Durability Prediction of Alumina- and YAG-Based CMC Combustor Liners

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 conducted since 1997 by Solar Turbines Incorporated has shown promise for SiC/SiC and Oxide/Oxide CMC combustor liners with protective coatings, with ∼15,000 hours and ∼25,000 hours of life, respectively. A study has been undertaken to evaluate the potential improvements in the durability of alumina-based CMC combustor liners by incorporating YAG (Y₃Al₅O₁₂)-based fibers, matrices and/or protective CMC surface coatings. It is hoped that this study will encourage future development work for these YAG-based materials for advanced gas turbine applications. Comparative data were generated over a temperature and stress range typical in gas turbine design. Upper Use Temperature (UUT) was compared for 100 MPa maximum stress for 30,000 hours of combustor liner life. The starting point was a previous study in which the authors had examined the combined effect of creep resistance and water vapor degradation on CMC durability. In the current study an improved database was utilized to generate creep rupture curves for the N720/A (Nextel 720 fiber/Al₂O₃ matrix) CMC. Combined creep and water vapor degradation was estimated for N720/A, N720/YAG, YAG/A and YAG/YAG CMCs with or without protective surface coatings. The creep rupture data indicated that in the absence of a protective coating the UUT for N720/A CMC combustor liners in engines with various pressure ratios is only ∼1080°C. An examination of literature data showed that YAG fibers offer a significant creep rupture strength benefit over Al₂O₃-based fibers. It was estimated that an improvement in UUT of ∼60°C is potentially achievable with a YAG fiber-based CMC compared to N720/A. It was estimated that water vapor degradation lowers the UUT of N720A CMC combustor liners from ∼1080°C to ∼1060–1070°C for engines with pressure ratios from 5:1 to 30:1. Literature data indicates that YAG has superior water vapor resistance over Al₂O₃ in a simulated gas turbine hot section environment. In this study, however, substituting YAG as a matrix material for Al₂O₃ in N720/A was not found to significantly improve the durability of this CMC at UUTs up to ∼1150°C, because creep rupture is the predominant degradation mode. The incorporation of an alumina-based FGI (Friable Graded Insulation) in a hybrid oxide CMC can significantly increase the UUT of N720/A. The FGI is a coating of low thermal conductivity that is several mm thick, overlaying the CMC. The incorporation of a 4-mm thick Al₂O₃ FGI was predicted to increase UUT capability of an N720/A CMC combustor liner from 1060 to 1270°C for an engine with 30:1 pressure ratio, and from 1070°C to as much as 1400°C for an engine with a 5:1 pressure ratio. It was estimated that the incorporation of a YAG FGI further increases the UTT capability by 150–230°C, compared to an Al₂O₃ FGI (the actual increase depends on the CMC composition and pressure ratio).