High-temperature low-friction properties of vanadium-alloyed AlCrN coatings

V-alloyed AlCrN hard coatings were deposited by reactive arc-evaporation in a commercial Balzers RCS coating system. The Al/(Al + Cr + V) atomic ratio in the target was kept constant at 0.7, while the Cr was stepwise substituted by V. Alloying V into the AlCrN coatings led to the partial transformation to a hexagonal phase of the otherwise metastable cubic lattice, whereas the hexagonal phase fraction was observed to increase with the V concentration in the coating. This results in a reduction of hardness (from 28 to 19 GPa) and biaxial coating stress (from -1700 to -900 MPa). The performed tribological tests at high temperature revealed a significantly lower coefficient of friction down to a value of 0.2 for Al(0.67)Cr(0.05)V(0.28)N at 700 degrees C. This behaviour is due to the formation and melting of the Magneli phase oxide V(2)O(5) forming a low-friction layer on the surface as indicated by Raman spectroscopy and scanning electron microscopy.