Surface treatment of carbon fibers by continuous gaseous system

The mechanical performance of carbon fiber-polymer composites strongly depends on interfacial adhesion, which is function of types of carbon fiber, surface chemistry, physical and chemical interactions, and mechanical interlocking. Untreated and unsized high strength carbon fibers were oxidized by continuous thermochemical treatment. Surface properties were investigated before and after treatment (chemistry, topography), as well as their mechanical properties. X-ray photoelectron spectroscopy revealed a significant increase of the oxygen atomic content from 3% to 17% and the analysis of the carbon peak showed that carboxylic acid functionalities and hydroxyl groups were generated. An observation of the fiber surface by scanning electron microscopy did not show any dramatic change of the fiber morphology and surface topography at a micron scale. No significant damage of the mechanical properties (tensile strength) was noticed either. The influence of the changes of the surface properties on interfacial adhesion of carbon fiber-epoxy and -vinyl ester matrix was evaluated using 90° flexural and short beam shear tests with unidirectional coupons. A significant increase of the 90° flexural and short beam shear strength showed that the interfacial adhesion between carbon fibers and resins used was improved. The observation of the fracture profile by scanning electron microcopy confirmed those results, as the rupture of the coupons after surface treatment was more cohesive.