Control of Thermal Spray Process through Observation on Individual Splat Behavior

概要

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In plasma spray process, the individual droplet behavior at impact is the fundamental element to understand the resulting coating microstructure and corresponding coating properties. In this study, the flattening behavior of the sprayed individual particle was systematically investigated by changing the substrate preheating temperature and ambient pressure in deposition chamber. The splat shape change transitionally from a splash shape to a disk shape by substrate preheating or ambient pressure reduction. A transition temperature, Tt, and transition pressure, Pt, were defined and introduced, respectively. Furthermore, the wetting behavior of water droplet and flattening behavior of thermal sprayed particles were studied on the substrate with different elapsed time in an air atmosphere after preheating. It is clearly found that the contact angle increase gradually with an increase of the elapsed time. More splash splats were observed on the substrate with increase of the elapsed time, which agreed with the contact angle measurement results well. Experiment results indicate that wetting of substrate by molten droplet may dominate the flattening behavior of the thermal sprayed particles. Good wetting may be generated by removing the adsorbed gas/condensation through substrate preheating or ambient pressure reduction. Based on the study above, a three-dimensional transition map was proposed as a controlling principle of the thermal spray process.