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

The effect of an alumina coating, obtained by mechanofusion, on stainless-s teel particles used in plasma spraying has been studied by examining spraye d 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 alu mina shell 4 mu m. The results show that without preheating the substrate t he splats of both types of powder are extensively fingered and become circu lar when the substrate surface is preheated over 200 degrees C. For the cas e of the stainless steel/alumina composite splats, Energy dispersive spectr oscopy (EDS) analysis of the distribution of the various elements shows tha t the alumina is either spread exactly on the stainless-steel splat or is d ispersed in pieces and frozen over the surface of the stainless-steel splat . The first case corresponds to well molten particles where, after their fl ight 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 alum ina 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 i s compared with a pure stainless-steel coating. Both hardness and cohesion are found to improve for the alumina-coated particles. (C) 2000 Elsevier Sc ience S.A. All rights reserved.