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Environmental challenges to the implementation of ceramic composites in gas turbines

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Gas turbine technology is at a cross-roads, with demands for increased engine performance and fuel flexibility translating into higher material temperatures, up to 1500°C in some designs, and more chemically aggressive environments for the hot gas path components. One promising strategy to overcome the limitations of Ni-base superalloys is based on the utilization of ceramic composites (CMCs). Oxide-based composites have superior environmental stability but are limited by the temperature capability of the fibers and thermomechanical concerns. Hence, much of the current effort is focused on SiCf/SiCm systems, which are critically dependent on the performance of two types of coating systems. Fiber coatings enable de-coupling of the fibers and matrix at their interfaces during crack propagation, while engineered surfaces protect the CMC from various forms of environmental attack. This presentation will discuss the challenges to the implementation of CMCs arising from issues related to the coating systems and the associated scientific issues. Notably, CMCs are susceptible to temperature embrittlement in the 700-900°C range due to the oxidative degradation of the fiber-matrix interfaces when cracks propagate in the matrix. Moreover, the environmental barrier coatings have a complex architecture because they must incorporate multiple layers for different functions, with attendant implications for the thermo-chemical and thermo-mechanical stability of the system. While feasible architectures have been identified, these systems are still vulnerable to extrinsic forms of damage associated with the penetration and impact of particulate materials in the gas stream. Strategies based on modified SiC matrices with self-healing capabilities and lower defect densities have been proposed to improve the robustness of the system against damage. The challenges involved in implementing these concepts will be discussed. Acknowledgments: The presentation benefits from various intramural and extramural collaborations and includes contributions from past and present graduate students (K.M. Grant, N.M. Larson, M.D. Novak, D.L. Poerschke, R.B. Reitz, E.M. Zaleski) and post-docs (S. Burk, R.W. Jackson, S. Krämer, N. Verma, J.Y. Yang). Research support provided by programs from the P&W Center of Excellence in Composites, the Office of Naval Research and the Air Force Office of Scientific Research. About the speaker: C.G. Levi received a Ph.D. in Metallurgical Engineering from the University of Illinois at Urbana-Champaign and has been in the faculty at UCSB since 1984, where he is Professor of Materials and Mechanical Engineering. The overarching theme of his research is the fundamental understanding of microstructure evolution in inorganic materials, and the application of this understanding to the design and synthesis of improved coatings, thin films, composites and monolithic systems, with emphasis on high temperature applications. Current areas of work include thermal and environmental barrier coatings for advanced gas turbine components, self-healing matrices and fibers for CMCs, environmental barrier layers for advanced nuclear energy systems, novel high temperature alloys and multi-phase functional materials. He is a Fellow of the American Ceramic Society (2012) and has received the TMS Morris Cohen Award (2014), the 2008 NIMS Award, the DLR Wissenschaftspreis (2004), the Alexander von Humboldt Forschungspreis (2002), the 1989 Grossman Award and the 1982 Howe Medal from ASM International.

This talk is part of the Engineering Department Bio- and Micromechanics Seminars series.

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