Mechanical properties and thermal stability of TiN/Ti B ₂ nanolayered thin films

This article describes a study of TiNTi B2 nanolayered coatings on sapphire and M2 tool steel substrates. Residual stress in as-deposited TiNTi B2 varied from tensile to compressive with increasing substrate bias. Increasing the density of nanolayer interfaces (i.e., decreasing bi-layer period) decreased the compressive stress; this effect was explained by diffusion of point defects to interfaces and/or an indirect effect of interfaces on stress via film structure. A thin TiN buffer layer substantially reduced the stress and improved adhesion on steel. Nanolayer film adhesion on steel was generally intermediate between that of monolithic TiN coatings, which was good, and Ti B2 coatings, which was poor. As-deposited nanolayers showed no hardness enhancement relative to rule of mixtures. X-ray diffraction results showed that the boride layers tended to be amorphous, especially for small layer thicknesses. After annealing at 1000 °C, nanolayer structure was retained, thin boride layers were at least partially crystallized, and hardnesses increased to as high as 49 GPa due to crystallization of the Ti B2 layers.