Zirconia ceramic (Zirconium oxide ceramic, ZrO2) is the main oxide ceramic of zirconium. It is usually a white, odorless and tasteless crystal. It turns yellow or gray when it contains impurities. It is insoluble in water, hydrochloric acid and dilute sulfuric acid. It is chemically inactive and has high melting point, high resistivity, high refractive index and low thermal expansion coefficient, making it an important high-temperature resistant material, ceramic insulation material and ceramic sunscreen agent, as well as the main raw material for artificial drills. The production of zirconia ceramics requires the preparation of powders with high purity, good dispersion properties, ultra-fine particles, and narrow particle size distribution. Pure ZrO2 has three crystal states under normal pressure: Monoclinic, Tetragonal, and Cubic.
| Crystal form | M-ZrO2 | T-ZrO2 | C-ZrO2 |
|---|---|---|---|
| Density (g/cm3) | 5.65 | 6.1 | 6.27 |
| Existence temperature (°C) | <950 | 1100-2370 | >2370 |
The above three crystal forms exist in different temperature ranges and can transform into each other. The monoclinic crystals form is stable below 1170°C. It transforms into the tetragonal crystals above this temperature and transforms into the cubic crystals when the temperature reaches 2370°C. Melting occurs at 2680~2700°C, and the entire crystal change process is reversible.
When cooling from high temperature to the transformation temperature of the tetragonal phase, due to the phase transformation hysteresis phenomenon, it takes about 1050°C, which is 100°C lower than the T phase to transform into the M phase (martensite phase transformation). At the same time, the change will produce a volume expansion of 5% to 9%. This volume change is enough to exceed the elastic limit of the ZrO2 crystals, causing the material to crack. In order to avoid this phase change, divalent oxides (CaO, MgO, SrO) and rare earth oxides (Y2O3, CeO2) are often used as stabilizers to form a solid solution with ZrO2 to generate a stable cubic phase structure (Note: These stabilizers effect can only be achieved when the difference between the radius of the oxide metal ion and the radius of the Zr4+ ion is less than 40%).
In summary, because the above three crystal states have different physical and chemical properties, in order to obtain the required crystal form and performance in practical applications, different types of stabilizers are usually added to make different types of zirconia ceramics, such as Partially Stabilized Zirconia (PSZ), when the stabilizer is Y2O3, MgO, CaO, it is expressed as Y-PSZ, Mg-PSZ, Ca-PSZ, etc. respectively. As tetragonal zirconia polycrystalline (TZP), When the stabilizers added are Y2O3 and CeO2, they are expressed as Y-TZP, Ce-TZP, etc. .
After adding different stabilizers and colorants, the color of the product will be different. Currently, the main ones on the market include the following:
1. The stabilizer is Y2O3: white zirconia, fine-grained microstructure mainly tetragonal phase; extremely high strength and toughness; service temperature below 1000°C; excellent chemical resistance; excellent wear resistance sex.
2. The stabilizer is Y2O3, and black colorant is added: black zirconia.
3. The stabilizer is MgO: yellow zirconia, used for high temperature applications; it is not prone to phase transformation at high temperatures; non-homogeneous microstructure to prevent grain boundary sliding; transformation is tougher; high fracture toughness.
4. The stabilizer is Y2O3, and vanadium zirconium blue material or Co3O4 is added as colorant: blue zirconium oxide.
