Phase composition of alumina–mullite–zirconia refractory materials

Refractories in the Al2O3–SiO2–ZrO2 system are widely used in many applications, for ceramic rollers in particular, and are characterized by high mechanical strength, excellent thermal shock resistance, resistance to corrosion by alkaline compounds and low creep at high temperature. Their performances greatly depend on the amount and chemical composition of crystalline and glassy phases, which were investigated by quantitative XRPD (RIR–Rietveld) and XRF in order to assess the effect of various Al2O3/SiO2 ratios of starting batches and different alumina particle size distributions. Refractories consist of mullite, corundum, zirconia polymorphs and a vitreous phase in largely variable amounts. The mullite percentage, unit cell parameters and composition vary with sintering temperature, being mostly influenced by the Al2O3/SiO2 ratio of the batch. Its orthorhombic unit cell increased its volume from 1400 to 1500 °C, while its stoichiometry became more aluminous. The corundum stability during firing is strongly affected by the Al2O3/SiO2 ratio, but not by the particle size distribution of alumina raw materials. Zirconia raw materials are involved in the high temperature reactions and about one-third of the available ZrO2 is dissolved in the glassy phase, ensuring excellent resistance to alkali corrosion, mainly depending on the fraction of coarse alumina. The phase composition of the vitreous phase increased with sintering temperature, being over 20% when the fractions of coarse alumina in the starting batch are between 0.2 and 0.5.