Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC–Co coatings

Abstract

WC–Co cermets have been used traditionally as wear-resistant materials. Recent work has shown that nanostructured cermets offer improved properties over their conventional counterparts. This work examines the performance of such conventional and nanostructured materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying. WC–Co coatings were deposited under identical conditions using both conventional sintered and crushed and nanocomposite powder feedstocks. Both powders consisted of tungsten carbide (WC) grains in a cobalt binder. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as tungsten hemicarbide (W 2C) and W and an amorphous Co-rich binder phase containing W and C. Due to differences in the morphology of the powder feedstock and the WC grain size, the nanocomposite coating contained a smaller fraction of unreacted WC than the conventional coating. Three body abrasive wear tests were performed using a modified dry sand rubber wheel apparatus with alumina and silica abrasives. A range of abrasive particle sizes and loads were used to assess the wear resistance of both coatings. It was found that the nanocomposite had a poorer wear resistance than the conventional coating under all the conditions examined. Wear was dominated by the loss of ductility in the Co-rich binder phase due to its amorphisation. The differences in the wear behaviour of the coatings could, thus, be explained in terms of differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates.