Influence of suspension characteristics on coatings microstructure obtained by suspension plasma spraying

The growing interest for SOFC leads research towards new materials but also towards processes which could be more effective and less expensive to produce fuel cells. Plasma spraying is one of these interesting processes due to its ability to manufacture the whole cell with the same process. The present study uses suspension plasma spraying (SPS) process which consists in injecting a suspension of sub-micrometer particles dispersed in a liquid into a DC plasma torch. This process allows producing finely structured coatings. In the present study, suspensions containing YSZ particles were atomized into a spray by a bi-fluid atomizer. The droplets were then radially injected into an Ar–H2 plasma jet where they were accelerated and heated. This paper aims at studying the importance of the suspension characteristics on the successive steps leading to the coating formation. Various suspensions based on three different media – methanol, azeotropic mixture of methyl–ethyl–ketone/ethanol and water – were evaluated. The amount of dispersant was adjusted in order to obtain well dispersed and stable suspensions. Plasticizers have been used to tailor the viscosity. Each suspension has thus been characterized versus stability, viscosity and surface tension. The influence of these characteristics on the droplets jet (size distribution and jet shape) issued from the atomizer-injector was assessed. In this purpose, two adimensional numbers (We, Re) were determined for the different atomization conditions and were correlated with the droplets jet. Finally, coatings were realised and characterized in terms of porosity by image analysis of SEM pictures. Powder velocity and temperature were evaluated implementing Accuraspray® and drop size distributions (after passing through the plasma jet) were determined as a function of the injection conditions.