Abstract:
In the formation of plasma-sprayed splats, the spreading behavior of molten droplets is essential for forming desirable lamella with good adhesion to the substrate. To understand the effect of the active element chromium on droplet spreading, pure Ni and Ni-20 wt.% (Ni20Cr) alloyed powders were plasma sprayed on mirror-polished 304 stainless steel substrates heated to different temperatures (below 200 °C). 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 characterized 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, some splashing still occurred to a measureable extent even at relatively high substrate temperatures, even above temperatures at which adsorbates (moisture) were totally removed from the surface. The splashing behavior 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.