Properties of mullite-zirconia composites prepared through reaction sintering kaolin, α-Al₂O₃, and ZrO₂

Mullite-zirconia composites were synthesized through reaction sintering Algerian kaolin, α-Al₂O₃, and ZrO₂. Phases present and their transformations were characterized using x-ray diffraction. Quantitative phase analysis was performed following the Rietveld method. Hardness and fracture toughness were measured by Vickers indentation. The flexural strength was measured using a Universal Testing Machine. It was found that the microstructure of samples sintered for 2 hours at 1600?C was composed of mullite grains which have whiskers' shape and ZrO₂ particles. In the composite containing 16 wt. % ZrO₂, the ratio of tetragonal zirconia transformed to monoclinic zirconia was relatively small and did not exceed 18%. However, in the composite containing 32 wt. % ZrO ₂ around 75% of the tetragonal structure changed to monoclinic structure. Also, it was found that the increase of ZrO₂ content from 0 to 32 wt. % decreased the microhardness of the composites from 14 to 10. 8 GPa. However, the increase of ZrO2 content from 0 to 24wt. % increased the flexural strength of the composites from 142 to 390 MPa then decreased it with further increase of ZrO₂ content. The fracture toughness increased from 1. 8 to 2. 9 MPa. m^1/2 with the increase of ZrO₂ content from 0 to 32 wt. %; and the rate of the increase decreased at higher fractions of ZrO₂content. The average linear coefficient of thermal expansion (within the range 50 to 1450°C) for samples containing 0 and 16 wt. % ZrO₂ sintered at 1600°C for 2 hours was 4. 7 x10 ^-6 K^-1 and 5. 2 x10^-6 K^-1 respectively.