Present knowledge in suspension plasma spraying

Suspension plasma spraying (SPS) consists in injecting a non-Newtonian liquid in a d.c. plasma jet where it is fragmented and then vaporized. The sub-micrometric or nanometric particles contained in the suspension are then accelerated, heated, partially or totally melted before flattening onto the substrate to form the coating. Such coatings are finely structured and present better thermo-mechanical properties than conventionally sprayed ones. Indeed, coating properties are strongly linked to: the interaction between the suspension and the plasma jet that depends on the liquid jet or injected drop average diameters and velocities, the suspension surface tension and its viscosity, the momentum density of the plasma jet and its fluctuations (linked to the arc root fluctuations). These phenomena control the liquid fragmentation followed by resulting droplets vaporization (two orders of magnitude slower) and acceleration; (1) the heat and momentum transfers to the solid particles contained within the droplets (related to their size distribution and propensity to agglomerate). These transfers are relatively complex since their particle average size is below a few tenths of micrometer, value for which the Knudsen effect and thermophoresis are promoted and since their dimensionless Stokes number close or lower than unity will make them to follow the fluid flow rather than impacting onto the substrate; (2) the rather short spray distance (30-50 mm) used to account for the very low inertia of the sprayed particles resulting in heat fluxes that can reach 30 MJ.m-2 (more than one order of magnitude higher than in conventional spraying) and thus play a key role on deposited layer transient temperatures; (3) the spray pattern for which it is very important to avoid the entrapment of untreated or poorly treated particles that very significantly affects the layer structure by promoting stacking defects. This paper presents what is our present knowledge in this field together with the available tools implemented to characterize the plasma-liquid interaction and the coating formation.