Microstructure and sintering behavior of mullite-zirconia composites

In the present work, the structure and sintering behaviour of mullite-zirconia composites were investigated. The composites were prepared by reaction sintering of Algerian kaolin, α-Al₂O₃, and stabilized zirconia (3Y-TZP). The raw materials were wet ball milled in a planetary ball mill followed by attrition. The green samples shaped using a uniaxial press were sintered between 1100°C and 1600°C for 2 hours. The density was measured by the water immersion method. Phases present and change of the average crystallite size of the mullite phase as a function of sintering temperature and ZrO₂ content were characterized through X-ray diffraction. Mulite grains had whiskers' shape; however, the increase of ZrO₂ content changed the morphology of grains to near spherical shape. The microstructure of the samples revealed uniform distribution of ZrO₂ particles; also, aluminium was uniformly distributed on all grains exception on zirconia grains. At least a relative density of 95 % was achieved for all samples at a relatively lower sintering temperature of 1500°C. In composites containing up to 16 wt. % ZrO₂, the zirconia phase retained its tetragonal structure and the transformed part did not exceed 3 %. However, with the addition of 32 wt. % ZrO₂ around 66 % of the tetragonal structure changed to monoclinic structure.