Mathematical Modeling and Numerical Simulation of Splat Cooling in Plasma Spray Coatings
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H. Fukanuma, R. Huang, Y. Tanaka, and Y. Uesugi (Submitted January 29, 2009; in revised form July 6, 2009) Particle deformation and cooling significantly affect the characteristics of thermally sprayed coatings, such as the adhesion and cohesion strength between a splat and a substrate and between splats, as well as the internal stresses of deposits. It is essential to understand these processes for the successful industrial application of thermal spray technology. However, to date, the microstructure of the boundary of a splat and the substrate has not been clarified, although much research has been conducted on splat formation and the cooling process. We have developed a microstructure model of the boundary between the splat and the substrate, based on splat morphology obtained from experiments. In the model, it is assumed that gaps, or voids, and contact areas are arranged on the splat boundary with the substrate in an orderly fashion. The model includes phase changes and heat resistance simulating the function of the microstructure during splat cooling. Assumptions in the model are that ambient gas trapped in the gaps, or voids, transfers heat only by conduction and not by convection or radiation. The results of the simulation indicated that the extent of gaps, or voids, significantly affects the rate of decrease of the average temperature of the splat surface, as well as the temperature distribution inside the splat.
Keywords
microstructure of splat/substrate boundary, modeling, splat morphology, splats cooling
1. Introduction The deformation of a splat and its cooling affect adhesion, cohesion, porosity, and residual stress of plasma sprayed deposits. It has been shown that adhesion and cohesion are influenced by the characteristics of the impinging particles such as velocity and molten state, as well as by the substrate characteristics, such as temperature and oxidative state (Ref 1). Bahbou et al. (Ref 2) have shown that substrate conditions influence the cooling rate of particles, as well as the flattening speed. Arata et al. (Ref 3) have reported that plasma sprayed deposits contain gaps (nonbonded interfaces) between splats. By measuring the curvature of a substrate in situ during plasma spraying, Kuroda et al. (Ref 4) have shown that the tensile stress depended on substrate temperature, as well as spray materials and characteristics. Research on splat morphology and flattening ratios as well as particle splash has been conducted (Ref 5-10). Fukumoto et al. (Ref 5, 6) have suggested that above a certain temperature, molten particles do not splash when they impinge on a smooth flat surface. Li (Ref 7) has H. Fukanuma and R. Huang, R&D, Plasma Giken Co., Ltd., Tokyo, Japan; and Y. Tanaka and Y. Uesugi, Department of Electrical and Electronic System, Kanazawa University, Kanazawa, Japan. Contact e-mail: [email protected].
Journal of Thermal Spray Technology
indicated that substances adsorbed on a substrate cause splat splashing because of evaporation resulting from heating by the molten droplet durin
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