Development of carbon/carbon composites by co-carbonization of phenolic resin and oxidised pan fibers

In order to develop cost effective carbon/carbon composites for general purpose applications, alternative reinforcements need to be tried to replace high modulus carbon fibers. Present studies deal with application of oxidised PAN fibers as reinforcement with char yielding phenolic resin and co-carbonisation of the composites. Polyacrylonitrile (PAN) fibers have been oxidised at 230 degrees C for different times in the presence of air. Characterisation of oxidised fibers for surface group analysis exhibit active groups such as carboxylic, etc., on the fiber surface increasing with time of oxidation. Pyrolysis of the fibers to 1000 degrees C causes anisotropic shrinkage, whereas the cured resins exhibit, to a greater extent, isotropic shrinkage. The shrinkage of the composites during carbonization has been found to be controlled by the shrinkage of the fibers and is observed to be anisotropic, higher in cross-section than in length direction. This results in the development of stresses in the matrix and at fiber/matrix interfaces. As a result, on examination of the composites under an optical microscope with polarised light, carbonized composites exhibit the development of anisotropic matrix structure initiating at the interface. On heat treatment to 2800 degrees C, the carbon matrix as well as the fibers exhibit well-developed graphitic structures with a sharp XRD peak at 2 theta = 26.4 degrees. Optical microscope and SEM examinations of the composites, show strong fiber/matrix bonding, although the two constituents are clearly distinguishable. In no case has the reaction between the resin and the oxidised PAN fibers leading to coalescence of the two or destruction of the fibers with the matrix during co-carbonization been observed. The composites at each stage have been evaluated for flexural strength. Carbonized composites exhibit low strength with catastrophic failure whereas the strength of the composites is found to increase on graphitization and the fracture is also changed to pseudoplastic. This is attributed to changing fiber/matrix interactions at different temperature of heat treatment of the composites. Copyright (C) 1996 Elsevier Science Ltd