Hardness and elastic modulus from nanoindentations in nominally pure and doped MgO single crystals

Using a nanoindentation technique, the hardness and Young's modulus were determined for nominally pure MgO single crystals and for MgO crystals doped with H, Li, Ni or Co impurities, subjected to different thermal treatments. The resulting defects were monitored by optical absorption spectroscopy. Undoped crystals have a hardness of 9.2 +/- 0.2 GPa. After thermochemical reduction up to about 2300 K in Mg vapour at 7 atm, resulting in a deficiency of anions, a hardness value of 10.4 +/- 0.1 GPa was obtained. The enhancement is attributed to O vacancies. For doped crystals, hardening was observed and was attributed to impurities, point defects, cavities and metallic precipitates. A constant value of 300 GPa for Young's modulus was obtained in all cases, indicating that the elastic properties are not influenced by either impurities or intrinsic defects.