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Abstract

Despite being introduced into restorative dentistry nearly 30 y ago and having evolved into the most widely used ceramic restorative system, zirconia has only recently become available for chairside, same-day treatments, thanks to the development of high-speed sintering technology. However, recent studies have revealed that high-speed sintering not only severely compromises the translucency of dental zirconias (particularly in 3YSZ and 5YSZ) but also alters the translucency hierarchy among these compositions. Building on these critical findings, we hypothesized that a composition between 3YSZ and 4YSZ, such as 3.5YSZ, may be better suited for high-speed sintering protocols, offering both excellent strength and translucency. To test this hypothesis, 30 disc-shaped 3.5YSZ were uniaxially pressed, followed by cold-isostatic pressing and bisque firing. These presintered discs were then subjected to high-speed sintering using a commercial 18-min zirconia speed-fire protocol. Translucency parameters (TP and TP₀₀; n = 18), contrast ratio (CR; n = 18), and biaxial flexural strength (n = 18) tests were conducted, along with density (n = 10), microstructural (n = 3), and compositional (n = 3) characterizations. Our findings show that high-speed sintered 3.5YSZ exhibited superior TP and CR values (P < 0.0001) compared to existing YSZ counterparts, while maintaining the high-strength characteristic of 3YSZ (P = 0.7420). The clinical implications of this newly developed composition, as well as future directions for fabricating this material with improved translucency and strength for efficient chairside workflows, are discussed.

Keywords

3.5Y-TZP high-speed sintering theory of reflection and refraction translucency parameter biaxial flexural strength Weibull analysis

Introduction

According to a leading dental laboratory, 81% of the millions of crowns produced annually in the United States are made of monolithic zirconia (Glidewell 2022). This dominance is attributed to the superior mechanical and optical properties of modern dental zirconia, particularly yttria-stabilized zirconia (YSZ) (Sen and Isler 2020; Cesar et al. 2024). However, this excellence is achieved through compromises among various zirconia compositions. In general, dental YSZ compositions range from 3YSZ and 4YSZ to 5YSZ. Under conventional sintering conditions, which typically require 8 to 12 h per cycle, strength decreases while translucency increases as the Y content rises (Kwon et al. 2018; Zhang and Lawn 2018). Figure 1A illustrates the dependence of flexural strength and translucency parameter on Y content for a widely used dental zirconia system—the Zpex series (Tosoh Corporation)—fabricated using conventional sintering protocols (Lim et al. 2022).

Figure 1.

Biaxial flexural strength (σ) and translucency parameter (TP) of widely used dental 3YSZ, 4YSZ, and 5YSZ compositions following conventional (Lim et al. 2022) and high-speed (Alshahrani et al. 2024) commercial sintering protocols. (A) Conventional sintering. (B) High-speed sintering. YSZ, yttria-stabilized zirconia.

Dentistry is increasingly adopting digital workflows to achieve greater time efficiency, enhanced accuracy, improved aesthetic potential, better patient comfort, reduced costs, and broader access to dental care (Rekow 2020; Cheng et al. 2021). Nowadays, the entire workflow can be completed within a few hours, making chairside, 1-visit treatment feasible (Kaizer et al. 2017; Al-Haj Husain et al. 2022). However, the slow conventional sintering protocols that yield the predictable properties of YSZ hinder the use of these materials for chairside treatments. To overcome this barrier, high-speed sintering furnaces and protocols have been developed, which can be fully integrated into the chairside workflow (Jerman et al. 2020). YSZ crowns and bridges (up to 3 units) can be sintered in just 18 min. While effectively reducing sintering time by an order of magnitude, a recent study has shown that high-speed sintering preserves the strength of YSZs but compromises their translucency (Alshahrani et al. 2024). Furthermore, although its influence on the strength’s dependence on Y content remains consistent, high-speed sintering alters the translucency hierarchy. As shown in Figure 1B, for the same zirconia system (Zpex series, Tosoh Corporation) subjected to the high-speed sintering protocol, the highest translucency is observed in 4YSZ, in contrast to 5YSZ under conventional sintering conditions (Alshahrani et al. 2024).

Previously, we demonstrated that the reduced translucency of high-speed sintered YSZ is primarily caused by the presence of a few percent of nanometer-scale porosity (on the order of 70–240 nm), which scatters light but has a minimal impact on flexural strength (Alshahrani et al. 2024). To mitigate this issue and preserve zirconia strength, 2 potential strategies exist: modification of the high-speed sintering protocol or tailoring the YSZ composition. Our lab is currently pursuing both approaches, but here, we focus on the latter.

Our analysis of the relationship between translucency, strength, and Y content under high-speed sintering conditions (Fig. 1B) suggests that a composition between 3YSZ and 4YSZ may offer an optimal balance of both properties. Accordingly, this study hypothesizes that a 3.5YSZ composition may be more suitable for the current high-speed sintering protocol than existing YSZ formulations, offering both excellent strength and translucency. In the following sections, we test this hypothesis by examining the translucency and strength of an experimental 3.5YSZ material relative to our previously studied 3YSZ and 4YSZ compositions, using identical fabrication and characterization protocols. Additionally, we investigate the dependence of the translucency parameter on clinically relevant thicknesses of 3.5YSZ and use the resulting data to validate theoretical predictions.

Optical Theory

In dentistry, the translucency parameter of a restorative material is typically measured using a spectrophotometer, with the specimen placed over either a black or a white backing. The dental spectrophotometer comprises collimated light sources (wavelength range: 400–700 nm) that emit light onto the surface of the restorative material and a detector that measures the reflected light across the visible spectrum, as illustrated in Figure 2A. This process can be described by the principles of light reflection and refraction within the framework of geometrical optics (Born and Wolf 1999).

Figure 2.

Schematic diagrams of translucency parameter (TP) measurements using a dental spectrophotometer. (A) Experimental setup with the specimen (solid layer) placed over either a black or a white backing. (B) Illustration of multiple reflections of incident light within a solid layer, based on the principles of reflection and refraction in geometrical optics.

When incident light strikes the surface of a solid layer of thickness d, part of it is reflected (R₁), and the remainder (1 – R₁) is refracted into the material (Fig. 2B). A fraction of the refracted light, T₁(1 – R₁), reaches the bottom surface. Here, T₁ denotes the internal transmittance of the solid layer and can be described by the exponential attenuation of light with depth in the material (Fernández-Oliveras et al. 2013):

T_{1} = e^{- μ d},

(1)

where µ is the effective transport coefficient, accounting for both scattering and absorption.

The detector captures the combined reflections from both the top and bottom surfaces, which can be estimated as

R = \frac{R_{1} + R_{1} {(1 - R_{1})}^{2} T_{1}^{2}}{1 - R_{1}^{2} T_{1}^{2}},

(2)

where the surface reflectance R₁ is given by

R_{1} = 1 - 2 n n_{a i r} / (n^{2} + {n_{a i r}}^{2}) .

(3)

Here, n and n_air ≈ 1 are the refractive indices of the solid and air, respectively (Zhang 2014).

Experimental

Specimen Preparation

An experimental powder composition, 3.5YSZ (Upcera Dental America), formulated for dental restorations, was evaluated. The raw powders were synthesized using a common hydrolysis process combined with spray drying, a method widely adopted by Tosoh and other leading ceramic powder manufacturers. The powders were uniaxially pressed in a hardened steel die, followed by cold-isostatic pressing at 250 MPa to produce disc-shaped specimens (Ø17 × 3 mm; n = 30).

Bisque firing was performed according to a protocol developed by Tosoh, involving a dwell temperature of 960°C for 120 min, with heating and cooling rates of 1°C/min. High-speed sintering was conducted in a dental induction furnace (SpeedFire; Dentsply Sirona) using a protocol developed for Katana One zirconia crowns and 3-unit bridges.

Both surfaces of the high-speed sintered specimens were sequentially lap-polished using 15-, 6-, 3-, and 0.5-µm diamond-impregnated metal pads under continuous water irrigation in an automatic polisher (Buehler). The final dimensions of the mirror-polished specimens were Ø13.75 ± 0.02 mm × 1 ± 0.02 mm.

Of the 30 specimens, 18 were allocated for biaxial flexural strength testing and Weibull analysis, with subsets also used for translucency (n = 12) and density (n = 10) measurements. The remaining specimens were used for translucency–thickness analysis (n = 6), phase analysis via X-ray diffractometry (XRD; n = 3), and microstructural analysis via scanning electron microscopy (SEM; n = 3).

Density Measurement

Bulk density was measured using Archimedes’ method on an analytical balance (Mettler Toledo; n = 10). The theoretical density ( $ρ_{Theo}$ ) for 3.5YSZ was calculated following the method described by Lim et al. (2022), based on the measured tetragonal and cubic contents obtained via XRD. Relative density ( $ρ_{Rel})$ was determined by dividing the measured bulk density ( $ρ_{Bulk}$ ) by $ρ_{Theo}$ .

Microstructural Analysis

The microstructure of specimens (n = 3) was examined on polished and thermally etched surfaces using environmental field emission SEM (Quanta 250 FEG; Thermo Fisher Scientific). Thermal etching was performed at 1,200°C for 15 min, which is over 300°C below the sintering temperature to prevent unwanted grain growth. Imaging was conducted at a 30-kV accelerating voltage and a 10-mm working distance.

Representative SEM micrographs were used to determine grain size via the linear intercept method (ASTM Standard E112-13 2013), applying a correction factor of 1.56 (Wurst and Nelson 1972). For each material, 500 grains were measured.

Phase Analysis

Crystalline phases were characterized by XRD using CuKα radiation (λ = 1.5418 Å; MiniFlex 6G; Rigaku). The operating parameters were 40 kV and 15 mA. Scans were conducted over a 2θ range of 20° to 80° (n = 3), with a 0.02° step size and a 1°/min scan speed.

The tetragonal and cubic zirconia contents were estimated using a method developed by Mochales et al. (2011), in which the integrated intensities of the cubic peak I_c(400) and the tetragonal peaks I_t(004) and I_t(220) were used to calculate the volume fraction of the cubic phase (V_c):

V_{c} = I_{c} (400) / [I_{t} (004) + I_{c} (400) + I_{t} (220)] .

(4)

Translucency Measurement

The translucency parameters of the specimens (n = 18) were determined using a dental spectrophotometer (SpectroShade Micro; MHT) over ideal black and white backings (Carter et al. 2018). To ensure optical continuity, a drop of coupling liquid (refractive index: 1.8; Gem Refractometer Liquid; Cargille Laboratories) was placed between the specimen and the backing (Nogueira and Della Bona 2013).

To facilitate direct comparison with previously published data, the translucency parameter (TP) was calculated based on the color difference between specimens placed on black (B) and white (W) backings, using the following equation (Johnston et al. 1995):

T P = {[{(L_{B}^{*} - L_{W}^{*})}^{2} + {(a_{B}^{*} - a_{W}^{*})}^{2} + {(b_{B}^{*} - b_{W}^{*})}^{2}]}^{1 / 2}

(5)

where L* , a* , and b* represent the lightness, red-green, and yellow-blue coordinates, respectively, in the CIE color space (Carter et al. 2018).

For the reader’s information, the translucency parameter (TP₀₀) based on the CIEΔE₀₀ (1:1:1) color difference formula (Luo et al. 2001) was also calculated:

T P_{00} = {[\begin{array}{l} {(\frac{L'_{B} - L'_{W}}{K_{L} S_{L}})}^{2} + {(\frac{C'_{B} - C'_{W}}{K_{C} S_{C}})}^{2} + {(\frac{H'_{B} - H'_{W}}{K_{H} S_{H}})}^{2} \\ + R_{T} (\frac{C'_{B} - C'_{W}}{K_{C} S_{C}}) (\frac{H'_{B} - H'_{W}}{K_{H} S_{H}}) \end{array}]}^{1 / 2},

(6)

where S_L, S_C, and S_H are weighting functions, and K_L, K_C, and K_H are parameter factors for the lightness (L′), chroma (C′), and hue (H′), respectively. R_T is a rotation function that accounts for interactions between chroma and hue differences, particularly in the blue-purple region.

The contrast ratio (CR) of the specimens (n = 18) was obtained by dividing the luminance measured on a black backing (Y_B) by that on a white (Y_W) backing (da Costa et al. 2009):

C R = Y_{B} / Y_{W}

(7)

CR values range from 0 (complete transparency) to 1 (total opacity).

Biaxial Flexural Strength Measurement

The biaxial flexural strength of the specimens (n = 18) was measured using a piston-on-3-ball jig. Three hardened steel balls (Ø4 mm) were positioned 120° apart to form a support circle with a diameter of 10 mm. The hardened steel loading piston had a diameter of 1.4 mm. Tests were performed on a universal testing machine (Instron 68TM-5) at a crosshead speed of 1 mm/min to minimize slow crack growth. Biaxial flexural strength and the subsequent Weibull analysis were computed in accordance with ISO 6872:2024. A Poisson ratio of 0.32 was used for zirconia (Kim et al. 2010).

Statistical analysis

Statistical analysis was performed using GraphPad Prism 9 (GraphPad Software). The significance level was set at α = 0.05 for all tests. Data sets were first assessed for normality using the Shapiro–Wilk test. If the data failed the normality test, the nonparametric Kruskal–Wallis test, followed by Dunn’s post hoc test, was used. For normally distributed data, 1-way analysis of variance with Tukey’s post hoc test was applied.

Statistical comparisons of the Weibull moduli (m) between groups were conducted by evaluating differences in the slopes of 2 independent ln(strength)–failure probability data sets, as described by Howell (2007). P values were calculated using both pooled and unpooled error variances.

Differences in the predicted flexural strength values at a 5% failure probability (with 95% confidence bounds) were evaluated by calculating the confidence interval of their difference. If the interval included zero, the difference was considered statistically insignificant. Otherwise, it was deemed statistically significant.

Results

The properties of the high-speed sintered 3.5YSZ material are presented in Figure 3, Table 1, and Figure 4. Where appropriate, corresponding data for the 3YSZ and 4YSZ compositions—previously obtained in our laboratory using identical powder compaction, sintering, and characterization techniques—are included for direct comparison (Alshahrani et al. 2024).

Figure 3.

Microstructural, compositional, optical, and mechanical properties of 3.5YSZ subjected to high-speed sintering. (A) Secondary scanning electron microscopy image of a polished and thermally etched surface (1,200°C for 15 min), revealing a dense, submicron-sized microstructure. (B) X-ray diffractometry spectrum showing that the crystalline content consists primarily of the tetragonal zirconia phase, with a minor cubic phase and no detectable monoclinic phase. (C) The dependence of the translucency parameter (TP) of high-speed sintered 3.5YSZ as a function of specimen thickness. Theoretical predictions based on geometrical optics principles of reflection and refraction are shown as a broad gray band. The predicted TP range is calculated using the 400- to 700-nm wavelength range of the dental spectrophotometer, along with the corresponding refractive index (Zhang 2014) and the effective transport coefficient (Fernández-Oliveras et al. 2013) of yttria-stabilized zirconia. (D) Weibull failure probability (P) versus biaxial flexural strength (σ) for high-speed sintered 3.5YSZ, relative to previously studied 3YSZ and 4YSZ compositions, all fabricated and characterized using identical protocols (Alshahrani et al. 2024). YSZ, yttria-stabilized zirconia.

Table 1.

Summary of the Physical and Mechanical Properties of 3.5YSZ Relative to Previously Studied 3YSZ and 4YSZ, All Subjected to Identical Powder Compaction, Speed Sintering, and Characterization Protocols.

Number of specimens:Density: n = 10Pore size: n = 36Grain size: n = 500Translucency/contrast ratio: n = 18Biaxial flexural strength: n = 18	3.5YSZ	3YSZ	4YSZ
	All specimens were fabricated from powder compacts, cold-isostatically pressed at 250 MPa, and presintered at 960°C for 2 h, followed by high-speed sintering in a SpeedFire furnace at 1,560°C with a 5-min dwell time.
	Total sintering time: 18 min
Bulk density, ρ_Bulk (g/cm³)	6.07 ± 0.01	5.96 ± 0.05	5.87 ± 0.05
Theoretical density, ρ_Theo (g/cm³)	6.08	6.08	6.05
Relative density, ρ_Rel (%)	99.87 ± 0.21^a	97.99 ± 0.88^b	96.97 ± 0.76^c
Porosity, P (%)	0.13 ± 0.15	2.01 ± 0.28	3.03 ± 0.24
Pore size, ϕ (µm)	0.041 ± 0.010^a	0.077 ± 0.011^b	0.110 ± 0.012^c
Grain size, D (µm)	0.66 ± 0.02^a	0.64 ± 0.01^b	0.85 ± 0.05^c
t-ZrO₂, V_t (vol.%)	81.47	85.09	67.64
c-ZrO₂, V_c (vol.%)	18.53	14.91	32.36
Translucency parameter, TP (%)	24.89 ± 1.15^a	14.42 ± 0.74^b	20.38 ± 0.87^c
Translucency parameter, TP₀₀ (%)	18.88 ± 0.89	—	—
Contrast ratio, CR	0.53 ± 0.04^a	0.73 ± 0.02^b	0.62 ± 0.03^c
Biaxial flexural strength, σ (MPa)	1017 ± 117^a	994 ± 83^a	817 ± 75^b
Weibull modulus, m	9.8^a	14.3^b	12.8^a,b

For reference, the translucency parameters of the current 3.5YSZ material are presented in both TP and TP₀₀ forms. All measurements were performed using disc-shaped specimens of identical dimensions. Superscript letters represent statistical comparisons among the different materials sintered under the same protocol: the same letter denotes no significant difference, while different letters indicate statistically significant differences. For 3YSZ and 4YSZ, data were obtained from a previous study (Alshahrani et al., 2024), in which TP₀₀ values were not presented.

YSZ, yttria-stabilized zirconia.

Figure 4.

Translucency parameter (TP) versus biaxial flexural strength (σ) for high-speed sintered 3.5YSZ, relative to corresponding data previously obtained for 3YSZ and 4YSZ compositions, all fabricated and characterized using identical protocols (Alshahrani et al. 2024). (A) Means and σ values with 1 standard deviation. (B) Strength at 5% failure probability with 95% confidence bounds. The vertical dashed line indicates the 500 MPa threshold strength (σ) recommended for 3-unit bridges according to ISO 6872:2024. The horizontal gray box represents the translucency parameter (TP) threshold for esthetic zones, corresponding to a TP value of 18.7% measured for human enamel (Yu et al. 2009). YSZ, yttria-stabilized zirconia.

Figure 3 shows the microstructure, composition, optical properties, and mechanical properties of 3.5YSZ subjected to high-speed sintering. A representative SEM image of a polished and thermally etched surface demonstrates a dense, uniform, submicron-sized grain structure (Fig. 3A). XRD analysis revealed that the crystalline content consists primarily of a tetragonal zirconia phase, along with a minor cubic phase (Fig. 3B).

The TP values as a function of specimen thickness for high-speed sintered 3.5YSZ are shown in Figure 3C. For comparison, theoretical predictions of TP dependence on specimen thickness, calculated using equations (1) to (3), are also plotted as a wide gray band in Figure 3C. Since the light source of a dental spectrophotometer consists of a broad range of visible wavelengths, and both the refractive index and effective transport coefficient of zirconia vary with wavelength, TP values were predicted for wavelengths ranging from 400 to 700 nm, using corresponding refractive index data (Zhang 2014) and effective transport coefficients (Fernández-Oliveras et al. 2013).

The Weibull plot for the current 3.5YSZ material, along with those previously generated for 3YSZ and 4YSZ (Alshahrani et al. 2024), is shown in Figure 3D.

Detailed data concerning bulk density, porosity, pore size, grain size, phase content, translucency parameter, contrast ratio, biaxial strength, and Weibull modulus of high-speed sintered 3.5YSZ are summarized in Table 1. Data are presented as the mean ± standard deviation (SD), where applicable. For comparison, corresponding data for the previously studied 3YSZ and 4YSZ compositions, obtained using identical protocols and techniques, are also included (Alshahrani et al. 2024). As can be seen, under the 18-min high-speed sintering protocol, 3.5YSZ is 22% and 73% more translucent than 4YSZ (P < 0.0001) and 3YSZ (P < 0.0001), respectively. This finding is supported by a significantly lower CR value for 3.5YSZ relative to both 4YSZ (P < 0.0001) and 3YSZ (P < 0.0001). Conversely, the biaxial flexural strength of 3.5YSZ is statistically similar to that of 3YSZ (P = 0.7420), but 24% higher than that of 4YSZ (P < 0.0001). However, although the Weibull modulus of 3.5YSZ is statistically similar to that of 4YSZ (P = 0.0683, pooled error variances; P = 0.0561, unpooled error variances), it is significantly lower than that of 3YSZ (P = 0.0066, pooled; P = 0.0020, unpooled).

Finally, for clinical relevance, the relationship between σ and TP for high-speed sintered 3.5YSZ, along with the previously studied 3YSZ and 4YSZ compositions, is plotted in Figure 4. The data are based on the mean ± SD values from Table 1 and the predicted strength at 5% failure probability with 95% confidence bounds from Figure 3C. Once again, 3.5YSZ is clearly more translucent than 4YSZ, and even more so than 3YSZ. In addition, despite having a lower Weibull modulus than 3YSZ, the predicted flexural strength of 3.5YSZ at the 5% failure probability, taking into account the 95% confidence bounds, is statistically similar to that of 3YSZ (P > 0.05) and significantly higher than that of 4YSZ (P < 0.05). Here, the P value was determined for the difference between 2 values, each with a given 95% confidence interval.

Discussion

Building on the critical findings of recent studies (Fig. 1), we hypothesized that a composition between 3YSZ and 4YSZ (e.g., 3.5YSZ) may be better suited for high-speed sintering protocols, offering both high strength and translucency. Our current findings demonstrate that, under an 18-min high-speed sintering protocol, 3.5YSZ exhibits higher translucency (TP and CR) than both 3YSZ and 4YSZ, while maintaining the high flexural strength (σ) of 3YSZ (Table 1 and Fig. 3). Thus, our hypothesis is supported.

The superior translucency of 3.5YSZ compared to 3YSZ, and even exceeding that of 4YSZ, warrants further discussion. Detailed microstructural and compositional analyses revealed that high-speed sintered 3.5YSZ exhibits lower porosity than 3YSZ (P < 0.0001), and even more so than 4YSZ (P < 0.0001). Additionally, the average pore size in 3.5YSZ is also much smaller than that in 3YSZ and 4YSZ (Table 1). The lower porosity, coupled with a smaller pore size, particularly when porosity is below 0.3% and pore size is under 50 nm, can significantly reduce light scattering from pores, thereby enhancing the translucency (Zhang 2014). However, the lower cubic phase content and the slightly smaller (although still >100 nm) grain size of 3.5YSZ relative to 4YSZ (Table 1) could counteract some of these translucency gains. This suggests that the translucency of 3.5YSZ may be further enhanced by slightly increasing the sintering temperature to promote greater cubic phase content and grain growth, while maintaining high density. It should be noted, however, that higher sintering temperatures can lead to excessive grain growth, which can induce pore formation and ultimately compromise translucency. Therefore, with careful selection of the sintering temperature, the translucency of 3.5YSZ can be further improved without compromising its high strength.

Since clinical indications for the minimum wall thickness of zirconia crowns, veneers, inlays, and onlays vary from 0.4 to 1.0 mm (Kolakarnprasert et al. 2019), depending on the restoration type, tooth location, and zirconia composition, the dependence of TP on zirconia thickness justifies a detailed investigation. Although it is well established that light attenuation in a solid follows an exponential decay, the actual TP of a restorative material measured by a dental spectrophotometer does not decrease as rapidly as the attenuation function suggests (Swinehart 1962; Antonson and Anusavice 2001; Mayerhöfer et al. 2020). This discrepancy arises because the spectrophotometer measures reflectance from the tooth or restoration, closely mimicking how translucency is perceived by the human eye. In such cases, while the total reflectance varies with ceramic thickness, the primary reflectance (i.e., the strongest reflection, shown as R₁ in Fig. 2) remains largely unchanged.

To shed more light on the TP versus thickness relationship, we conducted theoretical modeling to validate our experimental observations. Utilizing geometric optics principles of reflection and refraction, we predicted TP values for zirconia as a function of layer thickness (equations (1)–(3)). However, when compared with experimental data, especially at low thicknesses (below ~0.4 mm), the theoretical model grossly underestimated TP values. This deviation can be attributed to the limitations of geometric optics, which neglects diffraction, diffuse scattering, partial reflections, and interference effects occurring at the interfaces between zirconia, the coupling medium, and the backing. In thin zirconia layers, light arising from these neglected phenomena can reemerge at the top surface. Regardless of the exit angle, the enclosed walls of the spectrophotometer redirect this light toward the detector, resulting in higher photon counts than predicted by the geometric optics model.

According to ISO 6872:2024, the recommended flexural strength for monolithic ceramic restorations is 300 MPa for single-unit and 500 MPa for 3-unit restorations. All 3 zirconia compositions therefore meet the requirement for single crowns (Fig. 4). However, for 3-unit bridges, while the mean flexural strength of each material far exceeds 500 MPa (Fig. 4A), the strength of 4YSZ at the 5% failure probability approaches this threshold (Fig. 4B)—raising clinical concerns, particularly since the reported strength values are based on highly polished specimens. In clinical practice, the internal surfaces and margins of restorations are never polished, and grinding-induced flaws can compromise structural integrity (Lawn et al. 2021; Abdulmajeed et al. 2024). Furthermore, slow crack growth inherent to oxide ceramics can further diminish long-term performance (Wiederhorn 1967; Zhang and Lawn 2004). In this context, the newly developed 3.5YSZ offers a favorable combination of high strength and enhanced translucency, thereby supporting broader clinical indications and greater longevity.

We acknowledge that only a single composition between 3YSZ and 4YSZ was presented here for demonstration purposes. Further investigation of other compositions and alternative additives is warranted. Moreover, high-speed sintering protocols have yet to be fully optimized. Thus, the development of zirconia as a high-strength, highly translucent restorative material for chairside, same-day treatments remains ongoing and merits continued research.

Conclusions

High-speed sintered 3.5YSZ exhibits lower porosity and finer pore sizes relative to 3YSZ and 4YSZ. 3.5YSZ demonstrates higher translucency than both 3YSZ and 4YSZ, while retaining the high strength of 3YSZ. The geometric optics model can be used to predict the dependence of TP on zirconia thickness; however, it significantly underestimates TP for thin restorations.

Author Contributions

M.L. Chin, contributed to data acquisition, analysis, and interpretation, drafted and critically revised the manuscript; Z. Li, contributed to design, data acquisition, analysis, and interpretation, critically revised the manuscript; G.G. Madureira, contributed to data acquisition, analysis, and interpretation, critically revised the manuscript; Y. Zhang, contributed to conception and design, data acquisition, analysis, and interpretation, drafted and critically revised manuscript. All authors gave their final approval and agreed to be accountable for all aspects of the work.

Footnotes

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: This work was carried out in part at the Singh Center for Nanotechnology, which is supported by the NSF National Nanotechnology Coordinated Infrastructure Program under grant NNCI-2025608. This work was supported by the US National Institutes of Health/National Institute of Dental and Craniofacial Research (grant number R01DE033545). The funding sources had no involvement in study design; data collection, analysis, and interpretation; manuscript writing; and decision to submit the article for publication.

ORCID iD

Y. Zhang

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A Novel Zirconia Composition for Speed Sintering with Enhanced Properties

Abstract

Keywords

Introduction

Optical Theory

Experimental

Specimen Preparation

Density Measurement

Microstructural Analysis

Phase Analysis

Translucency Measurement

Biaxial Flexural Strength Measurement

Statistical analysis

Results

Discussion

Conclusions

Author Contributions

Footnotes

Declaration of Conflicting Interests

Funding

ORCID iD

References