Processing and performance of yttrium-stabilized zirconia-based multilayer thermal barrier coatings

Thermal barrier coatings (TBCs) have been used since the 1960s to protect superalloy gas turbine engine components and improve engine efficiency. TBCs typically operate under severe high temperature environments reaching ∼1200–1300°C. Heat resistance and coating adherence after high temperature exposure are two critical issues controlling coating performance and lifetime. The current state-of-the-art TBC system, which contains a Y₂O₃-partially stabilized ZrO₂ (YSZ) top coat and a metallic bond coat, provides excellent thermal resistance. However, it commonly fails due to bond coat oxidation-induced phenomena that causes coating delamination and spallation.; This work investigates a new multilayer TBC that incorporates a thin oxygen barrier layer of yttrium aluminum garnet (YAG) into a typical YSZ TBC system. Processing studies reveal that a unique plasma-spray system, called Small-Particle Plasma Spray (SPPS), is effective in expanding the tailorability of coating microstructures and properties by introducing a new spray parameter, injector angle. Furthermore, the use of small particles permits the production of thin individual coating layers.; Thermal property measurements indicate that the presence of YAG further reduces the thermal conductivity in the as-sprayed state and does not detrimentally increase conductivity with long-term, high-temperature exposure. In addition, YAG locally improves the high temperature phase stability of the YSZ in the near-interface region of the multilayer coating. Oxidation resistance was found to noticeably improve with the addition of YAG. Although the bond coat oxidation rate in a YAG/YSZ multilayer was lower compared to that in a pure YSZ coating, coating spallation occurred prematurely in both cases due to limitations in the test configuration. Overall, the concept of using SPPS multilayers to tailor oxidation resistance for improved TBC performance has been demonstrated.