Anodic surface oxidation for pitch-based carbon fibers and the interfacial bond strengths in epoxy matrices

In order to investigate the mechanism of anodic surface oxidation, pitch-based high-modulus carbon fibers with different surface crystal sizes have been oxidized in 0.1 M NH4HCO3, and their differential double-layer capacity, Cd, and polarization resistance, Rp, in a buffer solution were measured by the coulostatic method. X-ray photoelectron spectroscopy and laser Raman spectroscopy were also employed to characterize the surfaces. The interfacial bond strengths in epoxy matrices were evaluated from the interlaminar shear strength (ILSS) or interfacial shear strength (IFSS). It was found that the values measured by the coulostatic method could be used for monitoring the effect of the anodic surface oxidation because there was a good correlation between Cd and ILSS. From those characterizations, we have proposed that the mechanism of anodic oxidation of pitch-based carbon fibers is selective oxidation and the appearance of prismatic surfaces in the crevices, without a change in the surface crystal sizes. Since the prismatic surfaces have many sites that are chemically active with respect to epoxy resin, even after the fibers have been deoxidized, pitch-based carbon fibers show strong adhesion to epoxy resin.