CREEP RESISTANCE AND WATER VAPOR DEGRADATION OF SIC/SIC CERAMIC MATRIX COMPOSITE GAS TURBINE HOT SECTION COMPONENTS

Ceramic Matrix Composites (CMCs) are potentially valuable structural materials for high temperature stationary components in the hot section of advanced gas turbines. Endurance field testing, totaling in excess of 88,000 engine operating hours, conducted since 1997 by Solar Turbines Incorporated has shown promise for SiC/SiC and Oxide/Oxide CMC combustor liners. In support of ongoing development life prediction modeling was used to generate creep rupture strengths for two SiC/SiC CMC model systems, incorporating polycrystalline Hi-Nicalon and stoichiometric Sylramic-iBN fibers, respectively. The creep rupture strengths were combined with water vapor degradation modeling, extrapolated from laboratory studies, to estimate the Upper Use Temperature (UUT) of CMC combustor liners in the hot section of gas turbines of various pressure ratios. In the absence of a protective Environmental Barrier Coating (EBC) the UUT is <1000 degrees C and the advantage of the superior creep rupture strength of the Sylramic-iBN fiber over the Hi-Nicalon fiber is largely lost. The selection of a suitable EBC restores the creep rupture strength of the SiC/SiC CMCs provided the EBC remains functional during the operation of the gas turbine over the expected service life of 30,000 hours. A heat transfer study was conducted to estimate the EBC surface temperature and CMC surface and average temperatures over the Turbine Rotor Inlet Temperature (TRIT) range of 1000-1400 degrees C. A comparison was made with a standard metal combustor liner with Thermal Barrier Coating (TBC) which can be used for TRITs up to similar to 1200 degrees C. It was learned that the SiC/SiC CMCs 455 with the standard Si/Mullite/BSAS EBC does not offer an advantage over the metal/TBC systems at a TRIT of 1200 degrees C because of the recession rates of the EBC constituents. Replacing the BSAS top coat with a rare earth disilicate or monosilicate has the the potential for a reduction in surface recession of the EBC by one to two orders of magnitude. It is estimated that rare earth disilicate- and monosilicate-based EBCs may enable adequate SiC/SiC CMC life for gas turbines with TRITs up to similar to 1350 degrees C and similar to 1400 degrees C, respectively.