Ceramic components exhibit superior high-temperature strength and durability over conventional component materials in use today, signifying the potential to revolutionize gas turbine engine component technology. Silicon-carbide fiber-reinforced silicon carbide ceramic matrix composites (SiC/SiC CMCs) are prime candidates for the ceramic hot-section components of next-generation gas turbine engines. A key barrier to the realization of SiC/SiC CMC hot-section components is the environmental degradation of SiC/SiC CMCs in combustion environments. This is in the form of surface recession due to the volatilization of silica scale by water vapor. An external environmental barrier coating (EBC) is a logical approach to achieve protection and long-term durability.
Key EBC requirements.
Long description..
At the NASA Glenn Research Center, research was undertaken in the Ultra-Efficient Engine Technology (UEET) Program to develop advanced, multilayer environmental barrier coatings (EBCs), having a temperature capability of 2700 °F (1482 °C) at the EBC surface and 2400 °F (1316 °C) at the EBC/CMC interface. The preceding figure indicates the key requirements for a successful EBC. These include water vapor stability, chemical stability between the multiple layers of the EBC and at the EBC/CMC interface, low thermal expansion and phase stability for minimizing the stresses, and environmental durability in combustion environments. It is also desirable for an EBC to have a low thermal conductivity to maximize its thermal insulation potential.
Two very promising EBC systems have evolved from this work: silicon/mullite+BSAS/rare earth silicates and silicon/mullite+BSAS/hafnia-based oxides (U.S. patent pending). Mullite is an alumina-silica system, of the form 3Al2O3-2SiO2, and BSAS is a barium-strontium-alumina-silicate system, of the form BaO-SrO-Al2O3-2SiO2. The photomicrographs show cross sections of melt infiltrated (MI) SiC/SiC CMCs coated with silicon/mullite+BSAS/scandium silicate+scandia EBC and silicon/mullite+BSAS/hafnia+mullite EBC, respectively, after 300 hr at 1400 °C with 1-hr cycles in a simulated combustion environment (90 vol% H2O-balance O2). Both coating systems show excellent performance with minimal oxidation, cracking, and chemical reactions.
Cross section of Si/mullite+BSAS/Sc2Si2O7+Sc2O3 EBC-coated SiC/SiC CMC after 300 hr at 1400 °C with 1-hr cycles in a simulated combustion environment (90 vol% H2O-balance O2) .
Cross section of Si/mullite+BSAS/HfO2+mullite EBC-coated SiC/SiC CMC after 300 hr at 1400 °C with 1-hr cycles in a simulated combustion environment (90 vol% H2O-balance O2).
The new EBCs meet or exceed the UEET Program goal. The coatings are stable in water vapor (no material recession) at 1500 °C, are chemically stable (no detrimental interfacial chemical reactions) at temperatures up to 1400 °C, and have demonstrated environmental durability in a simulated combustion environment at temperatures from 1316 to 1400 °C. The new EBC top layers possess thermal conductivity as much as a factor of 2 lower than that of zirconia-8 wt% yttria-the current state-of-the-art thermal barrier coating-making them excellent thermal barrier coatings as well. The new EBCs will be optimized, scaled-up, and applied on SiC/SiC vanes. The performance of the coated CMC components will be evaluated in Glenn’s high-pressure, high-velocity combustion burner rig during the upcoming year.
Find out more about the research of Glenn’s Environmental Durability Branch.
Cleveland State University contact: Dr. Kang N. Lee, 216-433-5634, Kang.N.Lee@grc.nasa.gov
Glenn contact: Dennis S. Fox, 216-433-3295, Dennis.S.Fox@nasa.gov
Authors: Dr. Kang N. Lee, Dennis S. Fox, Dr. Jeffrey I. Eldridge, Dr. Dongming Zhu, Dr. Narottam P. Bansal, and Dr. Robert A. Miller
Headquarters program office: OAT
Programs/Projects: UEET
Last updated: June 25, 2003
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