We report first-principles density-functional theory calculations to investigate the role oxygen impurites play in determining the strength of $\mathrm{TiN}\left(111\right)∕{\mathrm{Si}}_x{\mathrm{N}}_y∕\mathrm{TiN}\left(111\right)$ interfaces, as may occur in the superhard and highly thermally stable “$\mathrm{nc}\text{−}\mathrm{TiN}∕a\text{−}{\mathrm{Si}}_3{\mathrm{N}}_4$” nanocomposite. For nitrogen-rich conditions, our investigations predict that the interfacial region consists of a thin “$\beta$-like ${\mathrm{Si}}_2{\mathrm{N}}_3$” layer with the silicon atoms tetrahedrally coordinated to nitrogen atoms, while under nitrogen-poor conditions, an octahedrally bonded $\mathrm{Ti}\text{−}\mathrm{Si}\text{−}\mathrm{Ti}$ arrangement is preferred. The tensile strength of $\mathrm{TiN}$ in the $⟨111⟩$ direction is found to be notably higher than in the $⟨100⟩$ and $⟨110⟩$ directions ($90\mathrm{GPa}$, 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.

• Received 2 February 2006

DOI:https://doi.org/10.1103/PhysRevB.74.035424