Effect of Oxidation Temperature on Microstructure and Flexural Strength of 2.5D SiO2f/SiO2 Composites Fabricated Via Sol–Gel Method

In this paper, 2.5D SiO_2f/SiO₂ composites were prepared by sol–gel method, and the effects of different oxidation temperatures on the properties were systematically investigated, with the expectation of providing certain theoretical support for the practical applications. The initial CMC sample (ST0) was amorphous SiO₂. The increase of oxidation temperature promotes the crystallization of SiO₂, and after oxidation at 1600 °C, all the SiO₂ was transformed into cristobalite. There was a good bonding between the fibers and the matrix in ST0. As the oxidation temperature increased, the fiber-matrix bonding became stronger and the matrix shrinked more severely, leading to the presence of holes between fiber bundles and a large number of cracks within the fiber bundles. Volume effects due to phase transitions also increase crack generation. After oxidation at 1600 °C, no interface existed. The fibers melted with the matrix, and the fibers failed completely. As the oxidation temperature increased, the density gradually increased, which was attributed to the shrinkage of the matrix. In addition, the phase transition also led to an increase in density. Of course, the porosity gradually decreased. The weight loss rates indicated that the ST0 exhibited high oxidation resistance in different oxidation atmospheres. Initially, the ST0 had ductile fracture and the flexural strength reached 86.4 MPa. As the oxidation temperature increased, the matrix and fibers were damaged, leading to a gradual decrease in the flexural strength of the sample and exhibiting a typical brittle fracture mechanism and microstructure. In order to ensure that the ST0 could be used for a long time in a high-temperature oxidation atmosphere, it was necessary to ensure that no damage occurs to the fibers and the matrix, including the stability of the crystal phase.