Fabrication And Properties Of Three Dimensional Carbon Fiber Reinforced Mullite Matrix Composites

Mullite is a promising ceramics in aeronautical and space technologies because of its low density, good high-temperature resistance and excellent resistance of oxidation and corruption. Unfortunately, the weaknesses of mechanical properties at room temperature strictly limit the application. Three dimensional braided continuous carbon fibers(3D-Cf) were introduced as the reinforcement based on the literatures of mullite composites. High solid content diphasic mullite sols and sol-gel method were used for the sake of the special structure of 3D-Cf.Two kinds of sols were prepared in the sorts of blending and synthesis. Solid content, ceramic yield, mullitization behavior, preparation efficiency and mechanical properties were compared. Synthetical sols were chosen for better bend strength of composites, which was 2.4 times that of the other one.Studies were carried out on mullitization and sintering shrinkage behaviors of synthetical sols. Mullitization is almost completed at 1573 K. The sols show ceramic yields of 11-16wt% according to Al-Si ratio after heat treatment at 1673 K. Mullitization degree and density of sintering body increase with increasing temperature.3D-Cf/mullite composites were prepared through repeating 18 cycles of infiltration-drying-sintering, which show 26%-44% in total porosity, 1.7-1.9g/cm3 in density, 200-245 MPa in flexural strength, 38-41 GPa in modulus and non-catastrophic fracture model. In general, porosity is raised with the elevation of Al-Si ratio or temperature. At the same time, the density of mullite matrix is significantly improved with increasing temperature. As a result, the mechanical properties of the composites show a little fluctuation over Al-Si ratio or temperature.Oxidation resistance of composites was characterized in static air environment between 1273 K and 1873 K for 30 min. The composites prepared at 1473 K and 1673 K maintain at least 76% of the original strength with a weight loss rate of about 2%. The viscous flow sintering of SiO2 at 1473 K or above is responsible for the good anti-oxidation property. However, the composites prepared at 1273 K lose approximately 10% in weight after oxidation and the retention rate ranges from 9% to 73%.The research results in this dissertation indicate that 3D-Cf/mullite composites with comparably high mechanical properties can be fabricated in high efficiency by using high solid content mullite sols. However, porosity is high in the composites. So, it is suggested that the improvement in infiltration, drying and sintering processes should be furtherly conducted in order to reduce porosity, and then enhance mechanical properties of 3D-Cf/mullite composites.