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.
Carbon/silicon carbide composites prepared by chemical vapor infiltration combined with silicon melt infiltration
In order to reduce processing costs and improve the thermal stability of three-dimensional carbon fiber-reinforced silicon carbide composites, a chemical vapor infiltration combined with silicon melt infiltration method was developed for fabricating composites. According to the size of the pores in the preform, chemical vapor infiltration (CVI) and silicon melt infiltration (SMI) were mainly used to infiltrate small pores between fibers in a bundle and large pores between bundles, respectively. In the chemical vapor infiltration process, a pyrolytic carbon interfacial layer and a silicon carbide barrier layer were deposited on the surface of the carbon fiber. Then the pre-coated preform was infiltrated with pitch which was pyrolysed to form a porous carbon matrix in the pores. Finally, the preform was infiltrated with silicon melt to obtain composites. The influence of the interface thickness on the mechanical properties and the failure behavior of the composites were investigated, and the optimum thickness of the pyrolytic carbon layer was obtained. Experimental results also revealed that CVI+SMI composites exhibited good thermal stability of the mechanical properties and failure behaviors after the composites were annealed at high temperatures.