Microscale Surface Defects Influence on Thermally Sprayed Alumina Droplets Deformation Dynamics

During plasma spraying, interaction between splats and surface micro-sized 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 (SEM) analysis. Droplets impacting directly onto a micro-sized surface protuberance show no signs of pre-mature splashing behavior. The microscopic features (˂5µm) 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.