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Abstract
The deposition process of WC – Co and Cr3 C2 – NiCr cermet coatings through a high velocity oxyfuel (HVOF) spray process was examined using powders of different carbide particle sizes that ranged from 1.5 m m to over 15 m m. The effect of carbide particle size on the thickness of flattened particles was investigated. The carbide size in the coating was compared to that in the original feedstock. Results showed that most of the carbide particles remained in a solid state after passing through the flame, and the deposition of the cermet coating involves lateral flattening of a solid – liquid two phase droplet rather than a single liquid droplet, as in plasma spraying of metallic and ceramic materials under optimised conditions. It was found that the size of carbide particles in a sprayed coating depends greatly on the corresponding carbide size in the starting powder. The large solid carbide particles in a two phase droplet tend to rebound easily during HVOF spraying when the droplet impacts on a substrate surface, while small carbide particles in the droplet may follow the flattening of the liquid binder phase and be easily retained in the deposit. Using a powder consisting of small carbide particles leads to substantial retention of carbide particles in the coating and consequently an improved wear performance. A model is proposed to explain the effect of carbide particle size on the splat formation by a solid – liquid two phase droplet.
