The formation of fibrous SiC in SiC/Al 2 O 3 composites from the silica–carbon–aluminum system

The formation mechanisms of β-SiC fibers in SiC-Al2O3 composites (β-SiCf/Al2O3) from the silica-carbon-aluminum system are investigated in this paper. The initial mixtures were prepared by the milling of white ash (WA), black ash (BA) and aluminum powder in predefined compositions. The associated chemical reactions in the evolution of the reactants were examined by Differential Scanning Calorimetry (DSC), and the final products were characterized by X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). It is found that the mechanisms of forming SiC fibers are different under different micro-circumstances; and the relationship between the morphologies of the β-SiC and the corresponding formation mechanisms is discussed. The density of the green bodies is found to play a key role in affecting the SiC formation mechanism, thus the morphology of the SiC; the green bodies formed under 150MPa compression pressure were found to facilitate the SiC fiber growth mainly by the Solid–liquid-Solid (SLS) mechanism; and those formed above 300MPa were found to produce primarily SiC particles first, then further annealed to quasi one-dimensional structure at a higher temperature (1500°C). Straight SiC fibers were also found locally around free voids, and were interpreted by the vapor–solid (VS) and vapor–liquid–solid (VLS) growth mechanisms.