Plastic Deformation Mechanism Of Nano-polycrystalline Diamond Under Indentation

The hardness of nano-polycrystalline diamond is higher than that of single-crystal diamond,and it is expected to be widely used under high stress conditions such as ultra-high-pressure apparatus.Therefore,studying the plastic deformation and the corresponding mechanisms of nano-polycrystalline diamond has aroused great interest.In this thesis,molecular dynamics method is used to study the plastic deformation of nano-polycrystalline diamond under indentations.Different diamond models are constructed,and rigid indenters are pressed into the surfaces of these diamond materials to obtain the hardness as well as the relationship between the loading force and indentation depth.Based on the analysis of the internal microstructures of the nano-polycrystalline diamond during the indenting process,the plastic deformation mechanisms under the rigid indenter loading were revealed.The influences of twin boundary,symmetric tilted grain boundary,grain size and temperature on the plastic deformation of nano-polycrystalline diamond were obtained.The main work of this thesis is as follows:1.The hardness and plastic deformation mechanisms of nano polycrystalline diamond with different grain sizes(2 nm,5 nm,7 nm and 10 nm)under the loading of spherical rigid indenter were studied.The hardness and plastic deformation mechanism of nano polycrystalline diamond at different temperatures(300,700,1100 and 1500 K)were also obtained.2.The hardness and plastic deformation mechanisms of different nano-twinned diamond models(NTD-i,i = 3,6,12,24,36 and 48)under Vickers indenter loadings were studied,and the tendency of hardness with the decreasing of twin size was predicted.3.The hardness of diamond models with different symmetrical tilted grain boundaries(? = 12.69 °,28.07 °,36.87 °,53.13 ° and 73.74 °)under Vickers indenter loading was studied,and the corresponding plastic deformation mechanisms wererevealed.These results are not only beneficial to reveal the plastic deformation mechanism of nano polycrystalline diamond under indentations,but also to understand the deformation law under complex stress state such as the pressure stress,and provide a theoretical basis for the design of nano polycrystalline diamond with higher hardness.