Flattening and solidification behavior of in-flight droplets in plasma spraying and micro/macro-bonding mechanisms

The microstructure and bonding strength of plasma sprayed coating are highly dependent on flattening and solidification behavior of in-flight droplets. In the present study, the flattening and solidification behavior of subsonic/supersonic droplets after impacting on a flat substrate was comparatively studied. Results show that a dimensionless Somerfeld number (K) of flattening droplets can be used as the splashing threshold. Due to the extremely rapid spreading and solidification of supersonic-droplet, some large-sized bubbles and grains that usually formed at the bottom of flattened subsonic-droplet were effectively restrained, leading to the improvement of splat/substrate micro-shear strength. The effect of grain size and growth feature on the macro-bonding property was further elaborated. It was found that the simultaneous increase of both velocity and temperature of in-flight droplets was an effective approach for the improvement of bonding strength of as-sprayed coating.