Role of oxygen in TiN„111... /SixNy /TiN„111... interfaces: Implications for superhard nanocrystalline nc-TiN/a-Si3N4 nanocomposites

We report first-principles density-functional theory calculations to investigate the role oxygen impurites play in determining the strength of TiN 111 /SixNy /TiN 111 interfaces, as may occur in the superhard and highly thermally stable “nc-TiN/a-Si3N4” nanocomposite. For nitrogen-rich conditions, our investigations predict that the interfacial region consists of a thin “ -like Si2N3” layer with the silicon atoms tetrahedrally coordinated to nitrogen atoms, while under nitrogen-poor conditions, an octahedrally bonded Ti-Si-Ti arrangement is preferred. The tensile strength of TiN in the 111 direction is found to be notably higher than in the 100 and 110 directions 90 GPa, similar to the weakest 111 bonding direction in diamond , and is likely connected to the observed enhanced hardness of these nanocomposites. For the structure favored under the technically relevant nitrogen-rich conditions, oxygen atoms are predicted to diffuse to the interface region and occupy nitrogen sites. This gives rise to a notable reduction in the calculated interface tensile strength, which could lead to a decreased hardness, in accord with recent experimental indications. For the structure favored under nitrogen-poor conditions, oxygen impurities are predicted to have little effect on the tensile strength.