Formation of mullite and mullite-corundum composites from kaolin using spark plasma sintering

Mullite, an excellent and versatile material, was synthesized, alone and in a composite with alumina, from natural kaolin—a primary product of a kaolin clay quarry. Kaolin powder for the mullitization process was inserted into a spark plasma sintering (SPS) apparatus in the form of raw kaolin, and in another experiment in the form of calcinated kaolin freed from bound water. Thermal properties, microstructure and phase composition were studied on the obtained samples. It was found out that the effect of calcination was in dehydration of kaolinite and amorphization of its phase composition. Thermal properties of the dehydrated and as-received kaolinite-based material were very similar. Materials with a perfect thermal insulation character were received. Dehydrated and as-received powders were mixed with alumina powder (corrundum phase) and fired by SPS. Mechanical properties–Vickers microhardness as well as parameters based on instrumented indentation were enhanced in the composite because of alumina content.