Influence of heat treatment on the abrasive wear behaviour of HVOF sprayed WC-Co coatings

WC-17 wt%Co powders have been sprayed by the HVOF method to form coatings approximately 200 mu m thick on steel substrates. The microstructure of the coatings has been analysed and found to consist of WC, W2C and an amorphous binder phase. Some of the carbide particles are found to decarburise and dissolve in the liquid metal binder during spraying, resulting in a brittle binder phase in the coating. The coatings also exhibit tensile stresses in the as-sprayed condition. Coatings have been heat-treated at a range of temperatures between 250 and 1100 degrees C. Heat treatment above 600 degrees C results in significant phase changes within the coating. However, heat treatment at all the temperatures examined resulted in changes in the integrity and residual stress state of the coating due to thermal expansion coefficient mismatch between the coating and substrate. The abrasive wear behaviour of the as-sprayed and heat-treated coatings demonstrates that heat treatment improves the wear behaviour. Indeed, heat treatment at temperatures as low as 250 degrees C has resulted in improvements in wear resistance of up to 35%. The abrasive wear behaviour has been correlated with the residual stress state. coating integrity and microstructure. (C) 1998 Elsevier Science S.A.