Effect of Ta content on microstructure, hardness and oxidation resistance of TiAlTaN coatings

• Solution strengthening caused by doping Ta is the main reason for hardness increase.
• The right amount of Ta alloying can increase the oxidation resistance.
• Too higher Ta content is not conducive to retard the formation of α-TiO2 during oxidation.
• The oxidation resistance is related with microstructure, phase transformation and stress.

Ti1 − x − yAlxTayN coatings with various Ta contents (y = 0.00–0.37) have been prepared by magnetron sputtering. Scanning electronic microscopy results show that the increased Ta content makes Ti1 − x − yAlxTayN coatings more compact and exhibit a finer columnar structure. X-ray diffraction results show that the preferred orientation of Ti1 − x − yAlxTayN coatings changes from (111) to (200) with increasing Ta content. Nanoindention results indicate that the hardness of the coatings sharply increases from 11 GPa without Ta to 31.2 GPa at y = 0.21. Solution strengthening caused by doping Ta is the main reason for the rapid increase of hardness. After oxidation in ambient air, Ti0·44Al0.56N coating is fully oxidized to α-TiO2 and γ-TiO2 at 600 °C. A significant increase of oxidation resistance for Ti0·37Al0.42Ta0.21N coating has been observed at 800 °C. However, a higher Ta content (y = 0.37) is not conducive to retard the formation of α-TiO2 phase at low temperature, but will increase lattice distortion. This can make the coating easy to spall during further oxidation.