Reaction-sintered mullite-zirconia composites by colloidal processing of alumina-zircon-CeO2 mixtures

Reaction sintered mullite-ZrO2 composites were prepared from a mixture of alumina and zircon with CeO2. These materials were made by colloidal processing (slip casting and pressure filtration) using 48-57 vol.% aqueous suspensions of a mixture of fine powders stabilized with a polyacrylate solution as dispersant. Rheological properties of the suspensions were optimized to achieve an homogeneous distribution of the components in a dense green body. The effect of CeO2 addition, dispersant concentration and solid loading on the density of dried compacts were examined. Compacts have been subsequently sintered at temperatures of 1400-1600DGC for 2 h and density, crystalline phase evolution, proportion of t-ZrO2 to m-ZrO2 and microstructure were determined. The use of CeO2 favored sintering and reaction simultaneously due to the presence of a transient liquid phase. Mullite and zirconia were developed at temperatures as low as 1450DGC whereas in mixtures without CeO2 their presence were not detected. Fraction of t-ZrO2 increased to a maximum at 1450DGC probably due to the very fine grain size. At 1550-1600DGC, mullite and zirconia formation was nearly complete and t-ZrO2 content slightly increased as compared with that without stabilizing oxide. Low substitution of Ce in ZrO2 may be attributed to the formation of an amorphous phase rich in CeO2 which decreased the amount as stabilizing.