Effect of Hf on structure and age hardening of Ti–Al-N thin films

Protective coatings for high temperature applications, as present e.g. during cutting and milling operations, require excellent mechanical and thermal properties during work load. The Ti(1 − x)Al(x)N system is industrially well acknowledged as it covers some of these requirements, and even exhibits increasing hardness with increasing temperature in its cubic modification, known as age hardening. The thermally activated diffusion at high temperatures however enables for the formation of wurtzite AlN, which causes a rapid reduction of mechanical properties in Ti(1 − x)Al(x)N coatings. The present work investigates the possibility to increase the formation temperature of w-AlN due to Hf alloying up to 10 at.% at the metal sublattice of Ti(1 − x)Al(x)N films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti(1 − x − y)Al(x)Hf(y)N alloys, as well as the ternary Ti(1 − x)Al(x)N, Hf(1 − x)Al(x)N and Ti(1 − z)Hf(z)N systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100 °C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti(1 − x − y)Al(x)Hf(y)N films starts at lower temperatures than the ternary Ti(1 − x)Al(x)N coating. However, the formation of a dual phase structure of c-Ti(1 − z)Hf(z)N (with z = y/(1 − x)) and w-AlN is shifted to ~ 200 °C higher temperatures, thus retaining a film hardness of ~ 40 GPa up to ~ 1100 °C, while the Hf free films reach the respective hardness maximum of ~ 38 GPa already at ~ 900 °C. Additional annealing experiments at 850 and 950 °C for 20 h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti(1 − x − y)Al(x)Hf(y)N.