Numerical simulation of plasma spray droplet spreading and fragmentation upon impact on a substrate at room and high temperature.

The mechanical and thermal properties of plasma spray coatings depend strongly on the flattening and solidification of individual splats, taking place in a few microseconds. Such a short time makes it difficult to thoroughly study the splat formation. In this study, a 3D axisymmetric model has been developed to simulate the impact, spreading, and fragmentation of a 50 micro-meter molten zirconia droplet on a smooth surface. Compressible Eulerian fixed-grid model has been combined with a volume of fluid method (VOF) to track the droplet interface. The droplet impact is simulated on substrates held either at room temperature or at 400° C. A novel approach is proposed to consider the effect of adsorbates on the substrate maintained at room temperature. It is observed that desorption of a small quantity of gas, initially adsorbed in several monolayers on the substrate surface, can lead to the liquid lift-off from the substrate. Consistent with experimental evidences, the computed droplet spreading diameter is found to be two to three times larger at the room temperature as compared to 400° C. Additionally, solidified core on the substrate held at room temperature is smaller than the solidified core formed on the substrate held at 400° C.