New Speed-Sintered CaO-Zirconia for Strong,Tough,Translucent Restorations

Abstract

The demand for rapid, reliable, and esthetic ceramic restorations continues to drive innovation in dental materials and chairside manufacturing technology. However, conventional yttria-stabilized zirconia presents an inherent trade-off between mechanical and optical properties, with moderate fracture toughness ≤5 MPa·m^1/2. This study investigates the feasibility of chairside speed-sintering of a 4.5 mol% CaO-stabilized tetragonal zirconia polycrystal (4.5Ca-TZP), focusing on its microstructure, mechanical reliability, translucency, and aging resistance. Nano-sized 4.5Ca-TZP powders were compacted and speed-sintered within 60 min at 1,250 °C to 1,350 °C. Four sintering groups were evaluated (n = 20/group for density; n = 1 for grain size; n = 10 for hardness; n = 3 for Rietveld refinement; n = 5 for fracture toughness, translucency, and aging; n = 30 for biaxial strength and Weibull analysis) with statistical differences set at α = 0.05. Speed-sintering gave rise to fully dense ceramics (≥99% relative density) with a homogeneous, fine microstructure (<200 nm) and negligible monoclinic content (≤2 vol%). The ceramics demonstrated excellent mechanical reliability, exhibiting a characteristic strength of ≥1.1 GPa and a Weibull modulus ≥11. Hardness slightly decreased with increasing temperature, whereas translucency reached its maximum (≈22) at 1,250 °C. Indentation testing revealed the absence of radial cracks in specimens sintered above 1,300 °C. No tetragonal-to-monoclinic transformation occurred after 20 h of hydrothermal aging at 134 °C, demonstrating aging resistance. Compared with conventionally sintered 4.5Ca-TZP, speed-sintered ceramics showed comparable translucency and strength with enhanced mechanical reliability but significantly lower fracture toughness of >5 MPa·m^1/2. Overall, this work demonstrates that 4.5Ca-TZP can be speed-sintered to achieve fully dense zirconia with performance exceeding that of conventional 3Y-TZP, providing a tougher alternative zirconia restoration. A computer-aided design/computer-aided manufacturing–milled crown fabricated as a proof of concept showed isotropic shrinkage, supporting its potential for chairside application, while further investigation of long-term resistance, large-scale manufacturing, and dimension limitations is required to confirm clinical applicability.

Keywords

ceramics CAD-CAM materials science(s)mechanical properties restorative dentistry restorative materials

Introduction

Stabilized zirconia ceramics, either in the form of tetragonal zirconia polycrystal (TZP) or partially stabilized zirconia (PSZ), are widely used in dentistry for fixed dental prosthesis (Denry and Kelly 2008; Zhang 2022; Nezir et al 2025). PSZ is characterized by a higher amount of cubic-zirconia (c-ZrO₂) compared to TZP, which mainly contains tetragonal-zirconia (t-ZrO₂). The most widely used dental zirconia are yttria-based compositions containing 3 to 6 mol% Y₂O₃. As the Y₂O₃ content increases, the proportion of c‑ZrO₂ rises. While t-ZrO₂ provides strength and toughness due to a phase transformation toughening effect, c-ZrO₂ is not transformable but has an isotropic refractive index with better optical properties (Zhang 2014). Therefore, a trade-off typically exists between mechanical properties and translucency (Zhang et al 2019).

Recently, a novel 4.5 mol% calcium-oxide stabilized zirconia (4.5Ca-TZP) emerged as a new candidate for dental applications (Vettorel et al 2025), possessing superior mechanical properties over Y-TZP ceramics, reaching a 4-point-bending (4PB) and piston-on-3-balls (P3B) strength above 1 GPa and a single-edge-V-notched-beam (SEVNB) fracture toughness of 8 to 10 MPa·m^1/2. Furthermore, the translucency was similar to that of conventional 3Y-TZPs, and no hydrothermal aging was observed after 20 h at 134 °C.

Zirconia dental restorations are manufactured via computer-aided design/computer-aided manufacturing (CAD/CAM) technologies, either by soft-machining of presintered blanks or hard-machining of fully sintered blanks. Due to the higher strength of fully sintered blocks, soft-machining is preferred with shorter milling times and the reduced wear of milling tools (Denkena et al 2017). Nevertheless, soft-machining is limited by necessary sintering after milling that can take up to 12 h using conventional sintering routes (Alshahrani et al 2024). To shorten the sintering time, speed-sintering (30–120 min) and high-speed sintering (<30 min) furnaces have been developed, allowing the completion of the restorative treatment chairside in just 1 visit, without the need for temporary restoration and a second appointment for the patient to apply the final restoration, providing improved time and cost efficiency and patient benefits.

In addition, the trend in clinical prosthodontics in recent years has shifted strongly toward monolithic restorations rather than veneered or layered systems because of reduced risk of chipping or delamination (Pjetursson et al 2021; Shihabi and Chrcanovic 2023). Depending on where the restoration will be placed, the balance between mechanical and optical properties should lean more toward better esthetic properties in the frontal area and toward improved mechanical reliability in the posterior area. Nevertheless, esthetics alone are insufficient. Restorations, especially in the posterior area (Rubió-Ferrer et al 2024), are subjected to mechanical loading, repeated chewing cycles, wear, and fatigue. Small flaws or micro-cracks act as stress concentrators and can propagate under cyclic stress, eventually causing catastrophic failure (Zhang et al 2013).

Therefore, zirconia ceramics that can be speed-sintered while combining high strength, high toughness, translucency, and long-term stability are in great demand. The main objective of this study was to explore the feasibility of producing fully dense 4.5Ca-TZP using a speed-sintering cycle compatible with chairside workflow, enabling a more complete understanding of how rapid thermal cycles influence the stability and performance of this new zirconia ceramic. Based on previous results (Vettorel et al 2025), the microstructure, mechanical properties, optical properties, and low-temperature degradation (LTD) of 4.5Ca-TZP were assessed and compared to conventionally sintered 4.5Ca-TZP and speed-sintered Y-TZPs/Y-PSZs as reference samples. A CAD/CAM-milled and speed-sintered single crown was fabricated as a proof of concept. The null hypothesis was that 4.5Ca-TZP can be successfully densified via speed-sintering while retaining properties comparable to conventionally sintered 4.5Ca-TZP.

Material and Methods

Materials

Zirconia discs with a 20-mm diameter and a 2-mm thickness were prepared from nano-sized 4.5Ca-TZP powder with 5 wt% organic binder (HSY-0480; Daiichi Kigenso Kagaku Kogyo), following classic pressing and cold isostatic pressing (CIPing). CIPed disks were thermally debinded at 500 °C for 5 h with a heating and cooling rate of 0.3 °C/min. After debinding, discs were pressureless speed-sintered (Programat CS4; Ivoclar) in air at different temperatures, namely, 1,250 °C, 1,300 °C, 1,325 °C, 1,350 °C, 1,400 °C, and 1,450 °C, for 30 min using the fastest heating rate of 130 °C/min from room temperature to 900 °C and 50 °C/min from 900 °C to the sintering temperature. A cooling rate of 70 °C/min from the sintering temperature to 800 °C was used, followed by furnace opening. The employed heating and cooling rates were chosen to conclude sintering under 1 h. The speed-sintering thermal cycle is compared to a conventional sintering cycle in Figure 1A. The sintered discs were precisely ground to the thickness necessary for the next characterization steps.

Characterization

The density (n = 20/sintering temperature) of the sintered ceramics was measured according to the Archimedes principle in ethanol. Relative density was calculated considering a theoretical density of 5.99 g/cm³.

X-ray diffraction (XRD) patterns (SmartLab SE; Rigaku) were collected on mirror-polished surfaces (n = 3/sintering temperature) from 20° to 90° 2θ. The phases were identified using the ICDD PDF 4+ database. The phase composition and lattice parameters were quantified by Rietveld refinement using Profex software (Doebelin and Kleeberg 2015). The tetragonality, t, was calculated as c/(a√2), with a and c being the lattice parameters of the t-ZrO₂ phase. The phase structures were refined as the t-ZrO₂ unit cell with a space group P42/nmc, m-ZrO₂ with P21/c, and c-ZrO₂ with Fm3m. The accuracy and precision of Rietveld refinement results were monitored using 2 selected figures of merit: the weighted profile residual (R_wp) and the goodness of fit (GoF). Refinement parameters were optimized for every measurement, resulting in an average R_wp of 1.9 and an average GoF of 2.1.

The microstructure was investigated by scanning electron microscopy (SEM; NanoSEM 450, FEI) on mirror-polished, thermally etched (200 °C below the sintering temperature for 30 min), and Pt-coated surfaces. The average grain size and grain-size distribution were measured on SEM photomicrographs by counting at least 1,000 grains using the linear intercept method with IMAGE-PRO software. The average grain size is corrected with a multiplication factor of 1.57 (Mendelson 1969).

The Vickers hardness (HV₁₀) (n = 10/sintering temperature) was measured (Model FV-700; Future-Tech) on mirror-polished disks with a load of 10 kg applied for 10 s. Optical microscopy (Axioskop 40 A Pol; Zeiss) was employed to assess the zirconia phase-transformation features around the Vickers indentations.

Strength was measured by biaxial P3B testing (n = 30/sintering temperature) on 1.2-mm-thick discs with a diameter of 15.2 mm at room temperature using a support circle with a diameter of 12 mm and 3.9-mm diameter steel balls. Discs were prepared and tested according to ISO 6872:2015 on a hydraulic testing machine (Instron 4467; Instron) at a loading rate of 0.5 mm/min.

Fracture toughness (n = 5/sintering temperature) was measured by the SEVNB method ( $K_{I C}^{S E V N B}$ ). Bending bars with 3 × 2 × 22-mm³ dimensions in the sintered condition were cut from a previously presintered block, considering the shrinkage, and then speed-sintered. A ~1-mm-deep U-groove was generated with a 150-μm-thick diamond-sawing blade in the center of the bar. Then, a ~300-μm-deep notch at the bottom of the U-groove was created with a femtosecond laser (SATSUMA; Amplitude). Specimens were annealed in air at 350 °C below the sintering temperature for 30 min to remove any residual stress generated during notch preparation. The annealed samples were loaded at a rate of 0.5 mm/min in a 4-point bending setup.

Translucency (n = 5/sintering temperature) was assessed on double-sided mirror-polished discs using a spectrophotometer (SpectroShade MICRO; MHT Optic Research). CIELAB coordinates (L*, a*, b*) were recorded on a black-and-white background and with a thin layer of Vaseline in between the specimen and the background for better optical contact. The translucency parameter (TP) was calculated according to the formula by Della Bona et al (2014).

Mirror-polished specimens (n = 5/sintering temperature) were autoclaved at 134 °C and 0.2 MPa in water vapor for 20 h. The amount of t→m phase transformation was determined after accelerated aging by XRD. The m-ZrO₂ vol% was calculated according to the formula by Garvie and Nicholson (1972) and Toraya et al (1984).

To compare the speed-sintering results, data from conventionally sintered 4.5Ca-TZP were added as a comparison in the figures and tables (Vettorel et al 2025).

Proof of Concept: Speed-Sintered CAD/CAM-Milled Crown

Blocks of 72 × 27 × 10 mm³ were prepared for CAD/CAM milling, following a similar route to the aforementioned disks. After thermal debinding, the blocks were presintered for 30 min at different temperatures from 850 °C to 1150 °C, and the relative density was measured geometrically. The presintering temperature for CAD/CAM blocks was determined according to the generally reported relative density of dental CAD/CAM Y-PSZ zirconia blocks between 50% and 60% (Ritzberger et al 2010; Amat et al 2018). Crowns were then milled from the presintered blocks by 3D Openminds (Schriek) and speed-sintered using the thermal cycle that resulted in the highest translucency, as determined by the discs. The density of the crowns before sintering and the sintering shrinkage were measured by scanning the milled and sintered crowns using μ-CT (TESCAN Unitom XL; TESCAN XRE). A tungsten target was installed on the reﬂection source together with a 1-mm-thick Cu ﬁlter to harden the X-ray beam. Almost 2,000 radiographic projections with 3 averages were acquired, each with an exposure time of 800 ms for the presintered crown and 1,080 ms for the sintered crown, while applying a tube voltage of 229 kV and a tube power of 15 W. The 3-dimensional (3D) image reconstruction, at a voxel size of 7 μm, was thereafter performed in the TESCAN reconstruction software Panthera. Further image processing, 3D visualization, and quantitative analyses were performed using Avizo software (Thermo Fisher Scientific). The presintered crown was isotopically shrunk in Avizo and quantitatively compared to the sintered one.

Statistical Analysis

Quantitative data for density, grain size, phase composition, hardness, strength, fracture toughness, and translucency parameter were presented by mean ± standard deviation, unless specified otherwise. Normality and homoscedasticity were assessed respectively with the Shapiro–Wilk test and the Levene test at a significance level of α = 0.05. If normality was satisfied, statistically significant differences between means of different groups were investigated by analysis of variance (ANOVA), followed by either Tukey’s post hoc test in the case of homoscedasticity or the Games–Howell post hoc test in the case of heteroscedasticity. If normality was not satisfied, statistically significant differences were investigated by the Kruskal–Wallis test, followed by Dunn’s post hoc test. Differences between means were considered statistically significant if P < 0.05. The bending-strength results were analyzed using Weibull statistics to determine the characteristic strength (σ₀) and the Weibull modulus (m), applying the maximum likelihood estimation with confidence bounds (α = 0.05).

Results

Microstructure and Phase Characterization

No statistically significant difference was observed when speed-sintering at different temperatures, while the average grain size increased with increasing sintering temperature, with the group speed-sintered at 1,350 °C reaching the statistically highest average grain size of 179 ± 77 nm (Table 1).

Table 1.

Density, Average Grain Size (with Correction Factor 1.57), and Rietveld Refinement Results for 4.5Ca-TZP Sintered at Different Temperatures.

Sintering Temperature (°C)	Relative Density (%)	d_avg (nm)	HV₁₀ (kg/mm²)	Phase Composition (vol. %)			Lattice Parameter t-ZrO₂ (nm)		Tetragonality of t-ZrO₂
Sintering Temperature (°C)	Relative Density (%)	d_avg (nm)	HV₁₀ (kg/mm²)	m-ZrO₂	t-ZrO₂	c-ZrO₂	c	a	Tetragonality of t-ZrO₂
SS 1,250	99.4 ± 0.3^a	124 ± 55^a	1,300 ± 8^a	/	81.7 ± 0.2	18.3 ± 0.2	0.5178	0.3601	1.0167
SS 1,300	99.4 ± 0.4^a	144 ± 58^b	1,250 ± 11^b	0.6 ± 0.1	80.4 ± 1.9	19.0 ± 1.8	0.5181	0.3601	1.0174
SS 1,350	*99.0 ± 0.3^a	179 ± 77^c	1,178 ± 7^c	2.0 ± 1.7	78.1 ± 2.1	19.9 ± 1.0	0.5183	0.3600	1.0182
CS 1,250	99.0 ± 0.2^b	124 ± 86^a	1,165 ± 11^d	8.1 ± 3.6	71.6 ± 2.5	20.4 ± 1.1	0.5187	0.3601	1.0184
CS 1,275	99.0 ± 0.2^b	155 ± 89^b	1,139 ± 11^e	14.2 ± 3.9	55.4 ± 7.9	30.5 ± 7.2	0.5190	0.3602	1.0187

Variables with different superscripts within each column are statistically different (P < 0.05). For CS 1,250 and CS 1,275, the data refer to conventionally sintered ceramics, as reported by Vettorel et al (2025).

CS, conventionally sintered; SS, speed-sintered; 4.5Ca-TZP, 4.5 mol% CaO-stabilized tetragonal zirconia polycrystal.

Density was measured on 3 samples with the ceramic sintered at 1,350 °C.

At different speed-sintering temperatures, SEM (Fig. 1B) and grain-size distributions (Fig. 1C) revealed an asymmetric distribution with a tail toward higher grain size due to the presence of larger c-ZrO₂ grains. Less than 0.2% of the grains exceeded 360 nm in the 1,250 °C and 1,300 °C groups (up to 400 nm) and less than 1.9% in the 1,350 °C group (up to 560 nm). Since 4.5Ca-TZP speed-sintered at least 1,400 °C spontaneously transformed into m-ZrO₂ during cooling, data are not reported for this group, while data for ceramics speed-sintered at 1,325 °C can be found in Appendix Table 1.

XRD patterns (Fig. 2A) exhibited similar diffraction features across different sintering temperatures with characteristic m-, t-, and c-ZrO₂ peaks. As the sintering temperature increased, the intensity of the m-ZrO₂ and c-ZrO₂ peaks increased. Quantification by Rietveld analysis (Table 1) revealed that m-ZrO₂ remained below 2.0 vol% for all speed-sintered Ca-TZP ceramics, increasing slightly with increasing sintering temperature from 0.0 to about 2.0 vol%. This was associated with a decrease in t-ZrO₂ from 81.7 to 78.1 vol% and a concomitantly increased c-ZrO₂ from 17.5 to 19.9 vol%. The tetragonality of t-ZrO₂, along with its lattice parameters, showed an almost perfect increasing linear correlation with sintering temperature (Pearson R² = 0.98).

Mechanical and Optical Properties

An inverse relationship was observed between hardness and sintering temperature (Table 1). Photomicrographs of indentations (Fig. 2B) for the Ca-TZP speed-sintered at 1,250 °C showed radial cracks starting from the corners of the indentation, without transformation bands. In contrast, no radial crack pattern was formed when speed-sintering at higher temperatures, with the formation of thin, regular, and close to the imprint transformation bands at 1,300 °C and more pronounced transformation bands at 1,350 °C.

Based on the Rietveld refinement results and HV₁₀-indentation imprints, the ceramics speed-sintered at 1,250 °C and 1,300 °C were selected for further characterization (Table 2). Ceramics speed-sintered at 1,250 °C and 1,300 °C showed a characteristic strength >1.1 GPa and Weibull modulus >11 (Fig. 2C, Table 2). The reliability of Ca-TZP speed-sintered at 1,300 °C or 1,250 °C was similar to overlapped 95% confidence intervals (CIs) of their Weibull moduli, while ceramics speed-sintered at 1,250 °C showed a statistically significant higher strength of 1,401 ± 95 MPa but a lower $K_{I C}^{S E V N B}$ of 5.11 ± 0.43 MPa·m^1/2 (Table 2).

Table 2.

Mechanical Properties of 4.5Ca-TZP Sintered at Different Temperatures.

Sintering Temperature (°C)	$K_{I C}^{S E V N B}$ (MPa·m^1/2)	σP3B (MPa)	P3B		Translucency		Aging (vol%)
Sintering Temperature (°C)	$K_{I C}^{S E V N B}$ (MPa·m^1/2)	σP3B (MPa)	95% CI of m (/)	95% CI of σ0 (MPa)	Thickness (mm)	TP (/)	m-ZrO₂ before Aging	m-ZrO₂ after Aging
SS 1,250	5.11 ± 0.43^a	1,401 ± 95^a	11.2–18.3^{a b}	1,407–1,484^a	0.52 ± 0.06^{a b}	22.4 ± 1.3^a	/	/
SS 1,300	6.22 ± 0.49^b	1,110 ± 56^b	15.6–26.3^b	1,116–1,159^b	0.57 ± 0.04^a	18.3 ± 1.3^b	1.1 ± 0.0	1.7 ± 0.1
CS 1,250	8.24 ± 0.43^c	1,358 ± 200^{a c}	5.7–10.6^c	1,361–1,523^a	0.48 ± 0.02^b	16.3 ± 1.9^b	13.1 ± 3.3	16.1 ± 1.4
CS 1,275	9.73 ± 0.45^d	1,306 ± 179^c	6.0–11.9^{a c}	1,309–1,462^a	0.38 ± 0.03^c	21.2 ± 1.5^a	20.3 ± 0.2	15.6 ± 1.3

Results are reported as mean ± standard deviation, along with the 95% confidence intervals of the Weibull moduli and the characteristic strength (σ₀), translucency, and monoclinic ZrO₂ content before and after 20-h hydrothermal aging at 134 °C. Variables within each column with different superscripts are statistically different (P < 0.05). For CS 1,250 and CS 1,275, the data refer to conventionally sintered ceramics, as reported by Vettorel et al (2025).

CI, confidence interval; CS, conventionally sintered; P3B, piston-on-3-balls; SS, speed-sintered; TP, translucency parameter; 4.5Ca-TZP, 4.5 mol% CaO-stabilized tetragonal zirconia polycrystal.

Hydrothermal aging (Table 2) revealed no significant increase in m-ZrO₂ after accelerated aging in both groups.

The TP values (Table 2) were statistically significantly different when speed-sintered at 1,250 °C and 1,300 °C, reaching the highest value of 22.4 ± 1.3 at 1,250 °C. CIELAB coordinates can be found in Appendix Table 2.

Speed-Sintered CAD/CAM-Milled Crown

The relative density of the block after CIPing was 49%, and it increased exponentially with increasing presintering temperature (Fig. 3A) from 49% at 850 °C to 83% at 1,150 °C. A relative density of 58% was obtained at 1,050 °C, which was used for the CAD/CAM milling blocks. The relative density of the milled crown was 58.3%, as measured by micro–computed tomography (μ-CT), consistent with the geometrically measured values shown in Figure 3A. After speed-sintering at 1,250 °C, the crown reached a final relative density of 99.3%. The sintered crown appears translucent under normal light illumination (Fig. 3B). The total volumetric shrinkage was 41.8% (i.e., 16.6% isotropic shrinkage). The milled crowns retained their shape after sintering; 0.8% of the crown shrunk more than an isotropic shrinkage, with an average mismatch of 8 ± 5 μm, while 1.3% of the crown shrunk less than an isotropic shrinkage, with an average of −11 ± 7 μm (Fig. 3C). Defects smaller than the voxel size were not observed in the 3D-reconstructed CT volumes.

Discussion

The speed-sintering protocol used in this study showed the potential to produce fully dense, translucent nanocrystalline 4.5Ca-TZP with high strength, improved toughness, and aging resistance compared to conventional 3Y-TZP. Compared to conventionally sintered 4.5Ca-TZP (Vettorel et al 2025), the entire speed-sintering process was faster, reducing sintering times to below 60 min (Fig. 1A). Importantly, speed-sintering resulted in negligible m-ZrO₂, whereas this could not be avoided in conventionally sintered 4.5Ca-TZP (Table 1). This is because faster heating rates generate finer microstructures and more stable t-ZrO₂ grains (smaller tetragonality), and rapid cooling quenches the t-ZrO₂ before it can transform into m-ZrO₂. Slightly higher densities were achieved with the speed-sintering protocol, as confirmed by the statistically significantly higher hardness (Table 1) and the absence of small pores observed in SEM photomicrographs of conventionally sintered Ca-TZP. Accordingly, speed-sintering of 4.5Ca-TZP also exhibited high P3B strength of >1,300 MPa and translucency, similar to conventionally sintered equivalents. However, the SEVNB toughness of speed-sintered 4.5Ca-TZP was lower than that of conventionally sintered equivalent, but it remained higher than that of classic Y-TZPs. Therefore, the null hypothesis that 4.5Ca-TZP can be successfully densified via speed-sintering while retaining properties comparable to conventionally sintered 4.5Ca-TZP is partially accepted.

A very strong positive linear relationship (Pearson R² = 0.96) was found between the tetragonality and the $K_{I C}^{S E V N B}$ of conventionally sintered and speed-sintered 4.5Ca-TZP. Fracture toughness is also influenced by grain size exceeding the critical size, that is, the size above which t-ZrO₂ spontaneously transforms upon cooling, 100 to 150 nm for Ca-TZP systems (Pyda and Haberko 1987; Lamas et al 1998). A higher $K_{I C}^{S E V N B}$ is expected for 4.5Ca-TZP speed-sintered at 1,350 °C because of its higher tetragonality and transformation bands that closely mirror the ones of conventional sintering at 1,250 °C (Fig. 2B).

Although grain sizes obtained by speed and conventional sintering were not statistically different, speed-sintered 4.5Ca-TZP at 1,300 °C showed significantly lower strength (Table 2), attributed to reduced t-ZrO₂ transformability. Conventionally sintered Ca-TZP showed higher tetragonality and a broader grain-size distribution, leading to a larger fraction of grains exceeding the critical grain size and thus higher transformability, but at the expense of mechanical reliability (lower Weibull modulus and larger strength scattering, Fig. 2C). In contrast, speed-sintered 4.5Ca-TZP showed a narrower strength distribution and higher mechanical reliability. This is notable as speed-sintering of classic yttria-stabilized zirconia typically reduces reliability compared to conventional sintering (Cokic et al 2020), and a higher Weibull modulus is critical for the long-term performance in industrial applications. In addition, regardless of sintering protocol, the current 4.5Ca-TZP differs fundamentally from other recent CaO-ZrO₂ systems that either struggled to reach strength >1 GPa (Łabuz et al 2015) or relative density >99% (Arun et al 2021; Kumar et al 2024).

The TP values of 4.5Ca-TZP were not significantly affected by the speed-sintering and conventional sintering protocol, but the different translucency trends with sintering temperature between CS and SS can be explained by the balances between different parameters that influence the translucency (Klimke et al 2011; Zhang 2014; Roitero et al 2023). In conventional sintering, the higher translucency at 1,300 °C compared with 1,250 °C is attributed to the increased c-ZrO₂ content, which outweighs the effects of grain growth and higher tetragonality. In contrast, speed-sintered specimens showed similar c-ZrO₂ contents at 1,250 °C and 1300 °C, and the lower grain size and reduced tetragonality at 1,250 °C resulted in higher translucency (Table 2).

Compared to speed-sintered commercially available 3Y-TZPs (Jansen et al 2019; Cokic et al 2020; Alshahrani et al 2024), the speed-sintered 4.5Ca-TZP not only is tougher, stronger, and aging-free but also shows similar translucency. Furthermore, Ca-TZP can be sintered at lower temperatures than 3Y-TZPs thanks to its nano-sized crystallites and high surface area starting powder. The LTD resistance and superior mechanical and optical properties highlight this novel 4.5Ca-TZP as a promising substitute for 3Y-TZP in dental applications requiring a well-balanced combination of these characteristics. The potential of speed-sintered 4.5Ca-TZP for chairside dental restorations was further validated by a proof-of-concept crown (Fig. 3) that was successfully milled and speed-sintered with an almost perfect isotropic shrinkage, as only ~2% of the volume shrunk anisotropically with a maximum deviation under ±30 μm.

Despite the promising results of speed-sintered 4.5Ca-TZP for chairside restorations, several limitations should be noted. The sintering window for 4.5Ca-TZP is relatively small, as a 25 °C to 50 °C difference may lead to a different microstructure with different properties or even spontaneous transformation. For industrial production, sintering furnaces need to be precisely controlled to limit batch-to-batch variations. Speed-sintering large ceramic components like bridges is much more challenging than sintering small parts. Thermal gradients between core and surface will be larger, which may cause differential densification, warping, microcracking, and residual stresses. Furthermore, although speed-sintered and conventional sintered 4.5Ca-TZP have significantly better mechanical properties than those of Y-PSZ ceramics and lithium disilicates, their translucency is inferior. The application of 4.5Ca-TZP is expected mainly for posterior restorations where mechanical reliability is of utmost importance. Extended aging tests and activation-energy analysis will be needed to clarify this ceramic’s intrinsic aging behavior.

Nevertheless, this unique properties combination offered by 4.5Ca-TZP can be attractive for ultra-thin chairside restorations (Øilo et al 2019; Bataineh and Al Janaideh 2023; Benalcazar Jalkh et al 2024), with an average thickness below 0.5 mm, allowing minimal tooth structure removal and reducing the risk of potential dental pulp damage or decreased stability of the remaining tooth substance. Furthermore, the strength of speed-sintered 4.5Ca-TZP was well above the ISO 6872:2015 threshold of 800 MPa. Future research should address coloration, cementation, margin adaptation, machinability of thin restorations, wear resistance, and long-term fatigue to support clinical translation.

Conclusions

Speed-sintering of the new 4.5Ca-TZP ceramic produced a homogeneous, nano-sized microstructure that combines high flexural strength with a toughness significantly higher than that of classic yttria-stabilized zirconia, along with adequate translucency and better hydrothermal aging resistance. Compared to conventional sintering, speed-sintering that was completed in less than 60 min is well suited to fabricate ceramic with a fine microstructure and a narrow grain-size distribution, resulting in sintered zirconia with negligible m-ZrO₂ and enhanced reliability, but with significantly reduced fracture toughness. As demonstrated by the proof-of-concept fabricated zirconia crown through CAD/CAM soft machining and speed-sintering, this 4.5Ca-TZP is suitable for chairside monolithic posterior restorations, while limitations remain, including material sensitivity to sintering temperature, challenges with sintering large components, and low translucency for frontal restorations.

Author Contributions

All authors gave their final approval and agree to be accountable for all aspects of the work. A. Vettorel: Data curation, Formal analysis, Validation, Investigation, Visualization, Methodology, Writing – original draft. B. V. Meerbeek: Writing – review & editing, Resources, Project administration, Funding acquisition. J. Vleugels: Conceptualization, Resources, Writing – review & editing. F. Zhang: Conceptualization, Resources, Formal analysis, Supervision, Funding acquisition, Methodology, Funding acquisition, Project administration, Writing – review & editing.

Supplemental Material

sj-docx-1-jdr-10.1177_00220345261435597 – Supplemental material for New Speed-Sintered CaO-Zirconia for Strong, Tough, Translucent Restorations

Supplemental material, sj-docx-1-jdr-10.1177_00220345261435597 for New Speed-Sintered CaO-Zirconia for Strong, Tough, Translucent Restorations by A. Vettorel, B.V. Meerbeek, J. Vleugels and F. Zhang in Journal of Dental Research

Footnotes

Acknowledgements

The authors thank Wout Veulemans for grinding the P3B discs, Ben Mercelis (cutting and U-notching) and Mathieu Chabanel (laser notching) for preparing the SEVNB samples, Jeroen Soete for the Avizo quantitative analysis, and Daiichi Kigenso Kagaku Kogyo, Osaka, Japan, for supplying the 4.5mol% CaO-stabilized zirconia (HSY-0480) nanopowder.

A supplemental appendix to this article is available online.

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: research fund of KU Leuven under project C24E/23/037 and the Research Foundation—Flanders (FWO-Vlaanderen) fund under project FWO G057924N.

ORCID iDs

A. Vettorel

B.V. Meerbeek

J. Vleugels

F. Zhang

Data Availability

Data presented in this work are available at 10.5281/zenodo.18196421.

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