Thermal stability and oxidation resistance of sputtered TiAlCrN hard coatings

In recent years, TiAlCrN coatings attract broad research interests owing to their excellent mechanical and tribological properties. Here, we study the thermal stability and oxidation resistance of two Ti1 − x − yAlxCryN coating series, having up to 47 at.% Cr (of the metal fraction, hence, y ≤ 0.47) for Al/(Ti + Al) ratios of ~ 0.55 and ~ 0.65. All as-deposited Cr-containing coatings are single-phase face-centered cubic (c) structured with hardness values between 32.0 and 33.9 GPa. For Cr contents below y < 0.38, age hardening due to spinodal decomposition of the Ti1 − x − yAlxCryN solid solution is dominant upon annealing in inert atmosphere. But, for Cr contents y ≥ 0.41, the thermodynamically stable wurtzite AlN phase rapidly nucleates upon annealing (accompanied by the dissociation of CrN bonds) leading to a significant hardness reduction above 700 °C. Nevertheless, the oxidation resistance constantly increases with increasing Cr and Al content of our single-phased c-Ti1 − x − yAlxCryN coatings, because both favor the formation of a dense well-adherent oxide scale and retard the growth of Ti-rich scales. Therefore, our Ti0.18Al0.35Cr0.47N coating - exhibiting the best oxidation resistance among all coatings studied − only exhibits a 1.3-μm-thin oxide scale after 20 h exposure to ambient air at 900 °C.