Phase development and shrinkage of reaction-bonded mullite composites withsilicon carbide of different particle sizes

Mullitization temperatures and mechanical properties of reaction-bonded mul lite composites were investigated using silicon carbide (SIC) of two differ ent particle sizes (180 nm and 2.5 mu m) as one of the starting components. The smaller SiC particle size resulted in earlier mullitization, lower fin al densities, and lower strength of these composites. The sintering shrinka ge 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 s howed a linear sintering shrinkage of <1% and had a four-point bend strengt h of 430 MPa. Samples that were made from precursors with the coarse (2.5 p m) 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 attri buted to the oxidation of residual SiC during sintering and the trapping of gaseous oxidation products. Samples that were made from the fine (180 mn) SiC did not exhibit such a layer and showed isotropic shrinkage.