Microstructure and strength of fused high alumina materials with 2.5wt% zirconia and 2.5wt% titania additions for refractory applications

Abstract High alumina alumina–zirconia–titania (AZT) materials have a high potential to resist thermal shock due to complex microcracking. In this study, therefore, the microstructure, phase composition, porosity and strength were investigated dependent on the cooling rate of fused raw materials, the sintering temperature and a thermal shock treatment. X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectrometry, dilatometry, true density, porosity and strength analysis were conducted. By slow cooling of fused AZT, stabilized aluminum titanate formed in the raw material whereas by fast cooling the formation or stabilization was prevented. The formation of new aluminum titanate during sintering occurred above 1300 °C. Stabilization by solid solutions with e.g. titania—forming at sintering temperatures above 1450 °C—was superior to the stabilizing effect of crack growth retarding additives like zirconia. After sintering at 1450 °C the fast cooled material contained unstabilized aluminum titanate whereas the titanate in the slowly cooled AZT was stabilized. But the strength losses with thermal shock were comparable and both effects beneficial: the presence of stabilized aluminum titanate and the decomposition of the titanate. The additives deposited in the fast cooled material between and inside of the alumina grains. In the slowly cooled material they deposited mainly inter-granular. Consequently, in the fast cooled material the alumina grains cracked, leading to lower strengths. AZT may reach its full potential to resist thermal shock for reactant particle sizes below 63 μm, inter-granular deposition areas without exception and an open porosity below 20%. Sintering temperatures above 1300 °C were beneficial if the raw material contained unconverted reactants of the aluminum titanate formation.