Advanced Ceramics: Continuous Fiber Ceramic Composites
Continuous fiber ceramic composites (CFCCs) have been an active area of research for the past two decades. For much of that time, I was quite skeptical of the potential for commercial success of this class of materials. However, a recent report[1] from the U.S. Department of Energy’s Office of Industrial Technologies on the accomplishments of its CFCC Program has transformed me from a skeptic to an enthusiast.
What are CFCCs?
CFCCs consist of a reinforcing phase, which is essentially a continuous length of ceramic fiber or yarn, and a matrix that converts the fiber architecture into a rigid body and provides environmental protection to the fibers. The fibers and matrix may be of the same or differing compositions (i.e., SiC/SiC, or Al2O3/mullite).Because fibers are significantly stronger than matrix materials (a SiC fiber is greater than 2.5 GPa, while bulk, sintered SiC is typically ~500 MPa), one would think that the CFCCs would be stronger than monolithic ceramics of similar composition. Often they are weaker. In most processing methods for producing CFCCs, it is difficult to fully densify the matrix and develop high levels of multi-axial strength. However, if the fibers and the matrix are weakly bonded, as the matrix fails the fibers do not. They slide along the matrix—pull-out—and continue to strain. As successively stronger fibers fail, strength peaks and falls off. The result is a stress strain curve that looks remarkably like a metal, with a proportional limit and a large strain to failure (for a ceramic)—in short, a “psuedo-ductile” ceramic, offering the prospect of large structural ceramic components that will not fail catastrophically.
Why Skeptic to Enthusiast?
First, ten years ago fibers that could carry appreciable loads at temperatures above superalloys (i.e., ~1100°C) were not readily available. Today they are. Second, CFCCs require coatings on the fibers that maintain a weak interfacial bond between fiber and matrix, as well as protect the surface of the fiber from environmental degradation at high temperature. Today, interfacial coatings have demonstrated efficacy for thousands of hours in several high temperature applications. Third, it was difficult and costly to manufacture useful shapes out of CFCCs. It still is today, but significant progress has been made.Finally, 10 years ago very few industrial applications existed where CFCCs were identified as providing a potentially cost effective capability. Today there are a significant number. One example of the pace of development is a SiC/SiC CFCC. When oxidized for 100 hours at 1200°C with a precracked matrix, this material went from a RT retained strength of only ~60 MPa and a brittle failure mode in 1994, to a strength of ~290 MPa and retention of its “metals-like” stress strain behavior by 1999. Dramatic improvements such as this have led to some equally dramatic application demonstrations.
Applications
Radiant Burner Screens. Radiant burners are used widely in industrial processes. Oxide/oxide gas-fired infrared radiant burners for removing water in making paper are more effective than metallic burners. The trick is that the oxides can be doped with rare earth compounds to tune the spectral emittance of the burner to match the optical absorption of the wet paper.Particle Separation from Hot Gas Streams. Combustion gas streams of municipal incinerators run at ~800-900°C and contain entrained hard particulates. Recent tests of SiC/SiC CFCC show an erosion rate 1⁄13 that of metal parts. After ~2100 hours of running in an incinerator, stress strain curves for exposed and unexposed material were virtually identical. In coal fired gas turbines, hot gas filters are required to remove erosive particulates from the combustor output. Multi-filter arrays of porous oxide/oxide tubes have survived over 1900 hours of pilot plant tests, enduring conditions that would cause monolithic ceramics to fail.
Gas Turbine Combustors. Uncooled ceramic combustors in industrial gas turbines reduce emissions. SiC/SiC combustors have demonstrated over 5000 hours of engine test durability, and a field test aimed at demonstrating 8000+ hours capability is underway.
Other industrial applications include gas turbine shrouds, immersion tubes for aluminum melting, heat treating furnace fans, and heat exchanger tubes.
Markets
Development of a sizable commercial market for CFCCs will require processing improvements leading to component cost reduction, while maintaining reliability. This now appears to be a good bet.For Further Reading
Freitag, D.W. and Richerson, D.W., Opportunities for Advanced Ceramics to Meet the Needs of the Industries of the Future, Office of Industrial Technologies, U.S. Department of Energy, December 1998, report # DOE/ORO 2076.Ceramic Fibers and Coatings, Publication NMAB-494, National Academy Press, Washington, DC, 1998.
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