Variation of the micromechanics of impact fracture of MgAl$_2$O$_4$ in response to the applied hot isostatic pressing

The MgAl$_2$O$_4$ ceramics produced by the preliminary clinkering of synthesized powder was exposed to the hot isostatic pressing (HIP). Samples were subjected to a pointed impact loading, which excited the acoustical and electromagnetic emissions (AE and EME, respectively). In non-HIP-treated samples, the energy distribution in time series of AE and EME pulses followed an exponential law, while after an application of the hot isostatic pressing procedure, the emissions of both types exhibited the power law energy distributions in time sweeps. A variation of the specificity of the impact-induced AE/EME energy release was explained by a transition from the nucleation/decay of microcracks in non-HIP-treated ceramics to the self-organized dislocation folding directed to grain boundaries in the material characterized by the density that was close to that of a MgAl$_2$O$_4$ single crystal.