DYNAMICS OF PARTICLE DEFORMATION DURING PLASMA SPRAY COATING

摘要

Molten droplets during plasma spray process either fragment during impact or form intact, disk-like splats. Splat break-up produces coatings with low adhesion strength and high porosity. Fragmentation of splats can be caused by two entirely different mechanisms. If contact between the droplet and substrate is poor, the splat spreads into a thin sheet liquid and ruptures, producing a small central splat surrounded by a ring of debris. If contact is good and solidification is very rapid, a solid ring forms around the edges of the spreading droplet that obstructs the outward flowing liquid and destabilizes it, resulting in a splat with long peripheral fingers. If the solid layer grows by a significant amount during spreading, but not enough to destabilize the flow, it restrains the splat from spreading and becoming thin enough to rupture, and produces a disk shaped splat with no fragmentation. Numerical models of droplet impact can be used to simulate all three types of impact, with the rate of solidification varied by changing the value of thermal contact resistance between the splat and substrate. To predict which splat shape is likely for a given set of impact conditions, a dimensionless solidification parameter is defined, taking into account droplet diameter and velocity, substrate temperature, thermal contact resistance and thermo-physical properties of the splat and substrate. Examination of plasma-sprayed nickel, molybdenum, and zirconium droplets landing on glass, steel and Inconel substrates confirmed that the solidification parameter can be used to predict splat morphology.