Determination of Dominating Factors on Flattening Behavior of Plasma Sprayed Ceramic Particles

Abstract

Recently, we have shown that the splat morphology of most metallic materials thermally sprayed strongly depend on the substrate temperature and usually the change occur drastically near the transition temperature ; Tt. In the present study, the effects of substrate material, powder material and PVD coating material on the flattening of the plasma sprayed ceramic particles were investigated.
Commercially available several kinds of powders, that is, Al₂O₃, TiO₂ and YSZ, were plasma sprayedand collected on the mirror polished substrate surface. The substrate materials are AISI304stainless steel, brass and glass, and those were held at a designated temperature before spraying. To investigate the effect of wetting on the flattening of the particle, Au, Ti and Al coated substrates by PVD were also prepared.
The splat morphology collected on the room temperature substrate was splash type. As the substratetemperature increased, the central solidification area of the splash splat gradually enlarged, and finally the splat morphology changed to the disk type over Tt range. This transitionis maybe caused by both the improvement of wettability at splat/substrate interface and suppression of rapid solidification with the substrate temperature increasing.
The different transition behavior was observed on the common substrate material with each PVD coating. It is well known that the standard formation free energy of oxide can be closely related to the static wetting of melt material on the substrate. If this relation can be applicable to thedynamic wetting like thermal sprayed particles on the substrate, it can be estimated from the results obtained that the better wetting promotes the occurence of the disk splat. The observationresults of the bottom surface microstructures of the splats supported well this hypothesis.
Consequently, it was clarified that the wetting played an important role on the flattening behavior of the plasma sprayed ceramic particles.