Status Quo of Unstable Solidification Process of Plasma Sprayed Droplets Impinging to Substrate

As is known, phases, defects, residual stress, deposition efficiency and mechanical properties of plasma sprayed coatings depend dramatically on the fabrication parameters and substrate pretreatment state. In other words, slight changes might lead to distinguished differences between coatings. Therefore, it is of great importance to perform researches on the formation process of plasma sprayed coatings microscopically. Basically, plasma sprayed coatings are constituted of enormous molten or semi-molten droplet, which are heated and accelerated by plasma torch. Hence, the flattening and solidification behavior of single droplet after impacting on the substrate are the most elementary process of the formation of coatings. The factors influencing the flattening and solidification behavior can be divided into two categories, namely, droplet properties: velocity, temperature, size, material properties, molten status, etc; substrate status: roughness, preheated temperature, surface chemistry status, deposition of adsorbates and condensates, wetting properties and interfacial thermal contact resistance between droplet and substrate, etc. With different droplet properties and substrate conditions, the droplets might spread into five different types, i.e., disk-like, fragmented, finger-like, flower-like and bubble-like. Each type represents specific deposition mechanism and quality. Therefore, a series of parameters can be used to quantificationally characterize the morphologies of splats, such as perimeter, area, volume, depth, circularity, eccentricity, solidity and flattening degree.