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

a r t i c l e i n f o Based on previous reports, showing that zirconium alloying to Ti 1 − x Al x N 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 Ti 1 − x − y Al x Zr y N hard coatings. For moderate Al-contents, Ti 0.49 Al 0.44 Zr 0.07 N, their structure is single-phase cubic and their hardness remains at ~35 GPa upon annealing in vacuum to 900 °C, when prepared with − 40 and − 80 V bias. Contrary, the coatings deposited with −120 V bias experience already for annealing temperatures above 700 °C a hardness reduction from the as deposited value of ~40 GPa. The higher Al-containing Ti 0.39 Al 0.54 Zr 0.07 N coatings are mixed cubic and hexagonal wurtzite type structured, and with increasing bias potential the cubic phase fraction increases. Whereas the coatings prepared with −120 V bias exhibit an almost constant hardness of ~25 GPa 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 Ti 0.49 Al 0.44 Zr 0.07 N coatings are able to withstand an oxidation treatment for 20 h in ambient air at 850 °C due to the formation of a dense, protective Al 2 O 3 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. Hard coatings are designed to meet the requirements for increasing the wear resistance of today's machining applications such as drilling, milling, or cutting. Therefore, in addition to wear resistance also high hardness, oxidation resistance, and thermal stability are necessary [1, 2]. TiN was one of the first industrially applied hard protective coatings. The limited oxidation resistance and hardness were soon further improved by Al-alloying [1–4]. Especially single-phased cubic Ti 1 − x Al x N coatings exhibit a pronounced age-hardening effect during annealing to approximately 900 °C and hence this material is also well known for its high mechanical strength …