The Influence of Process Equipment on the Properties of Suspension Plasma Sprayed Coatings

As plasma spray coating facilities are scaled up from the lab to mass manufacturing, changes in process equipment may be required.  These equipment changes may affect coating properties, even if the same torch is used.  To investigate the influence of process equipment configuration on coating properties, suspension plasma sprayed yttria-stabilized zirconia (YSZ) coatings were made in two different experimental facilities.  The coatings were targeted for use as electrolytes in solid oxide fuel cells (SOFCs).   Mettech Axial III torches were used in both facilities, but they had different suspension feed systems, substrate holders, and robots.  The lab-scale facility used a pressure-based suspension feed system and a rotating drum substrate holder with no active heating/cooling system.  The torch was mounted to a robot that traversed only in the vertical direction.  The manufacturing-scale facility used a pump-based feeder and a stationary substrate holder that included an active heating/cooling system to regulate substrate temperature.  The torch robot traversed in the horizontal and vertical directions.  This second setup is more representative of mass production coating facilities, and enables the fabrication of larger-area coatings and reduces the amount of overspray compared to the lab-scale facility. 

The equipment differences affected the suspension and solids feed rates, torch velocities relative to the substrate, heat inputs to the substrate per torch pass, substrate temperature fluctuations per pass, average substrate temperatures, and the number of passes needed to deposit coatings.  In general, it was found that deposition efficiencies were much higher at the manufacturing-scale facility compared to the lab-scale facility.  Also, at the lab-scale facility, the plasma conditions that produced the lowest coating permeability had poor deposition efficiency.  Conversely, at the manufacturing-scale facility, it was possible to achieve both high deposition efficiency and low permeability with the same conditions.  Coating microstructural differences between facilities are also presented and discussed.