Effect of Ambient Pressure on Flattening Behavior of Thermal Sprayed Particles

Cu splats were thermally sprayed onto the mirror polished SUS304 substrate surface at various ambient pressures ranging from 6.66 to 101.33 kPa. The effect of ambient pressure on the flattening behavior of the particle was systematically investigated. It was observed that only around 10% or less of disk-shaped splats deposited at atmospheric pressure. The splat shape on the flat substrate had a transitional changing tendency from a splash splat to a disk one with a decrease of the ambient pressure. The tendency of splash splat change with increasing the ambient pressure agreed with the BET curve, which indicates that adsorption/desorption of the adsorbed gas/condensation plays an important role on the flattening behavior of thermal sprayed particles. Moreover, a part of substrates were polished to a mirror finish and heated to 573 K for 10 min, then elapsed to air atmosphere for different duration of up to 1 h. The fundamental static wetting behavior of the once heated substrate surface by a water droplet was investigated. The contact angle measurement results agreed well with the splat morphologies. No chemical change and surface topography change took place with the elapse time increasing. Hence, the occurrence of desorption caused by reducing the ambient pressure or by substrate preheating provided good wetting. Wetting of substrate surface by molten particles may dominate the flattening behavior of thermal sprayed particles.