In situobservation of crack propagation in magnesium oxide ceramics

In situ TEM straining experiments were performed for MgO ceramics in order to investigate the crack propagation behaviour. Using a micro-indenter driven accurately by a piezo actuator, cracks were introduced into MgO single crystals and polycrystals. In MgO single crystals, cleavage fracture along the (010) plane occurred, and the atomic structure of the crack walls were found to contain a number of square-shaped atomic steps. In contrast, cracks in polycrystalline MgO propagated along the grain boundaries, and crack deflection was observed at the triple grain junctions. It was found that a number of dislocations were generated around the triple junctions where the crack was deflected. The above results indicate that dislocation nucleation and/or emission play an important role in the crack propagation processes, even in brittle MgO.