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turbine engine applications using CC composites include exhaust nozzle flaps and seals, augmenters, combustors, and acoustic panels.

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Turbine rotor prototype

• Higher temperature pert
without cooling

• Low weight
JJjf • Potential low cost

Carbon-carbon microstructure » Nonstrategic materials

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Figure 12. One-piece, bladed, carbon-carbon turbine rotor (ref. 27).

Carbon-carbon material systems using coatings, TEOS, and additions to the basic CC recipe have improved the oxidation resistance of products made of CC composites by an order of magnitude. The ACC composites are being used in products such as the nozzle in the F-100 jet engine afterburner, turbine wheels operating at >40 000 rpm, nonwetting crucibles for molten metals, nose caps and leading edges for missiles and for the Space Shuttle, wind-tunnel models, and racing car and commercial disk brakes (ref. 28).

Pushing the state of the art in CC composites is the piston for internal combustion engines (refs. 27 and 29). The CC piston would perform the same way as any piston in a reciprocating internal combustion engine while reducing weight and increasing the mechanical and thermal efficiencies of the engine. The CC piston concept features a low piston-to-cylinder wall clearance; this clearance is so low, in fact, that piston rings and skirts are unnecessary. These advantages are made possible by the negligible coefficient of thermal expansion of this kind of CC (0.54 x 10-6 cm/cm/°C (0.3 x 10-6 in./in./°F)).## Carbon-carbon material maintains its strength at elevated temperatures allowing the piston to operate at higher temperatures and pressures than those of a comparable metal piston. The high emittance and low thermal conductivity of the CC piston should improve the thermal efficiency of the engine because less heat energy is lost to the piston and cooling system. The elimination of rings reduces friction, thus improving mechanical efficiency.

Besides being lighter than conventional pistons, the CC piston can produce cascading effects that could reduce the weight of other reciprocating components such as the crankshaft, connecting rods, flywheels, and balances, thus improving specific engine performance (ref. 29).

Conclusions

Carbon-carbon composites offer a unique combination of properties. In nonoxidizing environments, they retain room temperature mechanical properties at >2225°C. For applications in oxidizing environments, current coatings limit maximum use temperatures to «1600°C. High thermal conductivity and low thermal expansion of carbon-carbon composites make them excellent candidates for applications involving thermal shock.

Because of the variety of fibers, weaving patterns, and lay-up procedures that can be used for carbon-carbon composites, their mechanical properties can be tailored over a wide range to fit the application.

Continuing research on carbon-carbon materials in the United States emphasizes an understanding of material behavior. Of particular importance to both researchers and fabrication personnel are methods of improving matrix properties (particularly in-plane shear and out-of-plane tensile strengths) and improving oxidation-resistant coatings with higher use temperatures, longer lifetimes, and less costly fabrication methods.

References

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""Carbon-carbon composites can have a range of thermal expansion coefficients, depending on the processing techniques.

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15. Buckley, John D.: Static, Subsonic, and Supersonic Oxidation of JT Graphite Composites. NASA TN D-4231, 1967.

16. Strife, James R.; and Sheehan, James E.: Ceramic Coatings for Carbon-Carbon Composites. Ceramic Bulletin, vol. 67, no. 2, 1988, pp. 369-374.

17. Buch, J. D.: Graphite Crystals—A General Model for Diverse Carbon Forms. Metal Matrix, Carbon, and Ceramic Matrix Composites, John D. Buckley, ed., NASA CP-2357, 1984, pp. 119-135.

18. Rummler, D. R.; and Sawyer, J. W.: Properties and Potential of Advanced Carbon-Carbon for Space Structures. Metal Matrix, Carbon, and Ceramic Matrix Composites, John D. Buckley, ed., NASA CP-2357, 1984, pp. 149-170.

19. Ransone, Philip O.; and Ohlhorst, Craig W.: Interlaminar Shear and Out-ofPlane Tensile Properties of Thin 3-D Carbon-Carbon. Metal Matrix, Carbon, and Ceramic Matrix Composites, John D. Buckley, ed., NASA CP-2357, 1984, pp. 137-148.

20. Webb, Richard D.: Oxidation-Resistant Carbon-Carbon Materials. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 149-162.

21. Gray, Paul E.; and Engle, Glen B.: Wettability of Carbon/Carbon Composites and Carbon Fibers by Glass Sealants Used in Oxidation Inhibition. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 163-174.

22. Johnson, A. C.; and Finley, J. W.: Carbon/Carbon Composites for Advanced Spacecraft. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 175-190.

23. Sawyer, J. W.; and Moses, P. L.: Effect of Holes and Impact Damage on Tensile Strength of Two-Dimensional Carbon-Carbon Composites. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 245-260.

24. Maahs, Howard G.; and Ransone, Philip O.: Mechanical Property Evaluation of 2-D Carbon-Carbon Panels Fabricated From a Specialty-Weave Fabric. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 261-276.

25. Ohlhorst, Craig W.; and Ransone, Philip O.: Effects of Thermal Cycling on Thermal Expansion and Mechanical Properties of Advanced Carbon-Carbon Composites. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 277-288.

26. Ransone, Philip O.; and Maahs, Howard G.: Effect of Processing on Microstructure and Mechanical Properties of 3-D Carbon-Carbon. Metal Matrix, Carbon, and Ceramic Matrix Composites1985, John D. Buckley, ed., NASA CP-2406, 1985, pp. 289-303.

27. Miller, T. J.; and Grimes, H. H.: Research on Ultra-High-Temperature Materials—Monolithic Ceramics, Ceramic Matrix Composites, and Carbon/Carbon Composites. Advanced Materials Technology, Charles P. Blankenship and Louis A. Teichman, compilers, NASA CP-2251, 1982, pp. 275-291.

28. Klein, J.: Carbon-Carbon Composites, Advanced Mater. & Process., vol. 130, no. 5, 1986, pp. 64-68.

29. Taylor, Allan H.: Carbon-Carbon Pistons for Internal Combustion Engines. NASA Tech Briefs, vol. 9, no. 4, Winter 1985, pp. 156-157.

Bibliography

A. Becker, Paul R.: Leading-Edge Structural Material System of the Space Shuttle. American Ceram. Soc. Bull., vol. 60, no. 11, Nov. 1981, pp. 1210-1214.

B. Rummler, Donald R.: Recent Advances in Carbon-Carbon Materials Systems. Advanced Materials Technology, Charles P. Blankenship and Louis A. Teichman, compilers, NASA CP-2251, 1982, pp. 293-312.

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