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 beta-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.