Splat Formation Mechanism in Thermal Spraying*

In order to understand the splat formation process of individual splat deposited by thermal spraying, commercially available Nickel powders with diameter of several tens micrometers were thermally sprayed onto mirror polished AISI304 substrate surface. The deposited splat shows transition phenomenon from splash type to disk one in flattening on collision onto substrate surface according to both substrate temperature and ambient pressure change. That is, transition temperature, Tt and transition pressure, Pt can be defined as critical values for the transitions. The three-dimensional map was set up to control the thermal spraying process. The observation on the bottom surface morphology on single splat indicates that the initial solidification may induce the splashing occurrence. Moreover, as the simulation of the real thermal spraying process, free falling experiment was conducted in this study. The thermal history of the free falling Cu droplet onto AISI304 substrate was investigated, indicating the flattening pattern is decided just after collision onto solid surface, which is enough earlier to finalization of the flattening.