Microstructure of pitch fiber-phenolic/CVI matrix carbon-carbon composite

Conventional and polarized light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the microstructure of a two-dimensional mesophase pitch fiber, phenolic resin char plus chemical vapor infiltrated (CVI) matrix carbon-carbon composite. Optical and SEM results indicate that the pitch fiber bundles are bonded by interbundle resin char. Each individual fiber within a bundle is surrounded by two distinctive CVI carbon layers. The majority of the intrabundle resin has shrunk from the fiber surface during fabrication, so that the large gaps among fibers are later filled by CVI. The numerous fibrils within individual fibers are highly aligned to the direction of fiber axis. TEM shows that both CVI carbons are anisotropic, with basal planes predominantly parallel to the fiber surface, and both exhibit a turbostractic structure. Compared with the first laminar CVI, the second CVI carbon has less perfect alignment of basal planes and fewer inter-basal-plane microfissures. A major portion of the interface between fiber and first CVI is physically separated. The interfaces between the two CVI layers as well as between CVI resin char are much better bonded. Dark field and selected area diffraction techniques have confirmed the excellent alignment of basal planes to the fiber axis and a fairly good three-dimensional order in the fiber crystallites. Most resin char exists in the form of large pockets among fiber bundles and has an amorphous structure.