Visualization and analysis of the impact of plasma -sprayed particles

Plasma-sprayed molten particles (38--63 mum diameter) were photographed during impact (velocity 70--220 m/s) and spreading on flat solid surfaces. The surfaces were maintained at either room temperature or heated above 350°C. Most surfaces were smooth. Several surfaces were preoxidized, while others had a structured pattern of columns to simulate rough surfaces. Droplets approaching the surface were identified by a photodetector, which triggered a charge-coupled device (CCD) camera to photograph the spreading splat. A rapid two-color pyrometer was used to collect thermal radiation from particles during flight and impact to follow the evolution of their temperature and size. Particles that impacted the surfaces at room temperature ruptured and fragmented, leaving a small central solidified core on the substrate. On heated surfaces, there was limited splashing and a circular, disk-like splat remained on the surface. Splats on rough surfaces splashed and formed finger-like projections due to changes in the surface topology. Mathematical models were developed to estimate the area of the splat in contact with the non-heated surfaces and to estimate the thermal contact resistance at the splat-substrate interface. The models supported the hypothesis that only a portion of the splat is in good contact with the non-heated surface at room temperature, while the rest of the fluid is separated from the substrate by a gas barrier. The experimental and model results were applied to a 3D numerical model to simulate particle impact and spreading.