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=We(0.5)Re(0.25) (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 K-f=0.5a(1.25)Re(-0.3)K (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 K-f, K-f(c), corresponds to the critical condition for splashing. In the present study, the relations between K and T-t for Ni, Cu and Fe particles were examined to observe the transition phenomenon of the splat morphology. A strong linearity between K and T-t for each material was recognized. Consequently, by introducing the measurement results in a free-falling experiment as a simulation of thermal spraying, K-f(c) 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 K-f < K-f(c) and the splash-splat at K-f > K-f(c). (C) 2002 Elsevier Science B.V. All rights reserved.