The effect of electrochemical treatment on the morphology and surface chemistry of pitch-precursor carbon fibers

Tonen pitch-precursor, high modulus carbon fibers were subjected to electrochemical oxidative treatment in an ammonium sulfate solution to improve interfacial adhesion with an epoxy matrix. The embedded single fiber tensile test was utilized to characterize interfacial shear strength between the treated fibers and the Shell EPON 828 epoxy matrix. The results indicated that the treatment enhanced the fiber-matrix bond strength significantly. Similar observation was also made by polarized light study.;X-ray photoelectron spectroscopy was performed to study the effect of the treatment on the surface chemistry of the fibers. The treatment increased oxygen content and introduced nitrogen on the fiber surface. However, there were no significant changes in the oxygen functionalities. A novel amplification scheme was developed to reveal the numbers of surface functional groups capable of reaction with solution phase via an electrochemical assay method involving cyclic voltammetry. The voltammetry results indicated an insignificant role of carboxylic functional groups in improving the fiber-matrix interfacial bond strength.;Finite element analysis was employed to explain the fiber tensile and compressive fractures observed in the embedded single fiber tensile test specimens after fabrication. The analysis demonstrated that significant stresses could develop in the embedded fiber during heating and cooldown in the fabrication stage.;Scanning electron microscopy and atomic force microscopy were employed to evaluate the effect of the treatment on the morphology of the Tonen fibers. The microscopy results indicated that the treated fibers had smooth surface contours without any pitting, etching or other surface erosion. The treatment did not seem to have any deleterious effect on fiber morphology. Raman spectroscopy was also conducted on the fibers to examine the changes in fiber morphology. The intensity of the Raman lines indicated that the treatment had only subtle effects on the fiber morphology, in the form of increased edge defects and dislocations, broken symmetry in the graphite lattice, reduced crystallite size and increased crystal boundary. Ultramicrotomy and transmission electron microscopy were applied to verify the removal of the weak outer layers of the high modulus fibers during the electrochemical treatment. However, these attempts were unsuccessful.