The preparation of Mg-PSZ mainly includes the steps of solid solution and annealing heat treatment. A solid solution is to make ZrO2 and MgO stabilizers fully solid solutions to form a cubic zirconia solid solution. Therefore, the sintering temperature is consistent with the solid solution temperature. According to the MgO-ZrO2 binary phase diagram, the smaller the amount of MgO stabilizer, the higher the solid solution temperature of the cubic phase region, and thus the higher the sintering temperature. Usually, 8% (mol) MgO stabilizer is selected, and the solid solution temperature of the cubic phase region is about 1800℃. Generally, solid solution treatment in the cubic phase region is required for 2~4 h. If it is lower than 8% (mol) MgO stabilizer, the cubic phase solid solution temperature is higher than 1800℃. When the MgO content increases to 9.5% (mol), the solid solution temperature can be reduced to 1750℃, but with the increase of MgO content, the tetragonal phase particles in the obtained PSZ decrease.
The cubic solid solution is then rapidly cooled to room temperature, with a cooling rate of 500°C/h. This quenching rate is too fast to precipitate an appropriate amount of tetragonal phase to reach equilibrium, but it can promote the uniform nucleation of very fine tetragonal precipitates.
Reheating to 1400°C for annealing heat treatment coarsens the tetragonal phase nuclei and inhibits MgO from entering the cubic phase matrix. Very fine tetragonal phase precipitates appear in the cubic phase matrix, and the tetragonal zirconia precipitates are lens-shaped. The cubic phase grain size is about 50μm, and the tetragonal phase grain size is about 0.5μm. Recooling to room temperature allows small t-2rC2 (less than 0.2μm) to remain in a metastable form during cooling, and coarser tetragonal phase particles may spontaneously transform into monoclinic phase during cooling.
In fact, the above process is often simplified in the preparation of commercial Mg-PSZ. A more commonly used process is to cool the cubic phase diagram with the furnace after sintering, or to quickly cool from the sintering solution temperature to the annealing temperature for heat treatment. This heat treatment system can form more tetragonal phase heterogeneous nucleation, so a thicker grain boundary tetragonal phase will be formed, which will turn into a monoclinic phase when cooled. Moreover, the precipitates of heterogeneous phase nucleation will also form in the grains. The size of this precipitate will grow rapidly during the subsequent 1400℃ heat preservation. The precipitates formed in this way will also be large enough to turn into a monoclinic phase when cooled, thereby reducing the total amount of metastable tetragonal phase retained. The metastable tetragonal phase plays a role in phase transformation toughening.