Abstract
In the formation of plasma sprayed splats, the spreading behaviour of molten droplet is essential for forming desirable lamella with good adhesion to substrate. To understand the effect of active element chromium on droplet spreading, pure Ni and Ni-20Cr alloyed powders with the size of 45~63μm were plasma sprayed on mirror polished 304 stainless steel heated to different temperatures (below 200°C) through electrical resistance heaters. The substrate heating resulted in very little change in the surface roughness. However, there was a measureable change in the surface chemistry of the outermost few nanometers, which became increasingly enriched in Fe at higher temperatures. The splat morphologies were characterised and the transition temperatures were estimated. The results show that the transition from splashed to disk splats was not solely dependent on the temperature of the substrate. In some cases, splashing still occurred to a measureable extent even at relatively high substrate temperatures, even above temperatures at which adsorbates (water) were totally removed from the surface. The splashing behaviour could be correlated to a combination of the change in the surface chemistry of the substrate and the presence of active elements in the coating materials.
