Understanding of suspension DC plasma spraying of finely structured coatings for SOFC

Suspension plasma spraying was used to achieve a dense and thin (∼30 μm) yttria stabilized zirconia (YSZ) coating for the electrolyte of solid oxide fuel cells (SOFCs). A suspension of YSZ powder (d₅₀∼1 μm) was mechanically injected in direct current (dc) plasma jets. The plasma jet acted as an atomizer and the suspension drops (d∼200 μm) were sheared, long before they started vaporizing, into many droplets (d∼2 μm). The solvent of the latters was then very rapidly (a few microseconds) evaporated and decomposed by the plasma jet. The solid particles enclosed in each droplet were then accelerated and melted before impacting on the substrate where they formed splats. The thermal inertia of particles with sizes below 1 μm being low, the standoff distance was much shorter than in conventional plasma spraying (40-60 against 100-120 mm). Thus, the heat flux from the plasma to the coating reached 20 MW·m^-2 when spraying YSZ suspensions with Ar-H₂ or Ar-H₂-He plasma jets. It allowed keeping the whole pass (about 0.8-μm-thick) completely molten resulting after its solidification, for YSZ, in a fully dense coating (20-30-μm-thick) with a granular microstructure.