A new low-friction concept for Ti1−xAlxN based coatings in high-temperature applications

Ti1−xAlxN hard coatings are well established in commercial applications—especially for extreme conditions like dry machining—due to their excellent oxidation and abrasion resistance. In order to enhance the tribological properties of these coatings at temperatures higher than 500 °C, V was incorporated into the coating which offers the possibility of forming lubricious phases based on easily shearable oxides (often also referred to as Magnéli phases, e.g. VnO3n−1). Thus, the aim of this work was to prepare Ti–Al–V–N coatings with different contents of V to verify the concept of lubricious oxide formation while maintaining or even improving the excellent mechanical properties of Ti1−xAlxN. An unbalanced DC magnetron sputtering system was used to deposit coatings from powder metallurgically prepared Ti–Al–V targets with 2, 5, 10 and 25 at.% V in an Ar+N2 discharge. The coatings were characterized with respect to their chemical composition, structure and microhardness. The incorporation of V increases the coating hardness from 35 to 46 GPa due to solid solution hardening. Tribological properties were investigated using high-temperature ball-on-disk tests against alumina and stainless steel balls. Increasing the testing temperature from ambient to 700 °C yields a reduction of the friction coefficient from about 0.8 to 0.43. Finally, it can be shown the formation of lubricious oxides due to alloying Ti1−xAlxN coatings with V is responsible for the improvement of their tribological properties especially at elevated temperatures.