Nano-indentation Process Of Multi-scale Quasi-continuous Media Method

Nanostructural materials have been the subject of intensive research in recent years due to its unique mechanical properties, such as high strength, hardness, superior wear resistance, high tensile ductility and superplasticity at relatively low temperatures. Research has shown that these unique mechanical properties are closely related to internal structure of nanomaterials and deformation mechanism. Nanoindentation has become a standard technique for evaluating the mechanical properties of thin film. The load-displacement response obtained from nanoindentation test can be used to predict the material properities, understand the nature of the elastic-plastic deformation mechanism, and reveal the relationships between the microstructure and the macroscopic mechanical properties. However, nanoindentation is a complicated contact problem, which can be strongly influenced by surface roughness, substrate effects, grain boundaries effects, indenter geometry, crystalline anisotropy and indentation size effect. The repeatability and reproducibility of nanoindentation test result are poor, even if the experimental equipment and condition are the same. Therefore, it is very important to study the incipient plasticity during nanoindentation by multiscale atomic simulation.To further study the microscopic failure process and the elastic-plastic deformation mechanism of thin film, the quasicontinuum method (QC) is employed to elucidate the details of incipient plasticity during nanoindentation. The influences of initial defect, crystalline anisotropy, indenter size, interface between adjacent layers and interfacial structure on nanoindentation are discussed, respectively. The main contents of this paper are as follows:(1) Effects of initial defect on nanoindentation. The nanoindentation processes under influences of the initial defect are investigated about dislocation nucleation, dislocation emission, Peierls stress, and load necessary for dislocation emission. The results demonstrate that the load versus displacement response curves experience many times abrupt drops with the emission of dislocations beneath the indenter. The initial defect is found to be insignificant on nucleation and emission of the 1st and 3r dislocation dipoles, but has a distinct effect on the 2nd dislocation dipole. The nucleation and emission of the 2nd dislocation dipole is postponed obviously because of the effect of initial defect, and then crack propagation is accompanied. The strain energy of thin film and Peierls stress of dislocation dipole beneath the indenter are increase with deformation processes due to the severe lattice distortion in the thin film. Before the cleavage occurs, the load necessary for the 2nd dislocation dipole nucleation and emission increases in nanoindentation with initial defect, on the contrary, it decreases after the cleavage occurred. The nanohardness and Peierls stress in this simulation show a good agreement with relevant theoretical and experimental results.(2) Effects of crystalline anisotropy and indenter size on nanoindentation. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load-displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and Rice-Thomson dislocation model solution.(3) Effects of interface between adjacent layers on nanoindentation. The nature of strengthening and weakening effects of interface on Cu/Ag bilayer film and the underlying deformation mechanisms during nanoindentation are revealed from both the numerical and theoretical aspects. The investigations show that there is not only a strengthening effect of interface on Cu/Ag bilayer film system, but also a weakening effect. Concerning the strengthening effect, it is governed primarily by the resistance to the glide dislocation transmission, such as Image force, Peierls-Nabarro force and the repulsive force between the glide dislocation and the misfit dislocation. The bigger resistance will lead to the stronger strengthening effect. With regard to the weakening effect, it is produced by the stress concentration and local misfit strain in the core region of the misfit dislocations due to the nucleation and propagation of misfit dislocations along the interface, which can impair significantly the binding strength between adjacent layers. However, compared with the weakening effect induced by the nucleation and motion of misfit dislocations, it must be emphasized that the strengthening effect of interface on Cu/Ag bilayer film system is predominant. The multiscale simulation results are in good agreement with the experimental results and dislocation theory model.(4) Effects of interfacial structure on nanoindentation. The influences of interfacial structure on nanoindentation of Cu-Ag bilayer film system are analyzed systematically about glide dislocation nucleation and emission, nanohardness and strain energy. The results show that the nucleation and emission of glide dislocations are postponed obviously because most of the indentation energy is absorbed by the interface between adjacent layers. The mechanical property of Cu-Ag bilayer film system strongly depends on the performance of upper thin film. The interfacial structure is found to have distinct effect on critical load and critical indentation depth for first glide dislocation emission and nanohardness of bilayer film system. Due to the softening effect of interface, a reverse Hall-Petch phenomenon is typically observed in Cu-Ag bilayer film system. The complicated dislocation configurations caused by different interfacial structure during nanoindentation can lead to a greatly change in strain energy. In particular, during the interaction between glide dislocations and interface, the strain energy-displacement curve will represents an abrupt jump.