Formation of plasma-sprayed oxide coatings

This paper is an overview of the present knowledge of the phenomena that affect the formation and quality of plasma-sprayed zirconia and alumina coatings. The characteristics of particles in flight and their condition at the point of impact, the flattening of the molten particles, the cooling of the resulting splats after impact, and the formation of coating by the accumulation of successive splats are discussed, along with the laboratory and industrial instrumentations that have been developed to study these phenomena. A threshold substrate temperature has been identified above which the splats on smooth surfaces (Ra < 0.1 mu m) have a perfect disk shape and excellent contact with the underlying material. The formation of the coating and its adhesion/cohesion values depend on residual stresses which are related to the initial condition of the surface to be coated, the preheating temperature and the thickness of material deposited in successive passes of the plasma torch over the coated surface.