Investigation of dominating factors on flattening behavior of plasma sprayed ceramic particles

In previous works, we have shown that splat morphology of the most metallic materials, which are thermally sprayed, depends strongly on the substrate temperature and that a drastic change usually occurs near the transition temperature, Tt. In this study, the effects of the substrate, powder and pre-coating by PVD on the flattening of the plasma sprayed ceramic particles were investigated. Several kinds of ceramic powders were sprayed onto the mirror polished substrate surface. The substrates were held at a designated temperature before spraying. To investigate the effect of wetting on the flattening of the particle, substrates coated with Au, Ti and Al by PVD were also prepared. The splat morphology was a splash type at room temperature. As the substrate temperature increased, the central solidification area of the splash splat gradually enlarged, and finally the splat morphology changed to a disk type at temperatures greater than Tt. This transition may be caused by the improvement of the wettability at the splat-substrate interface and the suppression of rapid solidification as the substrate temperature increased. Different transition behavior was observed on the common substrate material with various PVD coatings. It is well known that the standard free energy of formation of the oxide can be closely related to the static wetting of melt material on the substrate. If this relationship is applicable to dynamic wetting as plasma sprayed particles on the substrate, our results indicate that better wetting promotes the occurrence of the disk splat. Observation of the bottom surface microstructure of the splats supports this hypothesis. Consequently, it has been clarified that the wetting plays an important role on the flattening behavior of the plasma sprayed ceramic particles. (orig.)