Colloidal processing of glassy-phase-free mullite from heterocoagulated boehmite/silica nanocomposite sol particles

The synthesis of high purity mullite has been of great interest over the last three decades as it is an important candidate material for high-temperature applications where creep and thermal shock resistance are the key requirements. The most critical issue for a ceramic component that will be used at temperatures of 1200-1400 deg C is the presence of any residual intra- or inter-granular glassy phase, which will normally soften at lower temperatures than the use temperature thus reducing the creep resistance of the material. Although many advanced processing techniques, such as sol-gel synthesis, precipitation, hydrothermal processing, etc., have been described extensively in order to produce stoichiometric 3:2 (orthorhombic) mullite, the concept of eliminating or minimizing the glassy phase within the final sintered matrix remained not fully explored. The colloidal processing of nanosized particles has been proposed as an alternative method for producing high quality, defect-minimized ceramic components. Uniformly dense green microstructures and, thereby, fully dense sintered parts can be produced from stable nanosized colloidal suspensions if the particles are consolidated in a controlled manner. The present study investigates the effects of controlling the starting precursor microstructure on the formation and location of silica-rich glassy phases within the final sintered mullite matrix in compacts obtained by a high load pressure filtration technique.