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.