Photographing impact of plasma-sprayed particles on rough substrates

Plasma-sprayed nickel and molybdenum particles (*55 lm diameter) were photographed during spreading on silicon wafers that were patterned with micron-sized columns to make a textured rough surface. Impact on grit-blasted glass was also studied. The surfaces were maintained at either room temperature or at 350 C. As the droplets approached the surface they were sensed by a photodetector and, after a known delay, a fast chargecoupled device (CCD) camera was triggered to capture time-integrated images of the spreading splat. A rapid twocolor pyrometer was used to collect the thermal radiation from the spreading particles to record the evolution of their temperature and calculate splat cooling rates. It was found that micron-sized columns on the textured surfaces impeded fluid flow during spreading of splats, promoting splashing. When the column height was on the order of the splat thickness, increasing the space between each column increased the splat cooling rate as the columns penetrated into the liquid splat, providing a larger surface area for heat transfer. On the grit-blasted glass surfaces it was found that as the surface roughness increased, the maximum spread diameters of the molten droplets decreased, while the splat cooling rates increased. Impact on non-heated and heated roughened glass with similar roughness values produced splats with approximately the same maximum spread diameters, skewed morphologies, and cooling rates. On smooth glass, the splat morphologies were circular, with large maximum spread diameters and smaller cooling rates on non-heated smooth glass. An established model was used to estimate the splat-substrate thermal contact resistances. On highly roughened glass, the thermal contact resistance decreased as the glass roughness increased, suggesting that splat-substrate contact was improved as the molten metal penetrated the spaces between the large asperities.