The Effect of Oxidation at Elevated Temperature on Tribological Behavior of WC-Co HVOF Spraying Coatings

Friction and wear performance of WC-12Co and WC-17Co HVOF spraying coatings within the temperature range of 25~800℃ in atmospheric environment was studied by a ball-on-disc tribometer. The worn track morphologies, phases of products after rubbing tests were measured by scanning electron microscopy, X-ray diffraction and Raman spectroscopy. Combining with the oxidation tests of coatings the effect of oxidation at elevated temperature on the tribological behaviors was investigated. The results show that at 350 and 450℃, the wear of coatings was influenced by cobalt oxides and revealed the mild oxidative wear. At 500℃, WO₃ increased with the depletion of WC hard phase, which was beneficial to the antifriction properties and unbeneficial to the wear resistance. The friction behavior could promote CoWO₄ formation. Cobalt tungstate was able to reduce the friction and wear and enabled the coating to maintain good tribological properties even at 600℃. Above 600℃, the coatings oxidized seriously and their properties deteriorated rapidly, which caused serious wear. Thus WC-Co coatings should not be used as wear-resistant coatings above 600℃. The coating with higher cobalt content demonstrated better high temperature wear resistance.