The applications of magnesium aluminate spinel (MgAl 2 O 4 ) ceramics often require optical transparency, suggesting minimal macroscopic flaws otherwise too strong light scattering would make them opaque. Yet the flexural strength typically in 200–350 MPa range is unsatisfactory, suggesting rather large critical flaw size for fracture in brittle materials. Such inconsistency makes MgAl 2 O 4 an interesting system to study ceramic processing, strengthening, and toughening. Here, we reported progress in pressurelessly sintered zirconia toughened MgAl 2 O 4 ceramics with 700 MPa flexural strength, surpassing those reported in the literature by hot pressing and other techniques. The new advance was achieved by colloidal processing and two-step sintering, for best microstructural control with minimal microscopic defects, uniform two-phase distribution, and suppressed grain growth. We found ZrO 2 acts as sintering aid for MgAl 2 O 4 , effectively lowering the sintering temperature by 150 °C. The optimized two-step sintering was conducted first at 1325 °C without holding reaching a critical relative density of 93%, and then at 1225 °C for 20 h for full densification. The record-low pressureless sintering temperature of 1225 °C plus the two-phase inter-pinning refine the grain size to 184 nm for MgAl 2 O 4 and 155 nm for zirconia, offering microstructural benefits for strengthening. The zirconia grains stabilized by matrix constrain and partial Mg 2+ alloying are mostly tetragonal and mechanically transformable, offering active toughening mechanism to reduce flaw sensitivity. The fracture is intra-granular for MgAl 2 O 4 and mixed intra- and inter-granular for zirconia. Our work demonstrates the possibility to further strengthen MgAl 2 O 4 ceramics towards 1 GPa for advanced and new applications.
Cheng et al. (Mon,) studied this question.