A numerical study of thermal shrinkage influence on the impact dynamics of alumina droplets thermally sprayed on flat, grit-blasted and laser treated surfaces

Abstract

Thermal spray splat morphology and its microstructure are of significance to the coating resulting properties. Depending on the material, the changes in density as the droplet solidifies during splat development and solidification could have a substantial role in the particle deformation dynamics, formation of porosity and subsequent adhesion to the flat or rough target surface.In the current study, the effect of shrinkage on the final splat shape is numerically investigated for collisions occurring under several high-speed impact conditions using the Volume-of-Fluid (VOF) method on real surface topography. This influence is also studied for the interaction of two successive impacting particles. The developed models study the splat impact, development and dynamics of heat transfer on flat, laser-patterned, grit-blasted surfaces and numerically designed features. The results of droplet material variable density cases are compared with those assuming constant density. Experimentally obtained single splat depositions are also used to validate the developed models and obtained numerical results.Findings show that micro substrate surface features lead to splat stretching, porosity and limited finger creation subsequent to improved solidification through interfacial contact area increase. Macro surface characteristics, however, lead to splashing and separation from the substrate surface driven by fluid instabilities. Irrespective of the interface topography, shrinkage processes reduce droplet spreading and accentuate splat feature discontinuities such as porosity.