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.

NiCr was plasma sprayed at room temperature on aluminum and stainless steel substrates that had been hydrothermally treated in deionized water for 30 minutes. A major difference was observed in that splat formation occurred only on the stainless steel. A numerical model was developed to simulate the impact of molten nickel splats on the treated substrates. The simulation closely matched experimental results in terms of splat morphology, porosity, delamination, and specific locations of substrate melting. Moreover, it confirmed that splat morphology is influenced, not by oxide thickness, but rather by water release from the dehydration of oxyhydroxide at the outermost surface. The insulating layer of released water inhibits heat transfer from the splat to the substrate, which reduces solidification rates, resulting in further spreading and thinning of splats. These findings shed light on splat spreading and solidification and provide insights on the effect of substrate surface chemistry on thermal spray splat morphology.