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Thermal Conductivity and Stability of HfO 2-Y 2 O 3 and La 2 Zr 2 O 7 Evaluated for 1650 ° C Thermal / Environmental Barrier Coating Applications

HfO2-Y2O3 and La2Zr2O7 are candidate thermal and environmental barrier coating (T/EBC) materials for gas turbine ceramic matrix composite (CMC) combustor applications because of their relatively low thermal conductivity and high temperature capability. In this paper, thermal conductivity and high temperature stability of hot-pressed and plasma-sprayed specimens with representative partially-stabilized and fully-cubic HfO2-Y2O3 compositions and La2Zr2O7 were evaluated at temperatures up to 1700 °C using a steady-state laser heat-flux technique. Sintering behavior of the plasma-sprayed coatings was determined by monitoring the thermal conductivity increases during a 20-hour test period at various temperatures. Durability and failure mechanisms of the HfO2-Y2O3 and La2Zr2O7 coatings on mullite/SiC hexoloy or SiC/SiC CMC substrates were investigated at 1650 °C under thermal gradient cyclic conditions. Coating design and testing issues for the 1650 °C thermal/environmental barrier coating applications are also discussed. INTRODUCTION Thermal and environmental barrier coatings (T/EBCs) will play a crucial role in advanced gas turbine engine systems because of their ability to significantly increase engine operating temperature and reduce cooling, thus greatly help to achieve engine low emission and high efficiency goals. In particular, T/EBCs are being developed for low emission SiC/SiC ceramic matrix composite (CMC) combustor applications by extending the CMC liner and vane temperature capability to 1650 °C (3000 °F) in oxidizing and water vapor containing combustion environments. In order to achieve the engine design and performance goals, the coating systems under development must provide vital thermal and environmental protections for the Si-based components at the gas temperatures exceeding 1760 °C (3200 °F) in the harsh combustion environments, while maintaining the CMC substrate below its temperature limit of 1316 °C (2400 °F). The advanced 1650 °C T/EBC system is required to have significantly increased phase stability, lower lattice and radiation thermal conductivity, and improved sintering and thermal stress resistance under the engine high-heat-flux and severe thermal cycling conditions. Advanced heat-flux testing facilities [1–3] and approaches [3–7] have been established for the 1650 °C coating development. The simulated combustion watervapor environment is also being incorporated into the heat-flux test capabilities [3]. In order to develop a T/EBC system for the 1650 °C Si-based CMCs, a multi-functional coating concept may be considered. Figure 1 shows an advanced coating design concept for the 1650 °C T/EBC system for CMC combustor applications. The top layer is a high-temperature capability ceramic thermal barrier coating. This top coating layer will provide the major thermal protection for the sub-coating systems and CMC substrate, and also act as the firststage radiation barrier by reducing the transmission of the infrared thermal radiation from the combustion gas environment and form the higher temperature coating surface. The second layer is an energy dissipation layer that will act as a “bond coat” to tailor the thermal expansion mismatch and reduce the thermal stress of the coating system. The third layer is a secondary radiation barrier and thermal control layer. The sophisticated coating compositions and structures will provide combined high reflectivity and diffusion barrier functions. The fourth layer will be an improved stability environmental barrier coating system for the CMC substrate.