The Synthesis of Single Tetragonal Phase Zirconia by Sol-Gel Route

Zirconia (ZrO2) is a widely used ceramic material with high melting point, high strength, and good wear and erosion resistance. ZrO2 exhibits three crystallographic phases: monoclinic (m), tetragonal (t), and cubic (c). At room temperature, zirconia usually keeps monoclinic phase. Phase transformation of ZrO2 occurs during heating or cooling processes, with the following transformation equation (1) [1, 2]

There are two types tetragonal phase of ZrO2, transformable and non-transformable (metastable tetragonal phase). The nontransformable tetragonal phase ZrO2 is remarkable resistance and does not undergo the phase transformation to monoclinic phase. It is usually used for thermal barrier coatings due to the good thermomechanical properties [3, 4]. Transformable tetragonal phase ZrO2 is usually called toughened ceramic. It can be obtained at room temperature by doping with a suitable content stabilizer. The most common stabilizer is Y2O3. Martensitic phase transformation t to m occurs once the stress status within materials changes. This phase transformation has great technological importance. It can absorb the energy of crack propagation, thus restraining it, which is the basis for the transformation toughening of ceramic components. A large amount of surface and interfacial energy is introduced when decreasing the crystallite size to the nanoscale, hence the free energy of phase transformation is different from the micron scale. Therefore, those unstable phases of ZrO2 can exist in the stable or metastable state at room temperature. Nowadays, low fracture toughness is one of the serious drawbacks of new ceramics, especially for thermal barrier coatings [5-8]. So it is necessary to explore toughening methods for these kinds of ceramics. However, although stabilised or partially stabilised zirconia with yttria (YSZ) has been widely studied by other researchers, either synthesized by solid state reaction, or wet chemical methods such as via sol-gel method, there is lack of studies on using Pechini sol-gel method, which using citric acid as complexing agent and ethylene glycol as surfactant, particularly for the transformable tetragonal phase ZrO2. The traditional sol-gel method cannot be widely used due to the rare and expensive organometallic precursor and metal alkoxides. By using Pechini sol-gel method, metal salts can be used as precursor that much cheaper than those materials. Furthermore, despite ZrO2 stabilized with different contents of Y2O3 has already been studied, there is no study on the phase status of Y2O3 with a narrow range content as reported in this paper 3, 3.5, and 4mol%. Within this range, single tetragonal phase ZrO2 can be synthesized under 1000oC. Moreover, single tetragonal phase ZrO2 is difficult to be synthesized. They normally synthesized by sol-gel method that consists of both monoclinic and tetragonal phases, plus a little cubic phase. Therefore, In this paper, single transformable tetragonal phase ZrO2 has been synthesized successfully using Pechini sol-gel method. The calcination mechanism, phase characterization, and Raman shifts were studied.