Phase Development and Shrinkage of Reaction‐Bonded Mullite Composites with Silicon Carbide of Different Particle Sizes

Abstract

Mullitization temperatures and mechanical properties of reaction‐bonded mullite composites were investigated using silicon carbide (SiC) of two different particle sizes (180 nm and 2.5 μm) as one of the starting components. The smaller SiC particle size resulted in earlier mullitization, lower final densities, and lower strength of these composites. The sintering shrinkage of these composites was investigated. Low‐to‐zero shrinkage was rendered possible via volume expansions that were associated with the oxidation of SiC and aluminum in the green material. Green bodies that contained 55 vol% aluminum were compacted to 70% of the theoretical density. The materials showed a linear sintering shrinkage of <1% and had a four‐point bend strength of 430 MPa. Samples that were made from precursors with the coarse (2.5 μm) SiC were covered by a porous outer layer after firing in air. This layer led to anisotropy in shrinkage. The porosity of this outer layer was attributed to the oxidation of residual SiC during sintering and the trapping of gaseous oxidation products. Samples that were made from the fine (180 nm) SiC did not exhibit such a layer and showed isotropic shrinkage.