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Figure 9. Photomicrograph of lattice fringe image of interface between PAN fiber in pitch matrix.
shear relative to each other. Because of this, such layers will tend to preferentially orient themselves parallel to any surface across which they move. White (ref. 27) has also pointed out that wetting of a fiber surface by the mesophase will result in a similar effect. Regardless of the absolute mechanism of orientation, the result is that in a carbon matrix derived from pitch, each fiber will be surrounded by a matrix of highly aligned, strong-stiff graphitic basal planes.
Figure 10 is an optical micrograph of a 3-D CC composite composed of PAN (T-300) fibers and a coal tar pitch matrix. The composite was processed using a multiple high pressure impregnation carbonization (HPIC) technique at 30 ksi (206 MPa) pressure to densify the composite. Figure 10 position A shows a transversely oriented fiber bundle surrounded by the interbundle matrix (position C). The composite contains a significant amount of porosity (positions D and E) of which there are two types. The larger voids (position D) were formed as a result of volatilization during carbonization/graphitization cycles, whereas the cracks/fissures (position E) were formed due to shrinkage of the matrix carbon during cool down from the processing temperature (ref. 3). A large proportion of the voids are infilled with a resinous carbon added to complete the densification process.
Figure 10. Photomicrograph ofCC composite (PAN-based fibers plus coal tar pitch matrix).
The development of the mesophase within the interbundle matrix of the composite is not always uniform. The structure of some of the matrix regions is coarse-grained mosaic whereas the structure of other regions is fine-grained mosaic. This variation in the optical textures of the binder-coke causes differences in mechanical properties and chemical reactivity of the composite (refs. 85 and 86).
Variations in processing conditions have reportedly affected the matrix orientation. For instance, Cranmer et al. (ref. 87) concluded that the alignment of the graphitic planes under low-pressure carbonization is primarily controlled by the flow motion of mesophase. High pressures have been reported to favor a transversely aligned matrix (ref. 88). Other factors also seem to be important because Murdie et al. (ref. 89) have recently reported that both parallel and transversely oriented graphite planes were produced in a specimen carbonized under atmospheric pressure. Although local variations in crystallographic orientation are observed, pitch matrix composites generally exhibit microstructures in which the fibers are surrounded by coaxial, graphitic sheaths.
A schematic representation of the alignment of the mesophase pitch matrix within a unidirectionally reinforced material is shown in figure 11 (ref. 90). In this figure, the prismatic edges of the graphitic planes are schematically shown as flow lines distributed between the individual fibers. A similar flow pattern was observed in a PAN-pitch composite by etching the composite surface with chromic acid or atomic oxygen (ref. 59).
The example shown in figure 12 is an SEM micrograph of a PAN-pitch composite etched in this way. This micrograph clearly shows that the intrabundle matrix platelets (the basal planes of the platelets are parallel to the platelet broadface) are parallel to the fiber surface. As can be seen from this transverse section, the fiber bundles appear fully dcnsified, although the interface between the fiber and matrix appears to be quite reactive since it has been etched preferentially. The presence of a central core in the PAN fiber shown in this figure has been discussed in the literature and is attributed to graphitization of partially stabilized PAN fibers (ref. 91).
The interface between an individual fiber and the interbundle mesophase matrix is shown in figure 13. In this case, two rather large fissures are shown that occur
Figure 12. SEM micrograph of transverse section of fiber bundle showing fiberlintrabundle matrix interface after etching with atomic oxygen at 100°Cfor3hr.
at some distance from the fiber surface. It appears, therefore, that in some areas the bonding between the graphite-like layers formed within the pitch matrix is weaker than that between the pitch and fiber. This feature is quite different from that which occurs in pitch fiber-reinforced phenolic resin matrix materials where shrinkage during first carbonization causes the matrix to pull away from the surface of the fiber.
Figure 14 is a scanning electron micrograph of the same composite showing the interface between the interbundle matrix and a longitudinally oriented fiber. This micrograph shows that near the fiber-matrix interface, the matrix structure is parallel to the fiber surface in the longitudinal orientation for a considerable distance. This alignment only exists for a distance of 2 /xm to 6 /xm from the fiber surface and becomes more random in the bulk of the interbundle matrix. Zimmer and Weitz (ref. 92) has indicated that even when a strong magnetic field is applied to attempt reorientation of the mesophase, the mesophase is still aligned parallel to fiber surfaces for radial distances up to 6 pm. The results of this SEM study indicate that each fiber is surrounded by a highly oriented thin sheath of matrix. This sheath