Effect of CrN addition on the structure, mechanical and thermal properties of Ti-Al-N coating

Optimization of Ti–Al–N coating by incorporation of fourth substantially alloying elements attracts extensive attention in order to meet the severe application requirements at elevated temperature. Here, we investigate the effect of CrN addition on structure, mechanical and thermal properties of Ti–Al–N coating with cubic structure. Alloying with CrN promotes the spinodal decomposition of Ti–Al–N to form Al-depleted and Al-enriched domains and also the formation of hexagonal AlN during thermal annealing. Cr-containing coating has an increase in hardness from the as-deposited value of ~ 34.4 GPa to the maximum value of ~ 38.7 GPa with Ta = 900 °C, whereas the hardness of Ti–Al–N coating only increases from ~ 31.2 GPa to ~ 34.1 GPa. Thermal annealing of Ti–Al–Cr–N coating results in the N-loss to form h-Cr2N with Ta = 1000 °C and finally Cr with Ta = 1200 °C. Also addition of CrN into Ti–Al–N coating retards the transformation of metastable an-TiO2 into stable r-TiO2 and thereby the growth of porous Ti-oxide rich sublayer, which is beneficial to its oxidation resistance. Only a thin scale of ~ 0.55 μm for Cr-containing coating is shown after annealing in air at Ta = 850 °C for 10 h, whereas Ti–Al–N coating is already completely oxidized. Consequently, Ti–Al–Cr–N coated inserts have a better machining performance than Ti–Al–N coated inserts during continuous turning.