Phase transformation and sintering behaviour of mullite and mullite-zirconia composite materials

Mullite is one of the most promising engineering materials for applications at elevated temperatures, but has poor mechanical properties at ambient temperature; therefore, it is usually reinforced with particles, fibres or whiskers to improve its properties. Among particles added to mullite are ZrO₂ particles which improve its fracture toughness through the well known process of phase transformation from tetragonal to monoclinic in zirconia particles. The aim of the present work is to explore the utilisation of Algerian kaolin, aα-Al₂O₃ and ZrO₂ to synthesise mullite-ZrO₂ composites through reaction sintering and investigate phase transformation and sintering behaviour of the composites. The raw materials were mixed through planetary ball milling followed by attrition milling. Compacted samples were sintered at temperatures between 1100 and 1600°C for 2 h. The bulk density was measured by the water immersion method. X-ray diffraction (Rietveld method) was used to characterise phases present in the sintered samples. It was found that the zirconia phase retained its tetragonal structure with the addition of up to 16% zirconia. The formation of primary mullite in all samples was complete at 1250°C. The cristobalite started to form at 1150uC, and disappeared at 1300°C in the samples of mullite, and at 1250°C when ZrO₂ was added. The zircon compound ZrSiO₄ started to form at 1250°C and completely disappeared at 1400°C. The increase in ZrO₂ ratio promoted the formation of grains with spherical shape.