Impact of oxygen content on the thermal stability of Ti-Al-O-N coatings based on computational and experimental studies

Recently, the favorable impact of O-addition on the oxidation resistance, toughness and machining performance of Ti _1-xAl _x N coatings has attracted a wide range of research interests. We systematically studied the effect of O-addition on structure, mechanical and thermal properties of Ti _1-xAl _x N coatings by experimental and theoretical methods. The hardness values of Ti _0.42Al _0.58N _0.95, Ti _0.41Al _0.59(O _0.08N _0.92) _1.02, Ti _0.41Al _0.59(O _0.12N _0.88) _1.06 and Ti _0.41Al _0.59(O _0.18N _0.82) _1.13 with face-centered cubic (c) structure are 31.8 ± 0.7, 31.9 ± 0.6, 32.1 ± 0.8 and 31.7 ± 0.8 GPa, respectively. Thus, incorporation of oxygen into Ti _1-xAl _x N has no impact on the hardness (within the investigated range) of the films in their as-deposited state. After annealing, the peak hardness increases with the oxygen content and is 34.3 ± 0.7 GPa with 900 °C, 35.1 ± 0.8 GPa with 1100 °C and 35.9 ± 0.5 GPa with 1100 °C for Ti _0.42Al _0.58N _0.95, Ti _0.41Al _0.59(O _0.08N _0.92) _1.02 and Ti _0.41Al _0.59(O _0.12N _0.88) _1.06 , respectively. First-principles calculations combing the climbing image nudged elastic band (CI-NEB) method reveal that the incorporated oxygen retards the thermal decomposition by increasing the diffusion activation energy for Al diffusion. First-principles calculations further show that the most preferred decomposition products of c-Ti _1-xAl _x(O _yN _1-y) _z are c-TiO _yN _1-y and c-AlN. Moreover, although the highest oxygen-containing film, Ti _0.41Al _0.59(O _0.18N _0.82) _1.13 , contained a noticeable quantity of wurtzite (w-)AlN when annealed at 1200 °C, the sample exhibits the peak hardness of 35.5 ± 0.8 GPa. This, we attribute to the strengthening effect due to the precipitation of nano-sized α-Al ₂O ₃ and θ-Al ₂O ₃.