A material point method based investigation on crack classification and transformation induced by grit geometry during scratching silicon carbide

The microcracks and residual stress are commonly regarded as non-negligible obstacles while realizing non-damage surfaces in the grinding of brittle materials. In this paper, with the combination of stress field analysis, material point method simulation and high-speed scratch experiment, a variety of cracks in the classical indentation-induced crack system were decoupled to determine the inducing conditions and coupling modes of different cracks. The tests on pressureless sintered silicon carbide (S–SiC) exhibited that the grit geometry was crucial in inducing different cracks. The Hertzian and lateral crack systems were mainly determined by the rake angle of scratching grit and can transform from one to another near the critical value of −60°. Further, a two-point stress field model was proposed to prove that large contact width is the key factor to induce radial cracks. By contrast, large contact depth is related to the appearance of bottom debris. Furthermore, Raman spectroscopy was used to investigate the velocity effect of grit on cumulative damage. Besides the stacking disorder and residual stress, even chemical disorders such as the fracture of the Si–C bond and the formation of the homonuclear bond were found to be variable at the bottom of the scratch under different contact speeds and depths. Based on the above findings, a multi-layer superposition model was proposed to re-couple the effect of different cracks and predict the workpiece's damage/force state scratched by spherical grits. It proved that blunt grit could also realize low damage machining under specific conditions.