S. Chandra, Centre for Advanced Coating Technology, University of Toronto, Toronto, ON, Canada; R. Dhiman, Centre for Advanced Coating Technologies, University of Toronto, Toronto, ON, Canada
Splats formed during a thermal spray process are either highly fragmented or disk-like. To predict this change in splat morphology, a solidification parameter (Q), which takes into account important factors such as the substrate temperature, thermal contact resistance, and droplet and substrate properties, has been developed. Q 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 Q can be calculated from an analytical one-dimension heat conduction model. If the solid layer growth is very slow (Q << 1), the droplet spreads out to a large extent. Once it reaches maximum spread it becomes so thin that it ruptures. If, however, the solid layer thickness is significant (Q ~ 0.1 – 0.7), the droplet is restricted from spreading too far and does not become thin enough to rupture. Under such circumstances, disk-type splats are expected. When the solid layer growth is rapid (Q ~ 1 or greater), spreading of the droplet is significantly obstructed by the solid layer, producing splats with fingers around their periphery. Predictions from the model are compared with experimental data to confirm this proposed mechanism of splat fragmentation.
Summary: An analytical model is described to predict the splat morphology (splash-type or disk-type) observed in a thermal spray process. The model calculates a solidification parameter (Q), defined as the ratio of the solid layer thickness produced in the splat while it is spreading to the splat thickness. The value of the solidification parameter is indicative of the splat shape obtained in a thermal spray process. The model results were confirmed by experimental data from various sources.
