Carbon fibers and composites with epoxy resins: Topography, fractography and interphases Original Research Article

Interphases exist in hybrid materials and significantly influence their mechanical performance. To find a bridge between the microscopic and macroscopic mechanical properties, this work investigates the nanoscopic nature of surface/interphases in terms of topography, fractography, adhesion and stiffness. Here, we show that variations in both adhesive and attractive forces on oxidized high modulus (HM) and intermediate modulus (IM) carbon fiber surfaces appear to result from the coating layer. The coating layer is critical for adhesive interaction with two different epoxy resins. The HM fiber has the apparently higher roughness but lower surface area than the IM fiber on the scanning scale of 200 nm. The surface roughness on a few tens of nanometer scale has no significant contribution to interphase adhesion from ‘mechanical interlocking’. In contrast, the true contact area on the nanometer scale plays a dominant role in interfacial adhesion. Using force volume nanoindentation, the stiffness of the resin region near the finished fiber surface was found to not depend on the distance from the fiber surface. Our observations suggest an energy-geometry link between critical interphase energy release rate by micromechanical testing and detailed fracture surface features.