Thermal stability and oxidation resistance of Ti-Al-N coatings

Ti_1 - xAl_xN coatings are widely used for wear resistant applications due to their excellent mechanical and thermal properties, which depend to a great extent on the Al content. Here, we concentrate on a comparative study of the effect of Al content on crystal structure, thermal stability and oxidation resistance of Ti_1 - xAl_xN coatings. In agreement to earlier studies, thermal annealing of the individual cubic (c) and wurtzite (w) structured metastable Ti_1 - xAl_xN coatings induces decomposition into their stable phases c-TiN and w-AlN. The decomposition process for c-Ti_1 - xAl_xN involves an intermediate formation of cubic Al-rich and Ti-rich domains which results in a hardness increase to 34.7 and 34.4 GPa for x = 0.52 and 0.62 when annealed at 950 and 900 °C, respectively. In general, coatings with an Al content closer to the solubility limit, exhibit an earlier decomposition process, and hence an earlier peak-hardness. During exposure of the Ti_1 - xAl_xN coatings to ambient air at elevated temperatures Al₂O₃, TiO₂ and Al₂TiO₅ are formed. The oxidation resistance of as-deposited single-phase Ti_1 - xAl_xN coatings, cubic or wurtzite structured, increases with increasing Al content. However, coatings containing Al contents at the metastable solubility limit, which result in a mixed cubic-wurtzite structure, have the worst oxidation resistance of the Al-containing coatings investigated. The single phase wurtzite structured coating w-Ti_0.25Al_0.75N shows the best oxidation resistance, with only ~0.7 μm oxide scale thickness, after thermal exposure for 20 h at 850 °C in ambient air.