Investigation of dynamic wetting behavior in flattening of thermal sprayed particles and dominating relation between wetting and initial solidification

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Effects of both solidification and interface wetting on the flattering behavior of thermal sprayed particles are quite essential under the conditions of the particles relating factors kept constant. Dominating relation between wetting and initial solidification, however, has not been fully understood yet. As the flattering behavior of the thermal sprayed particles can be considered to be dynamic wetting phenomenon, dynamic wetting behavior in flattening of the thermal sprayed particles and dominating relation between wetting and initial solidification were investigated in this research.In the first year, several kinds of particles were plasma sprayed on the metallic flat substrate surface held at various temperatures, and the effects of both solidification and wetting at particles/substrate interface on the flattening behavior of the particles were investigated. The relation between transition temperature and thermal conductivity of the particles had the strong linearity, and the worse … More was the static wettability at interface, the steeper was the gradient of the line. This indicates that even in the ceramic particle, the rapid solidification at the bottom part of the splat occur when the substrate temperature is close to the transition temperature. However, as the splashing was observed in the material combinations of poor wetting, it was indicated that the most dominating factor on the flattening was wetting. Free falling experiments of the molten metal droplets was also conducted as a simulation of the thermal spraying. From the results, it was estimated that the unique porous microstructure on the bottom surface of the droplet was formed by the taking in a gas physically.In the second year, effect of dynamic wetting on the flattening behavior of the thermal sprayed particles and the dominating relation between wetting and initial solidification were investigated. The central disk part in the splash splat expanded gradually according to the increasing of the substrate temperature, and wholly scattered splat could be often observed when the wetting was bad. From these results, it may be concluded that the wetting seemed to be the most dominating factor in the flattening of the thermal sprayed particle on the substrate. The rapid initial solidification at the bottom part of the splat was thought to affect on the flattening of the particle when the wetting reached to some critical condition.In-flight measurement if the velocity and temperature of the particle was also conducted and transition temperature of the splat was measured. The transition temperature increased with increasing the velocity, temperature and diameter of the particles. Splashing parameter : K=WeィイD10.5ィエD1ReィイD10.25ィエD1 is usually used as a determinant of the splashing. The transition temperature increased with increasing Splashing parameter based on the measured data of the particle. However, from the relation with the transition temperature, it was found that only splashing parameter could not determine the transition phenomenon. Then, Flattening K value : Kf=0.5aィイD11.25ィエD1ReィイD1-0.3ィエD1K was defined and introduced in this study. It was concluded that The splashing seemed to occur when the Kf value of the splat reached to some critical values.From the free fallen experiment under low pressure, it is revealed that the atmosphere itself has an inhibitive action to the wetting at the droplet/substrate interface. From the comparison between the splat on the naked substrate and that on the gold coated substrate, it was found that the wetting at splat/substrate interface was improved through the morphological change of the substrate surface according to the increasing of the substrate temperature. From the observation results of the high-speed video, it was found that the flattening velocity of the splash splat is extremely higher than that of the disk splat. The Higher flowing of the splat seems to be caused by the ultra rapid cooling at the bottom part of the splat. Less