Effect of B-doping on the mechanical properties, thermal stability and oxidation resistance of TiAlN coatings

Abstract In this study, boron was introduced into TiAlN coating to improve its performance. Ti0.44Al0.56N, Ti0.42Al0.54B0.04N, and Ti0.36Al0.55B0.09N coatings were synthesized by arc evaporation, and the effect of B on structure, mechanical and thermal properties of TiAlN coatings were investigated by detailed experiments. Furthermore, we studied the formation energy (Ef) of cubic and wurtzite phases in Ti1-x-yAlxByN coatings by density functional theory (DFT) calculations. First-principle calculations indicate that the incorporation of B reduces the solid solubility of Al in cubic TiN, which is confirmed by the experimental results. Ti0.44Al0.56N coatings show a face-centered cubic structure while Ti0.42Al0.54B0.04N and Ti0.36Al0.55B0.09N exhibit a mixed structure of cubic and wurtzite phases. In addition, experimental results show that doping B by 4 at.% gives rise to an increase in hardness from 30.1 ± 0.4 GPa of Ti0.44Al0.56N to 32.7 ± 0.9 GPa of Ti0.42Al0.54B0.04N, while further increasing B content to 9 at.% triggers a dramatic decrease of the hardness down to 23.0 ± 0.6 GPa (Ti0.36Al0.55B0.09N). Alloying with B also significantly improves the oxidation resistance of TiAlN coatings by retarding the transformation of a-TiO2 to r-TiO2 and promoting α-Al2O3 formation.