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

Suspension plasma spraying was used to achieve a dense and thin (/spl sim/30 /spl mu/m) yttria stabilized zirconia (YSZ) coating for the electrolyte of solid oxide fuel cells (SOFCs). A suspension of YSZ powder (d/sub 50//spl sim/1 /spl mu/m) was mechanically injected in direct current (dc) plasma jets. The plasma jet acted as an atomizer and the suspension drops (d/spl sim/200 /spl mu/m) were sheared, long before they started vaporizing, into many droplets (d/spl sim/2 /spl mu/m). 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 /spl mu/m 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/spl middot/m/sup -2/ when spraying YSZ suspensions with Ar-H/sub 2/ or Ar-H/sub 2/-He plasma jets. It allowed keeping the whole pass (about 0.8-/spl mu/m-thick) completely molten resulting after its solidification, for YSZ, in a fully dense coating (20-30-/spl mu/m-thick) with a granular microstructure.