Improved thermal stability and oxidation resistance of Al-Ti-N coating by Si addition

Addition of Si is very effective in upgrading the machining performance and thermal properties of Al-Ti-N coating. Here, we concentrate on the thermal stability and oxidation resistance of Al-Ti-Si-N coating. Alloying with Si favors the growth of wurtzite phase, and thereby causes a drop in hardness from ~ 34.5 to 28.7 GPa. However, Si-containing coating retards the formation of w-AlN during thermal annealing, and thereby behaves a high hardness value of ~ 31.3 GPa after annealing at Ta = 1100 °C. After 10 h exposure in air at 850 °C, Al-Ti-N coating is fully oxidized. Incorporation of Si significantly improves the oxidation resistance of Al-Ti-N due to the combined effects with the promoted formation of Al-oxide rich top-scale and retarded transformation of anatase (a-) TiO2 into rutile (r-) TiO2, where only ~ 1.43 μm oxide scale is shown after oxidation at 1100 °C for 15 h. Noticeable is that the worst oxidation resistance of Al-Ti-Si-N coating in the temperature range from 800 to 1100 °C is obtained at 950 °C with oxide scale of ~ 1.76 μm due to the fast formation of r-TiO2. Additionally, a pre-oxidation at 1000 °C has a positive effect on the oxidation resistance of Al-Ti-Si-N coating, which is attributed to the formation of Al-oxide rich top-scale, and thus inhibits the outward diffusion of metal atoms and inward diffusion of O.