Effect of heat treatment on the microstructure and properties of VN alloy/Co-based composite coatings

Laser cladding VN alloy/Co-based composite coatings on a mild steel substrate were heat treated at 650 °C for 3 h and 5 h, respectively. Effect of heat treatment on microstructure and wear resistance of the composite coatings was investigated systematically by optical microscopy, X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscopy, micro-hardness testing and wear testing. The results showed that as-laser cladded and heat treated VN alloy/Co-based composite coatings were all composed of γ-Co, Cr23C6, σ-FeV, Co5.47N and VN phases. The intensities of γ-Co diffraction peaks were reduced gradually and the intensities of Cr23C6 and VN diffraction peaks were increased gradually with the increasing of heat treatment time. Lots of short rod-shaped dendrites and more uniform equiaxed grains exhibited in the composite coatings after heat treatment. The microhardness and wear resistance of VN alloy/Co-based composite coatings were reduced slightly after heat treatment for 3 h. The microhardness and wear resistance of the composite coatings were improved by 6.71% and 9.82%, respectively, for heat treatment for 5 h. The wear mechanism of three composite coatings was all the abrasive wear.