Parametric study of suspension plasma spray processing parameters on coating microstructures manufactured from nanoscale yttria-stabilized zirconia

A parametric study was conducted to determine the effect of suspension plasma spray (SPS) processing parameters, including plasma torch standoff, suspension injection velocity, injector location, powder loading in the suspension, and torch power, on the final microstructure of coatings fabricated from 80 nm diameter yttria-stabilized zirconia (YSZ) powders. Coatings made with different conditions were analyzed via stereology techniques for the amount of unmelted powder and spherical particles, which are undesirable features in a thermal barrier coating. Observation of unmelted powders, generally in the form of submicron-sized clusters of nanometer-sized powders, indicated insufficient plasma enthalpy or time in the plume to evaporate the liquid carrier in the fragmented droplet and melt the remaining powders. For coatings produced with a 4 cm standoff, a suspension injection velocity of 23 m/s, or a torch power of 21 kW, unmelted powders covered between 20 and 50% of the surface area. The presence of micron-sized spherical particulates was an indication of either partial powder melting or complete melting and re-solidification of a YSZ powder cluster prior to impact with the substrate. A 6 cm standoff, suspension injection velocity of 15 m/s, or 10 wt.% YSZ powder suspension each yielded coatings in which spherical particles comprised more than 8% of the coating surface area. The parametric study findings and the plasma-spray literature were employed to further modify the SPS experimental parameters to produce coatings that minimize unmelted powder and spherical particulates. The improved spray parameters for 80 nm diameter YSZ particles were found to be suspensions comprised of 8 wt.% YSZ powder, a 21 m/s suspension injection velocity (corresponding a ~ 50 ml/min volumetric flow), a 5 cm standoff, and a torch power of 50 kW. With these conditions less than 2% of the coating top surface area was covered by unmelted powder and spherical particulates.