Interactions between successive high-velocity impact droplets during plasma spraying

During plasma spraying, interactions between successive impacting particles/droplets are critical to the interfacial bonding properties between splats and the microstructural development of the bulk coating. The transient spreading process of two successive plasma- sprayed Ni20Cr droplets with different impacting spacing was numerically studied while the interfacial features between these two solidified splats were experimentally characterized by focused ion beam (FIB) microscope and transmission electron microscope (TEM). Droplets directly impacting onto the center of a previously deposited splat solidified quickly, inducing splashed fingers and smaller solidified grains. A higher droplet impact temperature could remelt the bottom splat and promote metallurgical bonding along the splat-splat interface more readily. For droplets impacting away from the previously solidified splat, the second molten droplet could either climb over the periphery of the previous splat inducing significant finger splashing, or spread beneath the curling-up splat modifying the solidification process. The solidification behavior of the subsequent droplet at the splat-splat interaction region played an important role in influencing the formation of inter-splat pores and grain growth.