Production and characterization of mullite-zirconia composites by slip casting method

During of study for the production of mullite/zirconia composite, the source of alumina was selected as alumina (Al2O3) and kaolin (Al2O3.2SiO2.2H2O) and zircon (ZrSiO4) was used as source of silica and zirconia. Studies were focused on both low temperature synthesis and phase transformations of zirconia as expected to be influenced by the addition of boron minerals such as colemanite (Ca2B6O11.5H2O). Phase and microstructure analyses were performed on samples produced by reaction sintering in addition to the mechanical property characterizations. For the reaction, zircon, alumina and kaolin were mixed in stoichiometric proportions; as colemanite in 7 wt%. Each composition was mixed for 6 h at 300 rpm by a planetary mill in methyl alcohol to obtain micrometer sized powder. To shape the prepared mixtures by slip casting; the aqueous slips were prepared with 45-55 wt.% powder, and 0.25 wt. ~% Darvan C, with regard to the additive used at pH 9. Cylindrical pellets (10 mm diameter) and rectangular rods with the size of 7.5 mm ×5 mm×55 mm were produced from well dispersed suspensions by slip casting in a plaster molds. All the specimens were sintered 1450, 1500 and 1550 0C for 5 h at a heating rate of 5 0C min−1. The results of the study indicate that 98 % of theoretical density was achieved as the solid concentration is increased up to 55 % in case for slip casted specimens with colemanite additive (MZK55 with 55 % solid concentration. Slip casted composites prepared with various solid concentrations, compositions where zircon exhibits decomposing at 1450 0C are the ones that comprise colemanite (Ca2B6O11.5H2O). Considering Energy Dispersive X-Ray Spectroscopy (EDX) results and the microstructure, it is believed that white grains represent zirconia whereas the gray ones belong to mullite in both slip casted composites. The interaction between mullite and zirconia grains is impotent in contrast with the samples sintered at 1500-1550 0C. To draw a comparison between the mullite-zirconia composites prepared via slip casting and studies at the literature from a mechanical standpoint, composites were in the literature have smilarly results in terms of bending strength, elastic modulus, hardness and fracture toughness.