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thickness (0.2 pm to 4.0 /xm) can vary within a fiber bundle owing to complex flow patterns generated during mesophase formation and different interfiber distances.
A TEM bright-field image showing a typical transverse morphology of an intrabundle matrix region in the same composite is shown in figure 15. As can be seen, the crystallites of the intrabundle matrix exhibit a flow-type morphology with basal planes oriented roughly parallel to the fiber surface. This geometry is similar to that described in Zimmer and White's model (ref. 93). The crystallites of the matrix near the fiber surface (position A) or between two closely spaced fibers exhibit better alignment with the fiber surface than those farther away from fibers. Microfissures, indicating weak bonding, are clearly revealed within the matrix and along the fiber-matrix interface.
A typical morphology of an interfacial region in the same composite is shown in figure 16. The bonding between the matrix and fiber appears discontinuous because some matrix graphite crystallites are well bonded to the fiber while other crystallites are poorly bonded. This interfacial morphology is similar to the fissuretype interface classified by Ragan and Marsh (ref. 86) in their study of bulk binder, coke-filler coke composites. This higher magnification micrograph shows more clearly the shape, size, and distribution of the numerous microcracks along and near the fiber-matrix interface (position B). The matrix crystallites adjacent to the interface are rather small and irregularly shaped, while those farther away from the fiber surface appear larger. The microcracks in the matrix, formed between and parallel to the graphitic platelets, generally have a sharp, lenticular shape. Near the fiber surface, these microcracks become smaller and denser.
The crystallites in the pitch matrix appear much larger and more graphitic than those in the turbostratic PAN fiber; this is clearly revealed in the lattice fringe images shown in figure 17. A rough estimate from these and other lattice images indicates that the crystallite length in the mesophase pitch matrix is at least 10 times larger than in the PAN fiber.
In summary, the microstructural investigation of pitch matrix composites indicates
1. The carbon produced from pitch precursor materials is highly graphitic and generally exhibits a flow-type morphology.
2. Although the bonding between the PAN fibers and the mesophase pitch matrix appears continuous at low magnifications, high-magnification TEM studies reveal numerous microcracks along the fiber-matrix interface, each separated by what appear to be well-bonded regions.
3. The microcracks within the matrix are smaller and denser when they are near the interface.
Figure 15. Photomicrograph of interfaces between PAN fibers in pitch matrix composite.
Figure 16. Photomicrograph of interface between PAN fiber and pitch matrix.
(b) Longitudinal pitch fiber.
Figure 17. Photomicrograph of comparison of lattice fringe images taken from (a) PAN fiber and (b) pitch fiber.
4. The crystallite size in mesophasic matrices is large compared to that in PAN fibers.
5. The crystallites in the matrix are aligned parallel to the fiber surface in a range approximately 0.2 pm to 6 pm from the fiber surface; thereafter, they tend to become more random.
Resin Matrix Composites
Carbon-carbon composites formed from reinforced resins are processed by carbonizing (and graphitizing) carbon fiber-reinforced phenolics (or other suitable resin matrices). A typical microstructure of a phenolic resin reinforced with pitch fibers is shown in figure 18. The composite shows good bonding characteristics between the fibers and the resin matrix. Polarized light microscopy, combined with a retarder plate, rendered the phenolic matrix purple. The color of the phenolic matrix structure did not change with the rotation of the microscopic stage, thus indicating that the resin is optically isotropic. A carbon fiber reinforced phenolic
resin of this type typically contains less than 3 percent macroporosity, and exhibit; a density of about 1.5 g/cm .
Carbonization of resin matrix composites involves heating the material in ar inert environment to a temperature of 1(XX)°C. During this heat treatment process, volatiles are emitted in the form of vapors and gases such as H2O. methane, and hydrogen. This process causes the resin to shrink in volume so that the porosity of the composite increases to 30 to 40 percent, while the bulk density typically decreases to between 1.2 g/cm to 1.4 g/cm , depending on fiber weave and volume fraction.
The resin contracts extensively on carbonization (illustrated in fig. 19). Shrinkage has been so excessive that the resin has shrunk away from almost all of the fibers present in this area. Interference colors indicated that most of the resin structure remained isotropic; however, in some regions of the intrabundle matrix, it was obvious that an anisotropic structure had developed. The development of this anisotropic structure was restricted to those areas where the distance between two fibers was very small, i.e., <2 /xm.