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turbine engine applications using CC composites include exhaust nozzle flaps and seals, augmenters, combustors, and acoustic panels.
Turbine rotor prototype
• Higher temperature pert
• Low weight
Carbon-carbon microstructure » Nonstrategic materials
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).
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.
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