Abstract
In plasma spraying, the dynamics of droplet deformation and cooling on the substrate determines the geometry of the resulting splat, the porosity formation and the quality of contact between the splat and the underlying layer, thereby controlling the properties of coatings. The process of splat formation depends on the droplet velocity, size, molten state, chemistry and angle of impact, and on the substrate material, roughness, temperature and surface chemistry. This paper concentrates on investigating the way the droplets impact onto the substrate. A fast-imaging camera is used to observe the impact of plasma-sprayed alumina droplets on cold and hot substrates. Three modes of impact are observed: splashing, deposition or rebounding. A droplet is said to splash when it disintegrates totally or partially into secondary droplets after impacting onto the surface while it is said to deposit if the liquid material spreads and solidifies on the surface with a minimum splashing. The results are expressed in dimensionless form as a correlation between the impact mode and the Sommerfeld parameter K. The latter quantifies the importance of the particle parameters at impact (temperature, velocity and size) and viscous and surface tension forces. It is found that, when K is higher than 57.7, the splashing mode prevails as already found in a variety of different fields for droplets impacting on surfaces.
