Phase formation, microstructure development, and mechanical properties of kaolin‐based mullite ceramics added with Fe ₂ O ₃

Abstract The effects of Fe ₂ O ₃ on phase evolution, density, microstructural development, and mechanical properties of mullite ceramics from kaolin and alumina were systematically studied. X‐ray diffraction results suggested that the ceramics consisted of mullite, sillimanite, and corundum, in the sintering range of 1450°C–1580°C. However, as the sintering was raised to 1580°C, mullite is the main phase with a content of 94%, and the corundum phase content is 5.9%. Simultaneously, high‐temperature sintering had a positive effect on the densification of the mullite ceramics, where both the bulk density and flexural strength could be optimized by adjusting the content of Fe ₂ O ₃ . It was found that 6 wt% Fe ₂ O ₃ was optimal for the formation of rod‐shaped mullite after sintering at 1550°C for 3 h. The sample's maximum bulk density was 2.84 g/cm ³ , with a flexural strength of 112 MPa. Meanwhile, rod‐shaped mullite grains with an aspect ratio of ~9 were formed. As a result, a dense network structure was developed, thus leading to mullite ceramics with excellent mechanical properties. The effect of Fe ₂ O ₃ on the properties might be attributed to the fact that Al ³⁺ ions in the [AlO ₆ ] octahedron were replaced by Fe ³⁺ ions, resulting in lattice distortion.