Molecular dynamics simulations of nanoindentation and nanoscratching of silicon carbide

Parallel molecular dynamics simulations were carried out to investigate the interaction between a diamond indenter and silicon carbide during nanoindentation and nanoscratching. The dependence of the critical depth and pressure for the elastic-to-plastic transition on indentation velocity, tip size, and workpiece temperature was studied along with the nature of the deformation due to indentation and scratching. The two most widely used polytypes---cubic silicon carbide (3C-SiC) and hexagonal silicon carbide (6H-SiC)---were considered while the Si-terminated (001) ((0001)) surface was used in each case. Simulations were implemented using the Tersoff SiC potential, which accurately reproduces the lattice and elastic constants of 3C-SiC and 6H-SiC. Nanoindentation experiments were also carried out for 6H-SiC.; For the 3C polytype, both the critical pressure and indentation depth for the elastic-to-plastic transition were found to decrease with increasing indenter size over the nanoscale range of indenter sizes used in our simulations. As a result, the measured hardness was found to be significantly higher than obtained experimentally for significantly larger indenter sizes. In addition, for indentation depths beyond the critical depth a phase transition to the rocksalt structure was observed. A similar phase transition was observed for the 6H polytype, but the transition pressure was found to be somewhat higher than for 3C-SiC. Both of these results are in good agreement with experimental results for bulk SiC. Thus, for nanoscale indentation of 3C and 6H-SiC, the onset of plastic behavior is related to the existence of a phase transition under the indenter tip. For the 6H case a weak dependence on indentation velocity was also observed. This claim was also confirmed by nanoindentation experiments, in which the strain rate sensitivity of mono-crystal 6H was investigated.; Simulations of the nanoscratching of 3C-SiC were also carried out. Significant anisotropy in the scratch hardness was observed while the value of the atomic scale friction coefficient was in good agreement with reported experimental results. In general it was found that scratching is accompanied by amorphization of the material around the indenter, and as a result the scratch hardness is smaller than the indentation hardness.