and grain boundary components of the average microrotation. A doubling of the grain size to 6 nm and addition of 1 at.% P eliminates stress-assisted grain growth in Ni. In the absence of grain coalescence, deformation is accommodated by grain boundary-mediated dislocation plasticity and thermally activated in pure nanocrystalline Ni. By adding solute to the grain boundaries, the temperature-dependent deformation behavior observed in both the lattice and grain boundaries inverts, indicating that the individual processes of dislocation and grain boundary plasticity will exhibit different activity based on boundary chemistry and deformation temperature.

Pressure-induced amorphization in the nanoindentation of single crystalline silicon

Large-scale molecular dynamics simulations of nanoindentation on a (100) oriented silicon surface were performed to investigate the mechanical behavior and phase transformation of single crystalline silicon.

Molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter.

We performed molecular dynamics simulation of nanoindentation on Cu/Ni nanotwinned multilayer films using a spherical indenter, aimed to investigate the effects of hetero-twin interface and twin thickness on hardness. We found that both twinning partial slip (TPS) and partial slip parallel with twin boundary (PSPTB) can reduce hardness and therefore should not be ignored when evaluating mechanical properties at nanoscale. There is a critical range of twin thickness λ (~25 Å < λ < ~31 Å), in which hardness of the multilayer films is maximized. At a smaller λ, TPSs appear due to the reaction between partial dislocations and twin boundary accounts for the softening-dominated mechanism. We also found that the combination of the lowered strengthening due to confined layer slips and the softening due to TPSs and PSPTBs results in lower hardness at a larger λ.

Nanoindentation study of cementite size and temperature effects in nanocomposite pearlite: A molecular dynamics simulation

We carry out molecular dynamics simulations of nanoindentation to investigate the effect of cementite size and temperature on the deformation behavior of nanocomposite pearlite composed of alternating ferrite and cementite layers. We find that, instead of the coherent transmission, dislocation propagates by forming a widespread plastic deformation in cementite layer. We also show that increasing temperature enhances the distribution of plastic strain in the ferrite layer, which reduces the stress acting on the cementite layer. Hence, thickening cementite layer or increasing temperature reduces the likelihood of dislocation propagation through the cementite layer. Our finding sheds a light on the mechanism of dislocation blocking by cementite layer in the pearlite.

Molecular dynamics simulation of nano-indentation of (111) cubic boron nitride with optimized Tersoff potential

We conduct molecular dynamics simulation of nanoindentation on (111) surface of cubic boron nitride and find that shuffle-set dislocations slip along <112> direction on {111} plane at the initial stage of the indentation. The shuffle-set dislocations are then found to meet together, forming surfaces of a tetrahedron. We also find that the surfaces are stacking-fault zones, which intersect with each other, forming edges of stair-rod dislocations along <110> direction. Moreover, we also calculate the generalized stacking fault (GSF) energies along various gliding directions on several planes and find that the GSF energies of the <112>{111} and <110>{111} systems are relatively smaller, indicating that dislocations slip more easily along <110> and <112> directions on the {111} plane.

Molecular dynamics simulation of deformation twin in rocksalt vanadium nitride

We perform molecular dynamics simulation of nano-indentation with a cylindrical indenter to investigate the formation mechanism of deformation twin in vanadium nitride (VN) with a rocksalt structure. We find that the deformation twins occur during the loading stage, and subsequently conduct a systematic analysis of nucleation, propagation and thickening of a deformation twin. We find that the nucleation of a partial dislocation and its propagation to form a stacking fault are premise of deformation twin formation. The sequential nucleation and propagation of partial dislocation on adjacent parallel {111} planes are found to cause the thickening of the deformation twin. Moreover, the deformation twins can exist in VN at room temperature.

Orientation-dependent mechanical behavior and phase transformation of mono-crystalline silicon

We perform a large-scale molecular dynamics simulation of nanoindentation on the (100), (110), and (111) oriented silicon surface to investigate the orientation-dependent mechanical behavior and phase transformation of monocrystalline silicon. The results show both the remarkable anisotropic mechanical behavior and structure phase transformation of monocrystalline silicon. The mechanical behavior of the (110) and (111) oriented surfaces are similar (has a high indentation modulus, low critical indentation depth for the onset of plastic deformation) but quite different from the (100) oriented surface. The mechanical behavior is carefully linked to the phase transformation. The formation of crystalline bct5 phase and β-Si phase is the fundamental phase transformation mechanism for (100) oriented surface. But, a large number of amorphous silicon can be found beneath the indenter for (110) and (111) oriented surface beside the bct5 phase and β-Si phase. The β-Si phase region is relatively small for (110) and (111) oriented surface, even cannot be detected for (111) oriented surface. This result highlights the dominating role of the amorphous transformation in the mechanical behavior of monocrystalline silicon. Additionally, our results indicate that the high pressure phases form a symmetrical, anisotropic pattern on the indented surface for all three oriented surface which is linked to the active {111}〈110〉 slip systems.

Molecular Dynamics Simulation of Ice Indentation by Model Atomic Force Microscopy Tips

We have performed extensive molecular dynamics simulations of nanoindentation of an ice slab with model atomic force microscopy (AFM) tips. We found the presence of a quasi-liquid layer between the tip and the ice for all explored indentation depths. For the smallest tip studied (R = 0.55 nm), the force versus indentation depth curves present peaks related to the melting of distinct monolayers of ice, and we were able to calculate the work (free energy) associated with it. For a larger tip (R = 1.80 nm) having a size not commensurate with the average monolayer thickness, we did not find a clear structure in force curves. This work can help guide the interpretation of experimental AFM indentation of ice and other crystalline solids. More specifically, it provides guidelines for tip sizes where layer-by-layer melting can be achieved and for the order of magnitude of forces that need to be detected.

Nanoindentation of Zr by molecular dynamics simulation

Molecular dynamics simulations of nanoindentation are used to study the deformation behaviors of single crystal Zr for four different surface orientations. The comparison of results for two different potentials, an embedded atom method potential and a charged optimized many body potential, reveals the influence of stable and unstable stacking fault energy on dislocation behaviors under nanoindentation. The load–displacement curve, hardness and deformation behaviors of the various surface orientations Zr are compared and the elastic and plastic deformation behaviors are analyzed.

Molecular dynamics simulations of nano-indentation and wear of the γTi-Al alloy

The γTi-Al alloy has great potential for applications in the field of aerospace engineering, as titanium aluminide (γTi-Al) exhibits outstanding mechanical and physical properties. In this study, we have performed large-scale molecular dynamics simulations of nano-indentation and wear on the γTi-Al alloy with a rigid diamond tip to investigate its mechanical properties as well as its wear mechanism. Our simulation results show that the Young’s modulus of the γTi-Al alloy calculated from nano-indentation is about 157.24GPa, which is in good agreement with other groups’ results. Our results also show that indentation speed has little influence on Young’s modulus in the range of 0.001–0.15Å/ps. In addition, the wear simulation results reveal that the material loss during sliding is linearly proportional to the sliding distance and the normal load, which is consistent with the empirical Archard’s law. Also, during sliding, there is a linear relationship between friction force and normal load. We hope that the present work will have a significant impact on the fundamental understanding of the mechanical properties and wear mechanism of γTi-Al material at the nano-scale.

The roles of crystallographic orientation, high-angle grain boundary, and indenter diameter during nano-indentation

Molecular dynamics simulation of nano-indentation of single-crystal and bicrystal FCC aluminum is performed. The role of crystallographic orientation during nano-indentation of single-crystal aluminum is assessed. Then, the influence of the presence of a grain boundary is analyzed by adding high-angle symmetric tilt boundaries of Σ5 /(100) and Σ5 /(100) parallel to the surface on which the indentation is performed. Furthermore, in both cases, the size of the indenter is changed to investigate how the surface curvature of the indenter affects the nano-indentation process. The results suggest that in each crystallographic orientation, the presence of a grain boundary increases the required force for indentation, while the distance of a grain boundary from the indentation surface could affect the increase in the required force. Simulations prove that the grain boundary acts as a source of generation and emission of dislocations and restricts the penetration of the indenter by limiting the slip band formation and plastic deformation. The dislocation emission from the grain boundary restricts the penetration of the indenter and limits the formation of the octahedral slip systems of type {111} and consequently increases the required force for indentation in bicrystals.

Depinning-controlled plastic deformation during nanoindentation of BCC iron thin films and nanoparticles

Nanoindentation is commonly employed to study mechanical properties of metallic specimen. In recent years, the use of this technique has been extended to study how mechanical properties of sub-micrometer scale specimens depend on size and dimensionality. In this work, we explore this dependence for α-Fe in molecular dynamics simulations of nanoindentation of thin films and nanoparticles. The simulations show that the strength poorly depends on the dimensions in the lateral directions and that the indentation curves of thin films and nanoparticles are alike. This is in contrast with recent observation in FCC metals. A detailed analysis of the dislocation mechanisms during the deformation reveals that ½ 〈111〉{110} dislocations are nucleated in six possible directions, but these remain pinned to their nucleation sites after nucleation. Load drops in the indentation curve, which corresponds to strain bursts, are attributed to depinning of these dislocations as full or half prismatic loops, i.e., the deformation is depinning-controlled. In nanoparticles, some of the dislocations depin partially, resulting in pure screw dislocations between the indenter and the lateral facet. The locality of the depinning-controlled deformation rationalizes the lack of size effect. Finally, we relate the dislocation mechanisms during the depinning-controlled deformation to the frequency and extent of strain bursts during nanoindentation of BCC specimens.

Mechanical and structure studies of Zr50Cu50 glass matrix composites during nano-indentation-a molecular dynamics study

In this paper we report molecular dynamics simulations of nano-indentation on Zr50Cu50 metallic glass matrix composite (14% crystalline volume fraction) at various strain rates. The objective of this paper is to investigate the effect of strain rate on the deformation behaviour and understand the deformation mechanism during deformation. Structural analysis during deformation is done by centro-symmetry parameter (CSP) studies. The load- displacement plots are drawn for the loading portion of indentation to analyze the deformation behaviour. It is found that strain rate has significant effect on the nature of the load- displacement plot. With increasing strain rate serrations decreased and flat load-displacement regime is observed with progress of indentation (~10 Å) at strain rate of 1 × 1011s−1. This could be due to atoms getting less time to get rearranged themselves so as to bear further load. Also, the structure studies by CSP indicated that, at low strain rates (2 × 1010s−1 and 5 × 1010s−1) there is significant plastic deformation of the crystallite as compared to that at higher strain rate value of 1 × 1011s−1 at a particular indentation depth. This indicates that there is load transfer from the glassy matrix to the crystallite much earlier at low strain rates. However, at indentation depths of 20 Å at all the strain rates amorphization of the crystallite is observed.

On the failure load and mechanism of polycrystalline graphene by nanoindentation.

Nanoindentation has been recently used to measure the mechanical properties of polycrystalline graphene. However, the measured failure loads are found to be scattered widely and vary from lab to lab. We perform molecular dynamics simulations of nanoindentation on polycrystalline graphene at different sites including grain center, grain boundary (GB), GB triple junction, and holes. Depending on the relative position between the indenter tip and defects, significant scattering in failure load is observed. This scattering is found to arise from a combination of the non-uniform stress state, varied and weakened strengths of different defects, and the relative location between the indenter tip and the defects in polycrystalline graphene. Consequently, the failure behavior of polycrystalline graphene by nanoindentation is critically dependent on the indentation site, and is thus distinct from uniaxial tensile loading. Our work highlights the importance of the interaction between the indentation tip and defects, and the need to explicitly consider the defect characteristics at and near the indentation site in polycrystalline graphene during nanoindentation.

Effect of boundary conditions on the MD simulation of nanoindentation

The present work investigates the effect of different boundary conditions on the simulation of nanoindentation using molecular dynamics (MD). The MD simulations of nanoindentation on Ni single crystal thin films are conducted using various boundary conditions and thicknesses. The coupling effects of indenter size and boundary conditions are studied using the spherical indenter with different radii. Silicon substrate is used as one of the boundary conditions. Three different atomic potentials including embedded-atom method (EAM), Tersoff, and Lennard–Jones (LJ) are incorporated to model the atomic interactions of Ni–Ni, Si–Si, and Ni–Si, respectively. The effect of boundary conditions on the elastic behavior of thin films with various thicknesses is investigated. The results show that the effect of boundary conditions on the elastic response is a function of the film thickness and indenter radius, such that as the thickness increases and the indenter radius decreases, the boundary conditions effects become less significant. Next, the nucleation and early evolution of dislocations in the simulated samples is addressed. Three different patterns of dislocation structures are observed which are governed by two mechanisms of indentation and bending. Finally, the incipient plasticity in thin films with different boundary conditions and thicknesses is investigated. The results show that the dislocation structure controls the mean contact pressure at the onset of plasticity.

Validation of classical force fields for the description of thermo-mechanical properties of transition metal materials.

It is demonstrated that classical force fields validated through the density functional theory (DFT) calculations of small titanium and nickel clusters can be applied for the description of thermo-mechanical properties of corresponding materials. This has been achieved by means of full-atom molecular dynamics simulations of nanoindentation of amorphous and nanostructured Ti and Ni-Ti materials. The theoretical analysis performed and comparison with experimental data demonstrate that the utilized classical force fields for Ti-Ti, Ni-Ni and Ni-Ti interactions describe reasonably well hardness and the Young's modulus of these materials. This observation is of the general nature and can be utilized for similar numerical exploration of thermo-mechanical properties of a broad range of materials.

Nanoindentation Simulation of Dislocation Evolution in Substrate/Film Systems

The dislocation evolution in substrate/film systems with Ag and Cu was investigated by the molecular dynamics simulation of nanoindentation. Film thickness in the range from 2 to 5 nm with the crystalline direction (001), which is the same as that of the substrate, was modelled in the present study. It was observed that the slip of the misfit dislocations in the interface is determined by indentation force and lattice constants of the substrate and the film. It was also shown that the crystal direction and the initial indentation position relative to misfit dislocation have a major role in the form of dislocation structures. The hardness of the bulk Cu is observed to be higher than that of the bulk Ag, as expected, however, it was also observed that soft film coating on the hard substrate cannot increase the hardness of the overall substrate/film system. On the contrary, the hard film, which is thicker than 4 nm, coating on the soft substrate increases the hardness of the overall substrate/film system.

Benchmarking of classical force fields by ab initio calculations of atomic clusters: Ti and Ni–Ti case

We propose a method for benchmarking classical force fields, which is based on the ab initio calculations of small atomic clusters. As a case study, we investigate the force fields which describe interactions between Ni and Ti atoms. By studying the stability of pure and Ni-doped titanium clusters within the classical and ab initio frameworks, we evaluate the accuracy of the existing classical interatomic potentials for the description of systems on the atomistic scale, where quantum effects play a crucial role. In order to check the ability of the force fields to reproduce correctly macroscopic parameters of transition-metal systems, we perform molecular dynamics simulations of nanoindentation of the crystalline Ni–Ti alloy. As a result of the analysis performed, we demonstrate that the existing classical force fields for Ti–Ti and Ni–Ti interactions describe adequately not only the macroscopic properties of the crystalline structure, but also point defects and local irregularities which may occur in a crystal.

Molecular dynamics simulation of nanoindentation of nickel-titanium crystal

We present the results of molecular dynamics simulations of nanoindentation of a bimetallic nickel-titanium crystal in the austenitic (cubic) B2 phase. By considering three different types of indenters, namely of square, conical and spherical shapes, we observe the dependence of deformations of the crystalline structure on the type of the indenter. Various load-displacement curves are observed for different indenter types. We perform the molecular dynamics simulations of a full indentation cycle, which includes the loading and unloading stages. On the basis of such simulations we evaluate mechanical properties of the material, namely we calculate hardness and reduced Young's modulus. We observe variation of the calculated parameters depending on the indenter type and discuss the origin of occurring discrepancies.

The evolution of machining-induced surface of single-crystal FCC copper via nanoindentation

The physical properties of the machining-induced new surface depend on the performance of the initial defect surface and deformed layer in the subsurface of the bulk material. In this paper, three-dimensional molecular dynamics simulations of nanoindentation are preformed on the single-point diamond turning surface of single-crystal copper comparing with that of pristine single-crystal face-centered cubic copper. The simulation results indicate that the nucleation of dislocations in the nanoindentation test on the machining-induced surface and pristine single-crystal copper is different. The dislocation embryos are gradually developed from the sites of homogeneous random nucleation around the indenter in the pristine single-crystal specimen, while the dislocation embryos derived from the vacancy-related defects are distributed in the damage layer of the subsurface beneath the machining-induced surface. The results show that the hardness of the machining-induced surface is softer than that of pristine single-crystal copper. Then, the nanocutting simulations are performed along different crystal orientations on the same crystal surface. It is shown that the crystal orientation directly influences the dislocation formation and distribution of the machining-induced surface. The crystal orientation of nanocutting is further verified to affect both residual defect generations and their propagation directions which are important in assessing the change of mechanical properties, such as hardness and Young's modulus, after nanocutting process.