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 degreesC, V was incorporated into the coating which offers the possibility of forming lubricious phases based on easily shearable oxides (often also referred to as Magneli phases, e.g. VnO(3n-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+N-2 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 degreesC 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. (C) 2004 Elsevier B.V. All rights reserved.