Preparation of C fiber reinforced alumina-mullite composite assisted by silica sol infiltration

3D C fiber preform reinforced alumina-mullite composite was manufactured. The materials was produced by precursor infiltration and pyrolysis and subsequently strengthened by silica sol infiltration route after coating the fiber preform with a 0.2μm thick carbon layer by chemical vapor infiltration. The composite was manufactured by infiltrating the preform with aluminium nitrate saturation sol, drying in an autoclave and subsequently heat treating at 950°C and pyrolysis of the aluminium nitrate. After ten cycles, the specimen was polished to remove the alumina matrix on the surface of the specimen, which generate "bottle effect" and prevent aluminium nitrate saturation sol from entering into the specimen. After another four cycles like above, the specimen was heated to 1150°C and 1300°C in Ar to transfer the amorphous alumina to γ-Al₂,O₃ and 2.2% α-Al₂O₃. The pore size and distribution in the specimens heat-treated in different temperature was determined. The specimen, which was heat-treated at 1150°C, was dipped in silica sol in vacuum condition, then drying in 100°C. Then the specimen containing silica was heated to 1400°C for 2h in Ar to impel the reaction between γ-Al₂O₃ and silica to create mullite. The stick-like mullite was observed which make for the strength of the composite. At the same time, the micro-cracks were observed on the unpolished cross-section matrix of composite, which was defected around the C fiber and doesn't damage the C fiber. It indicated the C interface was a weak interphase between the C fiber and alumina matrix. The three-point flexural strength and the strain of the as-fabricated composite were 180MPa and 2.2%.