V-alloyed ZrO2 coatings with temperature homogenization function for high-temperature sliding contacts

Abstract

The effect of V additions on reactively magnetron sputtered ZrO2 coatings was investigated with respect to their structural and mechanical properties. Coatings with different V-contents were co-sputtered from Zr and V targets using an Ar+O2 discharge. The X-ray diffraction patterns show a change from monoclinic (0–2.2at.% V) to cubic/tetragonal ZrO2 based phase (5.8at.% V) and finally X-ray amorphous structure at even higher V-content (17.4at.% V). Hardness and Young's modulus decrease beyond 5.8at.% V from 17.4 to 7.5GPa and from 230 to 150GPa, respectively. At 25°C, the coefficient of friction (COF) is ~0.2 for low V-contents and above 0.5 for high V-contents, respectively. Differential scanning calorimetry of powder samples showed a characteristic exothermic peak at ~600°C, which is due to the formation of the ZrV2O7 phase. This phase decomposes above 740°C in ZrO2 and liquid V2O5. The latter phase provides a self-lubricious layer in the sliding contact, decreasing the COF below 0.2 at 800°C for V-contents of 17.4at.%. The endothermic nature of melting of the V2O5 phase is suggested as the basis for thermal management abilities of tribological coatings, where the frictional heat generated in severely loaded sliding contacts and thus local flash temperatures are efficiently reduced.