Predicting splat morphology in a thermal spray process

Splats formed during a thermal spray process may be either highly fragmented or intact and disk-like. To predict this change in splat morphology, a dimensionless solidification parameter ( Θ), which takes into account factors such as the particle diameter and velocity, substrate temperature, splat and substrate thermophysical properties, and thermal contact resistance between the two, has been defined. Θ is the ratio of the thickness of the solid layer formed in the splat while it is spreading, to the splat thickness. The value of Θ can be calculated from simple analytical models of splat solidification and spreading. If solid layer growth is very slow ( Θ ≪ 1), the splat spreads out to a large extent. Once it reaches maximum spread, it becomes so thin that it ruptures, producing fragmented splats. If, however, the solid layer thickness is significant ( Θ ∼ 0.1–0.3), the splat is restricted from spreading too far and does not become thin enough to rupture, resulting in disk splats. When solid layer growth is rapid ( Θ > 0.3), it obstructs liquid from flowing outward during droplet impact, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data and found to agree well.