Role of oxygen inTiN(111)∕SixNy∕TiN(111)interfaces: Implications for superhard nanocrystallinenc−TiN∕a−Si3N4nanocomposites

Abstract

We report first-principles density-functional theory calculations to investigate the role oxygen impurites play in determining the strength of TiN(111)/Si{sub x}N{sub y}/TiN(111) interfaces, as may occur in the superhard and highly thermally stable 'nc-TiN/a-Si{sub 3}N{sub 4}' nanocomposite. For nitrogen-rich conditions, our investigations predict that the interfacial region consists of a thin '{beta}-like Si{sub 2}N{sub 3}' 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 direction is found to be notably higher than in the and directions (90 GPa, similar to the weakest 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.