Phase Evolution and Mechanical Performance of Zirconia Ceramics Synthesized Under High Temperature and High Pressure

Abstract

Achieving simultaneous enhancement of Vickers hardness and fracture toughness remains a critical challenge in designing oxide ceramic materials due to their partially antagonistic nature. In this study, we address this trade-off by tailoring the microstructure of zirconia (ZrO₂) ceramics through Y₂O₃ doping and high-pressure high-temperature (HPHT) sintering. Nanostructured composites were synthesized using 50 nm monoclinic ZrO₂ and varying Y₂O₃ contents (3, 5, and 7 mol%) under 5 GPa at temperatures ranging from 400 to 2000 °C. Among them, 3 mol% Y₂O₃-doped zirconia (3Y-PSZ) sintered at 1200 °C achieved a well-balanced mechanical performance, with a Vickers hardness of 11.6 GPa and a fracture toughness of 9.26 MPa·m^1/2. These results demonstrate that it is feasible to retain a high hardness while significantly enhancing toughness by controlling phase composition and grain refinement under HPHT conditions. This work offers valuable insights into microstructural optimization strategies for zirconia-based ceramics aiming to overcome the conventional hardness–toughness trade-off.

1. Introduction

ZrO₂ has garnered extensive attention across various fields, including bioceramics, cutting tools, refractories, and solid oxide fuel cells, owing to its outstanding mechanical properties and chemical stability [1,2,3,4]. At ambient conditions, ZrO₂ adopts the monoclinic (m) phase, while the tetragonal (t) and cubic (c) phases are stable only at elevated temperatures [5,6,7]. The m-phase exhibits inferior toughness and strength, whereas the t- and c-phase display enhanced mechanical performance. However, the transformation from the t- to m-phase during cooling leads to a volumetric expansion (~3–5%), causing microcracks and catastrophic material failure [8]. To address this challenge, stabilizing oxides such as Y₂O₃, CeO₂, MgO, and CaO are introduced to promote the metastable retention of t- and c-phase, facilitating transformation toughening [9,10,11,12,13,14,15,16]. Among these, yttria-stabilized zirconia (Y-PSZ) stands out due to its favorable combination of hardness, fracture toughness, and biocompatibility, which has led to its widespread use in dental restorations, orthopedic implants, and advanced ceramic cutting tools [17]. For instance, Alhotan et al. utilized a lithography-based ceramic manufacturing process to fabricate 3D-printed 3Y-TZP materials, achieving a fracture toughness of 6.07 MPa∙m^1/2 and a hardness of 14.2 GPa after multistep thermal treatments [18]. Similarly, Tovar-Vargas et al. reported that a composite of 10 mol% Ce-TZP and 1 mol% CaO sintered at 1450 °C under atmospheric pressure achieved a fracture toughness of 6.5 MPa∙m^1/2 and hardness up to 9.9 GPa [19].

Despite the significant progress in stabilizing ZrO₂, achieving a favorable balance between high hardness and enhanced fracture toughness remains a critical scientific challenge. Various strategies have been proposed to overcome this trade-off, including the optimization of dopant content, grain size refinement, the incorporation of second-phase reinforcements (e.g., Al₂O₃, SiC), and the adoption of advanced sintering techniques. HPHT sintering has emerged as a particularly effective approach for enhancing mechanical performance. Elevated pressure can suppress long-range atomic diffusion and inhibit grain coarsening, leading to an increase in grain boundary density and a reduction in dislocation mobility. This in turn improves yield strength, tensile strength, and fracture toughness [20,21]. Moreover, the application of high pressure facilitates densification, thereby further contributing to the enhancement of mechanical properties [22,23].

Previous studies have shown that high pressure significantly accelerates the phase transition kinetics of ZrO₂, thereby enhancing its structural evolution [24,25]. For example, Vahldiek et al. synthesized the t-phase at 1.5–2 GPa and 1200–1700 °C [26], while Kulcinski demonstrated that the t-phase could be formed directly from the m-phase at room temperature under pressures exceeding 3.7 GPa, although the transformation was entirely reversible upon pressure release [27]. Despite advances in understanding ZrO₂’s phase transitions under pressure, limited attention has been paid to simultaneously optimizing both hardness and fracture toughness.

To bridge this gap, this study systematically investigates the effects of varying Y₂O₃ content (3, 5, and 7 mol%) and sintering temperatures (400–2000 °C) under 5 GPa on the microstructure and mechanical properties of ZrO₂ ceramics. A ZrO₂ bulk ceramic exhibiting maximized fracture toughness without compromising hardness was successfully achieved by sintering at 1200 °C with 3 mol% Y₂O₃ doping. This work provides new insights into the design of high-performance ZrO₂-based ceramics under high-pressure conditions and offers a theoretical foundation for the development and application of advanced ceramic materials.

2. Materials and Methods

The ZrO₂ ceramics were consolidated via HPHT sintering using a Chinese domestic large-volume cubic press (GY560, SPD-6 × 2700 kN, Guilin Guiye Machinery Co., Ltd., Guilin, China) under a constant pressure of 5 GPa and within a temperature range of 400–2000 °C. m-ZrO₂ nanopowders with an average particle size of ~50 nm (purity ≥ 99.99%, Beijing Innochem Technology Co., Ltd., Beijing, China) and Y₂O₃ powders (purity ≥ 99.99%, Beijing Innochem Technology Co., Ltd.) were used as starting materials. Y₂O₃ was added to the ZrO₂ powder in stoichiometric amounts to achieve doping levels of 3, 5, and 7 mol%. The well-mixed powder was uniaxially pre-pressed under 10 MPa into cylindrical compacts with a diameter of 6 mm, which were then loaded into boron nitride (BN) ceramic tubes for sintering. The heating assembly consisted of a BN tube serving as a high-pressure liner, a graphite tube as the heating element, and pyrophyllite as the pressure-transmitting medium (all purchased from Zibo Jingyi Ceramics Co., Ltd., China, Zibo, China). Prior to assembly, all components were baked at 100 °C for 12 h to remove residual moisture. A schematic diagram of the assembly setup is shown in Figure 1a, while the experimental pressure–temperature profile is illustrated in Figure 1b. The pressure was ramped to 5 GPa within 20 min, and the temperature was raised to the target value over another 20 min, followed by a 10-min dwell time at the set temperature. The experimental synthetic pressure was based on the high-pressure phase transformation point of Bi, Ba, and Tl, and the chamber temperature was calibrated by a high-pressure in situ resistance measurement and monitored using Pt-30%Rh/Pt-6%Rh (0–1800 °C) and W-Re3/25 (1800–2300 °C) thermocouples.

Phase composition was characterized by X-ray diffraction (XRD, D8 ADVANCE, Bruker, Ettlingen, Germany, Cu-Kα radiation, and λ = 0.15418 nm). Raman spectra were obtained using the Monovista CRS+ (Monovista CRS+, Spectros)equipped with a 532 nm laser. Microstructure and grain morphology were examined using the Scanning Electron Microscope (FE-SEM, SU-70, Hitachi High-Technologies, Tokyo, Japan). Grain size distributions were determined by analyzing 100 g from SEM images. Vickers hardness (HV) was measured under a load of 19.6 N with a dwell time of 10 s using an HMAS-D tester (Shanghai Microcre Light-Mach Tech Co., Ltd., Shanghai, China), while fracture toughness was calculated based on crack lengths obtained under higher loads (>19.6 N) using a KB5 tester (KB Prüftechnik, Hochdorf-Assenheim, Germany). Young’s modulus was determined using a nanoindenter (G200, KLA Corporation, Chandler, AZ, USA) under a maximum load of 1000 mN. The HV [14] and fracture toughness (K_IC) [28] were calculated using the corresponding standard equations:

$$HV={\displaystyle \frac{1885.4P}{d^2}}$$

(1)

$$K_{IC}=0.004985{\left({\displaystyle \frac{E}{HV}}\right)}^{1/2}{\displaystyle \frac{P}{c^{3/2}}}$$

(2)

where P is the applied load (Kg), d is the average diagonal length (mm) of the indentation, c is the average length (mm) of the radial cracks, and E (230 GPa) is the elastic modulus of the ZrO₂-based ceramic composites.

3. Results and Discussion

3.1. Structure and Surface Morphology

Figure 2 presents the XRD patterns of 3Y-PSZ synthesized under 5 GPa at various temperatures. As shown in Figure 2a, with the increasing sintering temperature from 400 °C to 2000 °C, the intensities of the t- and c-phase peaks gradually increased, while the m-phase peaks weakened, suggesting a progressive phase transformation from the m-phase to the t- and c-phase. When 3Y-PSZ is sintered above 1000 °C, Y³⁺ ions are likely to fully dissolve into the ZrO₂ lattice, stabilizing either the t- or c-phase. As a result, the absence of distinct Y₂O₃ diffraction peaks in the XRD pattern is plausible. A localized enlargement of the 26–34° region, displayed in Figure 2b, shows that the weak t-phase peaks [29] appeared at 2θ ≈ 30.3° after 800 °C. Between 800 °C and 1500 °C, the t-phase fraction steadily increased from 31% to 62%, as calculated by the ratio of integrated intensities of selected reflections corresponding to the m-, t-, and c-phases, respectively. After 1700 °C, a new diffraction peak corresponding to the (311) plane of the c-phase emerged (Figure 2c), indicating the coexistence of m-, t-, and c-phase. At 2000 °C, the combined fraction of t- and c-phase increased significantly, with the c-phase becoming more dominant and the total high-symmetry phase content (t + c) reaching approximately 70%.

Table 1. Phase composition (m: monoclinic, t: tetragonal, and c: cubic) of 3Y-PSZ ceramics under high-pressure sintering (5 GPa) at varying temperatures.

Pressure (GPa)	Temperature (°C)	Time (min)	Phase	Phase Content
5	400	10	m	m: 100%
	600		m	m: 100%
	800		m + t	m: 68% t: 32%
	1000		m + t	m: 61% t: 39%
	1200		m + t	m: 38% t: 62%
	1500		m + t	m: 38% t: 62%
	1700		m + t + c	m: 52% t + c: 47%
	2000		m + t + c	m: 30% t + c: 70%

summarizes the sintering temperature, pressure, holding time, phase composition, and phase fractions for the 3Y-PSZ samples.

As shown in Figure 3, the XRD results for 5 mol% and 7 mol% Y-PSZ samples (5Y-PSZ, 7Y-PSZ) reveal similar phase evolution trends. Notably, the t-phase first appeared at 400 °C in both samples, indicating that higher Y₂O₃ content lowers the onset temperature for phase stabilization. By 1500 °C, m-phase peaks disappeared entirely, indicating a complete transformation into t- and c-phases. This further verifies that the increasing Y₂O₃ concentration effectively reduces the temperature required for the m- to c-phase transition. The progressive enhancement of t- and c-phase peaks, along with the gradual fading of m-phase peaks with the increasing temperature, further confirms the high-pressure-assisted phase transformation toward more stable high-symmetry phases. This phenomenon is attributed to the substitution of Zr⁴⁺ by larger Y³⁺ ions, which generates oxygen vacancies to maintain charge neutrality. The vacancies disrupt the long-range order of the oxygen sublattice and promote structural disorder, thereby reducing the free energy barrier for the formation of high-symmetry phases. In addition, lattice expansion caused by Y³⁺ incorporation helps to alleviate internal strain, making the c-phase thermodynamically favorable at lower temperatures under HPHT conditions.

Due to the overlap of the (101)_t and (111)_c diffraction peaks and the highly similar lattice parameters of the t- and c-phase, conventional XRD characterization cannot effectively differentiate between these two structures [30]. Therefore, Raman spectroscopy was employed to further analyze the phase composition of the ZrO₂ samples. The t- to c-phase transformation is primarily influenced by the relative displacement of one crystallographic axis compared to the other two, along with adjustments of the oxygen ion positions within the fluorite structure [31]. Notably, the Raman spectrum of the c-phase exhibits a broad band between 530 and 670 cm⁻¹, which is distinct from the sharp peaks characteristic of the m- and t-phase [32]. Figure 4 shows the Raman spectra of (3, 5, and 7)Y-PSZ samples, respectively, synthesized at temperatures ranging from 1000 °C to 2000 °C under 5 GPa. Sharp Raman peaks at m-180 cm⁻¹, m-191 cm⁻¹, and m-475 cm⁻¹, along with strong peaks at ~335 cm⁻¹, ~348 cm⁻¹, and ~382 cm⁻¹ (as shown by the labeled peaks in the figures) are characteristic of the m-phase, while the peaks at t-148 cm⁻¹ and t-263 cm⁻¹ are unique to the t-phase [33,34]. In Figure 4a, the intensity of the m-phase peaks (m-180 cm⁻¹, m-191 cm⁻¹, and m-475 cm⁻¹) decreases with the increasing sintering temperature from 1200 °C to 1500 °C, while the t-phase peaks at (148 cm⁻¹ and 263 cm⁻¹) become pronounced. This trend indicates a progressive phase transformation from m- to t-phase. The c-phase is characterized by a broad Raman band in the 530–670 cm⁻¹ region. In Figure 4a, this band becomes noticeably broader at 1700 °C compared to 1500 °C, which is consistent with the emergence of a c-phase (311)_c diffraction peak in the corresponding XRD patterns. At 2000 °C, the intensities of the m-phase peaks further decrease, while the combined fraction of t- and c-phase increases from 47% to 70%, resulting in a three-phase composition. Similarly, Raman broadening is observed at 1500 °C for 5Y-PSZ (Figure 4b) and 1200 °C for 7Y-PSZ (Figure 4c), confirming the formation of the c-phase at lower temperatures with higher Y₂O₃ content. These results suggest that the formation of the c-phase is significantly promoted under 5 GPa, and the critical temperature for its appearance decreases with increasing Y₂O₃ content; approximately 1700 °C for 3Y-PSZ, 1500 °C for 5Y-PSZ, and 1200 °C for 7Y-PSZ.

Figure 5 and Figure 6 present the microstructural evolution and grain size distributions of 3Y-PSZ synthesized under 5 GPa. SEM images (Figure 5) reveal dense, crack-free microstructures with a homogeneous dispersion of the m- and t-phase. Grain size analysis (Figure 6) shows minimal growth at 400–600 °C (from 125 nm to 126 nm), consistent with the absence of t-phase XRD signals (Figure 2), confirming the phase stability below 800 °C. At 800 °C, the emergence of the t-phase is accompanied by a moderate grain coarsening to 143 nm. Further heating to 1000 °C leads to distinct t-phase stabilization and accelerated grain growth (243 nm) driven by enhanced atomic diffusion and grain boundary migration driven by interfacial energy minimization. Progressive heating to 1200–2000 °C yields non-monotonic growth: 503 nm at 1200 °C, 488 nm at 1500 °C, and 1.83 μm at 1700 °C, followed by an anomalous refinement to 443 nm at 2000 °C. This reversal suggests a competitive interplay between thermal-induced coarsening and high-pressure-induced lattice strain, highlighting the complex balance between thermodynamic and mechanical factors in extreme-condition sintering.

3.2. Mechanical Properties

Figure 7 presents the Vickers hardness (under a load of 19.6N) and fracture toughness of (3, 5, and 7)Y-PSZ, respectively, synthesized at different temperatures under 5 GPa. For 3Y-PSZ (Figure 7a), the samples sintered at 400 °C and 600 °C remained in the m-phase with slightly coarsened grains, showing low-average hardness values of 4.6 GPa and 6.6 GPa. With the increasing temperature, both hardness values and fracture toughness improved, reaching maximum values of 11.6 GPa and 9.26 MPa·m^1/2 at 1200 °C, coinciding with the stabilization of the t-phase and grain refinement. However, further temperature elevation to 1700 °C resulted in significant grain growth (1.83 μm), leading to a decrease in hardness (7.8 GPa) consistent with the Hall–Petch effect [35]. Interestingly, at 2000 °C, the increasing content of the t- and c-phase, as confirmed by Figure 4, along with anomalous grain size refinement, contributed to a partial recovery in hardness to 11.2 GPa.

Similar trends were observed in the 5Y-PSZ and 7Y-PSZ samples, although both exhibited slightly lower fracture toughness compared to the 3Y-PSZ counterpart. For 5Y-PSZ (Figure 7b), the hardness increases with the increasing temperature as the t-phase fraction grows, reaching an inflection point at 1200 °C, where grain coarsening began to offset the strengthening effect. As the temperature rose further, the early appearance of the c-phase, due to higher Y₂O₃ content, led to a temporary increase in hardness, followed by a rapid decline caused by excessive grain growth and dominance of the c-phase. In 7Y-PSZ (Figure 7c), a similar trend was observed. However, the second hardness peak occurred earlier, at 1200 °C, consistent with the further reduction in the c-phase onset temperature induced by higher dopant levels. In summary, among all compositions, the 3Y-PSZ exhibited the best mechanical performance, achieving the highest hardness of 11.6 GPa and fracture toughness of 9.26 MPa·m^1/2 at 1200 °C.

To further elucidate the effects of the stabilizer concentration on the fracture behavior of ZrO₂ samples, Vickers hardness indentation and crack propagation analyses were carried out on 3Y-, 5Y-, and 7Y-PSZ samples sintered at 5 GPa and 1200 °C, as shown in Figure 8. In 3Y-PSZ, indentation under a 98 N load (Figure 8a) produced only short radial cracks. Higher magnification imaging (Figure 8b) reveals thin, blunt-ended cracks with limited extension and signs of crack branching, indicating strong resistance to crack propagation and excellent fracture toughness. The maximum toughness of 9.26 MPa·m^1/2 achieved in this work is over 12% higher than the previously reported value of 8.2 MPa·m^1/2 for t-phase ZrO₂ fabricated via the TSS process [36]. This enhancement is attributed to the improved densification achieved through the HPHT method, which effectively activates the stress-induced m- to t-phase transformation toughening mechanism.

In 5Y-PSZ, the cracks are longer and more prominent (Figure 8c). A few deflected or arrested transgranular cracks are observed (Figure 8d), suggesting a partial loss of toughening mechanisms. Correspondingly, 7Y-PSZ (Figure 8e,f), exhibits a long, smooth, and uninterrupted crack path with minimal deflection or bridging, reflecting a significantly reduced crack resistance. As the Y₂O₃ concentration increases, the metastable t-phase becomes more stabilized and less likely to transform under stress, reducing the effectiveness of transformation toughening. In particular, the c-phase, which emerges prominently in 7Y-PSZ, is non-transformable and contributes little to fracture resistance. As a result, the contribution of transformation toughening diminishes, and cracks tend to propagate in a straighter, more brittle manner, with reduced deflection or crack-bridging effects. Quantitatively, this is reflected in the decreasing fracture toughness values: 9.26 MPa·m^1/2 for 3Y-PSZ, 7.33 MPa·m^1/2 for 5Y-PSZ, and 5.21 MPa·m^1/2 for 7Y-PSZ. These observations confirm that 3Y-PSZ exhibits the highest fracture toughness among the studied compositions.

4. Conclusions

Nanocrystalline m-ZrO₂ doped with 3, 5, and 7 mol% Y₂O₃ was successfully sintered under 5 GPa at 400 to 2000 °C, resulting in the partial or complete transformation to t- and c-phase depending on the dopant content and temperature. XRD and Raman analyses revealed that higher Y₂O₃ concentrations effectively reduced the transition temperatures for both t- and c-phase. SEM and hardness measurements further showed that 3Y-PSZ sintered at 1200 °C achieved the best performance, achieving a hardness of 11.6 GPa and a fracture toughness of 9.26 MPa·m^1/2, about 12% higher than that of conventionally sintered ZrO₂. This enhancement is attributed to pressure-induced transformation toughening. These findings demonstrate the effectiveness of HPHT processing and the critical role of Y₂O₃ in optimizing the hardness–toughness balance. This work contributes valuable experimental evidence for assessing the scalability and industrial applicability of the HPHT process and provides a foundation for future investigations into alternative dopant or co-doping strategies. Further studies on long-term phase stability, creep behavior, and advanced microstructural characterization are essential for fully optimizing the performance and reliability of ZrO₂-based ceramics.