Splat formation and stacking behavior of particles in high velocity oxygen-fuel spraying of WC-Co coatings

Lamellar structure, splat formation of molten droplets, coating microstructure, transfiguration of carbide particles and deposition yield of the powders in high velocity oxygen-fuel flame spraying of WC-Co coatings using powders with different morphologies were examined. The ratio of the particle diameter to the thickness of splats determined in this experiment is 7 to 17. These values are in good agreement with the calculated values according to the splat formation model proposed by Jones et al. The formation of coating deposits is followed by the expanding flow of the molten binding phase accompanying solid WC particles. The smaller size of WC particles in the spray powder allows the fluidization and flattening together with the molten binding phase, on the contrary, the flattening behavior of particles are restrained by the size of WC particles in the case that the size of spray powder is large. Susceptivity to flattening of the spray powder tends to result in the compactness or the high density of the coating. Consequently, spray powders composed of the smaller size of WC particles allow the formation of the denser coatings. The selection of the spray powder composed of smaller-sized primary WC particles and the moderately larger size of cermet particles is effective for the higher deposition yield of WC-Co spraying.