Sliding wear behaviour of conventional and nanostructured HVOF sprayed WC–Co coatings

Abstract

WC–Co hardmetals have been used traditionally as wear resistant materials and a significant body of research has demonstrated that sintered hardmetals with WC particles in the nanoscale range 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 using both conventional (sintered and crushed) and nanostructured powder feedstocks. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as W 2C and W and an amorphous Co-rich binder phase containing W and C. The nanostructured coating exhibited higher levels of these decomposition products due to the enhanced kinetics of dissolution of the small WC particles into the molten binder during HVOF spraying. The performance of the coatings were examined under sliding wear conditions against an alumina ball and compared with the behaviour of a conventional sintered WC–Co hardmetal. It was found that the nanostructured coating exhibited the highest wear rate of the three materials examined. Microstructural examination showed that wear was dominated by the loss of ductility in the Co-rich binder phase (due to decomposition during spraying) leading to subsurface fracture and the generation of wear debris.