Plasma spraying of stainless-steel particles coated with an alumina shell

The effect of an alumina coating, obtained by mechanofusion, on stainless-steel particles used in plasma spraying has been studied by examining sprayed particles in mid-flight and their resulting splats and coatings. The mean size of the injected powders is about 65 mu m and the thickness of the alumina shell 4 mu m. The results show that without preheating the substrate the splats of both types of powder are extensively fingered and become circular when the substrate surface is preheated over 200 degrees C. For the case of the stainless steel/alumina composite splats, Energy dispersive spectroscopy (EDS) analysis of the distribution of the various elements shows that the alumina is either spread exactly on the stainless-steel splat or is dispersed in pieces and frozen over the surface of the stainless-steel splat. The first case corresponds to well molten particles where, after their flight in the plasma jet, all the alumina shell has flowed to the tail of the particle; the second case is related to particles which have still an alumina shell uniformly distributed around the stainless-steel core. Finally, a composite stainless steel/alumina coating sprayed on a rough (R-a similar to 6.7 +/- 0.3 mu m) stainless-steel substrate preheated to 400 degrees C is compared with a pure stainless-steel coating. Both hardness and cohesion are found to improve for the alumina-coated particles. (C) 2000 Elsevier Science S.A. All rights reserved.