Nanoindentation Simulations Research Articles

Orientation dependence of the nano-indentation behaviour of pure Tungsten

Coupling of nano-indentation and crystal plasticity finite element (CPFE) simulations is widely used to quantitatively probe the small-scale mechanical behaviour of materials. Earlier studies showed that CPFE can successfully reproduce the load-displacement curves and surface morphology for different crystal orientations. Here, we report the orientation dependence of residual lattice strain patterns and dislocation structures in tungsten. For orientations with one or more Burgers vectors close to parallel to the sample surface, dislocation movement and residual lattice strains are confined to long, narrow channels. CPFE is unable to reproduce this behaviour, and our analysis reveals the responsible underlying mechanisms.

Investigating the softening of weak interlayers during landslides using nanoindentation experiments and simulations

Based on nanoindentation experiments and numerical simulations, an approach is developed to investigate the softening of weak interlayers, which might trigger landslides. The components of weak interlayers in natural slopes are mainly rock fragments and clay, which lead to the difficulty of preparing intact samples for traditional rock mechanical experiments. Additionally, most rock slopes are microsensitive fractured geological media. The slope failures are significantly influenced by the anisotropic and rheological mechanical evolution of weak interlayers, which is strongly related to their microstructure. In this work, the shale in weak interlayers is investigated using nanoindentation experiments, so the experimental samples can be very small and possess arbitrary shapes. The experimental results at the microscale are upscaled using the Gaussian mixture model and Mori-Tanaka scheme, which are then used in numerical modeling to investigate the failure of a slope in southwestern China. According to the results, after water immersion, the friction angle of shales is almost constant, while the elastic modulus and cohesion decrease significantly. The shear strength of weak interlayers decreases significantly so that the plastic zones develop along the weak interlayer, which leads to landslides.

Advanced nanoindentation simulations for carbon nanotube reinforced nanocomposites

Mechanical properties of Carbon Nanotube (CNT) reinforced composites are obtained utilizing finite element (FE) method-based indentation simulations considering large strain elasto-plastic behavior of elements. This study includes nanoindentation simulations for chemically non-bonded CNT/matrix interface, including the length scale effect of nanocomposites. In order to investigate the mechanical properties of CNT reinforced nanocomposites, a number of FE models for nanoindentation tests have been simulated. Sample nanocomposites are examined to determine the suitable types of CNTs and their effectiveness as a reinforcement of different potential matrices. The Parametric study is conducted to obtain the influence of wall thickness, relative positioning, and volume fraction of CNT and strain hardening parameter of matrix on the mechanical properties of nanocomposites. The obtained results indicate that, properties such as modulus of elasticity and hardness of the nanocomposites are largely dependent on wall thickness of CNT and strain hardening parameter of the matrix. This study also suggests, the minimum wall thickness of CNT to avoid local buckling in nanocomposite which is required to be at least 0.2 nm for a diameter to thickness ratio of 5.0. Moreover, a matrix having a value of strain hardening parameter near 0.1 is expected to be significantly effective for nanocomposite.

Understanding the response of aluminosilicate and aluminoborate glasses to sharp contact loading using molecular dynamics simulation

Experimental studies have shown that glass systems with high boron content exhibit superior crack resistance under sharp contact loading. However, the underlying mechanism is still not fully understood. In this context, we carried out classical molecular dynamics simulations on sodium aluminosilicate and sodium aluminoborate systems to investigate the effect of boron on the response of glass to nanoindentation. A rigid V-shaped indenter is used to indent the glass sample with a fixed loading rate, during which the indenter interacts with the glass via a repulsive force field. The indenter angle and tip radius are varied to study the effect of indenter sharpness, as what has been done in experiments. These simulated nanoindentation tests reveal how the stress/strain field and the glass structure evolve with deformation underneath the indenter. It was found that a large number of boron atoms in the plastic zone change from three- to fourfold coordination during the loading process, and most of them revert back to the threefold coordination state during the unloading process. Our study shows that this “reversible” boron coordination change plays a critical role in increasing the damage resistance of glass.

Effects of Langevin friction and time steps in the molecular dynamics simulation of nanoindentation

ABSTRACT In the simplest form of molecular dynamics (MD), the system evolves according to Newton's equations of motion with its total energy conserved. Moreover, to model systems at constant temperatures, a modified set of equations of motion, called a thermostat, can be used. Langevin thermostat with one parameter called the Langevin friction coefficient is a popular thermostat, but problems may arise when it is applied to nonequilibrium systems without tuning the friction coefficient. Additionally, the size of MD time step also plays a pivotal role in determining accuracy and efficiency. In this study, we consider the effects of time step and Langevin friction on MD simulations. First, we evaluate the effectiveness of the Langevin thermostat for different combinations of time step and friction coefficient. Second, nanoindentation of Nickel crystals are carried out for five different combinations of Langevin friction and time steps. The simulation results reveal noticeable differences in mechanical properties as the dynamics changes from a constant energy to constant temperature simulations with increasing friction coefficients. The structural characteristics are also quantified using the common neighbour analysis, centrosymmetry parameter and dislocation analysis, all of which show a similar trend of more atoms being deformed with increasing friction.

Nanoindentation of ZrH2 by molecular dynamics simulation

Classical molecular dynamics simulations of nanoindentation on the (100) and (110) planes using a spherical indenter are performed to investigate the deformation behavior of δ-ZrH2. The Charged Optimized Many Body (COMB) potential is used to describe the interatomic interactions. The effect of the indenter speed and the thickness of the active layer to the nanoindentation are evaluated. We find that the nucleation and movement of {100} <110> dislocations are the main mechanisms of the inelastic deformation during nanoindentation on both the (100) and (110) planes. In addition, the load-displacement curve, hardness and deformation processes extracted from δ-ZrH2 nanoindentation on the (100) and (110) planes are analyzed.

Efficient and Reliable Nanoindentation Simulation by Dislocation Loop Erasing Method

Nanoindentation is a useful technique to measure material properties at microscopic level. However, the intrinsically multiscale nature makes it challenging for large-scale simulations to be carried out. It is shown that in molecular statics simulations of nanoindentation, the separated dislocation loops (SDLs) are trapped in simulation box which detrimentally affects the plastic behavior in the plastic zone (PZ); and the long-distance propagation of SDLs consumes much computational cost yet with little contribution to the variation of tip force. To tackle the problem, the dislocation loop erasing (DLE) method is proposed in the work to alleviate the influence of artificial boundary conditions on the SDL–PZ interaction and improve simulation efficiency. Simulation results indicate that the force–depth curves obtained from simulations with and without DLE are consistent with each other, while the method with DLE yields more reasonable results of microstructural evolution and shows better efficiency. The new method provides an alternative approach for large-scale molecular simulation of nanoindentation with reliable results and higher efficiency and also sheds lights on improving existing multiscale methods.

Numerical and experimental investigation on ductile deformation and subsurface defects of monocrystalline silicon during nano-scratching

Nano-scratching is an indispensable approach for investigating the ductile deformation and subsurface defects produced during nano-machining of hard, brittle material. In this study, three-dimensional molecular dynamic (MD) simulations of nano-indentation and nano-scratching were conducted on monocrystalline silicon using a diamond indenter with various tip radii. Under the same scratching speed, crystal orientation, and depth, comparative analyses were performed among the results of nano-scratching with indenter tip radii of 2, 4, and 6 nm. The interactions between C and Si atoms and the interactions among Si atoms are described in terms of the Tersoff empirical potential. The effects of the tip radius on the ductile deformation, scratching force, and subsurface defect thickness during the nano-scratching process were analysed in depth. Then, nano-scratching experiments using a self-developed device were conducted to verify the simulation results through a comparison. The Raman spectroscopy and electron backscattered diffraction techniques were used to detect the crystal structure evolution and subsurface defect thickness. The experimental results were observed to be in agreement with the MD simulation results.

Exploring the internal structure of soot particles using nanoindentation: A reactive molecular dynamics study

The mechanical properties and internal structure of soot nanoparticles is investigated using reactive molecular dynamics simulations of nanoindenting model soot particles. The particles that are provided as inputs to the simulations are generated using reactive molecular dynamics to create 3D networks of crosslinked coronene, circumanthracene and core-shell mixtures of coronene and circumanthracene. The results of the simulated nanoindentation experiments are analysed as a function of the degree of crosslinking (defined as the number of crosslinks per monomer in the particles), the size and the core-shell structure of the particles. In the case of homogeneous particles (i.e. those without a core-shell structure), the simulations show a unique relationship between the degree of crosslinking (CL) and the simulated hardness, Young’s modulus and deformation ratio. In the case of particles with a core-shell structure, a unique relationship was only found by considering the core-shell ratio and the degree of crosslinking in both the core and the shell. Our results allow for interpretation of the nanoindentation experiments as suggesting crosslinks are present in mature soot particles and preliminary evidence that crosslinks also are present within the interior of soot particles.

Atomistic-level study of the mechanical behavior of amorphous and crystalline silica nanoparticles

Molecular dynamics method was used to simulate and investigate the mechanical behavior of amorphous and crystalline silica during nanoindentation and uniaxial compression tests in the nanoscale. Also, the hardness and elastic moduli were experimentally checked by nanoindentation tests. The MD nanoindentation simulation results show that the amorphous silica has an indentation modulus less than the crystalline one, and the elastic strain of the amorphous silica is higher than crystalline silica. In contrast, the amorphous silica has a lower hardness. The uniaxial compression test results show that both structures have three deformation stages during the tests. The first stage corresponds to the linear elastic region; the second stage indicates the elastic-plastic region for crystalline silica, and yielding stage in the amorphous silica. The last stage corresponds to the fracture region for crystalline structure and densification for the amorphous structure. By comparing the area under stress-strain curves of the two structures, it can be concluded that the absorbed energy under external stress in the amorphous silica is three times greater than that in the crystalline one. Moreover, the nanoindentation tests of SPS samples made of amorphous and crystalline silica were performed, and the results confirmed that the crystalline silica has higher hardness and indentation modulus than the amorphous silica. Also, comparing the indentation moduli obtained by simulation and experimental tests shows that the indentation modulus predicted by simulation is in good agreement with that obtained based on experimental results.

Strength of Graphene-Coated Ni Bi-Crystals: A Molecular Dynamics Nano-Indentation Study.

Nanoindentation simulations are performed for a Ni(111) bi-crystal, in which the grain boundary is coated by a graphene layer. We study both a weak and a strong interface, realized by a and a twist boundary, respectively, and compare our results for the composite also with those of an elemental Ni bi-crystal. We find hardening of the elemental Ni when a strong, i.e., low-energy, grain boundary is introduced, and softening for a weak grain boundary. For the strong grain boundary, the interface barrier strength felt by dislocations upon passing the interface is responsible for the hardening; for the weak grain boundary, confinement of the dislocations results in the weakening. For the Ni-graphene composite, we find in all cases a weakening influence that is caused by the graphene blocking the passage of dislocations and absorbing them. In addition, interface failure occurs when the indenter reaches the graphene, again weakening the composite structure.

Atomic Simulation of Nanoindentation on the Regular Wrinkled Graphene Sheet.

Surface landscapes have vague impact on the mechanical properties of graphene. In this paper, single-layered graphene sheets (SLGS) with regular wrinkles were first constructed by applying shear deformation using molecular dynamics (MD) simulations and then indented to extract their mechanical properties. The influence of the boundary condition of SLGS were considered. The wrinkle features and wrinkle formation processes of SLGS were found to be significantly related to the boundary conditions as well as the applied shear displacement and velocity. The wrinkling amplitude and degree of wrinkling increased with the increase in the applied shear displacements, and the trends of wrinkling wavelengths changed with the different boundary conditions. With the fixed boundary condition, the degree of graphene wrinkling was only affected when the velocity was greater than a certain value. The effect of wrinkles on the mechanical characterization of SLGS by atomic force microscopy (AFM) nanoindentation was finally investigated. The regular surface wrinkling of SLGS was found to weaken the Young’s modulus of graphene. The Young’s modulus of graphene deteriorates with the increase in the degree of regular wrinkling.

The effect of crystal anisotropy and pre-existing defects on the incipient plasticity of FCC single crystals during nanoindentation

Molecular dynamics simulation is used to identify and quantify the initial plastic deformation mechanisms of gold as a model face-centered cubic (fcc) metal in the nanoindentation process. The coupling effects of crystallographic orientation and internal structural defects on the resulting load distribution at the onset of plasticity are investigated to clarify the anisotropic characteristics of material responses to crystallographic orientation. Homogenous defect nucleation is studied by correlating the indentation force-displacement curve with the instantaneous defect structure. In the absence of pre-existing defects, nanoindentation deformation is dominated by nucleation of Shockley partial dislocations regardless of crystal orientation. Various forms of dislocation propagation are observed in different crystal orientations. The elastic-plastic transition point appears later for the [111]-oriented surface than the ones for [001]-, and [011]-oriented surfaces. The relation of hardening and dislocation density shows that conventional Taylor hardening captures the plasticity after a certain amount of indentation depth in the presence of enough dislocation density. The crystal's sensitivity to the presence of internal structural faults is strongly dependent on the crystallographic orientation. Nanoindentation simulations in the presence of sessile dislocation loops in the structure show that the most significant reduction in the pop-in load happens for the [111] oriented sample. Our simulations suggest that the indentation near a defect can lead to small, subcritical events that lead to a smoother "pop- in" at the onset of plasticity. Since internal defects in materials are nearly inevitable, a defect-based model can be useful to understand the stochastic pop- in loads in nanoindentation tests.

Study on irradiation effect in stress-strain response with CPFEM during nano-indentation

Abstract The influence of irradiation on the constitutive stress-strain response of A508-3 steel is investigated by a series of nano-indentation simulations with Crystal Plasticity Finite Element Model (CPFEM). In this regard, the crystal plasticity constitutive model based on the densities of dislocations and irradiation defects for body-centered cubic (BCC) crystal of concern is summarized and numerically implemented at first. Moreover, the method that converts hardness vs. half-angle (θ) into stress vs. strain is introduced briefly. Based on the model, numerical results are challenged in comparison with experimental data and found in excellent agreement with the reported results, from which, the validity is verified. Meanwhile, we further analyze the depth-dependent irradiation damage distribution of ion irradiation and offer the equivalent value of irradiation damage in terms of displacement per atom (dpa) in the uniform fashion, which can definitely deepen the comprehension of the depth-dependent irradiation hardening of ion-irradiated A508-3 steel and guide relevant experimental work.

Dislocation Reaction Mechanism for Enhanced Strain Hardening in Crystal Nano-Indentations

Stress–strain calculations are presented for nano-indentations made in: (1) an ammonium perchlorate (AP), NH4ClO4, {210} crystal surface; (2) an α-iron (111) crystal surface; (3) a simulated test on an α-iron (100) crystal surface. In each case, the calculation of an exceptionally-enhanced plastic strain hardening, beyond that coming from the significant effect of small dislocation separations in the indentation deformation zone, is attributed to the formation of dislocation reaction obstacles hindering further dislocation movement. For the AP crystal, the exceptionally-high dislocation reaction-based strain hardening, relative to the elastic shear modulus, leads to (001) cleavage cracking in nano-, micro- and macro-indentations. For α-iron, the reaction of (a/2) &lt;111&gt; dislocations to form a [010] Burgers vector dislocation obstacles at designated {110} slip system intersections accounts for a higher strain hardening in both experimental and simulated nano-indentation test results. The α-iron stress–strain calculations are compared, both for the elastic deformation and plastic strain hardening of nano-indented (100) versus (111) crystal surfaces and include important observations derived from internally-tracked (a/2) &lt;010&gt; Burgers vector dislocation structures obtained in simulation studies. Additional comparisons are made between the α-iron calculations and other related strength properties reported either for bulk, micro-pillar, or additional simulated nano-crystal or heavily-drawn polycrystalline wire materials.

Pile-up and heat effect on the mechanical response of SiGe on Si(0 0 1) substrate during nanoscratching and nanoindentation using molecular dynamics

Molecular dynamics (MD) simulations are performed to study the mechanical behavior of SiGe thin film during nanoindentation and nanoscratching processes. The MD simulations of deposition and annealing process are conducted to create the Si80Ge20 thin film deposited on Si(0 0 1) substrate. MD simulations of nanoindentation and nanoscratching processes are carried out to study the influence of the temperature on deformation behaviors and mechanical properties of this material. Moreover, the influences of the scratching depth, tool velocity, and temperature on the deformation behavior during nanoscratching process are also surveyed. It is found that the elastic and plastic deformations occur and the shear transformation zones (STZs) are independently developed in the contact region. The force, hardness, and Young’s modulus of the Si80Ge20 thin film deposited on the Si substrate decrease as increasing the temperature during the indentation process. The pile-up, friction force, and normal force grow as increasing the scratching depth. As increasing the scratching speed, the pile-up increase while the normal force and the friction force reduce due to the effect of heat and strain rate are more efficient than the effect of the pile-up. As increasing the temperature, the height of pile-up increases while the normal force and the friction force decrease as a result of a higher temperature leads to softening the workpiece because the thermal expansion causes a reduction of the bonding strength of atoms.

Mechanical characteristics of Ni[sbnd]Ti[sbnd]Cu alloys from experiments and molecular dynamics simulations

In this article, we report investigations of the mechanical characteristics of NiTiCu alloys using experimental compression, nanoindentation, and molecular dynamics (MD) simulations. The results show that an amorphous pillar is harder to compress than a nanocrystalline one, with the hardness of the NiTiCu alloys we used ranging from 2.51 to 4.98 GPa. In the MD simulations, the substrate has a greater effect at the bottoms of the shorter pillars. Adhesion occurred at the upper surfaces of nanopillars that were 200 nm long. We also discuss the effects of size on the deformation response of the NiTiCu nanopillars.

Atomistic Simulations of Nanoindentation Response of Irradiation Defects in Iron

Radiation response of a material is a consequence of defects’ evolution in any radiation damage event. The radiation-induced defects can significantly alter the mechanical properties of a material. Radiation damage initiates from incident neutron by bombardment on solid material causing production and evolution of Frenkel defects. Since voids are formed due to aggregation of a large number of vacancies that cause dimensional changes and hence irradiation-induced swelling. In order to characterize the effect of irradiation defects, we have performed molecular dynamics (MD) simulations to investigate nanoindentation response of point defects and voids in Fe and their effects on mechanical parameters. The radial effect of voids and their interaction mechanism is also explored by nanoindentation simulation. It has been found that most of the dislocation produced are and during nanoindentation in all simulated models. There will be an increase in dislocation density which will harden the material and reduce its toughness. The mechanical parameters such as hardness H and reduced elastic modulus Er of irradiation defects are calculated from P-h curves. It is found that both H & Er of the point defects and voids are lower than the perfect model.

Modelling of Nanoindentation of TiAlN and TiN Thin Film Coatings for Automotive Bearing

Bearings are critical components for the transmission of motion in machines. Automotive components, especially bearings, will wear out over a certain period of time because they are constantly subjected to high levels of stress and friction. Studies have proven that coatings can extend the lifespan of bearings. Hence, it is necessary to conduct studies on coatings for bearings, particularly the mechanical and wear properties of the coating material. This detailed study focused on the mechanical properties of single-coatings of TiN and TiAIN using the finite element method (FEM). The mechanical properties that can be obtained from nano-indentation experiments are confined to just the Young’s modulus and hardness. Therefore, nanoindentation simulations were conducted together with the finite element method to obtain more comprehensive mechanical properties such as the yield strength and Poisson’s ratio. In addition, various coating materials could be examined by means of these nanoindentation simulations, as well the effects of those parameters that could not be controlled experimentally, such as the geometry of the indenter and the bonding between the coating and the substrate. The simulations were carried out using the ANSYS Mechanical APDL software. The mechanical properties such as the Young’s modulus, yield strength, Poisson’s ratio and tangent modulus were 370 GPa, 19 GPa, 0.21 and 10 GPa, respectively for the TiAlN coating and 460 GPa, 14 GPa, 0.25 and 8 GPa, respectively for the TiN coating. The difference between the mechanical properties obtained from the simulations and experiments was less than 5 %.

Mechanical Properties of Soot Particles: The Impact of Crosslinked Polycyclic Aromatic Hydrocarbons

ABSTRACT In this paper, we estimate the degree of crosslinking within soot particles making use of reactive molecular dynamics simulations of mechanical properties of crosslinked polycyclic aromatic hydrocarbons (PAH). Representative systems of PAH (pyrene, coronene, ovalene and circumpyrene) with a density similar to soot and with varying degrees of crosslinking were built. Uniaxial tensile test simulations were carried out on the systems and the yield stress of each sample was calculated. The hardness was estimated from the yield stress using an empirical conversion constant and the obtained values were compared with nanoindentation experiments of soot particles. The results show that mature ethylene and diesel soot particles are expected to present a degree of crosslinking between 2.1–3.0 and 3.0–3.5, respectively, to have a value comparable to the hardness found experimentally. Finally, an MD simulation of nanoindentation of a particle of crosslinked coronene molecules provided an alternative means to compute the empirical constant used to convert the yield stress in hardness. These results reveal the importance of crosslinking reactions during soot maturation that give rise to a structure in which the majority of aromatics are aliphatically linked in a 3D network.