Effect of bond coat and post-heat treatment on the adhesion of air plasma sprayed WC-Co coatings

• Carbide phases subsequently decarburize to η-carbides controlled heat treatment.
• Adhesion of WC-Co coating has increased by applying interlayer bond coat.
• Post-heat treatment at 1150 °C increased the adhesive strength of the coating layer.

In this study, WC-12wt%Co and NiCr feedstock powders were sprayed by the air plasma spray (APS) method to form conventional WC-Co and double-layer NiCr/WC-Co coatings on steel substrates. The microstructure of as-sprayed samples with and without NiCr bond coat was analyzed and found to consist of WC, W2C and an amorphous phase. The conventional WC-Co coating samples were also heat treated at 650, 900 and 1150 °C for 1 h. Heat treatment at all temperatures examined resulted in phase changes within the coating layer by formation of WxCoyC, η-carbides. Adhesive strength of the as-sprayed and heat-treated coatings also investigated. Results indicated that adhesive strength has improved by applying bond coat before spraying of overlay WC-Co coating. The results also showed that by increasing heat treatment temperature from 900 °C to 1150 °C, adhesive strength increased. In this case, post-heat treatment process at 1150 °C and or applying NiCr bond coat gave the higher adhesive strength, which was due to the control of interfacial porosities and formation of partially metallurgical bonding in interface of coating and substrate.