Hardness of α- and β-Si3 − nCnN4 (n = 0, 1, 2, 3) crystals

The hardness of α- and β-Si3 − nCnN4 (n = 0, 1, 2, 3) crystals were calculated by using our microscopic model of hardness combined with first principles calculations. The results show that the hardness progressively increased with the increase of carbon content. Both C-rich α- and β-SiC2N4 phases are novel superhard materials whose hardness is at the range of 57–61 GPa, which can rival to that of c-BN. The two Si-rich SiC2N4 phases are also superhard materials, but their hardness is just higher than 40 GPa. The C–N bonds with short length and lower ionicity play key role to the increase of hardness. The Si–N bonds in the presented SiCN phases have shorter bond length than the binary silicon nitride and also contribute to the increase of hardness.