Improving the manufacturability and assessing the performance of rare earth zirconates for thermal barrier coatings

Abstract

Key aero-engine components are subject to gas stream temperatures above the melting point of their metal alloy, a demanding environment that requires the deployment of thermal barrier coatings (TBCs) for their operation. Electron-beam physical vapour deposition (EB-PVD) can produce TBCs with unique columnar microstructures, conferring them the strain compliance required to survive in the cyclic, high-strain, high-thermal load environment experienced by the rotating parts of the high-temperature turbine. Rare earth zirconates (REZs) are proposed as substitute materials of partially-yttria-stabilised zirconia (PYSZ) to operate at higher temperatures due to their ability to withstand CMAS (calcium‑magnesium-alumino-silicate) attack. On the other hand, the lower toughness of REZs makes them more susceptible to erosion damage during service, and some manufacturability issues have been noticed in previous studies. The current paper evaluates the manufacturability, CMAS resistance and erosion resistance of PYSZ and three REZ systems: gadolinium (GZ), neodymium (NZ) and lanthanum (LZ) zirconate. For the first time, successful NZ TBCs have been produced by EB-PVD, presenting a similar morphology, ease of manufacture and CMAS resistance to GZ, but inferior erosion resistance. LZ has compositional banding, lacks columnarity in La-rich regions, and has the lowest CMAS and erosion resistance of the REZs investigated. Co-evaporation of lanthana and PYSZ ingots increased compositional homogeneity of LZ, but the increased La content of the unoptimised process makes the coatings hygroscopic. The erosion resistance of the REZs is 6–10 times lower than PYSZ, but the study of their failure mechanisms indicates a potential improvement strategy by altering the coating morphology.