Verification of the flattening behavior of thermal-sprayed particles and free-falling droplets through controlling ambient pressure

Aiming at clarifying the role of ambient pressure on flattening behavior of molten particles on flat substrate surface, commercially available Cu powders were thermally sprayed onto mirror-polished AISI304 substrate using low-pressure plasma-spraying technique. The splat shapes on flat substrates underwent a transition from a distorted shape with splash to a disk-shaped one with a decrease of the ambient pressure, because there was no chemical modification and surface topography change took place, and the adsorption/desorption of the adsorbates and condensates on substrate surface played an important role on the splat flattening and solidification process. As a simulation of the splat formation process, the flattening behavior of millimeter-sized free-falling Cu droplet on the flat AISI304 substrate was investigated under designated ambient pressures. Better heat transfer and enhanced cooling rate were obtained under lower pressure in the deposition chamber due to the more intimate contact. Smaller contact angle of molten Cu droplet onto AISI304 substrate was maintained at lower pressure condition, which indicated that more favorable wetting can be obtained under such experiment condition. Consequently, the improved wetting generated by removing the adsorbates and condensates though reducing the ambient pressure might dominate the flattening behavior of the molten droplet on flat substrate surface.