Microstructural investigation and thermal shock behaviour of mullite-cordierite refractory materials C. Leonelli, D. N. Boccaccini, M. Romagnoli, P. Veronesi, I. Dlouhy*, A. R. Boccaccini** Dipartimento di Ingegneria dei Materiali e dell'Ambiente, Università di Modena e Reggio Emilia, Italy *Institute of Physics of Materials, ASCR, Brno, Czech Republic **Department of Materials, Imperial College London, UK ABSTRACT The influence of the mineralogical composition and phase distribution on crack initiation and propagation in cordierite-mullite refractory materials used in fast firing of porcelain whiteware is investigated. Two different refractory compositions, characterised by different silica to alumina ratios, were studied. Propagation of cracks introduced by Vickers’ indentations was observed by scanning electron microscopy. Chemical analysis by EDS was used for phase identification together with X-ray diffraction analysis. Thermal and mechanical properties were determined by means of standard laboratory techniques. The fracture toughness of selected samples was measured by the chevron notched specimen technique. Preliminary assessment of thermal shock resistance was obtained by water-quenching tests from 1250ºC. Microstructural features and crack propagation behaviour were correlated and used to draw conclusions on the behaviour of the two different refractory compositions under thermal shock. 1. INTRODUCTION Cordierite-mullite composites find increasing applications as refractories in the context of recent developments in fast-firing techniques of ceramic products [1]. These refractories exhibit in general a complex microstructure characterised by crystalline phases of different thermal expansion coefficients and elastic moduli and a residual silicate glassy phase. There is a current lack of understanding about the effect of microstructural features, including residual thermal stresses, on the overall performance of the materials at high temperature and under thermal shock conditions. Many authors have proposed thermal resistance parameters to characterise the thermal shock behaviour of refractory materials [2-5]. These parameters can be calculated using thermal and mechanical properties of refractories measured at room temperature. It has been demonstrated [6] that the sensitivity to thermal shock damage of many refractory materials decreases as temperature increases. This suggests that the evaluation of thermal shock parameters based on properties measured at room temperature is probably an efficient (cost effective) procedure in determining the smallest difference between different refractory materials under thermal shock. The aim of this work is to investigate the microstructure and microcrack propagation behaviour at room temperature and to correlate them with the observed thermal shock behaviour when refractory plates are subjected to actual duty cycles. Two different types of refractory plates were chosen, one exhibiting a fast microcrack propagation behaviour and the second showing early microcrack formation, but delayed microcrack propagation. Thermal shock tests by the water quenching method were carried out and the relevant mechanical properties measured on samples after each number of thermal shock cycles.