Microstructure of Carbon/Carbon Composites

The use of carbon/carbon composites for thermostructural applications requires a basic knowledge of the structural characteristics of the composites that control their engineering properties. These include macroporosity, microstructure of the matrix including interbundle and interfilament matrix, microcracking and interfaces at different levels, fiber orientations, demaged fibers etc. By understanding details of the composites’ structure and mechanism of microstructural development, one can control the matrix microstructure and improve its mechanical properties, especially the fiber strength utilization. Microstructural features of carbon/carbon composites may be classified according to various scales of magnification. At the largest useful scale, the millimeter scale, microstructure relates to composite architecture. It comprises the distribution of fibers and matrix, especially interbundle matrix, voids, and pores. At the microscale, it reveals features such as interfilament matrix and at the nanoscale it relates to fiber/matrix interface. Figure 5.1 shows a broad classification of some of the characteristics. For 2D and MD carbon/carbon composites, Jortener [1] classified the former as minimechanical features and the latter as micro-mechanical features. The origin and characteristics of these depend on (i) the reinforcement configuration, (ii) type of carbon fiber used, (iii) matrix precursor, (iv) fabrication routes, and (v) processing conditions.