Mechanical properties of magnesia-spinel composites

A set of dense magnesia-magnesium aluminate spinel composites has been prepared by hot-pressing magnesia powder using 0 to 30 wt.% of spinel powder of mean particle size 3, 11 and 22 μm. Bend stength, modulus, and fracture toughness, have been measured. Strength and modulus decrease with increasing spinel content, and for a given loading, spinel particle size, as a result of microcracking caused by thermal expansion mismatch between the magnesia matrix grains and the spinel particles, the effects of which may be intensified by recrystallization of spinel. Fracture is predominantly transgranular for the pure magnesia, and intergranular for the composite materials. Possible reasons for the differences in behaviour between this system and the SiC–Al2O3 system with a similar thermal expansion mismatch are examined.