Influence of Heat Treatment on Phases, Microstructures and Mechanical Properties of Laminated Fabric (SiC and Aluminosilicate)/Mullite Matrix Composites

Abstract

Oxidation resistance of SiC satin fabric (25-29vol%) or aluminosilicate plain fabric (25-33vol%)/mullite matrix porous composites was investigated at 1000-1200°C in air for 24h using thermogravimetric analysis. The composites were produced by a polymer impregnation and pyrolysis (PIP) method using a mullite precursor solution of the mixtures of Si(OC2H5)4 and Al(NO3)3. The oxidation rate of the sintered SiC fibers with a stoichiometric SiC composition and well-crystallized microstructure was controlled by the diffusion process of gases through dense SiO2 layer formed on the SiC surface. The activation energy for the oxidation of the SiC fibers was 299kJ/mol. This value was close to the activation energy for the diffusion of oxygen atom in silica glass. The SiC satin fabric/mullite matrix composite heated in an Ar atmosphere showed pseudoductility due to the delamination along the fabric layers. The four point-flexural strength decreased slightly with increasing heat treatment temperature. However, the composite heated in air at 1100-1200°C showed brittle fracture behavior without delamination because of the increased strength of interfacial bond between the oxidized SiC fibers and mullite matrix. On the other hand, the amorphous aluminosilicate fibers/mullite matrix composite showed little weight change during the heat treatment at 1000-1200°C. Crystallization of mullite occurred in the mullite precursor-derived amorphous solid and aluminosilicate fibers at 1100 and 1200°C in air, respectively. However, no microstructural change was observed in the heat-treated composite. The significant pseudoductility and strength of the aluminosilicate plain fabric/mullite matrix composite were maintained after the heat treatment. The fracture energy increased with increasing heat treatment temperature.