Fabrication of mullite/SiC and mullite/zirconia/SiC composites by ‘dual’ in-situ reaction syntheses

Mullite/SiC and mullite/zirconia/SiC composites were fabricated via ‘dual’ in-situ reaction synthesis processes. β-SiC particles were synthesized from carbon black/colloidal silica mixtures, while mullite was reaction-synthesized from alumina/amorphous silica powder mixtures. Dry-pressed pellets containing SiC precursor and powder mixtures for the matrix phase were subjected to a three-stage sintering: a synthesis run at 1500 °C for 6 min for the synthesis of SiC particles, a carbon removal run at 600 °C, followed by a densification run at 1500 °C for 2 h. The results showed that after the synthesis run, both SiC particles and mullite were already reaction-formed. The samples after densification run consisted of the expected phases: mullite and SiC phases (and tetragonal zirconia in mullite/zirconia/SiC composites). Nevertheless, the bulk densities of these composites were relatively low and drastically decreased with an increase in SiC amount. Although the densification run was intended for the final densification of the pellets, the bulk densities after densification run only increased slightly from those after synthesis run. The poor densification of the samples was due to the extensive formation of mullite during the synthesis run.