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.
Manufacture and properties of carbon fibre-reinforced C/SiC dual matrix composites
Abstract Carbon fibre-reinforced carbon and silicon carbide dual matrix composites (C/C-SiC) were fabricated by a combination of chemical vapor infiltration with liquid silicon infiltration. The structural characteristics, mechanical performance and tribological properties of the C/C-SiC composites and their wear mechanism at different braking speeds were investigated using a QDM150 friction testing machine, SEM and x-ray energy dispersive analysis. Results indicate that the C/C-SiC composites show an increased bonding strength at the fibre/matrix interface, and the value of flexural strength and compressive strength of the C/C-SiC composites can reach 240 and 210 MPa, respectively. The friction coefficients are between 0.41 and 0.54. Wear rates are not sensitive to the brake speed and remain constant at about 0.02 cm3/MJ, and the friction coefficient is stable. Frictional films with a thickness of 1∼3 μm are formed on the worn surface of the composites upon braking. The wear mechanism changes with increased braking speed, from abrasion at 8m/s, adhesion at 12 m/s and 16m/s to fatigue and oxidation at 20 m/s and 24 m/s, respectively.
Effect of humidity on the tribological behavior of carbon-carbon composites
The present work is a study of the effect of humidity on the tribological behavior of carbon-carbon composites, including two-dimensional PAN/pitch (designated TM), PAN/CVI (designated E), and pitch/resin/CVI (designated A) formulae. Results indicate that there exist close relationships among friction coefficient, wear rate, and worn surface morphology. Whenever a morphological transition has occurred, transitions in friction and wear also occur. Three different debris morphologies are identified. The smooth type I and type III debris films can lubricate, but not the powdery type II debris. The relative humidity level has a strong effect on the tribological behavior of all three composites. Low humidity and high sliding speed generally accelerate the occurrence of type I-to-type II transitions in friction, wear, and debris morphology, whereas high humidity and low speed enhance the formation of type III debris. At any humidity level, the type I morphology is always accompanied by low friction and wear. After the type I-to-type II transition, higher friction coefficient and wear rate are observed in the low humidity than in the high humidity. Under the present conditions, composite E appears to be the most sensitive to the humidity effect, while composite A appears the least sensitive.