Tribological Properties of Reactive Magnetron Sputtered V2O5 and VN–V2O5 Coatings

Next generation of advanced hard coatings for tribological applications should combine the advantages of hard wear resistant coatings with low-friction films. In this study, the tribological behaviour of vanadium pentoxide (V2O5) single-layer as well as VN–V2O5 bi-layer coatings was investigated in the temperature ranging between 25 and 600 °C. For VN–V2O5 bi-layer coatings, the V2O5 top-layers were deposited by dc and bipolar-pulsed dc reactive magnetron sputtering, where the V2O5 phase shows preferred growth orientation in (200) and (110), respectively. The V2O5 single-layer coatings were prepared by dc reactive magnetron sputtering with a substrate bias of −80 V which leads to a preferred (200) growth orientation. Tribological properties were evaluated using a ball-on-disc configuration in ambient air with alumina balls as counterpart. The structure of the as-deposited films and eventual changes after tribometer testing were identified using X-ray diffraction, Raman spectroscopy and scanning electron microscopy. The friction coefficient of VN–V2O5 bi-layer coatings deposited in dc and pulsed dc mode decreases from room temperature to 600 °C, where the pulsed dc VN–V2O5 coatings have a significantly lower coefficient of friction over the whole testing temperatures reaching a value of 0.28 at 600 °C. Up to 400 °C, V2O5 single-layer coatings showed almost the same coefficient of friction as pulsed dc VN–V2O5 bi-layer coatings but reached a value of 0.15 at 600 °C. It seems that thermal activation of crystallographic slip systems is necessary for V2O5 films to show a low-friction effect.