Abstract
Yttria-Stabilized Zirconia (YSZ) suspensions are currently popular in developing strain resistant columnar structured thermal barrier coatings by Suspension Plasma Spray (SPS) as a less costly alternative to conventional EB-PVD. Coatings produced by SPS have a disadvantage of reduced usable spray distance, compared to conventional APS, due to quenching of the plasma by the suspension liquids, which are most commonly alcohol-based. The reduced spray distance can interfere with the coating process for substrates with complex geometries such as turbine blades. This paper shows how spray distance can be increased by using larger suspension particle sizes that are not normally considered for SPS. Such large particle suspensions are shown capable of producing columnar or segmented YSZ coating microstructures that are similar to those produced by submicron particle suspensions, but at longer and more practical spray distances. Another limitation to SPS process technology is the delivery system of feedstock from the point of manufacture to the SPS feed hopper. Current commercial ready-to-use suspensions have limitations involving cost, transportation and storage that effect both the producers and the end-users. An alternative suspension delivery system may be applied to SPS feedstock materials, including current sub-micron and the coarser particle size cuts described herein. Discussed is a pre-formulated dry feedstock that is constituted into fresh suspension by the end-user with locally sourced liquid media and appropriate high-speed mixing equipment. This alternative delivery system for suspensions provides lower cost materials and process flexibility that is particularly suited to commercial scale SPS coating facilities.
