Microscale Surface Defects Influence on Thermally Sprayed Alumina Droplets Deformation and Splashing Dynamics

During plasma spraying, interaction between splats and surface microsized features can be critical to the splat dynamic progress and consequently to the coating microstructural development and interfacial bonding. The transient spreading of molten alumina impacting a flat substrate exhibiting micro-obstructions, commonly produced during surface machining, grinding and/or even polishing, is numerically investigated using a three-dimensional model comprising of splat solidification and shrinkage developments. Single isolated splats are also experimentally characterized using top surface scanning electron microscope analysis. Droplets impacting directly onto a microsized surface protuberance show no signs of premature splashing behavior. The microscopic features (<2.5 μm) are not able to generate flow instabilities to initially affect the splat inherent overall spreading. However, subsequent splat peripheral contact with target surface micro-obstructions, characterized by peak and valley features, induces peripheral lift, waviness, and instability. It follows that the ejected destabilized material shears/fractures during stretching triggering the formation of splash fingers. Solidification plays a major role in detracting the role of surface micro-obstructions, i.e., surface roughness, in splashing phenomena.