SLURRY EROSION BEHAVIOR OF THERMALLY SPRAYED WC-M COATINGS

Coatings of tungsten carbide-metal matrix cermets deposited using high velocity oxyfuel spraying were subjected to sand erosion tests. The experimental conditions were as following; flow velocity varying between 20-145 m s(-1), abrasive particle size distributed over 20-150 mu m with a mean at 120 mu m, and abrasive concentration 0.3 wt.% in tap water. Cermets of cobalt binder phase showed higher strength than those of nickel matrix, and addition of chromium improved the erosion resistance several times. Electron microscopy observations and microanalysis techniques were used to characterize the microstructure of coatings and the mechanisms of erosion. It was found that during powder processing and thermal spraying a substantial fraction of WC was melted and reacted with metal matrix to form ternary carbides or mixed W-C-M compounds which exhibit a nanocrystalline character (M is Co, CoCr, and Ni). Another part of WC led to decarburization and resulted in the appearance of W2C or metallic W within the coatings. The molten appearance of some single impact sites in conjunction with the results of extraction replicas from the eroded surface, indicated a local temperature rise within the surface layers during tangential erosion. The erosion results are discussed with respect to the microstructural modifications occurred during powder processing and thermal spraying.