TRANSPORT AND CHEMICAL RATE PHENOMENA IN PLASMA SPRAYS

Abstract
This paper is focused on the understanding of plasma-particle and particle-substrate interactions during plasma spraying. Rough estimates by using averaged values of the appropriate transport properties make it possible to determine the relative magnitudes of heat transfer, evaporation, mass transfer and chemical reaction when 30 μm iron particles are entrained in a plasma jet, operating in ambient atmosphere. When particles impinge on the substrate, they undergo severe deformation and rapid cooling. Particle spreading and morphology changes as well as solidification rate, control the nature of the particle-substrate interactions and thus the coating microstructure. The temperature of the substrate surface plays a major role in splat formation and splat shape. It is postulated that maintaining the substrate medium above a certain "transition" temperature ensures that the splat during formation remains in the liquid phase. When the substrate is below this temperature, the droplet begins to freeze after impact, part of the liquid mass splinters away and spreading of the droplet away from the point of impact is irregular. After impact of a metal droplet with the substrate, the principal mode of oxidation is by gas-solid reaction which follows a parabolic diffusion law.

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