Influence of nitric acid treatment time on the mechanical and surface properties of high-strength carbon fibers

High-strength carbon fibers were treated with nitric acid and periodically analyzed by several different methods to develop an understanding of overall property changes and how they relate to composite design. Fiber diameter, tensile strength, surface morphology, surface chemistry and surface energy were all evaluated as a function of treatment time and two distinct stages of change were identified; the first characterized by surface modification and the second by carbon material loss. Initially, the tensile strength, degree of surface oxidation and surface energy all increased. The surface oxidation consisted primarily of carbonyl and carboxylic acid types. Then in the second stage, both the tensile strength and surface oxidation reached stable levels and the fiber diameter began to rapidly decrease. The surface morphology and energy were the only properties that showed no obvious changes from one stage to the next. The surfaces were found to be smooth through all treatment times and the energy increased steadily throughout. It is believed that the variation of all of these properties is related to the fiber microstructure and how it varies through the cross-section of high-strength fibers. Specifically, high-strength carbon fibers are known to have better microstructural organization and alignment in the near-surface layer than within the interior.