Thermally Activated Deformation Mechanisms in MgO Investigated via High Temperature Scanning Indentation

Magnesium oxide (MgO) serves as a model material for plastically deformable ceramics. In previous studies, it has mainly been investigated via uniaxial compression, with studies often focusing on a particular deformation mechanism and temperature range. In this work, the novel high temperature scanning indentation (HTSI) nanoindentation method is applied to characterize the mechanical properties of single-crystalline MgO from room temperature to 800°C, allowing a quasi-continuous measurement of hardness, elastic modulus, the coefficient of strain rate sensitivity, and the activation volume. Results show that the hardness is controlled by hard 1/2<110>{100} slip, while the activation parameters are influenced by both 1/2<110>{100} and 1/2<110>{110} slip systems. The temperature dependence of the hardness follows two linear regimes, while the strain rate sensitivity shows complex behavior with a maximum of m = 0.05 at a temperature of 675°C. In the same regime, the activation volume remains approximately constant at V = 10–20 b³. At higher temperatures, it increases to V = 100 b³. The combined hardness and activation parameter data enable the determination of transition temperatures between different deformation regimes. With increasing temperature, MgO first shifts from a kink-pair to an obstacle-controlled regime at 240°C, with long-range dislocation interactions setting in at higher temperatures above 675°C.