Thermal stability and mechanical properties of arc evaporated Ti–Al–Zr–N hard coatings

Abstract

Based on previous reports, showing that zirconium alloying to Ti1−xAlxN hard coatings can increase their thermal stability, we study in detail the interaction of bias potential and Al content on structure, hardness, thermal stability, and oxidation resistance of arc evaporated Ti1−x−yAlxZryN hard coatings. For moderate Al-contents, Ti0.49Al0.44Zr0.07N, their structure is single-phase cubic and their hardness remains at ~35GPa upon annealing in vacuum to 900°C, when prepared with −40 and −80V bias. Contrary, the coatings deposited with −120V bias experience already for annealing temperatures above 700°C a hardness reduction from the as deposited value of ~40GPa. The higher Al-containing Ti0.39Al0.54Zr0.07N coatings are mixed cubic and hexagonal wurtzite type structured, and with increasing bias potential the cubic phase fraction increases. Whereas the coatings prepared with −120V bias exhibit an almost constant hardness of ~25GPa upon annealing to 900°C, their counterparts prepared with lower bias, experience even a slight increase in hardness, due to the formation of well defined crystallites. However, only single-phased cubic structured Ti0.49Al0.44Zr0.07N coatings are able to withstand an oxidation treatment for 20h in ambient air at 850°C due to the formation of a dense, protective Al2O3 based outer oxide scale. Their oxidation resistance decreases with increasing bias potential, due to the increased defect density and thus promoted diffusion. Based on our studies we can conclude, that although the droplet-size decreases and the as deposited hardness increases with increasing bias potential, their thermal stability and especially oxidation resistance decrease.