Microstructural investigations in cordierite–mullite refractories

The influence of the mineralogical composition and phase distribution on crack initiation and propagation in cordierite–mullite refractory plates used as substrates in fast firing of porcelain whiteware is investigated. Two different refractory compositions (termed REFO and CONC), 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. 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. It was found that the presence of α-quartz crystals and favourable residual stress field are responsible for room temperature fracture toughness in REFO samples. The CONC material contains a larger amount of residual glassy phase than REFO material, which should lead to better high-temperature mechanical properties and higher thermal shock resistance, as the glass phase may close (“heal”) propagating cracks.