Prime factors of importance in achieving tough and high thermal conductive carbon/carbon composites are proper choice of reinforcing carbon fibers, carbon matrix microstructure, density and macrostructure of the composites. Processing routes and the choice of carbon precursor greatly influence the density, macrostructure and matrix microstructure. The voids and macrocracks are normally fewer in pitch-derived C/C composites processed through HIP route than in those made by CVD route or phenolic resin derived carbon route. The latter exhibit low densities. These features also control the thermal transport mechanism in carbon-carbon composites. In order to have desired fiber/matrix bonding and to reduce cracking at the fiber/matrix interface, pyrolytic carbon coatings are also used at the surface of the fibers.
The matrix microstructure is also an important aspect governing the properties of carbon-carbon composites. From the strength point of view, a semicrystalline or randomly oriented carbon matrix is desired which can arrest crack propagation whereas for thermal and electrical properties, highly graphitic matrix is a prerequisite. The development of crystalline carbon matrix originates from CVD route or pitch route. Thermosetting polymer route result in amorphous carbon matrix. However, in composites made with high strength carbon fibers or surface treated carbon fibers, due to strong fiber/matrix bonding at polymer stage and differential pyrolysis shrinkage in fibers and matrix, thermal stresses are generated in the matrix which on heat treatment to 2000C and above cause stress graphitization of the matrix. The subject has been of great scientific interest, especially correlating fiber/matrix bonding and resulting orientation of graphite basic structural units in the matrix. The technological importance of this phenomenon has been to control the fiber/matrix bonding to that low priced carbon fibers can be used for making carbon/carbon composites for mechanical and general purpose application.
Oxidation protection of carbon/carbon composites:
Carbon is prone to reaction with oxygen at temperature of 450C and above. For long time application of these composites at elevated temperature in a normal environment, it is essential that these composites be protected against oxidation. Therefore studies on oxidation protection of carbon/carbon composites are as important as the development of the composites themselves. Oxidation protection systems for carbon/carbon composites are based on modification of matrix through addition of some oxidation inhibitors or/and deposition of ceramic coatings on the surface. These Si, Zr, Ta, Al etc.
With carbon fiber reinforcements in different forms and directions and thermosetting resins or thermoplastic pitches or hydrocarbon gases as matrix precursors, these composites can attain densities between 1.6-1.98g/cm3. Reinforcing fibers are more anisotropic in structure and properties than the carbon matrix. Accordingly, the properties of carbon-carbon composites are dependent on fiber volume content and fiber orientation. Unlike polymer matrices, carbon matrices contribute significantly to the automate properties of the composites, especially in case of pitch and CVD-derived carbon matrices. Carbon-carbon composites are a family of materials with choice of variation in fiber and matrix architecture, structure, microstructure, mechanical, thermal and physical properties etc. Hence these provide high performance materials for application in a number of sectors.