Improved mechanical properties, thermal stabilities, and oxidation resistance of arc evaporated Ti-Al-N coatings through alloying with Ta

Ti-Al-N is widely used as protective coating in various industrial applications. Here, we investigate the influence of Ta content - ranging from 0 to 28 at.% on the metal sublattice - on the mechanical properties, thermal stability, and high temperature oxidation resistance of arc evaporated Ti-Al-N coatings. We found an increase in hardness from 30.0 ± 1.0 GPa for Ti0.54Al0.46N to 35 ± 0.7 GPa for Ti0.44Al0.41Ta0.15N. Furthermore, age hardening due to spinodal decomposition of the supersaturated solid solution into cubic AlN-, TiN-, and TaN-rich domains lead to maximum hardness values of 39.5 ± 1.0 GPa upon annealing to 1100 °C for Ti0.38Al0.34Ta0.28N. This behaviour is directly linked with the extremely retarded formation of wurtzite structured (w) AlN. Even after annealing Ti0.38Al0.34Ta0.28N at the maximum temperature of 1100 °C, no crystalline w-AlN phase could be detected. Additionally, the incorporation of Ta to Ti-Al-N leads to a significantly higher oxidation resistance. While Ti0.54Al0.46N is already fully oxidised during exposure to ambient air at 900 °C, following a linear like oxide scale growth kinetic with 2.96 ± 0.48 μm/h, the Ti0.49Al0.44Ta0.07N and Ti0.44Al0.41Ta0.15N coatings provide the lowest parabolic-like oxide growth rates of 7.1 ± 0.4·10−2 μm2/h. The even higher-Ta-containing Ti0.38Al0.34Ta0.28N exhibits already a much higher oxide scale growth rate of 23.1 ± 1.7·10−2 μm2/h. Consequently, the Ta content for an optimised oxidation resistance needs to be balanced to the Ti content of the Ti-Al-N coatings.