The inhibited and uninhibited composites showed different degradation morphologies. Uninhibited CC composite oxidized in a manner that individual fibers were easily discernible. The fibers were loose and easily scraped from the surface. Inhibited CC composite oxidation morphologies displayed few individual fibers. Instead smooth fiber bundles were covered with boron oxide.
Both types of CC composite oxidized for a short time exhibit a pitted morphology. The pits might be caused by porosity, spalling, catalytic impurities, or oriented microcrystals. Porosity would result in these pits if oxidation uncovered preexisting voids in the carbon. Spalling along cracks in the carbon might cause small chunks to fall out and form the pits. Catalysis may cause a higher reaction rate to occur in areas where the impurities reside. Finally the orientation of carbon crystals may be such that the surface reaction is faster on certain exposed crystallographic planes. The faster reaction would result in the etching of these crystals and the pitted morphology.
Of these possible causes the catalytic processes are the most likely. Impurity levels in CC composite are larger than in many grades of commercial carbon where catalysis is known to occur. The impurities are leftover from the precursor in CC composite fabrication. Thus catalysis is the rule rather than the exception.
The morphologies of the uninhibited composites show that the matrix carbon is oxidized before the fibers. The preferential attack of the matrix phase has also been documented by Yauda et.al. In the present investigation the pole figure was constructed for CC139E after 2 days of oxidation in air at 625C. This exposure resulted in a 96% weight loss. The pole figure shows the persistence of the cross pattern generated by the 0-90° weave of fibers which documents that the fibers are the last carbon constituent remaining.
Inhibited CC composite have similar morphologies to uninhibited composites at short times. During oxidation the B and B4C inhibitor changes the oxidation morphology. The most prominent change is the coating of large bundles of fibers and matrix. The first stage involves the oxidation of inhibitors into molten boron oxide. The bundles are about 200-500 um. Large weight losses result in thicker boron oxide coatings on the fibers. At times, pores were evident in the B2O3 which may arise because of CO evolution. Finally it is important to note that, the B2O3 coated the fibers and, therefore, helped to retain the shape of the inhibited samples. The fact that the inhibited samples retain their shape implies that the boron oxide coats the fibers more than the matrix, i.e., the inhibitors protect the fibers to a large extent. The greater dimensional stability with oxidation is a benefit of the inhibitors. The apparent and He-measured densities for inhibited CC composite vary by a larger degree than the uninhibited composites. The means that the inhibited composite is forming significant open porotisy as it oxidizes.