Processing And Property Of Carbon Fibers With Surface Treatment Using Hydrogen Peroxide Under Microwave Radiation

Carbon fibers are high-performance engineering materials, which are widely used as reinforcing materials in various composites. Carbon fiber reinforcements with superior mechanical performance are particularly preferred for advanced composite applications. The interface between carbon fibers and matrix play s a critical role in controlling the overall properties of the composites. However, the poor adhesion between carbon fiber surfaces and polymer matrix is often caused by chemical inertness of carbon, so modifying carbon fibers surfaces is required.The thesis described research work on the modifications and characterizations of carbon fiber surfaces, including microbond test probing the interface shear strength. A new method of surface treatment for carbon fibers, i.e. oxidization with hydrogen peroxide under microwave radiation, was developed. The physical and chemical properties on fiber surfaces were investigated by field-emission scanning electron microscopy(FE-SEM), X-ray photoelectron spectroscopy(XPS), Raman spectrometry and laser microscopy. Besides, interface shear strength was measured by microbond test and some influential factors were investigated.The effects of various process parameters(microwave power, processing time and the concentration of hydrogen peroxide) on surface properties of carbon fiber samples were studied by FE-SEM, XPS, Raman and laser microscopy. FE-SEM revealed significant changes in the fiber surface morphology upon varied processing conditions. Some grooves and holes on fiber surface were observed. In addition, the treated fiber surfaces exhibited some special structural features, including nanoscale bumps and spots. XPS analysis revealed that both oxygen content and nitrogen content of the treated fiber surface rose, with the increase of oxygen contents varied slightly, depending on processing conditions and types of carbon fibers. The maximum oxygen content of a lab-made pitch carbon fiber was as high as 16.29 mol%, containing C-O, C=O and other oxygen functional groups. Raman spectroscopy showed increased R values(area ratio of D peak to G peak), indicating that microwave-induced oxidations increased structural disorder on carbon fiber surfaces. The increased roughness of fiber surfaces was also confirmed by laser microscopy. For all five types of carbon fibers studied in the work, significant changes in surface morphology and surface composition were observed, indicating potential wide applicability of this treatment method.In order to quantify the bonding strength between carbon fiber and epoxy resin matrix, the interface shear strength of carbon fiber and epoxy resin was determined by the microbond test. The forces at microdebonding between a resin microdroplet and carbon fiber were obtained from load-displacement curve, and the morphologies of debonded microdroplets and carbon fibers were both examined by scanning electron microscopy. The average interfacial shear strength, standard deviation and coefficient of variation for two fiber samples were calculated. The relationships between interfacial shear strength and the length and diameter of microdroplet, and carbon fibers diameter, were analyzed. The results showed that interfacial shear strength was approximately proportional to the ratio of length-to-diameter of the resin microdroplet, and inversely proportional to the carbon fiber diameter, to the ratio of the microdroplet length-to-carbon fiber diameter, and to the ratio of the microdroplet diameter-to-carbon fiber diameter.