In the late 1950’s, converting carbon fiber-resin matrix composites into carbon fiber-carbon matrix composites by slow pyrolysis of the resin was found to produce useful high-temperature materials. Densification of these materials by repeated cycles of resin or pitch impregnation and pyrolysis or by infiltration and decomposition of hydrocarbon gases produced bodies with thermal shock and fracture toughness properties far superior to those of earlier synthetic graphite materials.
To this day, CC composites are principally known as lightweight, very-high-temperature materials with superior thermal shock resistance, toughness, and ablation properties. These attributes and specific tribological properties are emphasized in present aerospace and military applications for CC composites such as rocket nozzles, heat shields, reentry vehicle nosetips, and aircraft brakes.
It is not because of a lack of structural capability that current applications do not involve long-term operation under substantial primary loads. When made with high-performance carbon fibers, CC composites can have strength and stiffness characteristics comparable to those of many metal alloys. Because a variety of fiber weaves are available, properties can be tailored for specific component applications.
Carbon-carbon composites with superior mechanical properties contain highly oriented fibers derived from mesophase pitch or PAN resin. Fibers of this type have been thermally stabilized for normal CC processing and have strengths in the 2000Mpa to 2400Mpa range and elastic moduli between 350Gpa and 400Gpa. Composite elastic moduli in the principal fiber directions usually reflect the fiber modulus according to the rule of mixtures. Strength utilization of the fibers is 25 percent to 50 percent of rule-of-mixtures predictions, depending on the fiber architecture and processing specifics.
Similar to many fiber composites, shear and cross-fiber tensile properties are often a major issue and frequently have a strong influence on design and materials selection. In general, the shear strengths and moduli of CC composites are low compared with those of more conventional materials. The same limitations exist for tensile properties in directions normal to the fiber directions because of processing that creates less than optimum bonding between the matrix and fibers.
One of the reasons CC composites are attractive is because they can exhibit relatively high values of fracture toughness. These materials belong to a special group of composites in which the failure strains of the matrix is much lower than that of the fibers. As a result, strong bonding between the matrix and fibers produces composites that are both brittle and low in strength because strong bonds promote failure of the fibers at the low strains where the matrix fails. Conversely, relatively weak bonds allow matrix cracking to occur without crack propagation throughout the fibers.
Matrix cracking can occur during CC processing as a result of matrix shrinkage during pyrolysis, the thermal expansion coefficient mismatch between the fibers and matrix, and when the composite is loaded in service. Strong bonding in carbonization can result in fiber damage and, combined with strong bonding that persists in the final product, can result in weak and brittle composites. In contrast, weak bonds leave fibers undamaged and allow the matrix to crack in service; however, they continue to transfer load to the fibers so the composite can utilize the high strength of the fibers. Shear displacement of the matrix relative to the fibers and eventual fracture of the fibers at points away from the primary zone of matrix failure create the fibrous pullout fracture surfaces indicative of toughness.
A good method of comparing the fracture toughness of two materials is the notched beam work-of-fracture test. This simple test measures the energy required to move a tensile crack through the material in a controlled fashion.
Carbon materials are unique in their retention of mechanical properties at high temperatures. High-performance fibers exhibit modest tensile strength increases as temperatures increase to about 2200C, while the tensile elastic modulus gradually decreases to about 50 percent of the room temperature value at 2200C. Carbon-carbon composites show similar behavior in the principal fiber directions. The matrix and matrix bonding to the fibers, however, play a large role in determining the effect of temperature on the shear, cross-fiber tensile, and compressive strengths. One reason for matrix cracking is the CTE mismatch with the highly oriented fibers. Cracking is extensive at room temperature. Cracks close as the temperature increases, providing a more coherent and, perhaps, adherent matrix. This phenomenon accounts for increases in shear and cross-fiber tensile strengths that observed with increasing temperature and, together with increasing fiber strengths, is consistent with observed increases in compressive strength. Moduli either decrease or increase with temperature, depending on the influence of matrix cracking relative to the basic thermal reductions in modulus of the constituents. This change appears to depend on fiber architecture and specifics of the CC processing.
The high-temperature time-dependent deformation of carbon fibers and CC composites is currently an active area of investigation. Although comprehensive scientific and engineering studies are needed to define creep mechanisms and effects of microstructure, fabrication processing, and thermal and mechanical history on deformation behavior, the available data clearly demonstrate the superior structural capability of CC composites at very high temperatures. For example, conservative interpretation of the data projects creep rates on the order of 10-3 percent per hour in the 2000C to 2200C range at stresses of 70Mpa to 140Mpa for unidirectionally reinforced CC composites.