Based On The Surface Of The Concave Type Nano Indentation Multi-scale Study

As the development of Micro-electronics and Microsystems, the properties of materials in nanometer scales have been the subject of research in recent years. The investigation shows that the properties of materials always perform different characters from those in macroscopic condition when its size is reduced to nanometer scale, such as high strength, hardness, high tensile ductility, superior wear resistance and superplasticity at relatively low temperatures. So it is significant to probe the property of nanostructual materials.Because traditional methods based on continuum theories do not work in investigating the mechanism of micro-deformation, as a standard technique for evaluating the mechanical properties of thin film, nanoindentation has been applied by many scientific workers. However, nanoindentation experiment can be strongly influenced by many factors, such as surface roughness, substrate effects, grain boundaries effects, indenter geometry and crystalline anisotropy. Furthermore, Microcosmic methods such as Molecular Dynamics can’t be used to simulate a large size of models in common PCs, which results from its large computational freedom. Consequently, the multi-scale methods coupling atomistic scale and continuum scale are widely applied in computer simulations. Quasicontinuum method belongs to the multiscale approaches that keep an atomistic description at highly deformed regions of a material while far away from this dislocation core a linear elastic continuum method is implemented. It uses continuum assumption to reduce the degrees of freedom and computational demand without losing atomistic detail in regions where it is required.Considering the great significance of studying a pit surface, Quasi-continuum Method is employed to investigate the influence of surface pit defect on the details of incipient plasticity in nanoindentation. The works of this thesis are as follows:(1) Multiscale simulations are performed to understand the nature of delay effect of dislocation nucleation with surface pit defect on the post yield process in nanoindentation. Compared with the nanoindentation on defect-free surface, eight different distances of adjacent boundary between surface pit defect and indenter are taken into account. The investigation shows that the delay effect of dislocation nucleation with surface pit defect is performed by2%～22%, which actually results from the emission of [110] perfect dislocation. The reason of such perfect dislocation emission is exactly a strain competition between the two places of1st and2nd largest strain concentration near the load surface. In addition, the spatial extent of influence of delay effect is found to be three times the half contact width of indenter, which is in good agreement with the experimental results.(2) The nanoindentation process of single Al thin film has been simulated in order to study the nature of distance influence of surface pit defect on nanohardness. Various distances between surface pit defect and indenter have been taken into account. It is found out that as the distance between surface pit defect and indenter increases, nanohardness increases in a wave type associated with a cycle of three atoms, which is closely related to periodic atoms stacking sequence on{111} atomic close-packed planes of face-centered cubic metal; when the adjacent distance between surface pit defect and indenter is more than16atomic spacing, there is almost no effect on nanohardness. In addition, the theoretical formula for necessary load of the first dislocation emission of Al film has been modified with initial surface pit defect, which has important directive to the investigation of nanohardness with surface defects.(3) Various width and height of surface pit defect have been taken into account to investigate the size effect of surface pit defect on the yield load of thin film. The results show that as the size coefficient of width (of height) increases, the yield load of thin film at first extremely slowly decrease, until the size coefficient of width equals approximately one unit (half unit), the yield load experiences an obvious drop. When the size coefficient of width (of height) reaches approximately two units (one unit), the yield load is almost the same with the one of the nanoindentation on a stepped surface. The variation of yield load has been proved by the estimation of Peierls stress, which is in an agreement with the experimental results. In addition, the height of surface pit defect makes more influence than the width on the yield load of thin film.