New criterion for splashing in flattening of thermal sprayed particles onto flat substrate surface

In the collision of a liquid droplet onto a flat solid surface, the splashing parameter K=We0.5Re0.25 (which is composed of both in-flight information and the physical properties of the droplet) has been used to evaluate the break-up phenomenon of the droplet. However, the transition phenomenon in the flattening morphology of a thermal sprayed particle from splash-shaped to disk-shaped with increasing substrate temperature cannot be explained by K. In order to evaluate the transition phenomenon, the splashing parameter on flattening Kf=0.5a1.25Re−0.3K (which includes the influence of both the in-flight information of the particle and the substrate temperature change) was proposed in this study as a general evaluation criteria for splashing in the flattening of thermal sprayed particles. A critical value of Kf, Kfc, corresponds to the critical condition for splashing. In the present study, the relations between K and Tt for Ni, Cu and Fe particles were examined to observe the transition phenomenon of the splat morphology. A strong linearity between K and Tt for each material was recognized. Consequently, by introducing the measurement results in a free-falling experiment as a simulation of thermal spraying, Kfc was estimated to be approximately 7. This value was almost constant, regardless of the splat material and the collision conditions. It is expected that the disk-splat is formed at Kf<Kfc and the splash-splat at Kf>Kfc.