Synthesis of mullite/zirconia composite materials doped with Boron minerals

During the course of this study for the production of mullite/zirconia composite doped with boron minerals, the source of alumina was selected as alumina (Al2O3) and kaolin (Al2O3.2SiO2.2H2O). 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) and ulexite (NaCaB5O9.5H2O). 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 and ulexite in 7and 8 wt%, respectively. Each composition was mixed for 6h at 300 rpm by a planetary mill in ethyl alcohol to obtain micrometer sized powder. Pellets and bars were reaction sintered with a heating rate of 50C/min at 1450, 1500 and 15500C for 5h.The bulk density of sintered composites was measured by Archimedes method. True density was also measured in powdered sample by using a pycnometer. Sintering shrinkage was measured by using digital milimeter. Addition of colemanite and ulexite at a certain level enabled the samples to be sintered at lower temperature hence lead to high density product. Powder samples were analyzed by XRD for phase formation. The results indicated the formation of mullite, monoclinic zirconia, and tetragonal zirconia major phases with minor amount of retained zircon and corundum in undoped samples processed at 1450 and 15000C. For the samples sintered at 1500 and 15500C, similar phase formation was observed with a slight difference in the intensity of major peaks.The fracture surfaces of all samples sintered at different temperatures were analyzed for microstructures using scanning electron microscopy (SEM, Jeol JSM 50-CF). The flexural strength and elastic modulus values were measured at room temperature (RT) on the fired rectangular bars by three point bending method using Instron 5581 Universal Testing Machine. The hardness (H) was measured using a Vickers microhardness tester (Future-tech); the fracture toughness (KIC) was examined by the Vicker?s Indention method following the equation of Anstis et al.The highest density, elasticity modulus, and flexural strength values were obtained in samples sintered at 15000C with colemanite doping. Similarly when doped with ulexite, highest values of density, elastic modulus, and flexural strength were obtained at 15500C.