COATING DEPOSITION BY THERMAL SPRAY PROCESSES
The formation of coatings by thermal spray deposition is reviewed. Different thermal spray technologies are described in which powders or wires are melted in a high-temperature gas jet and droplets of molten material (metal, ceramic, or polymer) are accelerated toward a substrate where they impact and spread into thin splats while merging with each other and solidifying to form a dense coating. The shapes of splats determine coating properties such as porosity and adhesion strength: disk-shaped splats produce less porous and stronger coatings than those that fragment after impact. Experiments using both large droplets and those produced by a thermal spray have been used to develop models of droplet impact and predict the shape of splats. The rate of heat transfer from molten droplets to the colder substrate determines the solidification rate and, therefore, splat shape. Thermal contact resistance due to contaminants adsorbed on the surface limits heat transfer between the splat and substrate. Preheating a substrate before applying a thermal spray coating removes adsorbed contaminants, reduces splat fragmentation, decreases porosity, and enhances adhesion strength.
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