Effect of atmosphere, temperature and carbide size on the sliding friction of self-mated HVOF WC–CoCr contacts

Friction and wear of self-mated HVOF 86WC–10Co4Cr containing different carbide sizes have been investigated after sliding in air and nitrogen gas at room temperature and 200°C. A statistic analysis of the parameters by ANOVA was performed. The surfaces and tribofilms were characterized by XPS after sliding. The results from both the tribological testing and the analysis of results by ANOVA showed that the two main parameters affecting friction and wear were temperature and atmosphere. Subsequent surface analysis highlights the complexity of interacting factors affecting friction and wear. Highest friction was measured for sliding in air at 200°C while lowest friction was measured for sliding in nitrogen. On the surfaces where friction was highest, largest amount of oxides after sliding were found. These mainly consisted of tungsten and chromium oxides as revealed by XPS analysis of the surfaces after tribological testing. The effect of primary carbide size on friction and wear is marginal although it cannot be totally dismissed. •The highest friction was found where the concentration of tungsten oxides on the surface was higher.•The tungsten oxide tribofilm is amorphous and does not lubricate reducing friction.•The presence of binary oxides (CoO–WOx) and mixing of Cr, Co and W oxides increases friction.•Sliding in nitrogen reduces specific wear rates and friction.