Influence of heat treatment on the microstructure and performance of high-velocity oxy-fuel sprayed WC–12Co coatings

In this study, a high-velocity oxy-fuel sprayed WC–12Co coating was heat treated at 550, 750, 900 and 1150 °C for 1 h in an argon atmosphere. Various properties of these coatings such as hardness, fracture toughness, abrasive wear resistance, phase composition and microstructure were investigated before and after heat treatment. We can conclude from X-ray diffraction patterns that the phase of the as-sprayed coating is composed mainly of WC, an amorphous phase, a small amount of W2C and trace metal W. However, with an increase in the heat-treatment temperature, the amorphous phase was found to be gradually transformed to eta phases such as Co3W3C, Co6W6C and Co2W4C, coupled with the decrease of W2C and WC phases. Element diffusion and new phase generation processes at the coating–substrate interface and within the coating were studied via energy dispersive X-ray spectrometry, scanning electron microscopy and transmission electron microscopy. The results indicated that the coating hardness increased at first with an increase in the heat-treatment temperature, followed by a decrease, whereas the fracture toughness shows almost the reverse tendency. In addition, wet sand rubber wheel abrasive wear testing results indicate that heat treatment of the WC–12Co coatings at 950 °C resulted in the highest wear resistance among all the coatings due to the formation of the hard, unbrittle, crystalline Co6W6C phase and the tough Co phase without consuming much of the WC phase.