Grain size-dependent hardness of transparent magnesium aluminate spinel

Most different transparent grades of sintered and single crystalline spinel were investigated by Vickers and Knoop tests with loads of 1 kg (HK1, HV1) and of 10 kg (HV10). The hardness ranking of all samples was HK1<HV10<HV1. All measuring approaches revealed a negligible grain size effect from about 5 µm up to the single crystals. For finer microstructures, a more dominant improvement was observed to be limited to grain sizes < 1 µm. Alumina-rich sintered and single crystalline MgO·nAl2O3 compositions with n >= 2 exhibited hardness values significantly below the common grain size-hardness relationship.