## Graphical abstract

## Abstract

## Objective

To investigate the influence of high-speed and conventional sintering on the flexural strength (FS) of three zirconia materials initial and after artificial aging.

## Methods

Milled zirconia specimens (3Y-TZP: ZI and Zolid; 4Y-TZP: Zolid HT+; Amann Girrbach AG; N = 288, n = 96/group) were sintered in a high-speed sintering protocol (final temperature 1580 °C, n = 48/subgroup) or a conventional sintering protocol (control group, final temperature 1450 °C, n = 48/subgroup). FS was tested initially and after artificial aging (10 h in an autoclave or 1,200,000 chewing cycles; n = 16/subgroup). Univariate ANOVAs, post-hoc Scheffé, partial eta-squared, Kolmogorov–Smirnov-, Kruskal–Wallis- and Mann–Whitney-U-test were performed (p < 0.05).

## Results

ZI showed the highest and HT+ the lowest FS, regardless of the sintering protocols and aging regimens (p < 0.001). High-speed sintered HT+ showed higher initial FS than the control group (p < 0.001). ZI (p < 0.001–0.004) and Zolid (p < 0.001–0.007) showed higher FS after thermo-mechanical aging. High-speed sintered HT+ showed higher FS in the initial stage (p < 0.001). The Weibull modulus of the three thermo-mechanically aged materials was negatively influenced by high-speed sintering.

## Significance

As shorter sintering times represent a cost and time efficient alternative, high-speed sintering is a valid alternative to conventional sintering protocols.

## 1

## Introduction

All-ceramic restorations are very popular and frequently used in the dental field due to their excellent aesthetic appearance, good chemical resistance and high biocompatibility . One disadvantage of all-ceramic restorations is however the high brittleness of dental ceramics, which limits their use from long span multi-unit fixed dental prostheses (FDPs) to small anterior restorations. Zirconia ceramics represent an exception to this rule , as zirconia possesses excellent mechanical properties due to the inherent transformation toughening mechanism in the tetragonal phase . The martensitic transformation is induced by high tensile stresses at the crack-tip causing a tetragonal to monoclinic crystal phase transformation with a local increase in volume of approximately 4–5% . This leads to a local compression at the crack-tip and further crack growth is inhibited . Due to its excellent chemical properties, such as a high inertness to outside influences, its resistance to wear, the observed high biocompatibility and described mechanical properties, zirconia has become a very attractive material for the fabrication of diverse dental restorations. As the first and second zirconia generations (3Y-TZP with 0.25 and 0.05 wt.% Al _{2} O _{3} ) possess a high fracture toughness and excellent flexural strength (FS; >1000 MPa), they can be employed for the manufacturing of three- and even multi-unit FDPs as an alternative to metal or metal-ceramic restorations . One disadvantage observed for 3Y-TZP is however it is very low light transmission, which leads to an opaque appearance, especially when compared with the high-end esthetic results achieved by employing glass-ceramics. As the low light transmission makes a matching to the natural teeth color challenging, supplying to the high esthetic requirements of nowadays patients can therefore prove to be difficult . 3Y-TZP was thus mostly used as a framework material for FDPs. Veneered zirconia restorations were however reported to present a high chipping rate of the veneering ceramic . This happens due to the overall lower flexural strength and brittleness of the veneering ceramic, whereby the tensile stress gradient in the veneered layer cannot be compensated and the veneer fractures . To tackle this problem, new zirconia compositions were developed to be employed in a monolithic setting . Through the most recent third and fourth generations of zirconia (5Y-TZP and 4Y-TZP), a more translucent material with a reduced amount of Al _{2} O _{3} , a higher yttrium content (5 mol% and 4 mol%) and a cubic-tetragonal instead of a tetragonal-monoclinic lattice structure was developed , as an improved translucency can be achieved by a higher cubic content . Optical properties are however inversely related to the flexural strength of zirconia , with 4Y-TZP and 5Y-TZP being fully stabilized materials that show no transformation toughening.

From a clinical point of view, conventional sintering times for zirconia are still very time-consuming (taking 4–12 h) when compared to other materials, which in turn extends treatment times leading to multiple treatment appointments due to the long sintering times, which restricts the use of zirconia for direct applications while raising expenses. Therefore speed- and high-speed sintering has recently gained attention as a possible alternative to conventional sintering, resulting in reduced clinical times, cost and increased patient comfort. The sintering procedure of zirconia also dictates the grain size and is a very important aspect to consider, as it gives the material stability, its final microstructure and thus influences both optical and mechanical properties . Currently, high-speed sintering furnaces and programs for sintering small FDPs within 10–20 min are developed and analyzed . There are already first studies, which report that high-speed sintering leaded to similar or even higher flexural strength than observed for conventional sintering protocols .

According to the present literature, both an increase in grain size and translucency, and a decrease in flexural strength (above 1550 °C ) has been observed for 3Y-TZP by increasing sintering temperatures and holding times . One study showed, that a higher biaxial flexural strength was obtained for 3Y-TZP with a reduced Al _{2} O _{3} content (0.05 wt.%) when high-speed sintered at 1590 °C when compared with sintering at 1570 °C . At the same time, a decrease in grain size and an increase in translucency have been reported for shorter sintering times . While 5Y-TZP presented comparable flexural strength values as 3Y-TZP and 4Y-TZP , little is known on the influence of the sintering parameters on the mechanical and optical properties of 4Y-TZP and 5Y-TZP materials. To align the production times of monolithic zirconia to monolithic lithium-disilicate ceramics , an optimization of zirconia ceramics’ sintering protocols is sought.

One further parameter affecting a material’s mechanical properties is low-temperature degradation (LTD), which is abetted by hydrothermal aging . LTD occurs in the presence of water at room temperature, where water molecules penetrate into micro cracks and induce tetragonal to monoclinic phase transformations, leading to reduced mechanical properties. Especially zirconia materials with a higher amount of Al _{2} O _{3} dopant (3Y-TZPs) and colorants are susceptible to this phenomenon . While some studies have shown the fully stabilized 4Y-TZP not to be affected by hydrothermal aging , that in turn affects the zirconia’s flexural strength , an alteration of flexural strength was observed for 3Y-TZP and 5Y-TZP .

As literature on the impact of the sintering times and temperatures, and the effect of hydrothermal aging in the form of treatment in an autoclave and chewing simulator on the third and fourth generation of zirconia are limited, this study aimed to advance our knowledge about the influence of both sintering and LTD on the mechanical properties of different zirconia materials. The null hypotheses of this in vitro study are that neither the choice of sintering protocol, nor the choice of zirconia material (3Y-TZP with 0.25 and 0.05 wt.% Al _{2} O _{3} and 4Y-TZP), nor the aging regimen affect the flexural strength.

## 2

## Materials and methods

## 2.1

## Specimen preparation

The three-point flexural strength of non-shaded zirconia materials ( Table 1 ) using two sintering protocols was tested after different aging regimens ( Fig. 1 ). In total, N = 288 rectangular specimens from three materials (n = 96/group) were milled using a five-axis milling machine (Ceramill Motion 2, Amann Girrbach, Koblach, Austria) and ground with SiC abrasive paper P2500 (Buehler, Illinois, USA) to achieve the desired specimen dimensions for sintering. The specimens were sintered according to the sintering protocols summarized in Table 2 .

Zirconia | Material | Abbreviations | Manufacturer | Shade | Y _{2 }O _{3 }Content (wt.%) |
Al _{2 }O _{3 }Content (wt.%) |
Lot. no. |
---|---|---|---|---|---|---|---|

3Y-TZP | Ceramill ZI | ZI | Amann Girrbach AG | White | 4.5–5.6 | ≤0.25 | 1712002, 1712100, 1712142, 1712101, 1712145, 1712130, 1712131, 1712128, 1712124 |

3Y-TZP ^{a } |
Ceramill Zolid | Zolid | Amann Girrbach AG | White | 4.5–5.6 | ≤0.05 | 1703001, 1703065, 1703063, 1703062, 1703065, 1703067, 1703066, 1703069, 1703070 |

4Y-TZP | Ceramill Zolid HT+ | HT+ | Amann Girrbach AG | White | 6.7–7.2 | ≤0.05 | 1708000, 1708074, 1708030, 1708005, 1708075, 1708157, 1708108, 1708067, 1708045 |

Furnace | Specimen number | Final temperature | Holding time | |
---|---|---|---|---|

High-speed sintering | Experimental Ceramill Therm RS, Amann Girrbach | 48/zirconia material | 1580 °C | 10 min |

Control group | Ceramill Therm 2, Amann Girrbach | 48/zirconia material | 1450 °C | 120 min |

The 48 specimens per zirconia material and sintering protocol were further divided into three aging regimens (n = 16/subgroup), namely for testing without aging-initial, after hydrothermal aging for 10 h at 134 ± 2 °C and 0.2 MPa (Euroklav 29-S, Schembera, Munich, Germany) and after thermo-mechanical aging with 1,200,000 chewing cycles. Chewing simulation included a changing of the distilled water ambient temperature between 5 °C and 55 °C for 6000 cycles, while mechanical loading was performed with a steel antagonist of 4 mm diameter at 10 N (SD Mechatronik, Feldkirchen, Germany) . All specimens had a final dimension of width 4.0 ± 0.2 mm, thickness 1.0 ± 0.2 mm and length 25 mm.

## 2.2

## Flexural strength test

The measurement of the three-point flexural strength was performed in a universal testing machine (1445 Zwick/Roell, Ulm, Germany). Cross-sectional dimensions of the specimens were measured with a digital micrometer (electronic micrometer Holex 421490, Hoffmann, Munich, Germany). Specimens were then placed centrally with the wide 4 mm area resting on the support. The distance between the supports rolls was 12 mm. Force was applied perpendicularly to the longitudinal axis of each specimen side with a crosshead speed of 1 mm/min until fracture. Both, the support rolls as well as the pressure fin had a 1 mm radius of curvature. Experiments were conducted in air and at room temperature (23 °C). The fracture load of each specimen was recorded and flexural strength was calculated using the following formula:

σ=3PL/2wb2

where: <SPAN role=presentation tabIndex=0 id=MathJax-Element-2-Frame class=MathJax style="POSITION: relative" data-mathml='σ’>?σ

σ

: flexural strength (MPa); <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax style="POSITION: relative" data-mathml='P’>?P

P

: fracture load (N); <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax style="POSITION: relative" data-mathml='L’>?L

L

: center between the outer support rolls; <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='w’>?w

w

: width of the specimen, lateral to the direction of applied force (mm); <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='b’>?b

b

: thickness of the specimen, parallel to the direction of the applied force (mm).

## 2.3

## Statistical analysis

Descriptive analysis was calculated and deviations from the normal distribution were tested using Kolmogorov–Smirnov. A global univariate ANOVA test with post-hoc Scheffé and partial eta-squared <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

was performed to determine the influence of zirconia material, sintering protocol and aging regimen on the flexural strength. Data was further analyzed using Kruskal-Wallis- and Mann–Whitney-U test. The Weibull modulus was calculated using the maximum likelihood estimation method . For all tests, the level of significance was set to p < 0.05. The statistical analysis was conducted with IBM SPSS Statistics 25 (IBM Corp., New York, USA).

## 3

## Results

Two of eighteen groups (11%) showed a deviation from the normal distribution. Therefore, the statistical comparison for this data was made using non-parametric tests.

In general, the choice of zirconia material (partial eta-squared <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.575, p < 0.001), presented the highest impact on flexural strength, followed by the aging regimen ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.170, p < 0.001) and sintering protocol ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-10-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.131, p < 0.001). The effects of the binary combinations zirconia material and aging regimen ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-11-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.143, p < 0.001), sintering parameter and aging regimen ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-12-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.049, p < 0.001) and the ternary combination of all three parameters ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-13-Frame class=MathJax style="POSITION: relative" data-mathml='ƞp2′>ƞ2?ƞp2

ƞ p 2

= 0.142, p < 0.001) on flexural strength were significant. Therefore, data were split for zirconia material, sintering protocol and aging regimen and analyzed with respect to the null hypotheses.

ZI showed the highest FS values regardless of the sintering protocol and aging regimen (p < 0.001, Table 3 ). Within high-speed sintered specimens, Zolid presented lower initial values compared to ZI (p < 0.001) and HT+ (p < 0.001). Furthermore, within the non-aged control group as well as the high-speed sintered and thermo-mechanically aged group, ZI (p < 0.001) showed the highest values, followed by Zolid (p < 0.001–0.015). The lowest values were observed for HT+ (p < 0.001–0.015). The remaining groups presented higher FS for ZI than for Zolid (p < 0.001–0.491) and HT+ (p < 0.001–0.491). High-speed sintered ZI presented higher Weibull modulus (m = 12.0) after hydrothermal aging than the remaining zirconia materials (m = 6.6–8.0). Within the non-aged control group, Zolid (m = 11.6) showed a higher Weibull modulus than ZI (m = 6.1).