Origin of high temperature oxidation resistance of Ti-Al-Ta-N coatings

Alloying Ti-Al-N coatings with Ta has proven to enhance their hardness, thermal stability, and oxidation resistance. However, especially for arc-evaporated Ti-Al-Ta-N coatings only limited information on the detailed influence of the elements on various properties is available. Therefore, we have developed arc-evaporated Ti1−x−yAlxTayN coatings with various Al (x = 0.50-0.65) and Ta (y = 0.00-0.15) contents. While the thermal stability of our coatings during annealing in inert He atmosphere increases with increasing Ta content, best results are obtained for specific Ta-Al ratios during oxidation. Single phase cubic Ti0.32Al0.60Ta0.08N yields a mass-gain of only ~ 5% after 5 h at 950 °C in synthetic air, whereas Ti0.35Al0.65N is completely oxidized after 15 min. This is in part based on the suppressed anatase and direct rutile TiO2 formation at a defined Ta-Al content. Consequently, the anatase-to-rutile transformation, generally observed for Ti1−xAlxN, is absent. This reduces the generation of pores and cracks within the oxide scale and especially at the nitride-oxide interface, leading to the formation of a protective rutile and corundum based oxide scale. This is also reflected in the pronounced decrease in activation energy for the protective scale formation from 232 kJ/mol for Ti0.35Al0.65N down to 14.5 kJ/mol for Ti0.32Al0.60Ta0.08N. Based on our results we can conclude that especially phase transformations within the oxide scale need to be suppressed, as the connected volume changes lead to the formation of cracks and pores.