Melt-infiltrated SiC/SiC composites are emerging as the leading material for shrouds, vanes, and combustor liners in commercial gas turbines because of their high thermal conductivity and excellent thermal shock resistance, creep resistance, and oxidation resistance in comparison to other ceramic matrix composites and to the nickel- and cobalt-based superalloys used in current engines. One risk that can significantly reduce the lives of the SiC/SiC composites in an engine environment is the lack of environmental durability in the presence of water vapor. Oxide coatings have shown promise in providing environmental protection for these SiC/SiC composites because of their resistance to corrosive environments up to 1250 °C. To further extend the thermal capability of SiC/SiC ceramic composite components to the hot-wall surface temperatures (up to 1650 °C), researchers are developing thermal and environmental barrier coatings (TEBCs) under the NASA Ultra-Efficient Engine Technology (UEET) Project.
At the NASA Glenn Research Center, baseline plasma-sprayed yttria-stabilized zirconia (YSZ: ZrO2-8wt%Y2O3) thermal barrier coatings (TBCs) and BSAS+mullite/Si layered environmental barrier coatings (EBCs) showed excellent adherence to SiC/SiC ceramic matrix composite substrates under thermal cycling in burner rig tests up to 1250 °C (ref. 1). However, the same coating system tested to higher temperatures (up to 1480 °C), especially under a temperature gradient, resulted in coating-delamination failures (as seen in the following figure) that were attributed to an increase in the crack-driving forces in the coating. Therefore, research is being performed to develop advanced TEBC coating systems that can last for the tens of thousands hours required for gas turbine applications. The ideal TEBC system should have low thermal conductivity, good high-temperature stability, and resistance to sintering.
Typical observed cracking in multilayered coating systems.
Long description of figure 1.
The effects of temperature gradients and coating configurations were analyzed using finite-element analysis models based on the observed cracking in a YSZ baseline TEBC system (ref. 2). The crack-driving forces as a function of crack length, coating layer thickness, top coating modulus, and sintering time were synthesized to provide insight into the material properties required in minimizing the cracking and delamination observed in the baseline TEBC protecting system for SiC/SiC composite material. The variation of the effective stress intensity factor (SIF) with sintering time was calculated as a function of the TBC elastic modulus. The analysis assumed a constant crack length of 0.5 mm in the TBC layer for a TEBC geometric construction of a 0.381-mm-thick TBC layer and a 0.254-mm-thick EBC under the thermal gradient condition. The effective SIF increased with increasing TBC modulus starting from 1 MPa-m1/2 for a TBC modulus of 5 GPa to well above 10 MPa-m1/2 for a TBC modulus of 50 GPa (above any measured fracture toughness KIC of plasma-sprayed YSZ; see the following graph). As the TBC layer sintered during exposure, the effective SIF increased with time upon cooldown. The crack-driving forces increased quickly within the first 50 hr of exposure and then increased more gradually with increasing time. The initial rise of the SIF was gentler for a low-modulus TBC material in the range of 5 to 10 GPa and was steeper for the higher modulus TBC (20 to 50 GPa). The results show that highly compliant materials are desirable in a TEBC system to accommodate the coefficient of thermal expansion mismatch and the temperature gradient.
Variation of the stress intensity factor with time for various TBC elastic moduli (ETBC). TBC thickness, 0.254 mm; crack length, 0.5 mm; KIC, measured fracture toughness of the TBC.
Long description of figure 2.
Find out more about the research of Glenn’s Durability and Protective Coatings Branch: http://www.grc.nasa.gov/WWW/EDB/
Ohio Aerospace Institute (OAI) contact:
Dr. Louis J. Ghosn, 216-433-3822, Louis.J.Ghosn@nasa.gov
Glenn contact:
Dr. Robert A. Miller, 216-433-3298, Robert.A.Miller@nasa.gov
Authors:
Dr. Louis J. Ghosn, Dr. Dongming Zhu, and Dr. Robert A. Miller
Headquarters program office:
Aeronautics Research, VSP
Programs/Projects:
UEET, low-emission combustors
Last updated: October 16, 2006
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