From the thermodynamic perspective, the driving force of phase transformation can be reduced by decreasing the grain size of TZP, which is favorable for the stabilization of the tetragonal phase and LTD resistance; while from kinetic perspective, more grain boundaries acting as nucleation sites for t-m phase transformation will be generated^[32]. Therefore, grain size will significantly affect the tetragonal phase stability and aging performance of the sample. A number of reports suggest that the critical grain size (GS) of stabilized tetragonal phase for pure ZrO₂ powder is 5‒10 nm at room temperature, under which the tetragonal phase is thermodynamically stable and would have good LTD resistance^{[27, 33]}. While for Y-TZP system, the critical GS is closely related with the amount of Y³⁺, yet it is still controversial in the detailed critical GS even for the same amount of Y^{3+[27, 29, 34]}. Denry and Kelly indicated that such controversy may be due to the different distribution Y³⁺ in the matrix, which will affect the t-m phase transformation of ZrO₂grains^[27]. However, to the best of our knowledge, there are virtually few undisputed systematical studies involving the relation between Y³⁺ distribution and GS of ZrO₂ to stabilize the tetragonal phase, which is definitely unfavorable to investigate the stabilization of tetragonal Y-TZP and its LTD resistance theoretically and experimentally. The critical GS of powder sample is quite different from that of solid sample, and the distribution of Y³⁺ in the matrix may also influence GS.

In fact, it is particularly difficult to rigorously control the GS of ZrO₂ and distribution of Y³⁺ at the same time through conventional methods^[38]. Till now, the liquid method, such as sol‒gel method and hydrothermal method are the most popular methods to prepare Y-TZP due to their simplicity and low cost, yet both methods have their restrictions. For sol‒gel method, although the homogenous distribution of Y³⁺ in ZrO₂ matrix can be achieved, it is very difficult to control the GS of samples especially to several or dozens of nanometers. While for hydrothermal method, the GS of samples can be controlled to as low as several nanometers owing to the relatively low reaction temperature, but the distribution of Y³⁺ in ZrO₂ matrix is uncontrollable due to the different precipitation consequence of ZrO₂ and Y₂O₃ during the hydrothermal process. Therefore, it is of great importance to overcome such restrictions to further enhance the LTD resistance of Y-TZP for dental applications.

Our group has been working on dental materials^{[38, 39, 40, 41]}, and we devised a novel “ complexing-hydrothermal” method to prepare ZrO₂-based materials by integrating complexing sol‒gel method and hydrothermal method. The flowchart of the so-called “ complexing-hydrothermal” is shown in Fig. 1. Zirconium nitrate and/or yttrium nitrate were used as reactants and ethylenediaminetetraacetic acid (EDTA) as chelating agent to prepare the EDTA-zirconium and EDTA-yttrium monomer complexes. With different time of gelation process, the mixed solution was adopted as the precursor of hydrothermal reaction. After the hydrothermal reaction, the pure ZrO₂ powders, whose GS is lower than 10 nm, and Y-TZP powders, whose GS is lower than 15 nm, were obtained by washing and drying the precursor powder. In addition, the stability of tetragonal phase could be enhanced even if no Y³⁺ stabilizer was added, which was favorable to enhance the LTD resistance of samples. The reactions related to the “ complexing-hydrothermal” process are as follows:

EDTA+Zr⁴⁺⇔ [EDTA-Zr]+H⁺​, EDTA+Y³⁺⇔ [EDTA-Y]+H⁺(1)

Zr⁴⁺+OH^-⇔ Zr(OH)₄​, Y³⁺+OH^-⇔ Y(OH)₃(2)

Zr(OH)₄⇔ ZrO₂+H₂O, Y(OH)₃⇔ Y₂O₃+H₂O (3)

where reactions (1) and (2) are the complexation reaction of metal ions and complexing agents, i.e., EDTA and OH^-, respectively. Reaction (3) is the hydrothermal reaction of the hydroxide of Zr⁴⁺ and Y³⁺. Unlike conventional hydrothermal process, whose reaction is just reactions (2) and (3), a strong complexing agent, i.e., EDTA, was adopted to complex both Zr⁴⁺ and Y³⁺. In this way, the competition of complexation reactions of reactions (1) and (2) can delay the precipitation of ZrO₂ and Y₂O₃ during the hydrothermal reaction, which can improve the control of GS and distribution of Y³⁺ in ZrO₂ matrix. The preliminary energy spectrum analysis also confirmed the speculation, although the ratio of Zr⁴⁺ and Y³⁺ was still nonstoichiometric. Therefore, further investigations on the competition and balance of different reactions are in need in this promising method to clarify the relation mechanism between GS and distribution of stabilizer in stabilization of tetragonal Y-TZP, and then to improve the LTD resistance of Y-TZP.

Fig. 1.

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	Fig. 1. Flowchart of complexing-hydrothermal method to prepare ZrO2-based powder.

It was reported that the LTD resistance of Ce-TZP was bigger than that of Y-TZP after the International Standard Organization (ISO) aging cycles test in water vapor conducted at 134 ° C and 2 ×  10⁵  Pa (2 bar) for 5 h^[42]. Unlike the stabilization mechanism of yttria, ceria (CeO₂) stabilizes the tetragonal zirconia by generating the O^2- vacancies due to the different ionic radius of Ce⁴⁺ and Zr⁴⁺. Generally speaking, the amount of CeO₂ should be in the range of 8‒12 mol% to obtain the stabilized tetragonal zirconia^[32]. However, the stabilization mechanism of the concentration of Ce⁴⁺ on tetragonal zirconia is still unknown. Furthermore, Ce⁴⁺ can be reduced to Ce³⁺ under the reaction of many factors, e.g., sintering, local stress or glucose in oral cavity. As a result, the phase stabilization ability of CeO₂ will deteriorate and the color of Ce-TZP will change, which are unfavorable for Ce-TZP crowns from both mechanical and aesthetic point of views. As an alternative, CeO₂ can be applied as a secondary stabilizer of Y-TZP to prepare Ce⁴⁺ doped Y-TZP, i.e., Ce/Y-TZP, which can improve the LTD resistance of Y-TZP without affecting the mechanical properties and aesthetics obviously. This promising system, however, cannot solve LTD of tetragonal zirconia in the long term.

It is of great importance to restrict the contact and reaction between water molecules and tetragonal zirconia to improve the LTD resistance. From this perspective, it can shield tetragonal zirconia from water effectively if we can coat Y-TZP grains with shell. The choice of shell materials should base on the following: the shell should restrict the penetration of water to affect the stability of tetragonal zirconia; the introduction of shell should not deteriorate the mechanical properties of the Y-TZP system remarkably; the biocompatibility of the shell materials should be good enough for the clinical application.

Azimi et al.^[43] found that surfaces of some rare earth oxides (e.g. CeO₂ and Er₂O₃) are hydrophobic as shown in Fig. 2. Accordingly, it is of particular interest if we can combine both the hydrophobicity of CeO₂ and its stabilization functions on Y-TZP to restrict or diminish the influence of water on tetragonal zirconia. In this respect, the core-shell Y-TZP@CeO₂ composite may be a promising dental material with better LTD resistance compared to Y-TZP or Ce/Y-TZP. To the best of our knowledge, there are few studies investigating TZP coated with Y₂O₃, which was based on physical mixing method^[44], and no report has been published on the study of the influence of hydrophobic shell on LTD resistance and mechanical properties of ZrO₂-based crowns. Fig. 3 is the schematic flowchart of our ongoing work, in which core‒shell structure is constructed by electrostatic attraction of Zr(OH)₄-Y(OH)₃ colloid particles and Ce⁴⁺, and Y-TZP@CeO₂ composite can be obtained by hydrothermal process.

Fig. 2.

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	Fig. 2. Hydrophobicity of surface of rare-earth elements oxides[43]. The inset is the photograph of water droplet on ceria surface.

Fig. 3.

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	Fig. 3. Schematic formation of Y-TZP@CeO2 composite.

Meanwhile, nanotechnology has been widely applied to prepare ZrO₂-based materials to improve the mechanical properties of the crowns^[45], which may bring the potential biological toxicity of the materials. Especially, the toxicity of CeO₂ is controversial^[46], which is yet to be clarified for the clinical application of Y-TZP@CeO₂ composite.