MICROSTRUCTURE, MICROCHEMISTRY, AND FLEXURAL STRENGTH OF MULLITE CERAMICS

The microstructure of mullite ceramics hot-pressed and sintered at different temperatures was studied using transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) with EDS, and electron probe microanalysis (EPMA). The specimens, consisting of stoichiometric mullite grains without glassy phase, are obtained by hot-pressing stoichiometric mullite powder at 1575-degrees-C for 1 h. Silica-rich glassy phases are observed using TEM at three-grain junctions of mullite grains in specimens heated at and above 1600-degrees-C. However, high-resolution transmission electron micrographs show no glassy phase at two-grain boundaries in all specimens. SEM with EDS analyses show that the average value of Al2O3 contents of mullite grains increases slightly with increasing temperature. These results are consistent with a published Al2O3-SiO2 phase diagram. The flexural strength of mullite specimens at room temperature depends on their microstructure, such as the grain size and grain size distribution of mullite grains. The strength is high at room temperature and up to 1200-degrees-C, and it decreases at and above 1350-degrees-C, irrespective of the presence of the glassy phase.