Two-step sintering and grain size refinement of MgAl2O4 ceramics

There are growing interests to process magnesium aluminate spinel (MgAl₂O₄) ceramics using commercially available powders and to achieve lowered pressureless sintering and hot isostatic pressing temperatures, refined grain size, and improved mechanical properties. We recently used deagglomerated Baikowski powders and centrifugal casting to lower the pressureless sintering temperature of MgAl₂O₄ ceramics to 1350°C and obtain 526 nm grain size for full density and 197 nm for 97% relative density with closed porosity, respectively. Here, we practiced two-step sintering to further lower the sintering temperature and refine the grain size. The results show positive yet limited effect of two-step sintering in the subject system. For the first sintering step, we found a relatively high critical density of 91% and holding is not needed. For the second sintering step, we found a temperature difference (between the first and second sintering steps) of 70°C to obtain full density and 115°C to obtain 95% relative density with closed porosity. The optimized two-step sintering conditions are (i) the first step at 1350°C for 0 h and the second step at 1280°C for 20 h to obtain 99.2% relative density and 429 nm grain size, and (ii) the first step at 1350°C for 0 h and the second step at 1235°C for 20 h to obtain 95.2% relative density and 184 nm grain size. Implications from isothermal grain growth experiments shall be discussed.