Transient contact pressure during flattening of thermal spray droplet and its effect on splat formation

The flattening process of an isothermal droplet impinging on flat substrate in plasma spraying is studied numerically using "Marker-And-Cell" technique that enables the evolution of the droplet/substrate dynamic contact pressure. The distributions of the pressures upon substrate surface during flattening are calculated under different droplet conditions. The correlation of the distribution of the peak contact pressure along substrate surface with the observed splat morphology is examined. The results show that the transient contact pressure is initially high and concentrates at a small contacting area and then spreads quickly with droplet flattening. The maximum pressure is located at the front of the droplet at an early stage of deformation, which drives the fluid moving quickly along substrate and results in lateral flow. The contact pressure is mainly associated to droplet density and velocity. The peak pressure reduces monotonically with flattening and becomes negligible at the region where the flattening degree is larger than 2. The magnitude of the pressure resulting from evaporating gas by rapid heating of the adsorbed water on the substrate surface is comparable to that of the dynamic contact pressure at the region where the flattening degree ranges from 1.5 to 2. It is suggested that the reduced contact pressure at the late stage of spreading and disturbance by the evaporation-induced pressure resulting from rapid heating of the surface adsorbents by flattening droplet may contribute significantly to the splashing of flattening droplet and the formation of a reduced disk-like splat.