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

Ti1-xAlxN 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 Ti1-xAlxN coatings. In agreement to earlier studies, thermal annealing of the individual cubic (c) and wurtzite (w) structured metastable Ti1-xAlxN coatings induces decomposition into their stable phases c-TiN and w-AIN. The decomposition process for c-Ti1-xAlxN 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 Ti1-xAlxN coatings to ambient air at elevated temperatures Al2O3, TiO2 and Al2TiO5 are formed. The oxidation resistance of as-deposited single-phase Ti1-xAlxN 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 AI-containing coatings investigated. The single phase wurtzite structured coating w-Ti0.25Al0.25N shows the best oxidation resistance, with only similar to 0.7 mu m oxide scale thickness, after thermal exposure for 20 h at 850 degrees C in ambient air. (C) 2011 Elsevier B.V. All rights reserved.