Ceramic Composites Still Face Technical Challenges
Dateline: 02/22/00By Laurel M. Sheppard
At the 24th Annual Cocoa Beach Conference & Exposition held the last week in January, sponsored by the American Ceramic Society's Engineering Ceramics Division (www.ceramics.org), ceramic matrix composites (CMCs) were a major focus. Nine sessions were devoted to the subject, which discussed oxidation resistant composites, novel composites and fibers, synthesis and processing. Another special symposium covered mechanical behavior of ceramics, including composites. Over 425 researchers and engineers were registered for the conference, many from government laboratories and universities. Final attendance may have been lower due to Mother Nature playing havoc with the weather during the week of the conference.
The opening session discussed opportunities for CMCs, as well as barriers that need to be overcome for successful commercialization. Carbon-reinforced silicon carbide is already being used as nozzle flaps and seals in certain military jets, and is being considered for the X-38, the next generation space shuttle vehicle. However, for further expansion into other aerospace applications (aircraft, satellites, and launch vehicles), Dr. David Lewis of the Naval Research Laboratory believes that a vertically integrated program with both manufacturers and users is needed to share the research and development required to reduce manufacturing costs.
Another major problem is the lack of design knowledge, which is needed to boost user confidence. This problem is being partially addressed by the development of Volume 5 of MIL-HDBK-17 (www.materials-sciences.com/MIL17), Composite Materials Handbook. The mission of Volume 5 is to develop a CMC handbook, which is the primary and authoritative source for characterization, statistically-based property and performance data of current and emerging CMCs.
Materials covered in Volume 5 include ceramics reinforced with continuous fibers or discontinuous phases, and C/C composites. Sections cover design and supportability, testing methods and data requirements/properties. The effort is being supported by the U.S. Army, Navy, Air Force, the Department of Energy and NASA, with voluntary participation from industry and academia. A first working draft was completed in October 1999, with a final draft expected to be completed by the summer of next year.
Energy Uses Show Promise
Land-based applications for CMCs are expected to have more potential than aerospace applications. One factor that could contribute to higher growth in this area, according to Doug Freitag of Bayside Materials Technology, is a change in demographics. As growth in underdeveloped countries continues, demand for power generation that relies on gas turbines will increase. Urban areas will also be concerned with controlling pollution. Among the barriers that need to be overcome are cost, a short time to market and maintenance/disposal concerns.
The NIST Advanced Technology Program, which has invested $3 billion in research (all fields) since 1990 with industry sharing 50% of the costs, has recently established a three-year program totaling over $7 million for developing CMCs for turbine engine components. Three companies are involved-Siemens-Westinghouse, Solar Turbines, and Engineered Ceramics.
Advanced coal-based power generation systems is another potential market. Since these systems require hot gas filtration under high-temperature high-pressure conditions, ceramics have been considered. Reliability of individual filter elements is a major factor in determining overall system reliability. Because conventional ceramics are prone to brittle failure, a DOE-funded program investigated using elements made from continuous fiber (aluminum oxide) CMCs and corrosion resistant metals (iron aluminide). CMC elements have survived thermal tests and have qualified for pilot testing. They are nearing commercialization and will be installed in a demonstration plant by 2004. The metal filters are already commercially available.
Japan is also investigating CMCs (C/C and SiC/SiC) for nuclear fission and fusion applications in a joint program with the European Union and Russia (the United States dropped out). The operating environment for these applications is severe-materials must withstand radiation, chemical reactions, high temperatures, plasmas and mechanical stresses. SiC/SiC gas cooling blankets are being considered since they could help lower electrical costs. However, Dr. Akira Kohyama of Kyoto University's Institute of Advanced Energy says that performance under irradiation remains an issue.
A Technology Roadmap
The United States Advanced Ceramics Association (www.advancedceramics.org), a trade association based in Washington, D.C. has outlined technology needs for continued commercialization of engineered ceramics, ceramic coatings and CMCs. In the composites area, the cost of precursors used for the matrix (and sometimes the fiber) is still too high. A fiber that can withstand temperatures of 1200-1500 degrees C must be developed. Better screening methods and tests, less expensive complex parts, and improved design times are also required. According to William Werst, Executive Director, national test beds are needed to reduce the time and cost of demonstrations. Low cost and flexible manufacturing processes will be key to CMCs' success, he concludes.
About the Author
Laurel M. Sheppard is President of Lash Publications International (www.lashpublications.com) and Technical Editor of Ceramic Industry (www.ceramicindustry.com). She has a B.S. in ceramic engineering and has written numerous articles on ceramic technology and manufacturing, as well as a market report on ceramic matrix composites for Business Communications Co. (www.buscom.com)