Molecular dynamics analysis on the effect of grain size on the subsurface crack growth of friction nanocrystalline 6H-SiC

The purpose of this study is to investigate whether the grain size of ceramic materials follows the classical Hall-Petch effect and the effect of grain boundaries and grains on the subsurface crack propagation of nano-polycrystalline 6H-SiC by molecular dynamics. We established an initial friction model of diamond on nano-polycrystalline 6H-SiC based on the Voronoi's polycrystalline construction method. Construct Tersoff and Vashishta potential functions. Molecular dynamics parameters were optimized according to 6H-SiC relaxation equilibrium state model, and initial friction model was modified by integrating potential function, boundary conditions, relaxation equilibrium, temperature control, time step and other parameters. Combined with the molecular defect analysis method, radial distribution function, and the influence of stress, strain, and dislocation on the surface crack theory, the shear strain and shear stress distribution trend and sub-surface damage rate of grain boundaries were calculated. It was found that the potential energy increases with the increase in the grain size. The atoms between grain boundaries show a "stress concentration" phenomenon and transfer between the grain boundaries. Under the action of friction, the crystal breaks, and the HP phenomenon is obvious. The degree of damage to the grain boundary is smaller than that of the grain. The larger the grain size, the smaller is the hardness of the crystal and higher is the possibility of the crystal fracture, which is obvious in crystals with larger grain size. The IHP effect was observed in the crystal with an average grain size of 16.0 nm. The results show that the HP and IHP effects also exist in ceramic materials, and the crossover mechanism between the grain boundaries and grains is another driving force for the change in the crystal structure.