Multi-scale And Finite Element Analysis Of Nano-scale Contact And Friction Behavior For Crystal Aluminum

The micro-scale contact behavior reveals new characteristics that different from macro contact problem and contact mechanical problems involved in MEMS directly relevant to the design, manufacture, life and other aspects of MEMS. However, the real-time dynamic process of atomic motion and migration which trigger a series of deformation mechanisms in material deformation and fracture at angstrom scale is impossibly observed. Therefore, numerical simulation methods are explored to investigate the atomic deformation. As it is known that the part of MEMS is usually around1to100micrometers, it is evident the research scope of molecular dynamic (MD) method is incapable of continuum scale. The limitations of MD method urge the continuous exploring on the multiscale method.This topic is a part of the project "study on multi-scale coupling analysis method of contact mechanical behavior of friction pair in MEMS ", which is supported by the National Natural Science Foundation of China. As a developed multiscale method, the quasicontinuum(QC) method is introduced to investigate the micro-mechanisms of mechanical behavior in contact pair with types of planar and planar surfaces, spherical and planar surfaces of sliding contact process and then demonstrate the feasibility of the application of the multi-scale method to the contact and friction behavior in micro/nano scale with taking the metal material crystal aluminum as the research object.First of all, the contact process with three different sized indenter by types of planar and planar surfaces is simulated using the QC method. Through analyzing the obtained respond load-depth curve, we researched the nucleation and emission of dislocations in the substrate. The Rice-Thomson theoretical results is made to validate the numerical results. In order to explore the common tribopair with types of spherical and planar in micro/nano scale, the repulsive force-field approach is utilized in QC method. Therefore, a muti-scale modeling of nano-contact process between rigid spherical indenter and surface of single crystal aluminium is established. The result shows that due to the fact that the direction of force on atoms that contact the indenter is constantly changing with the increase of contact depth, the corresponding load-depth shows that step increases which is different from that of the square indenter with the response of the underlying crystal. Due to the indenter geometry, the close packed planes under both sides of indenter slip partially lead to Shockley partial dislocations. In the process of disengagement, the elastic recovery is accomplished as the atoms move up with indenter. The residual depth is0.3nm, which is closely to the magnitude of the Burger vector,0.285nm.Aiming at researching the friction behavior between the spherical surface and planar surface, the QC method is also introduced to investigate the process of nanocontact and nanoscratch between a large tip and a single aluminum substrate with low computation cost, simulation results show that the elastic recovery of scratched surface starts to appear obviously when scratching distance is of3.8nm with the glide band expanding and localized strain sharply increasing in nanoscratch process. As a result, part of dislocations underlying surface extend to surface and form twinning deformation. Meanwhile, the crystal lattice deep beneath surface undergoes severe distortion and rotation. A comparison of friction coefficient for scratch process is also made between theoretical values and our numerical results. The effect of elastic strain energy feedback on friction coefficient in elastic recovery of scratched surface has been quantitatively analyzed.Then, the nanoindentation and nanoscratch experimental study on crystal aluminum is also done by utilizing nanoindenter and AFM(atomic force microscope), results show that the trend of load-depth curve is quite consistent with our multi-scale simulation result. In addition, the nanoscratch experiments indicate the friction force increase with the sliding velocity, which verify our previous conclusion of theoretical calculations.At last, in order to get the plastic mechanics characteristics of crystal aluminium used in our experiment, the finite element analysis software ABAQUS, which is famous for its strong capacity of non-linear analysis, is introduced to simulate the process of nanoindentation. With fitting curves of load-depth from ABAQUS to it from our respond experiments, we obtained the yield stress-plastic strain curve which shows the plastic mechanics characteristics of crystal aluminium in our experiments and analyzed the microscratch process by utilizing the ABAQUS.