Effects of surface treatment on the tensile and bonding properties of carbon fibers

In this article, the effects of electrochemical oxidation and sizing treatment of PAN-based carbon fibers (CFs) on the tensile properties, surface characteristics and bonding to epoxy resin were investigated. Electrochemical oxidation treatment of the carbon fibers was shown to improve the tensile strength by 9.8%, which is ascribed to the removed surface defects and impurities of the carbon fibers. Further sizing treatment exhibited a negligible effect on the tensile strength. Both oxidation and sizing treatments significantly improved the wettability and surface energies of the CFs by introducing oxygen-containing functional groups on the surface of the CFs. Microbond pull-out test was conducted to characterize the interfacial shear strength (IFSS) between a single carbon fiber and an epoxy droplet. As found, the oxidation treatment slightly increased IFSS from 71.24 MPa to 73.62 MPa. This is due to the formation of chemical bonds between resin matrix and carbon fibers, and the decrease of the mechanical interlocking. Further sizing treatment can significantly enhance the IFSS to 81.03 MPa due to the formation of vast chemical bonds. The effect of fiber treatment on the hygrothermal aging behavior of the bond between single carbon fibers and epoxy droplets was studied. It was found that the hygrothermal aging dramatically deteriorate IFSS of untreated fibers to resin matrix. However, the oxidation and sizing treatment can effectively improve the retention rate of IFSS.