Interfacial Indentation to Determine the Adhesion of WC-Co-NiCr Thermal Spray Coatings

Carbide coatings offer a high hardness, but due to their high brittleness they have limited applications. Thin films of carbides have potential as wear coatings if their adhesion to engineering material substrates, toughness, and durability can be increased. Addition of metals to ceramics is a well known method for increasing the properties of the coatings. This study examines adhesion characteristics of NiCr base alloy coatings with addition of different percentages of a WC–Co alloy (50 and 70 wt.%) with different thicknesses (0.2, 0.4 and 0.6 mm), deposited on plain steel by using HVOF process. In order to improve the mechanical properties of the coatings, post heat treatment was conducted in an inert atmosphere between 600°C and 1000°C.  Due to the self-fluxing properties of the NiCr alloys the samples also were post heat treated by using an oxyacetylene flame. The authors use Vickers interfacial indentation test to characterize adhesive fracture between substrates and coatings. This methodology allows the determination of apparent interfacial toughness which can represent the adhesion of a coating on its substrate.  Phase evolution and microstructural changes due to post heat treatment were investigated by optical microscopy, X-ray diffractometry and scanning electron microscopy. Significant improvement in coating-substrate interfacial toughness was observed by the addition of NiCr. It was found that fracture toughness was enhanced after flame post heat treatment and by increasing temperature to 800 °C.  Crack propagation at the interface was slowed down after these post heat treatments but decreased after heat treatment at 1000 °C. This behavior is discussed and related to the role played by the residual stresses in the coating.