| The level of fiber-matrix adhesion present in Tonen HM pitch-based carbon fiber/EPON 828 epoxy matrix composite materials was characterized. It was determined that the failure of a weak layer on the fiber surface was likely responsible for the poor base case fiber-matrix adhesion.; A number of fiber treatments, designed to enhance the strength of the fiber-matrix interfacial bond were evaluated. Two classes of fiber treatments were considered. The first group was designed specifically to improve the chemical bonding between the fiber and matrix. The second group of treatments was designed to have the capability to remove weak regions on the fiber surface, in addition to increasing the degree of chemical interaction between the fiber and matrix.; The treatments designed to enhance only the chemical bonding between the fiber and matrix were applied to unsized HM fibers which had previously received a proprietary, oxidative surface treatment by the manufacturer (i.e., HMS fibers). It was concluded that the effectiveness of these treatments was dependent upon the level of fiber-matrix adhesion present for the base fibers. When the base adhesion was poor, improvements coincident with the treatments were limited to a value near that for the basal plane shear strength of graphite. This is suggestive of weak graphitic layers on the fiber surface. Treatment of base fibers in which the adhesion was moderate, however, produced strong fiber-matrix adhesion. These enhancements were likely due to increases in chemical interactions between the fiber and matrix.; An electrochemical treatment in ammonium sulfate, potentially capable of removing weak graphitic regions on the fiber surface, and also increasing the fiber-matrix chemical interactions, was subsequently developed. This treatment was observed (using the embedded single fiber and short beam shear tests) to produce strong fiber-matrix interfacial bonding, even when the base case adhesion was weak. Furthermore, the experimental results agreed closely with the maximum composite shear strength which was predicted using a micromechanics analysis. Despite the impressive increases in the shear strength, however, the composite compressive strength was unaffected by the fiber treatment. This suggests that failure under compressive loadings may be due to the failure of the fibers themselves. |
