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An SEM micrograph of a graphitized pitch fiber/phenolic resin plus CVI matrix CC composite etched with atomic oxygen is shown in figure 20. Based on the orientation of the microcracks within the resin char (position B), the matrix appears highly oriented. The interface between the CVI and resin char is crenulated and can be expected to facilitate mechanical bonding through an interlocking mechanism. No evidence for a three-dimensional graphite structure existed within the resin char in this specimen, processed at relatively low temperatures. However, Hishiyama and his co-workers (ref. 94) reported that graphitization can indeed occur when phenolic resin precursor CC's are heated to very high temperatures.
Furfural alcohol matrices align relatively easily. A typical example of this alignment is shown in figure 21, an optical micrograph of a rayon fiber cloth impregnated with multiple cycles of furfural alcohol resin. The fiber bundles appear fully densified. The intrabundle matrix region exhibits a high degree of basal plane alignment parallel to the fiber surface. The interbundle matrix alsc exhibits a high degree of basal plane alignment parallel to the fiber bundle/matrix boundary (position F).
In an attempt to increase the yield of resin matrix composites and to reduce shrinkage, the addition of fillers has been investigated (ref. 95). Figure 22 is an optical micrograph showing the structure of the as-molded composite, composed of rayon fibers and phenolic/carbon-black matrix. The matrix exhibits a granular appearance owing to the presence of the carbon black (filler). In addition, filler rich (position C) and filler depleted (position D) regions exist, thus supporting the known observations of the difficulties associated with obtaining a uniform distribution of particles in filler containing resin matrices. The microcrack present at position E has propagated along individual fiber-matrix interfaces, indicating that the fibermatrix bond is weaker than the matrix strength. This fact is confirmed by the TEM study. These shrinkage cracks can extend across the total fiber bundle before stopping at the junction with neighboring fiber bundles of different orientation. At this magnification, there appears to be little difference in bonding characteristics and level of macroporosity between doped and undoped resin matrix composites.
A TEM bright-field image of a phenolic resin matrix doped with carbon black is shown in figure 23. Resin platelets aligned parallel to the faceted carbon-black surfaces are shown at position D. This alignment suggests that the carbon-black particles "grow" (position G). The carbon-black particles not only restrict shrinkage of the resin but also act as nuclei for the development of preferred orientation during processing. In some areas, preferred orientation does not develop as illustrated at position H.
The microcracks present in the specimen shown in figure 23 occur between the basal planes of the oriented resin and are concentric to the center of the carbon-black particles. Such cracks are not observed in neat (pure) resin composite matrices. It should be noted that although the presence of carbon black inhibits the shrinkage
Figure 23. Bright-field image of graphitized phenolic resin containing carbon-black particles.
of the matrix, it does not eliminate it. The matrix still shrinks away from the fiber surface during processing, although for much smaller distances. This shrinking effect is shown in the TEM micrograph of a resin-CVI hybrid matrix composite, (fig. 24(a), ref. 96). In this image, it can be seen that carbonization causes the fillerdoped resin to shrink away from the fiber surface for a distance of about 1 /xm. This space was subsequently infilled with a CVI layer. The interface between the fiber and CVI layer appears continuous while that between the CVI and the filler-doped resin contains short cracks.
The dark-field image of figure 24(b) indicates the continuity of the fiber-matrix interface and the position of the cracks along the CVI-resin interface. Also indicated in this micrograph are the highly aligned basal planes of the filler modified matrix regions. The diameter of these regions varies between 0.1 (im and 0.5 /xm. The selected area diffraction patterns indicate the highly oriented nature of the fiber (fig. 24(c)), the isotropic nature of the CVI deposit (fig. 24(d)), and the isotropic (the diffraction spots are due to the presence of the carbon black) nature of the resin (fig. 24(e)). It is interesting to note that an isotropic CVI layer bonds well to either pitch or PAN-based fibers, whereas CVI deposits with a laminar microstructure do not. The higher magnification TEM micrograph (fig. 25) indicates that the CVI-resin interfacial cracks can propagate along the interface.
In summary, the microstructural investigation of resin matrix composites indicates
1. The carbon produced from resin precursor materials is usually isotropic but can become highly oriented. However, the degree of preferred orientation generated depends on resin type and process conditions. Phenolic resins are more difficult to orient than furfural alcohols. Local regions of phenolic and furfural resins can become graphitic when heated to graphitization temperatures.
2. Photomicrographs suggest that the bonding between resin char matrices and pitch-based fibers is relatively weak. (Fiber-surface treatments may improve this bonding.)
Figure 25. Photomicrograph of interfacial cracks propagating through graphitized resin interface.
3. Addition of carbon black to phenolic resin causes the preferred orientation to develop, concentric with the original carbon-black particles during processing.
4. Microcracks are generated within the oriented resin char around the carbonblack particles that presumably contribute to the reduction of apparent shrinkage of the bulk resin.
CVI Matrix Composites
Infiltrating a carbon fiber preform with a reactive gas leads to the deposition of a carbon matrix and directly produces a CC composite. No subsequent thermal degradation step is necessary. However, pyrolytic carbon tends to deposit preferentially on the surface of the composite being fabricated, thereby blocking surface porosity. An example of this surface layer is shown in the optical micrograph (fig. 26). A very thick surface layer with associated growth cones is observed at position K. Infiltration can only be continued after terminating the densification process and removing the surface crust. Most isothermal depositions are carried out at a temperature low enough to limit the rate of surface deposition, yet high enough to assure an economically viable commercial process. The temperature of reaction has been shown to sensitively affect the density and the