Evaluation of the effect of low-temperature degradation on the translucency and mechanical properties of ultra-transparent 5Y-TZP ceramics

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Evaluation of the effect of low-temperature degradation on the translucency and mechanical properties of ultra-transparent 5Y-TZP ceramics Jiadi Shena, Haifeng Xieb, Xinyi Wub, Jiaxue Yangb, Mengyuan Liaob, Chen Chena,∗ a b

Jiangsu Key Laboratory of Oral Diseases, Department of Endodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, China Jiangsu Key Laboratory of Oral Diseases, Department of Prosthodontics, Affiliated Hospital of Stomatology, Nanjing Medical University, Nanjing, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Low-temperature degradation Zirconia Translucency Flexural strength Surface hardness Phase transformation

This study aimed to evaluate the effects of low-temperature degradation (LTD) on the translucency, flexural strength, and surface hardness of ultra-transparent 5Y-TZP ceramics. Three commercial zirconia materials were investigated, including 5Y-TZP and two 3Y-TZP samples. The LTD process was mimicked by hydrothermal aging at 134 °C for 20 h. The translucency parameters (TPs) corresponding to different thicknesses were analyzed. The surface volume fraction of the monoclinic phase was determined by X-ray diffraction (XRD). The three-point flexural strength was measured, and the surface hardness was measured by the indentation method. The translucency of all three ceramics showed a significant decrease with increasing thickness. A translucency gradient was present within a ceramic block of 5Y-TZP, whose translucency was higher than that of both 3Y-TZP products. Aging at 134 °C for 20 h did not reduce the TPs of the three ceramics. The XRD analysis showed a high monoclinic phase content in both 3Y-TZP surfaces after aging, while no monoclinic phase was detected in 5YTZP. Hydrothermal aging had no significant effect on the flexural strength; however, the flexural strength of 5YTZP was significantly lower than that of 3Y-TZP. No statistical difference in surface hardness was observed among the three products. In summary, 5Y-TZP presented higher LTD resistance than 3Y-TZP.

1. Introduction Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) ceramics have become increasingly common in clinical applications and dental restorations due to their excellent mechanical properties [1] and biocompatibility [2]. However, the early traditional Y-TZP ceramics are gray-white [3] and exhibit high opacity, which compromises the aesthetics of the restoration [4]. In order to achieve a more natural appearance and improve the superficial translucency of a restoration, veneering porcelain is usually applied on its surface [5]. Unfortunately, chipping and fracture of the veneering porcelain has been reported as a technical complication associated with the corresponding restorations [6,7]. Therefore, the need to improve the translucency of Y-TZPs for achieving acceptable aesthetic restorations and reducing the risk of porcelain fracture led to monolithic translucent Y-TZP restorations to be proposed and rapidly recommended for clinical use. The translucency of Y-TZP ceramics is affected by various factors, including chemical composition [8], crystal content [9], grain size [10,11], porosity [12], impurities [8], and sintering density [13]. Among the above methods, eliminating the impurities (e.g. alumina sintering additives) that dampen the translucency of Y-TZP is a

∗

commonly used method by some dental manufacturers (e.g. 3 M ESPE, USA and Amann Girrbach, Austria) [8], when keeping the yttria content to be constant. This is because alumina has a different refractive index (1.76) from zirconia (2.21) at 600 nm wave length. Therefore, scattering of light occurs when a light beam travels across the two phase boundaries. Increasing the yttria content is another common method used to improve the translucency [14]. Based on this strategy, Katana UTML (KU), an ultra-transparent multilayered Y-TZP recently introduced in the dental market, has been claimed to exhibit the highest translucency among current Y-TZP-based products. According to the manufacturer, the translucency of this product is close to that of natural human enamel. This improvement was achieved simply by increasing the content of yttria, which, in turn, introduced an increased amount of cubic zirconia and reduced light scattering at the zirconia grain boundaries. One known disadvantage of the Y-TZPs is represented by their hydrothermal or low-temperature degradation (LTD) [15], a superficial aging phenomenon that may take place in the presence of water. This process consists of a slow tetragonal to monoclinic (t→m) transformation of the grains at the surface [16]. The LTD leads to a progressive deterioration of mechanical properties [17], reducing the light

Corresponding author. Stomatological Hospital of Jiangsu Province, Han-Zhong Road 136th, Nanjing, 210029, China. E-mail address: [email protected] (C. Chen).

https://doi.org/10.1016/j.ceramint.2019.09.002 Received 4 July 2019; Received in revised form 16 August 2019; Accepted 1 September 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Jiadi Shen, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.002

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Table 1 Ceramic materials investigated in this study. Material

Abbreviation

Katana UTML

KU

Lava Plus

LP

Ceramill Zolid White

CZ

Manufacturer

Kuraray Noritake Dental, Japan 3 M ESPE, USA Amann Girrbach, Koblach, Austria

Sintering conditions

Batch No.

Heating

Hold

Cooling

10 °C/min to 1550 °C

2h

10 °C/min to Tamb

DPJLJ 1 PC

20 °C/min to 1000 °C + 10 °C/min to 1500 °C 8 °C/min to 1450 °C

2h

15 °C/min to 800 °C + 20 °C/min to 250 °C, then to Tamb 20 °C/min to Tamb

4525742

2h

760164

Tamb means ambient temperature.

samples (Lava Plus (LP), 3 M ESPE, USA and Ceramill Zolid (CZ) White, Amann Girrbach, Koblach, Austria). These ceramic blocks were wet-cut using a low-speed saw (ISOmet1000, Buehler Ltd., Lake Bluff, IL, USA). The dimensions of the cut specimens were determined taking into account the ~20% shrinkage occurring during dense sintering [28]. Because Katana UTML belongs to the multilayer zirconia system, each specimen has a fourlayer structure (enamel layer, transition layer 1, transition layer 2 and body layer), from the incisal to the gingival margin. The specimens were then wet-finished in a grinder/polisher machine with 600-grit silicon carbide paper to precisely control their size in the relevant tests performed in this study, including the translucency, flexural strength, and surface hardness measurements. Each type of specimen was sintered according to its manufacturer's instructions. After sintering, each specimen was wet-polished with a rotational polishing device (PG-1, BiaoYu Instrument, Shanghai, China) using 1000-, 2000-, 3000-, and 4000-grit silicon carbide abrasive papers, in order to control the final dimensional error to less than 0.01 mm. The investigated Y-TZP products are described in detail in Table 1.

transmission [18] and affecting the long-term stability [19]. As the aging gradually spreads from the surface to the core of the material, the phase transformation and volume expansion occurring in the LTD process result in internal stress and formation of microcracks, eventually damaging the structural integrity of the zirconia phase [20]. The LTD depends on the yttria [21] and cubic phase contents of YTZP [16]. A decrease in the stabilizer (such as yttria) content will make the zirconia ceramic more susceptible to LTD [21], whereas increasing the yttria content will slow down the t→m phase transformation and thus improve the aging resistance of Y-TZP [22]. Among the dental YTZP ceramics available in the market, 3 mol% yttria is usually doped into Y-TZP (giving 3Y-TZP ceramics) to stabilize the tetragonal phase of zirconia at room temperature [23]. The content of yttria in Katana UTML is about 5 mol% [24], higher than that of the 3Y-TZP and 4Y-TZP ceramics commonly used in dental clinics. As mentioned above, a higher yttria content is associated with a higher cubic phase content and better translucency [14,25]. However, some previous studies found that an increase in yttria content is usually accompanied by an improvement in translucency, unfortunately at the expense of reduced mechanical properties [26]. Moreover, no information is available on whether changes in yttria content have a clinically significant effect on the LTD resistance of Katana UTML. Usually, Y-TZP shows uniform translucency within a block. However, inspired by the use of computer-aided design and computeraided manufacturing (CAD-CAM) lithium disilicate glass-ceramics to mimic the translucency gradient of natural tooth [27], the KU multilayered zirconia system was developed to achieve a translucency gradient within a ceramic block including an enamel, a transition, and a body layer, corresponding to the incisal margin, middle part, and gingival margin of the tooth, respectively. The aim was to achieve a more realistic effect for restorations based on Y-TZP, which would represent another advantage besides the high translucency claimed by the manufacturer. Therefore, as the newest member of the multilayer zirconia ceramic system, Katana UTML also features a translucency gradient. However, further tests are needed to clarify whether its gradually changing translucency might affect the mechanical properties and the aging resistance of zirconia. Therefore, the current study investigates the LTD resistance of Katana UTML, by evaluating its translucency, monoclinic phase content, three-point flexural strength, and surface hardness properties, comparing them with those of two 3Y-TZP products used as control. The null hypothesis of this study was that the aging resistance of the present ultra-transparent 5Y-TZP product would be the same as that of the two 3Y-TZP control samples.

2.2. LTD simulation The LTD process was simulated by hydrothermal aging, which was performed in an autoclave (Vacuklav 24B, Melag, Berlin, Germany) at 134 °C and 0.2 MPa for 20 h, according to the International Organization for Standardization (ISO) standard 13356 [29]. Half of samples of the three ceramics (KULTD, LPLTD, and CZLTD) were subjected to hydrothermal aging respectively. 2.3. Translucency The final length and width of the tested specimens were both 10 mm, while their thicknesses were 0.5, 1.0, 1.2, and 1.5 mm. Five samples were tested for each thickness from each brand of ceramic. The translucency parameters (TPs) of the four layers (enamel layer, transition layer 1, transition layer 2 and body layer) of the 5Y-TZP (Fig. 1) and two 3Y-TZP products before and after aging were

2. Materials and methods 2.1. Y-TZP fabrication and sintering Three different commercial Y-TZP products in the form of machinable pre-sintered blocks were employed: ultra-transparent 5Y-TZP (Katana UTML (KU), Kuraray Noritake Dental, Japan) and two 3Y-TZP

Fig. 1. The four-layer structure (enamel layer, transition layer 1, transition layer 2 and body layer) of Katana UTML. 2

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where P is the loading force at the fracture point, L is the length of the support span (13 mm), and w and b are the width and depth of the beam specimen, respectively.

determined by calculating the color difference between readings against black (B) and white (W) backgrounds for the same specimen [30]. Five values were obtained for each thickness, and CIELAB values (L*, a*, and b*) were determined using a dental colorimeter (ShadeEye, Shofu, Japan). The TP was calculated according to the following equation [31]:

TP =

(L ∗B − L ∗W

)2

+ (a ∗B − a ∗W

)2

+ (b ∗B − b ∗W

)2

2.6. Surface hardness Five samples (those whose flexural strength was closest to the mean value) were selected among the fractured specimens after the threepoint flexural strength test. The surface hardness was measured by the indentation method. Indentations were created using a Vickers hardness tester (Wilson T2500, Buehler Ltd., USA) with a constant load of 9.807 N for 10 s. The diamond indentations were assessed using a microscope of the hardness tester at 500 × magnification. The Vickers hardness was calculated as:

(1)

where the subscripts B and W denote the color values obtained with the black and white background, respectively. 2.4. X-ray diffraction (XRD) The final dimensions of the specimens subjected to XRD analyses were 10 mm (length), 10 mm (width), and 1.0 mm (thickness). One sample of each brand ceramic with or without aging was submitted to XRD analysis, respectively. The percentages of crystalline phases (m: monoclinic; t: tetragonal; c: cubic) present on the Y-TZP surfaces before and after hydrothermal aging were determined by XRD (D8 ADVANCE, Bruker AXS, Germany), using Ni-filtered Cu K (λ = 1.5418 Å) radiation at ambient temperature, with a 2θ step size, angular range, and scan rate of 0.02°, 20°–80°, and 2°/min, respectively. The monoclinic phase fraction was calculated with according to the equation proposed by Garvie and Nicholson (1972) [32]:

Xm =

Im (−111) + Im (111) Im (111) + Im (−111) + It (101)

H = 0.1891F / d 2

where F is the load at fracture (N) and d denotes the mean half-diagonal length left by the indenter (mm). The Vickers hardness of each group of ceramics was recorded before and after aging, and mean and standard deviation values were calculated. 2.7. Scanning electron microscopy (SEM) The final dimensions of the specimens subjected to SEM were 10 × 10 × 1 mm3. Two samples before and after aging were selected from each brand of ceramic. All the specimens were wet-polished in a grinder/polisher machine with 1000-, 2000-, 3000-, 4000-grit silicon carbide paper, and ultrasonically cleaned in distilled water for 30 min. After drying, these samples were sputter-coated with Au and submitted to examination by a SEM (TESCAN MAIA3, Kohoutovice, Czech Republic) in secondary electron imaging mode, at magnifications of 20K × . Observation parameters were, accelerating voltage 10 kV; working distance 9 mm.

(2)

where Xm is the monoclinic fraction, while Im(-111), Im(111), and It(101) are the intensities of the monoclinic (−111) (2θ = 28.1°), monoclinic (111) (2θ = 31.17°), and tetragonal (101) (2θ = 29.9°) diffractions. The volume fraction of the monoclinic phase (Vm) was calculated according to Toraya et al. (1984) [33]:

Vm =

1.311Xm 1 + 0.311Xm

2.8. Statistical analysis

(3)

Statistical analyses of the TP, three-point flexural strength, and surface hardness measurements were performed using the SPSS 21.0 statistical software (IBM SPSS Inc., Chicago, IL, USA). After examining the normal distribution and homogeneity of the data via Levene tests, two-way analysis of variance (ANOVA) and post-hoc tests (Tukey's honestly significant difference, HSD, for multiple comparisons) were performed to examine the effects of aging on translucency, flexural strength, and surface hardness. The statistical significance level was set at α = 0.05.

2.5. Three-point flexural strength The final size of the tested beam specimens was 16 × 4 × 1.6 mm3. Twenty samples per group were selected for the flexural strength tests of the three products, and half of the samples in each group was subjected to in vitro aging before testing. Each specimen was placed on a three-point flexural test device (Instron 3365, ElectroPlus, USA) with a 13.0 mm distance between the supporting rods. A central load was applied at a crosshead speed of 1.0 mm/min until the samples fractured, and the maximum load was recorded. The flexural strength (FS) was calculated as:

FS =

3PL 2Wb2

(5)

3. Results 3.1. Translucency parameters The mean values and standard deviations of the TPs of samples of the three materials with different thicknesses, measured before and

(4)

Table 2(a) Mean (with standard deviation in parentheses) translucency parameters of tested ceramics with different thickness before aging. Ceramic

Katana UTML enamel layer Katana UTML transition layer 1 Katana UTML transition layer 2 Katana UTML body layer Lava Plus Ceramill Zolid White

Thickness 0.5 mm

1.0 mm

1.2 mm

1.5 mm

12.84(0.49)Aab 12.43(0.59)Aab 12.79(0.64)Aab 12.94(0.46)Aa 12.23(0.59)Ab 12.20(0.35)Ab

11.83(0.25)Ba 11.38(0.21)Bb 9.85(0.53)Bc 9.62(0.08)Bc 8.57(0.20)Bd 8.23(0.22)Bd

11.00(1.42)Ba 9.12(0.23)Cb 8.61(0.13)Cbc 8.44(0.12)Cbc 7.85(0.19)Cc 7.95(0.06)Bc

8.45(0.14)Ca 7.82(0.21)Dab 7.40(1.18)Db 7.12(0.15)Db 7.44(0.10)Cb 6.86(0.09)Cb

In each row (column), the TP values of groups marked with the same superscript uppercase (lowercase) letter are not significantly different (p > 0.05). 3

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Table 2(b) Mean (with standard deviation in parentheses) translucency parameters of tested ceramics with different thickness after aging. Ceramic

Thickness 0.5 mm

Katana UTML enamel layer Katana UTML transition layer 1 Katana UTML transition layer 2 Katana UTML body layer Lava Plus Ceramill Zolid White

1.0 mm Aa

1.2 mm ABa

11.23(0.73) 10.03(1.19)Bab 9.63(1.44)Bab 9.41(1.65)ABab 8.15(0.26)Bb 8.08(0.21)Bb

12.41(0.54) 12.32(1.76)Aa 12.10(1.38)Aa 12.23(2.78)Aa 11.94(1.13)Aa 11.69(2.08)Aa

1.5 mm Ba

7.61(1.59)Ca 7.57(0.56)Ca 7.20(1.43)Ca 7.20(1.50)Ba 7.12(0.44)Ba 7.02(0.26)Ba

10.14(1.00) 7.99(0.51)Cb 7.80(0.83)Cb 7.82(2.54)Bb 7.71(0.40)Bb 7.30(0.21)Bb

In each row (column), the TP values of groups marked with the same superscript uppercase (lowercase) letter are not significantly different (p > 0.05).

after aging, are shown in Table 2. The two-way ANOVA highlighted a significant difference between the TPs corresponding to different materials and thicknesses. The results of Tukey post-hoc tests showed that the TPs of the three products decreased significantly with increasing thickness, regardless of the aging treatment. The enamel layer of KU showed a higher TP than the two 3Y-TZP products, regardless of the thickness; however, no significant difference was found between the four layers of KU.

Table 3 Mean ( ± SD) values of flexural strength and surface hardness of the zirconia materials before and after aging. Material KU LP CZ KULTD LPLTD CZLTD

3.2. XRD analysis

Flexural strength (MPa) 551.23 817.07 875.92 529.20 898.32 893.42

± ± ± ± ± ±

a

57.23 71.78b 58.32b 25.61a 134.45b 143.20b

Surface Hardness (Hv) 1425.60 1508.60 1476.60 1420.20 1501.80 1420.20

± ± ± ± ± ±

34.63a 178.23a 138.80a 28.42a 95.48a 84.22a

The groups marked with the same superscript letter are not significantly different (p > 0.05).

Representative XRD patterns obtained from the three tested products are shown in Fig. 2. The monoclinic phase of zirconia (m-ZrO2) was not detected before aging in any of the specimens. After 20 h of hydrothermal aging, a strong peak corresponding to the monoclinic phase (Im(-111), 2θ = 28.2°) was observed on the left of the main tetragonal peak (It(101), 2θ = 30.2°) in specimens of LP and CZ groups, while no monoclinic peak was detected in the KU samples. The results of Vm detected a lower m-ZrO2 content on the LP than the CZ surface (14.89 vs. 22.81 vol%, respectively). The XRD patterns of each product were also used to highlight differences in cubic phase (c-ZrO2) content. As shown in Fig. 2(a), KU contained a relatively high amount of c-ZrO2 regardless of aging, while LP and CZ presented a low cubic phase content. In addition, Fig. 2(b) shows the appearance of a broader peak with a shoulder, which composed of t-ZrO2 (002) and c-ZrO2 (200) peaks around 2θ = 35° for the KU and KU samples suffered from LTD. However, two clearly separated peaks, corresponding to t-ZrO2 (002) and t-ZrO2 (200) or t-ZrO2 (020), were found for the LP and CZ samples.

irrespective of aging. KU showed the lowest flexural strength regardless of aging, with no statistical difference before and after aging, indicating that LTD did not result in significant changes in flexural strength.

3.4. Surface hardness The surface hardness results for the three products before and after aging are presented in Table 3. The Levene test showed that the surface Vickers hardness data were normally distributed and exhibited equal variance. Regardless of aging, no statistical difference in surface hardness was found among the three products.

3.5. SEM Representative SEM images before and after 20-h hydrothermal aging treatment are shown in Fig. 3. The microstructure and crystalline grain size of the three Y-TZP products could be observed clearly in SEM. Regardless of aging, the KU samples showed a larger grain size than the other two 3Y-TZP. Besides, the grains-boundary continuity became obscure in both 3Y-TZP after aging.

3.3. Three-point flexural strength tests The flexural strength values before and after aging are shown in Table 3. The Levene test showed that the flexural strength values were normally distributed and exhibited equal variance. There was no significant difference in flexural strength between LP and CZ samples,

Fig. 2. Representative X-ray diffraction patterns of three different translucent zirconia materials before and after LTD. (a) 2θ = 20–80°; (b) 2θ = 27–36°. While the as-sintered (untreated) specimens did not contain monoclinic ZrO2 before LTD, the LP and CZ specimens showed (−111) monoclinic peaks after LTD. A: Katana UTML; B: Katana UTML LTD; C: Lava Plus; D: Lava Plus LTD; E: Ceramill Zolid White; F: Ceramill Zolid White LTD.

4

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Fig. 3. Typical SEM images (20k × magnification) of three products before aging (a–c) and hydrothermal aging for 20 h (d–f). Labels a-c and d-f represent Katana UTML, Lava Plus and Ceramill Zolid White, respectively.

4. Discussion

boundary areas to reduce scattering [42]. The differences in grain size and distribution can be caused by different sintering protocols [43]. A higher sintering temperature results in a larger grain size [21]. KU was sintered at a higher temperature (1550 °C) than the other two 3Y-TZP (1500 °C for LP; 1450 °C for CZ). Therefore, the grain size of KU is larger than that of other two 3Y-TZP, which also contributed to the higher TP. The LTD is an unavoidable process for Y-TZP materials. The metastable tetragonal phase undergoes a spontaneous phase transformation in humid atmosphere without mechanical stress; this transformation begins at the surface through stress corrosion [44]. Once the whole surface of the material is degraded, the transformation propagates inside the Y-TZP bulk in the form of microcracks [45], leading to roughening of the surface area [46] and deterioration in the overall mechanical properties of the material [47], such as a 20–40% reduction in fracture load [17]. Hydrothermal treatment at 134 °C is the most common method used to simulate the accelerated low-temperature degradation in vitro [29]. It was reported that 1 h of autoclaving at 134 °C theoretically corresponds to 3–4 years in vivo [16]. Based on this estimation, the LTD caused by 20 h of autoclaving in the present study would correspond to at least 60 years at body temperature. The present XRD analysis showed that the monoclinic phase content of LP and CZ samples increased significantly after 20 h of aging, whereas no monoclinic phase was detected in 5Y-TZP. This result is consistent with previous studies [48,49]. Tests under more drastic LTD conditions showed that KU did not form a monoclinic phase even after hydrothermal aging at 134 °C for 100 h [49]. These results suggest that 5YTZP has a higher aging resistance than 3Y-TZP. This effect is attributed to the higher yttria content of the former material, which helps preventing the low-temperature degradation of Y-TZP [38,50]. The XRD patterns reveal that KU contained a higher amount of cubic phase before and after aging, compared with LP and CZ; these results are similar to those obtained for KU in a previous study [22]. The yttria content was ~5 mol% in KU and ~3 mol% in LP and CZ. As mentioned above, the c-ZrO2 content increases with increasing yttria content, which contributes to the higher translucency [51] and aging resistance [26,52] of higher-yttria materials. In addition, m-ZrO2 was detected on the LP and CZ surfaces after aging. The aging process caused superficial modifications that can be observed in SEM images, which supported the present XRD results. Previous studies reported the occurrence of the t→m phase transformation on the Y-TZP surface after aging, which negatively affects the mechanical properties of the material [17]. Therefore, both effects discussed above indicate that the aging resistance of 3Y-TZP was inferior to that of 5Y-TZP. In addition to the yttria content [21], other factors such as grain size [53], alumina additive content [8], and residual stress [54] also determine the resistance of Y-TZP against LTD. Further studies are needed

All-ceramic restorations have different thicknesses depending on the location, shape, and anatomical characteristics of the tooth involved [34,35]. Thickness is an important factor in the translucency of Y-TZPs [36]: a significant decrease in translucency was previously observed with increasing ceramic thickness [37,38]. A monolithic Y-TZP restoration such as a crown requires a tooth reduction ranging from 0.5 to 1.5 mm or more to contain the thickness of the ceramic layer. In order to understand the behavior of 5Y-TZP ceramics of different thicknesses in clinical applications, in this study we investigated four representative thicknesses, i.e., 0.5, 1.0, 1.2, and 1.5 mm. The present results show that the translucency of sintered 5Y-TZP specimens decreased significantly when their thickness increased. This behavior was consistent with that of the two 3Y-TZP products examined in the current study, as well as with that previously reported for other 3Y-TZP ceramics [24,39]. The TP values of KU obtained in this work were close to those reported in previous studies [40]: apart from the sample with 0.5 mm thickness, the translucency of the four layers of KU, going from the incisal to the gingival margin, showed a gradual decrease. This created a translucency gradient from the incisal to the gingival margin of the prosthesis, reproducing the optical characteristics of natural teeth. However, no statistical difference in translucency between the four layers was observed for the 0.5 mm-thick product. The minimum applicable thickness of monolithic Y-TZP in the oral environment is approximately 0.5 mm [41]. Although affecting the permeability appearance and the appearance of layering positively, the high translucency of thinner Y-TZP samples may be insufficient to cover the abnormal color of discolored teeth and metal post-core substrates. Nevertheless, the TP of KU was higher than that of the two 3Y-TZP products, which suggests that the novel ultra-transparent 5Y-TZP has superior translucency properties to the 3Y-TZP ceramics. Nevertheless, when the sample thickness reached 1.5 mm, the superior translucency properties of KU were lost, indicating that a 1.5 mm-thick restoration will not benefit from the aesthetic improvements associated with the enhanced translucency of the material. The TP of 3Y-TZP decreased sharply when the thickness was higher than 1.0 mm, whereas no further significant changes were observed when it reached 1.5 mm; this trend suggests that traditional 3Y-TZP exhibits the best translucency with a 0.5 mm thickness, whereas the aesthetic improvements will be significantly reduced for a thickness of 1.0 mm. Based on the present SEM observation, the different crystal structure contributed to the higher TP of KU. The present SEM images about the grain size of these Y-TZP products are in accordance with the previous findings [11]. Cubic phase can be conducive to improve translucency, due to the optical isotropy and larger grain size, which decreases grain5

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China [2016YFA0201704], the Natural Science Foundation of Jiangsu Province of China [BK20190262], the Qing Lan Project of Jiangsu Higher Education Institutions, China and the Priority Academic Program Development of Jiangsu Higher Education Institutions [grant 2018-37].

to evaluate the contribution of each of these factors. Previous studies reported that the translucency of Y-TZP ceramics could be affected by aging [49,55,56]. Hydrothermal aging may alter the crystal structure of Y-TZP and induce reactions within the grain boundaries, thus affecting the transmission of light [57]. The co-existence of different phases (monoclinic/tetragonal/cubic) after aging may have contributed to increase the difference between the refractive indices of the various phases, and hence to reduce the translucency [36,58]. The present study also evaluated the effect of the LTD on the translucency. No significant differences were observed between the different materials however, which was inconsistent with the previous studies discussed above, and at least suggesting that the aging conditions adopted in the present study would not impair the translucency of 3Y-TZP, as well as 5Y-TZP. The flexural strength is a measure of the resistance of a material under bending, which is the most common form of stress in prosthetic dentistry. This parameter is thus an important indicator for evaluating the clinical performance of restorations [59]. In this study, the flexural strength of KU before and after aging were significantly lower than those of 3Y-TZP; this indicates that 5Y-TZP is not the preferred material for application in posterior teeth, which are subjected to higher occlusal loading. This is mainly because restorations of posterior teeth are usually selected based on their ability to withstand biting forces, rather than on aesthetic criteria. This study also found no statistical difference in mean flexural strength of the two 3Y-TZP before and after aging. Combined with the XRD analysis, the results showed that the monoclinic phase content of LP and CZ increased after aging, suggesting that phase transformation toughening (PTT) would not lead to a detectable improvement in flexural strength after aging of these two materials [60,61], as well as not lead to a negative effect. In the absence of the PTT mechanism, the 5Y-TZP ceramic with increased cubic content cannot achieve enhanced mechanical properties [19], and fortunately, aging did not decrease the mechanical properties of 5Y-TZP either. The surface hardness parameter is used for the evaluation of the occlusal wear resistance [60], which reflects the ability of a Y-TZP surface to resist deformation or damage [62]. Previous studies of the effects of hydrothermal aging on surface hardness gave inconsistent results. No statistical differences between the surface hardness values of the three products were found in the present study. This may indicate that the surface hardness of both the 3Y-TZP and 5Y-TZP restorations was not influenced by the complex oral environment.

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5. Conclusions Based on the present study, the following conclusions can be drawn: i. The translucency and flexural strength of 5Y-TZP are higher and significantly lower, respectively, than those of 3Y-TZP. ii. The translucency of 5Y-TZP and 3Y-TZP significantly decreases with increasing ceramic thickness. Moreover, the superior translucency of 5Y-TZP disappears when its thickness reaches 1.5 mm. iii. 5Y-TZP presents higher LTD resistance than 3Y-TZP. Declarations of interest None. Acknowledgements The authors thank Mr. Taoran Ma (Kuraray Noritake Dental, Japan) and Mrs. Fang Li (Jingyi Denture, China) for providing the Katana UTML and Ceramill Zolid White blocks respectively, and thank Mrs. Mei He and Mrs. Min Li (Yzj Dental lab, China) for providing the Lava Plus blocks and ceramic processing assistance. This study was supported by the National Natural Science Foundation of China [grant 81400539]; the National Key Research and Development Program of 6

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