Most metallic and ceramic splats exhibit temperature-dependent behavior when they strike a metal substrate and assume a shape that suggests temperature-dependent wetting properties. In this study, the authors investigate the relationship between high-temperature oxidation, substrate morphology, and the flattening behavior of free-falling droplets. Substrate surfaces are examined by means of atomic force microscopy, which shows that changes in morphology of just a few nanometers can have an effect on flattening behavior. Paper includes a German-language abstract.

In the collision of a liquid droplet onto a flat surface, a splashing parameter (K=We 0.5 Re 0.25 ) has been used to evaluate the flattening behavior, and a critical value of K (K C ) was introduced as a criterion for splashing. In order to evaluate K of thermal sprayed particle, in-flight measurement for the velocity and temperature of the particle was conducted in this study. As a flattening pattern of the thermal sprayed particle changed significantly with a substrate temperature, the transition temperature (T t ) was also measured by changing the substrate temperature. Both K and T t showed a tendency of monotonous decreasing with increasing the spray distance and a strong linearity was recognized in a K-T t relationship. This straight line corresponds to a critical value for the splashing. However, as the influence of substrate temperature on the flattening is essentially independent of K, a new criterion for the splashing in the flattening of the particle was proposed in the study. That is, a splashing parameter on flattening was proposed, which considers the ratio of the flattening velocity to the impact velocity of the particle.

A free falling experiment was conducted as a simulation of a thermal spray process. A flattening behavior of the freely fallen metal droplet impinged onto a flat substrate surface was fundamentally investigated. The substrates were kept at various designated temperatures, and the substrates coated with gold by PVD were also prepared in order to investigate the effect of a wetting at the splat/substrate interface on the flattening behavior of the droplet. A falling atmosphere was atmospheric pressure nitrogen to prevent the oxidation of the melted droplet, and the experiments under low-pressure condition were also conducted. A transition of the splat morphology was recognized in atmospheric pressure nitrogen experiments, that is, the splat morphology on a room temperature substrate was a splash type, whereas that on a high temperature substrate was a disk type. The cross-section microstructure of the splat obtained on the room temperature substrate was an isotropic coarse grain, whereas that on the high temperature substrate was a fine columnar. The grain size changed transitionally with increasing the substrate temperature. Transition temperature on the gold-coated substrate was higher than that on the substrate without coating. The cross-section microstructure of the splat obtained under low-pressure was a fine columnar even on the room temperature substrate. The results indicate that the metal droplet wets better under low-pressure condition than in atmospheric pressure nitrogen condition, and the wetting has a significant role in the flattening of the droplet.