The beneficial effect of high-temperature oxidation on the tribological behaviour of V and VN coatings

The mechanisms for frictional self-adaptation of advanced hard coatings with optimum tribological properties are based on the formation of thin reaction films in sliding contacts. The aim of this work was to characterize VN and V coatings deposited by unbalanced magnetron sputtering and to determine the influence of oxide phase formation on their friction and wear properties during high-temperature exposure in ambient air. Dry sliding experiments were performed in the temperature range between 25 and 700 degrees C. Structural changes and oxide phases were identified by X-ray diffraction and Raman spectroscopy after tribometer testing. The friction coefficient of the V coatings decreases from about 1.05 at RT to 0.27 at 700 degrees C, while the VN coating has a lower coefficient of friction (similar to 0.45) at RT which further decreases at temperatures above 400 degrees C. The onset of rapid oxidation of V and VN coatings, as evidenced by differential scanning calorimetry (DSC) and thermo-gravimetric analysis (TGA), occurred at 400-450 degrees C with the formation of vanadium oxides, which were identified by XRD as V2O5, VO2 and V6O13. For both coatings evaluated, a significant decrease of the friction coefficient at temperatures above 400 degrees C was observed reaching a value of 0.25-0.27 at 700 degrees C, due to the formation of V2O5, giving rise to solid lubrication, and subsequent melting of this phase at temperatures above 600 degrees C, leading to liquid lubrication.