Column Formation in Suspension Plasma-Sprayed Coatings and Resultant Thermal Properties

The suspension plasma spray (SPS) process was used to produce coatings from yttria-stabilized zirconia (YSZ) powders with median diameters of 15 μm and 80 nm. The powder-ethanol suspensions made with 15-μm diameter YSZ particles formed coatings with microstructures typical of the air plasma spray (APS) process, while suspensions made with 80-nm diameter YSZ powder yielded a coarse columnar microstructure not observed in APS coatings. To explain the formation mechanisms of these different microstructures, a hypothesis is presented which relates the dependence of YSZ droplet flight paths on droplet diameter to variations in deposition behavior. The thermal conductivity ( k th ) of columnar SPS coatings was measured as a function of temperature in the as-sprayed condition and after a 50 h, 1200 °C heat treatment. Coatings produced from suspensions containing 80 nm YSZ particles at powder concentrations of 2, 8, and 11 wt.% exhibited significantly different k th values. These differences are connected to microstructural variations between the SPS coatings produced by the three suspension formulations. Heat treatment increased the k th of the coatings generated from suspensions containing 2 and 11 wt.% of 80 nm YSZ powder, but this k th increase was less than has been observed in APS coatings.