Microstructural development and mechanical properties of alumina-hexaluminate composites As-sintered and after aging in aqueous and physiological solution

Addition of calcium, strontium or barium nitrates to alumina powders promotes the crystallization "in situ" of calcium, strontium and barium hexaluminates during the sintering of green compacts. Calcium and strontium hexaluminates develop platelike structures randomly dispersed in the alumina matrix whereas barium hexaluminate does not. The strength and toughness of as-fired samples containing calcium or strontium hexaluminates are greater than those of the matrix. Materials containing barium hexaluminate have lower values. These composites maintain the same properties after aging for 30 d at 35 degrees C in aqueous or physiological solution and no chemical reaction between the ageing media and the hexaluminates was observed on examination by scanning electron microscope.