Influence of Hf on the structure, thermal stability and oxidation resistance of Ti-Al-N coatings

• Alloying with 3 at.% Hf into Ti-Al-N coatings induces an increased hardness.
• Hf-addition has a positive effect on the thermal stability of Ti-Al-N.
• Hf-addition improves the oxidation resistance of Ti-Al-N at 800 and 850 °C.
• Ti-Al-Hf-N coating behaves worse oxidation resistance at 1100 °C.

Ti1−xAlxN coatings with cubic structure are widely applied in advanced machining and other high temperature applications due to their excellent mechanical and thermal properties. Recently, alloying to tailor the properties of Ti-Al-N coatings attracts considerable interest. Here, we investigate the effect of Hf on structure and thermal properties of Ti-Al-N coating with cubic structure. Alloying with 3 at.% Hf slightly promotes the spinodal decomposition of Ti-Al-N, and retards the formation of wurtzite AlN during thermal annealing. The optimized oxidation resistance at 850 °C for 10 h is obtained by Hf-containing coating with oxide scale of ~ 0.81 μm due to the retarded transformation of metastable anatase to stable rutile TiO2, whereas Ti-Al-N coating is fully oxidized. However, the thermal gravimetric analysis and differential scanning calorimetry results shows that Ti-Al-Hf-N exhibits faster mass gain and higher exothermic heat flow between ~ 1053 and 1137 °C than Ti-Al-N, indicating its inferior oxidation resistance, which is certified by a fast oxidation of coated sample on Al2O3 substrates at 1100 °C.