Zirconium oxide changes from a tetragonal phase to a monoclinic phase during thermal cycling, accompanied by volume changes, which can cause cracking and damage to the material. Therefore, zirconia needs to be stabilized or partially stabilized in all engineering applications. The most commonly used ZrO2 stabilizers are CaO, MgO, Y2O3, and CeO2. In addition to these commonly used Ca, Mg, Y, and Ce ions, most rare earth element oxides can form solid solutions with ZrO2. As long as the added rare earth oxide cation radius differs from the Zr4+ ion radius by no more than 40%, it can stabilize zirconia.
Among them, the best wear resistance is 3mol yttria-stabilized zirconia ceramics, which are often used to make various plungers, shafts, grinding media, etc.
The following will discuss the solid solution characteristics and phase diagrams of commonly used stabilizers Y2O3, CeO2, and ZrO2. It is worth pointing out that in all cases, the exact location of the phase boundary is controversial. This is because the purity of the material is different, the components evaporate at high temperatures, and it takes a long time to reach equilibrium.
1. Y2O3-ZrO2
The Y2O3-ZrO2 phase diagram commonly used at present was proposed by Scott (1975) because it is in good agreement with the experiment. The most important feature of this phase diagram is that Y2O3 has a large solubility in the tetragonal phase diagram solution until about 2.5% of Y2O2 is dissolved into the solid solution intersecting the low eutectoid temperature line. If the grain size is small enough, a complete tetragonal ZrO2 ceramic (called tetragonal zirconia polycrystal, TZP) can be obtained. By sintering well-dispersed ultrafine powder at 1400~1550℃, the grain growth rate can be controlled to obtain fine-grained tetragonal ZrO2 ceramics.
In addition, there is a large coexistence region (c+t) of cubic and tetragonal phases in the phase diagram, which makes it possible to form partially stable zirconia (PSZ) by sintering at a higher temperature (up to 1700℃), ensuring that enough tetragonal phase enters the solid solution to produce metastable tetragonal ZrO2 grains, and then slowly cools the sintered sample to make the tetragonal phase enter the cubic lattice.
2. CeO2-ZrO2
The CeO2-ZrO2 phase diagram refers to (Tani, 1983). This system has a wide range of tetragonal phase regions, the CeO2 solubility limit is 18% (mol), and the eutectic temperature is (1050±50) ℃, which is slightly higher than the Y2O3-ZrO2 system, but the grain size in the tetragonal phase region can still keep them all in tetragonal phase structure. Similar to the situation in the Y2O2-ZrO2 system, the sintering temperature is also close, and the sintering temperature of CeO2-ZrO2 is generally 1550℃. However, good ultrafine powder must be used to ensure that fine tetragonal phase grains are produced in the sintered ceramic.
In Ce-TZP materials, the addition of CeO2 in the range of 12%~20% (mol) can obtain a complete tetragonal phase
structure. Compared with Y-TZP, Ce-TZP allows a wider range of solid solution compositions, which is beneficial for process control.
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