Influence of oxide phase formation on the tribological behaviour of Ti–Al–V–N coatings

Ti–Al–V–N coatings are potential candidates for dry machining applications due to the combination of superior mechanical properties of the Ti1−xAlxN phase and the lubricating effects of vanadium oxides formed between 500 and 700 °C. The aim of this work was to prepare Ti–Al–V–N coatings with a high V content (25 at.% V in the Ti–Al–V target) to evaluate the influence of the oxides formed, on the friction behaviour during tribological tests up to 700 °C. The coatings were deposited by DC magnetron sputtering of a powder-metallurgically produced Ti–Al–V target in an Ar + N2 discharge. High temperature ball-on-disc tests were used to investigate the tribological properties against alumina balls. Up to temperatures of 500 °C only minor changes in tribological properties compared to Ti1−xAlxN coatings could be observed. Increasing the testing temperature to 600, 650, and 700 °C yields a continuous reduction of the friction coefficient from around 1 to 0.27, respectively. However, during the experiment at 700 °C the friction coefficient increases to a constant value of 0.45. Thus, main emphasis was laid on the examination of the formed oxide phases to elucidate their relation to the changing friction coefficients. Scanning electron microscopy investigations, X-ray diffraction and Raman spectroscopy shows that first a V2O5 phase is formed which is responsible for the reduction of the friction coefficient. The further oxidation to form TiO2 and AlVO4 on the surface and especially in the wear track (due to the higher local flash temperatures) controls the ongoing oxidation.