Molecular Dynamics Investigations Of Temperature Effects On The Nano-indentation/Scratching Response Of Typical Single Crystal Materials

Nano-indentation /scratching are the main testing technology in testing the micro mechanical properties of materials. Molecular dynamics(MD) can be used to simulate the nano-indentation/scratching process to investigate the mechanical properties of materials in nanoscale, to analyze the micro deformation and failure mechanism and to explain the new phenomenon of material’s mechanical response at micro scale. Recently, as an important supplement to the test methods of nano-indentation/ scratching, the MD investigations have become research hotspots for domestic and foreign scholars.The material mechanical properties are affected by various factors and the temperature is one of an important factor. The research of temperature effects on material mechanical behaviors and failure mechanism has attracted widespread attentions from domestic and foreign scholars. However,it is still not deep enough for the research on the evolution mechanism of typical single crystal materials’ micro mechanical properties(such as single crystal silicon and copper) at different temperatures, especially the research of material performance in cryogenic temperatures is rare.Therefore, focusing on the above problems, MD models are conducted to simulate the process of nano-indentation of monocrystalline silicon and nano-scratching of single crystal copper at different temperatures. The mechanical properties, plastic deformation, phase transformation and the generation and evolution of defects are researched. The main aspects of the research are included as following:In the paper, MD simulations of nano-indentation on monocrystalline silicon are conducted at temperature 10, 100, 200 and 300 K respectively. Then it studies the mechanical properties and phase transformed mechanism at cryogenic temperatures and room temperature, and the deformation mechanism is explained. The nano-indentation performed on non-periodic boundary condition is investigated; the potential function without adhesion is used to depict the interaction between indenter atoms and specimen atoms. Then the simulation results are compared with previous ones. It finds that the changing of boundary condition and potential function will affect the mechanical properties of silicon to a certain extent, such as the number of transformed atoms and the surface hardness, however,it not affects the deformation mechanism of the material and the general conclusion that the temperature effects on mechanical behaviors of single crystal silicon.Through the distribution of transformed silicon atoms at different temperatures, the temperature effect on anisotropic property is analyzed that the material anisotropic property is more significant at cryogenic temperature. The nano-indentation is performed on three typical silicon crystal surfaces(010)(110)(111) at four different temperatures. The elastic modulus of three typical silicon crystal surfaces at four temperatures are calculated, especially 84.2、102.7、110.9GPa;86.4、103.4、110.8GPa;92.7、108.9、121.1GPa;95.7、111.4、112.2GPa, and it is found that the elastic modulus difference between the crystal surfaces increase with temperature decreasing, indicating that the anisotropic property enhances at low temperature.MD simulations of nano-scratching test on single crystal copper are also carried out at different temperatures. The relationship between the fluctuation of load displacement curve and the plastic deformation of materials as well as the evolution of internal defects in materials is studied qualitatively. The internal dislocation is extracted by CNA and DXA technology, and the evolution of dislocation is analyzed. The temperature effects on surface topography and the internal defects are researched. It also finds that the temperature has effects on slip surfaces and slip directions of the internal dislocations but will not change the slip system of the crystal structure. Due to the combined influence of dislocation and temperature, the changes of the scratching force and normal force at different temperatures are not significant, as well as the change of friction coefficient.The research work of this paper has important significance in revealing the temperature effects on mechanical response of the typical single crystal material, and in the prediction analysis of the service performance under different temperature conditions.