Physical Mechanisms in Plasma Spray Processing of Suspensions

Suspension plasma spraying (SPS) is increasingly studied to produce finely structured coatings with columnar microstructures promising for thermal barrier coatings in aerospace application, especially. However, this process involves many parameters and complex phenomena with large spans of timescales and space scales, such as droplet breakup, liquid droplet evaporation and various physical phenomena occurring within the suspension droplet, making it difficult to master. The aim of this work is to supply a prior analysis to select the dominant physical phenomena and their sequence according to droplet properties and plasma conditions. Thus, this study provides a detailed analysis of these mechanisms, their significance and characteristic timescales, as well as the effects of plasma flow and droplet suspension properties on droplet/submicronic particle behavior. Additionally, the main governing forces acting between the different continuous and discrete phases (plasma, liquid, submicronic particles) are investigated. The results show that droplet breakup occurs much faster than other mechanisms, while the diffusion of submicron particles within droplets, mainly controlled by convection, is the longest. Furthermore, the importance of considering the lift force on droplet transport within the plasma stream is highlighted, particularly in the high-gradient region of the plasma jet.