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 Ti1-xAlxN 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 Ti1-xAlxN 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 Ti1-xAlxN films. Ab initio predictions on the phase stability and decomposition products of quaternary Ti1-x-yAlxHfyN alloys, as well as the ternary Ti1-xAlxN, Hf1-xAlxN and Ti1-zHfzN systems, facilitate the interpretation of the experimental findings. Vacuum annealing treatments from 600 to 1100 degrees C indicate that the isostructural decomposition, which is responsible for age hardening, of the Ti1-x-yAlxHfyN films starts at lower temperatures than the ternary Ti1-xAlxN coating. However, the formation of a dual phase structure of c-Ti1-zHfzN (with z = y/(1 x)) and w-AlN is shifted to similar to 200 degrees C higher temperatures, thus retaining a film hardness of similar to 40 GPa up to similar to 1100 degrees C, while the Hf free films reach the respective hardness maximum of similar to 38 GPa already at similar to 900 degrees C. Additional annealing experiments at 850 and 950 degrees C for 20 h indicate a substantial improvement of the oxidation resistance with increasing amount of Hf in Ti1-x-yAlxHfyN. (C) 2011 Elsevier B.V. All rights reserved.