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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· n Al 2 O 3 compositions with n ≥2 exhibited hardness values significantly below the common grain size–hardness relationship.
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Krell et al. (2010) studied this question.
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