Deformation and fracture of corundum ceramics with a multilevel pore structure

The behavior of Al$_2$O$_3$ ceramics having a 50% volume of pore space under compression is studied. The porous structure includes three types of pores: large porosity with an average size of 120 $\mu$m, small porosity with an average size of 2 $\mu$m, and extended porous channels (about 150 $\mu$m) formed as a result of zonal separation during sintering. It is shown that at a load of ceramics with such a multilevel pore structure, the microdamage accumulates over the entire volume of the sample. It causes decreasing the scale level of fracture from microscopic (in the case of ceramics with unimodal pore structure) to meso- and microscale (for ceramics with a multilevel pore structure). Residual deformation in such material appears at small loads of about 0.3$\sigma_c$ due to displacements of blocks and their groups during deformation before the ultimate strength.