Adhesion of carbon fibers to amine hardened epoxy resin: Influence of ammonia plasma functionalization of carbon fibers

The modification of carbon fiber surface properties represents a powerful tool to improve the adhesion between fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented work the surface of untreated carbon fibers, taken from the production process directly after carbonization, is functionalized by a low pressure ammonia plasma treatment. Compared to the untreated fibers an enhanced concentration of surface nitrogen functionalities and an increased surface energy are observed. This leads to an improved wetting behavior, similar to that of carbon fibers activated by anodic oxidation. No changes of the fiber surface topography on micro- and on nanoscale are induced by the plasma treatment. Compared to the untreated fiber no increase of interfacial fracture toughness between the plasma treated fibers and an amine hardened epoxy resin is detected by single fiber push-out tests. Anodically oxidized fibers, in contrast, show a significant increase of interfacial fracture toughness. The results suggest, that adhesion of carbon fibers to an amine hardened epoxy resin is dominantly enhanced by interactions between surface oxygen functionalities and the components of the resin. In contrast, nitrogen surface functionalities appear to be of minor importance for fiber matrix adhesion in carbon fiber reinforced amine hardened epoxy resin.