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 (kth) 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 kth values. These differences are connected to microstructural variations between the SPS coatings produced by the three suspension formulations. Heat treatment increased the kth of the coatings generated from suspensions containing 2 and 11 wt.% of 80 nm YSZ powder, but this kth increase was less than has been observed in APS coatings.