Spreading and solidification of a highly undercooled Y3Al5O12 droplet impinging on a substrate

Because of the containerless state, an in-flight droplet in thermal spray coating (TSC) has been presumed to experience a large undercooling prior to impact on a substrate. In the present investigation, using a Y3Al5O12 (YAG) droplet as a model material for TSC, the spreading and solidification of the droplet impacted on a substrate were investigated over a wide range of undercoolings by means of both levitation and high-speed imaging techniques. The maximum spread of the droplets upon impact decreased with increasing undercoolings. The rebound of the droplets observed at high temperature also disappeared with increasing undercoolings. These results suggest that the spreading behavior and the final splat shape are strongly influenced by undercoolings because of their increase in viscosity.