Molecular dynamics simulations on nanoindentation mechanisms of multilayered films

Molecular dynamics (MD) simulations were carried out to study the effects of indention deformation, contact, and adhesion on Al, Ni, and Al/Ni multilayered films. The results show that when the indention depth of the sample increased, the maximum load, plastic energy, and adhesion increased. Jump-contact behavior was observed at the beginning of the loading process. Force relaxation and adhesion took place at the holding depth and during the unloading process, respectively. The glide bands of the interface were on the {1 1 1} 1 1 0 slip systems and the maximum width of the glide bands was about 1 nm. The mechanical responses of the indented films are also discussed.     

基于分形理论的两粗糙表面接触的黏滑摩擦模型EI北大核心CSCD

两粗糙表面的接触本质上是大量微凸体的接触,具有复杂的力学行为,连接界面的力学建模是重要的科学问题。从微观角度出发,对单个微凸体进行接触分析,并考虑了微凸体相互作用造成的基底面的下降,根据分形理论积分,建立了整个接触面的法向接触模型。利用该模型,可确定在给定法向预紧载荷下微接触截面积的概率密度函数;根据Mindlin模型、Masing准则和分形理论,建立了两粗糙表面接触的切向载荷与切向位移的关系,并研究了不同参数对系统能量耗散的影响。数值仿真结果表明,能量耗散随分形维数D增大而增大,随分形粗糙度参数G及法向预紧力增大而降低。     

Simulating scratch behavior of polymers with mesoscopic molecular dynamics

Part replacement and repair is needed in structures with moving parts because of scratchability and wear. In spite of some accumulation of experimental evidence, scratch resistance is still not well understood. We have applied molecular dynamics to study scratch resistance of amorphous polymeric materials through computer simulations. As a first approach, a coarse grain model was created for high density polyethylene at the mesoscale. We have also extended the traditional approach and used real units rather than reduced units (to our knowledge, for the first time), which enable an improved quantification of simulation results. The obtained results include analysis of penetration depth, residual depth and recovery percentage related to indenter force and size. Our results show there is a clear effect from these parameters on the tribological properties. We also discuss a "crooked smile" effect on the scratched surface and the reasons for its appearance.     

A combined method of molecular dynamics with micromechanics improved by moving the molecular dynamics region successively in the simulation of elastic--plastic crack propagation

Molecular dynamics is applicable only for a small region of simulation. To simulate a large region it is necessary to combine molecular dynamics with continuum mechanics. Previously we proposed a new model in which molecular dynamics was combined with micromechanics. A molecular dynamics model was applied to the crack tip region and a micromechanics model to the surrounding region. In that model, however, crack propagation simulation must be stopped when the crack tip reaches the boundary of the two regions. In this paper the previous model is improved by moving the molecular dynamics region successively with crack propagation. The improved model may be applied to simulate limitless crack propagation. In order to examine the validity of the improved model, we simulate α-iron. The calculation cost with the improved model is less than a tenth of that of the previous model although the results are equal to each other. The crack tip opening displacement calculated with this model is almost equal to the analytical solution derived by Rice. This revised version was published online in July 2006 with corrections to the Cover Date.     

Thickness and chirality effects on tensile behavior of few-layer graphene by molecular dynamics simulations

The mechanical response of few-layer graphene (FLG), consisting of 2-7 atomic planes and bulk graphite is investigated by means of molecular dynamics simulations. By performing uniaxial tension tests at room temperature, the effects of number of atomic planes and chirality angle on the stress-strain response and deformation behavior of FLG were studied using the Tersoff potential. It was observed that by increasing of the FLG number of layers, the increase of bonding strength between neighboring layers reduce the elastic modulus and ultimate strength. It was found that, while the chirality angle of FLG showed a significant effect on the elastic modulus and ultimate tensile strength of two and three graphene layers, it turns to be less significant when the numbers of layers are more than four. Finally, by plotting the deformation behavior, it was concluded that FLGs present brittle performance.     

Lattice dynamics and molecular dynamics simulation of complex materials

In this article we briefly review the lattice dynamics and molecular dynamics simulation techniques, as used for complex ionic and molecular solids, and demonstrate a number of applications through examples of our work. These computational studies, along with experiments, have provided microscopic insight into the structure and dynamics, phase transitions and thermodynamical properties of a variety of materials including fullerene, high temperature superconducting oxides and geological minerals as a function of pressure and temperature. The computational techniques also allow the study of the structures and dynamics associated with disorder, defects, surfaces, interfaces etc.     

Atomistic simulation of nanoindentation behavior of amorphous/crystalline dual-phase high entropy alloys

High-entropy alloys(HEAs)are a new type of multi-principal metal materials that exhibit excellent me-chanical properties.However,the strength-ductility balance in the HEAs remains a challenge that needs to be addressed.The amorphous/crystalline(A/C)structure is a new design strategy to achieve high strength and excellent ductility of the HEAs.Here,the influences of amorphous layer spacing,indenter velocity,and indenter radius on the mechanical properties and microstructure evolution of the A/C dual-phase CoCrFeMnNi HEAs under nanoindentation were investigated by molecular dynamics(MD)simula-tion.The results indicate that the plastic deformation mechanism of the monocrystalline HEAs is mainly dominated by the nucleation and slip of dislocations,while the plastic deformation mechanism of the dual-phase HEAs is mainly dominated by the interaction between dislocations and amorphous phases.The results show that the average indentation force of the dual-phase HEAs increases with the increase of the amorphous layer spacing.The amorphous layer in the HEAs can hinder the expansion of disloca-tions,limiting them to the crystalline matrix between the two amorphous layers.The results also indicate that Young's modulus of the HEAs increases with the increase of the indentation velocity and indentation radius.However,the hardness of HEAs is positively correlated with the indenter velocity,and negatively correlated with the indenter radius.It should be noted that the critical indentation depth and critical in-dentation force for the plastic deformation of the dual-phase HEAs decrease with the increase of indenter velocity,which is opposite to that of the single-phase crystalline HEAs.     

Molecular dynamics simulation of incipient plasticity of nickel substrates of different surface orientations during nanoindentation

The present study aims to elucidate the anisotropic characteristics in material responses for crystallographic nickel substrates with (001), (011) and (111) surface orientations during nanoindentation. Molecular dynamic simulation is applied to compensate for the experimental limitation of nanoindentation, particularly for pure nickel substrates. Defect nucleation and evolution in Ni single crystal of these three crystal orientations was examined. Hardness and Young’s modulus are also extracted in different orientations. The Young’s modulus of (111) crystallographic orientation is the largest, while that of (001) surface is the smallest. The sensitivity of the yield point for face centred cubic crystals depends on the crystallographic orientation. The (001) crystallographic orientation reaches the yield point first, while the (111) crystallographic orientation is the most difficult in which to achieve yield. Using a visualisation method of centrosymmetry parameter, the homogeneous nucleation and early evolution of dislocations were investigated, deepening understanding of incipient plasticity at the atomic scale. The present results suggest that defect nucleation and evolution are the root of curve jitter. The indentation depth of the elastic–plastic transition point varies in the different crystallographic orientation models, and appears latest in the (111) model. The strain energy of the substrate exerted by the tip is stored by the formation of homogeneous nucleation and is dissipated by the dislocation slide in the {111} glide plane. The three nickel substrates with different crystallographic orientations exhibit different forms of dislocation propagation.     

An investigation of the transport and sorption properties of xenon in ferrierite and zeolite-L using molecular dynamics

Summary Molecular dynamics has been used to study the diffusion of xenon in ferrierite and zeolite-L. It was found that at 298 K and a loading level of 1.33 atoms per unit cell, diffusion down the 10-ring channel in ferrierite is a more facile process than down the wider 12-ring channel in zeolite-L (D = 8.90 x 10^-9 m²/s for ferrierite vs. 1.78 x 10^-9 m²/s for zeolite-L). This effect can be rationalised by consideration of the effect of channel shape on the diffusion pathway. Under the same conditions, the heat of sorption was calculated to be more favourable for ferrierite (U_ads = -25.7 kJ/mol vs. -20.0 kJ/mol).     

Nanoindention study of indium nitride thin films grown using RF plasma-assisted molecular beam epitaxy

In this study, we used an RF plasma-assisted molecular beam epitaxy (RF-MBE) system to grow single-crystalline indium nitride (InN) films onto aluminum nitride (AlN) buffer layers on Si (111) substrates. We then used nanoindentation techniques and reflection high-energy electron diffraction (RHEED) to study the influence of the c-axis-oriented InN films on the mechanical performance. From morphological observations, we compared the stiffness and resistance against contact-induced damage of the InN films in the presented shrinkage of the area. InN films prepared at growth temperatures of 440, 470, and 500 °C had nanohardnesses (H) of 3.6 ± 0.2, 4.5 ± 0.25, and 9.1 ± 0.8 GPa, respectively, and Young’s moduli (E) of 97.4 ± 1.2, 147.7 ± 1.8, and 176.0 ± 2.3 GPa, respectively.     

Interaction of lattice dislocations with a grain boundary during nanoindentation simulation

Interaction of dislocations with a Σ = 5 (210) [001] grain boundary was investigated using molecular dynamics simulation with EAM potentials. The results showed that the dislocation transmitted across the grain boundary during nanoindentation and left a step in the boundary plane. Burgers vector analysis suggested that a partial dislocation in grain I merged into the grain boundary and it was dissociated into another partial dislocation in grain II and a grain boundary dislocation, introducing a step in the grain boundary. Simulation also indicated that, after the transmission, the leading partial dislocation in the grain across the boundary was not followed by the trailing partials, expanding the width of the stacking fault. The results suggested that the creation of the step that accompanied grain boundary motion and expansion of the stacking fault caused resistance to nanoindentation.     

Critical size, recovery, and mechanical property of nanoimprinted Ni-Al alloys investigation using molecular dynamics simulation

The nanoimprinting process of nickel-aluminum (Ni-Al) alloys is studied using molecular dynamics (MD) simulations based on the many-body tight-binding potential. The effects of the temperature, loading and unloading velocities, holding/dwelling time, and composition of Ni-Al alloys are evaluated in terms of molecular trajectories, imprinting force, potential energy, stress, slip vector, and elastic recovery ratio. Simulation results show that the imprinting force increases with decreasing temperature and increasing loading velocity and Ni content. The average potential energy of a specimen decreases and its stress increases with increasing loading velocity. Slip planes of (1 1 0) and (1 1 0) form during Ni-Al alloy imprinting. During unloading, the adhesion force increases with increasing unloading velocity. The formability of a Ni-Al alloy can be enhanced by increasing the Ni content. Elastic recovery for a pattern can be avoided by decreasing the imprinting temperature and increasing the holding time. At a critical line width of 6.5 nm, elastic recovery ratios can be maintained in a range of 8-11%.     

Mechanical properties of carbon nanotube by molecular dynamics simulation

The mechanical properties of single-walled carbon nanotube (SWCNT) are computed and simulated by using molecular dynamics (MD) in this paper. From the MD simulation for an armchair SWCNT whose diameter is 1.2 nm and length is 4.7 nm, we get that its Young modulus is 3.62 TPa, and tensile strength is 9.6 GPa. It is shown that the Young modulus and tensile strength of armchair SWCNTs are 12 order higher than those of ordinary metal materials. Therefore we can draw a conclusion that carbon nanotubes (CNT) belong to a particular material with excellent mechanical properties.     

A molecular dynamics approach in simulations of fluid-solid particles flow

In the paper a discrete system of particles carried by fluid is considered in a planar motion. The volumetric density of particles is assumed to be small enough such that they can be treated within the framework of a molecular dynamics model. The fluid is then considered as a carrier of particles. The Landau-Lifshitz concept of turbulence is used to describe the fluctuating part of fluid velocity. This approach is applied to simulate different regimes (laminar and turbulent) and various states of particle motion (moving bed, heterogeneous flow, and homogeneous flow) using only two parameters, which have to be determined experimentally. These two parameters, found for a particular pipe and for a particular velocity from a simple experiment, then can be used for other pipe diameters and different velocities. The computer simulations performed for the flow of particles in pipes at different flow velocities and different pipe diameters agree favorably with experimental observations of the type of flow and critical velocities identifying transitions from one type to another. Received: 8 January 1999     

Orientation and strain rate dependent tensile behavior of single crystal titanium nanowires by molecular dynamics simulations

Molecular dynamics simulation was employed to study the tensile behavior of single crystal titanium nanowires(NWs)with1120,1100and[0001]orientations at different strain rates from 10~8s~(-1)to10~(11)s~(-1).When strain rates are above 10~(10)s~(-1),the state transformation from HCP structure to amorphous state leads to super plasticity of Ti NWs,which is similar to FCC NWs.When strain rates are below 10~(10)s~(-1),deformation mechanisms of Ti NWs show strong dependence on orientation.For1120orientated NW,1011compression twins(CTs)and the frequently activated transformation between CTs and deformation faults lead to higher plasticity than the other two orientated NWs.Besides,tensile deformation process along1120orientation is insensitive to strain rate.For 1100orientated NW,prismaticaslip is the main deformation mode at 10~8s~(-1).As the strain rate increases,more types of dislocations are activated during plastic deformation process.For[0001]orientated NW,1012extension twinning is the main deformation mechanism,inducing the yield stress of[0001]orientated NW,which has the highest strain rate sensitivity.The number of initial nucleated twins increases while the saturation twin volume fraction decreases nonlinearly with increasing strain rate.     

Large-scale molecular dynamics simulations of hyperthermal cluster impact

Using multimillion-atom classical molecular dynamics simulations, we have studied the impact dynamics of solid and liquid spherical copper clusters (10–30 nm radius) with a solid surface, at velocities ranging from 100 m/s to 2 km/s. The resulting shock, jetting, and fragmentation processes are analyzed, demonstrating three distinct mechanisms for fragmentation. At early times, shock-induced ejection and hydrodynamic jetting produce fragments in the normal and tangential directions, respectively, while sublimation (evaporation) from the shock-heated solid (liquid) surface produces an isotropic fragment flux at both early and late times.     

Nanometric cutting of copper: A molecular dynamics study

Molecular dynamics (MD) simulations were carried out to study the nanometric cutting of copper. In our approach, the many-body EAM potential was used for the atoms interaction in the copper workpiece. The effect of the tool geometry on the cutting process was investigated. It is observed that with negative rake angle, the chip becomes smaller due to the larger plastic deformation generated in the workpiece. It is shown that as the rake angle changes from −45° to 45°, the machined surface becomes smoother. Besides, both the cutting forces and the ratio of normal force to tangential force decrease considerably with the rake angle changing from negative to positive. In addition, MD simulations with the two-body Morse potential instead of the EAM potential were also carried out to study the effect of different potentials on the simulation results. It is found that there is no big difference in the simulated chip formation and the machined surface under the two different potentials. However, the Morse potential results in about 5–70% higher cutting forces than the EAM potential. It is recommended that the EAM potential should be used for the MD simulations of nanometric machining processes.     

Nanomechanics of imperfectly straight single walled carbon nanotubes under axial compression by using molecular dynamics simulation

The buckling characteristics of several curved forms of single walled carbon nanotubes (SWCNTs) were studied in this work via molecular dynamics simulation method. The structural morphology of the CNT was modified to induce curvature along the tube axis. The nature of mechanical properties of these classes of CNTs under compression deviates from the ideal ‘perfectly straight’ CNTs. We found that the inclusion of curvature along the tube axis can significantly impact the performance of the carbon nanotube under compression. These curvilinear CNTs, when combined with other CNTs to form a bundle, will have a greater weakening effect on the mechanical performance of the CNT bundle.     

Molecular dynamics and atomistic based continuum studies of the interfacial behavior of nanoreinforced epoxy

In this study, we investigate the interfacial mechanical characteristics of carbon nanotube (CNT) reinforced epoxy composite using molecular dynamics (MD) simulations. The second-generation polymer consistent force field (PCFF) is used in the MD simulations. In particular, we compare MD results with those obtained by atomistic-based continuum (ABC) multiscale modeling technique, which makes use of the appropriate constitutive relations derived solely from interatomic potentials. The results of our comparative investigation suggest that (i) the ABC multiscale model and MD simulation provides almost identical predictions for the interfacial properties of the nanocomposite for smaller diameter of CNTs, (ii) the ABC model slightly over predict the interfacial properties of the nanocomposite for larger diameter of CNTs, and (iii) the MD simulations represents the real nanocomposite structure with the minimum assumptions compared to that of the ABC multiscale model but with much greater computer requirements and limited length scale.