ICMCTF 2023 Session A2-2-WeM: Thermal and Environmental Barrier Coatings II

Environmental barrier coatings (EBCs) have enabled the implementation of SiC/SiC ceramic matrix composites (CMCs) in gas turbines by protecting CMCs from H₂O-induced volatilization. Improving the reliability of CMC components requires long-life EBCs and accurate EBC lifing. Steam oxidation-induced failure is the most frequently observed EBC failure mode. NASA previously reported that modifying Si / Yb₂Si₂O₇ EBC by adding Al₂O₃ or Al₂O₃-containing compound, such as mullite (3Al₂O₃·2SiO₂) and YAG (Y₃Al₅O₁₂), in Yb₂Si₂O₇ reduces the EBC parabolic oxidation rates by up to about 20 times at 1316^oC in steam. Oxidation is a thermally activated process and therefore the oxidation rates vs. temperature relationship typically follows the Arrhenius equation. This paper reports the results of follow-on studies that expanded the test temperature to 1250 ^oC and 1350 ^oC to understand the temperature dependency of oxidation rates, which is a key component for EBC lifing.

SiC ceramic matrix composites (CMCs) are desired for use in combustion environments to achieve higher turbine operating temperatures, although CMCs require environmental barrier coatings (EBCs) for protection from the gas environment. EBC systems are known to primarily fail through coating delamination via growth of a thermally grown oxide (TGO) at the EBC – silicon bond coat interface. The TGO undergoes a phase transformation during thermal cycling, which results in stresses that may encourage EBC spallation. Uncoated Si and SiC substrates were exposed to air and steam to form cristobalite TGOs of a thickness that remained intact upon cooling to room temperature. These TGOs were then thermally cycled across the phase transformation temperature for cristobalite and characterized with Raman microscopy to map the α↔β transformation. The stress in the silicon and SiC was simultaneously measured with Raman microscopy. Finite element modeling was used to predict the CTE mismatch stress and transformation stress in the TGO which was compared to the measured Raman stress. This research was funded by the Advanced Turbine Program, Office of Fossil Energy and Carbon Management, U.S. Department of Energy.

Continued growth within the aviation sector is often seen as running counter to the shared objective of carbon-neutral flight. That growth trend brings not only higher demand and corresponding emissions, it also brings exposure to more aggressive and challenging operating environments for gas turbine engines. Accordingly, sustainable propulsion systems must build on recent learning from harsh environmental exposure while provisioning for a future state of more efficient, lower carbon engine technology sets encompassing a suite of higher thermal efficiency, hybrid-electric, and alternate fuel solutions. A common thread across this complex future turbine engine landscape is the need for continued improvement and development of thermal and environmental barrier coatings used in the hottest areas of the engine to enable both performance and durability. An overview and historical perspective of Pratt & Whitney work in this area will be presented, along with a system-level vision for the continued role of coating development in helping to deliver on a more sustainable net-zero future.