Study on carbon fiber surface treatment based on surface oxidation method
Surface conditions of carbon fibers severely effect on the bonding strength between the fibers and the matrix, even on the mechanical properties of the composites when carbon fibers were used as reinforcement on the preparation of ceramic matrix composites. Surface treatment of carbon fibers was carried out based on the air oxidation method and the liquid oxidation method with concentrated nitric acid (68%) as the oxidizing solution. It could remove impurities from the surface of carbon fibers and increase the degree of the surface concavoconvex, so the active surface area were increased and then improved the bonding strength between fibers and matrix. The surface topographies of carbon fibers after surface oxidation were characterized by SEM. The results illustrate that, impurities on the surface of carbon fibers are decreased and there are many longitudinal grooves evenly distributed on the surface in favor of increasing the bonding strength between the fibers and matrix.
An investigation into the surface heterogeneity of nitric acid oxidized carbon fiber
Abstract The carbon fiber surface plays a critical role in the performance of carbon fiber composite materials and, thus it is important to have a thorough understanding of the fiber surface. A series of nitric acid treated intermediate modulus carbon fibers with increasing treatment level was prepared and characterized using a range of surface sensitive techniques including Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-ray Photoelectron Spectroscopy (XPS) and Raman spectroscopy. The results, which were found to be consistent with increasing treatment levels, were compared to the literature. Raman spectral mapping has been used to investigate the heterogeneity of the carbon fiber surface after nitric acid oxidation. The mapping enabled the effects of surface treatment on carbon fiber to be investigated at a spatial resolution unattainable by XPS and provided chemical structure information not provided by SEM or AFM. The highest level of treatment resulted in the most heterogeneous surface. Raman mapping, while time consuming, can provide valuable information which can lead to an enhanced understanding of the heterogeneity of the carbon fiber surface.
Short Fibre Reinforced CMC Materials for High Performance Brakes
Carbon fibre reinforced Ceramic Matrix Composites (CMC) on the basis of woven fabrics are well known from the development of highly complex and thin walled structures for heat shields of reusable spacecrafts. Since 1988, DLR has developed so called C/C-SiC materials via a three step manufacturing route based on the infiltration of liquid silicon into porous carbon/carbon. Especially for automotive brake disks, the costs of the carbon fabrics and of the common processing techniques for the CFRP preform, like resin transfer moulding or autoclave technique, are too high for a serial production. The most promising way to reduce the costs for the brake disks and to simplify the CFRP manufacture is using a short fibre reinforcement and applying hot pressing techniques. In different tribological test campaigns the performance and the excellent wear resistance of brake disks with short fibre reinforcement were proven. Due to their high thermal stability and their low weight, a great leap in brake technology could be achieved, combining non-fading characteristics with better driving dynamics. This paper describes the manufacture of different short fibre reinforced C/C-SiC materials as well as patented processing techniques, especially for ventilated brake disks. Also, principal material properties and some specific problems in measuring mechanical strength values are reported.