Hardness of alpha- and beta-Si3-nCnN4 (n=0, 1, 2, 3) crystals

The hardness of alpha- and beta-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 alpha- and beta(3)-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. (C) 2008 Elsevier B.V. All rights reserved.