  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 {\deg}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.
