## Abstract

## Objectives

Testing and comparing of different non-shaded zirconia materials (3Y-TZP, 4Y-TZP and 5Y-TZP) on optical and mechanical properties.

## Materials and methods

Zirconia materials (N = 320, Opaque O, Translucent T, Extra Translucent ET, High Translucent HT) were investigated on translucency, Martens parameter, biaxial flexural strength, Chevron-Notch-Beam (CNB) fracture toughness (K _{IC} ) and grain size. The grain size was analyzed using a scanning electron microcopy (SEM). Univariate ANOVA, post-hoc Scheffé, partial eta-squared, Kolmogorov-Smirnov-, Kruskal-Wallis- and Mann-Whitney-U-tests (p < 0.05) were performed. The reliability of flexural strength was calculated with two-parametric Weibull analysis and 95 % confidence level.

## Results

The translucency of ET and HT increased with the thermo-mechanical aging (p < 0.001). The zirconia material and aging had no impact on the Martens hardness and the indentation modulus. ET showed the highest flexural strength values after initial and thermo-mechanical aging (p < 0.001 – 0.683). All four materials showed the highest flexural strength after thermo-mechanical aging after 1.2 Mio cycles. Thermo-mechanically (1.2 Mio cycles) aged HT presented the highest Weibull modulus (m = 15.0) regardless of aging. Within initial groups, T (p ≤ 0.001) showed the highest fracture toughness, followed by O (p ≤ 0.001), ET (p < 0.003) and HT (p ≤ 0.001).

## Significance

Translucency of ET and HT increases with thermo-mechanical aging. Chevron-Notch-Beam (CNB) is a valid alternative to the single-edge-V-notched beam (SEVNB) method for testing fracture toughness.

## 1

## Introduction

The most common restoration technique used for many years was porcelain-fused-to-metal system, with the aesthetics appearance becoming more and more important, all-ceramic restorations took over. However, the range of indications for leucite ceramic is limited to anterior restorations due to brittleness, low flexural strength and fracture toughness. As an alternative for esthetic posterior dental restorations and long span all-ceramic fixed partial dentures (FDPs), porcelain veneered zirconia framework became popular [ ]. The large restorations from this material are possible because of the excellent flexural strength and fracture toughness, hardness, high biocompatibility and wear resistance of zirconia with a major drawback of the chipping within the veneer ceramic [ ]. This happens because of the notably lower flexural strength and brittleness of the veneering ceramic, thus the overall tension of the restoration cannot be compensated and the material fractures [ ]. Therefore, monolithic zirconia restorations became the latest trend. Over the years from first 3Y-TZP (Y-TZP: yttria-stabilized tetragonal zirconia polycrystal) zirconia material with extraordinary flexural strength but poor optical properties, further three materials were developed. The second 3Y-TZP zirconia generation showed some difference in the degree of light transmittance, by only varying the Al _{2} O _{3} content, which was reduced from 0.25 wt.% to 0.05 wt.% [ , , , ]. The lower content of alumina makes the tetragonal phase of the material less stable and thus more susceptible to low-temperature degradation [ ]. This phenomenon is facilitated in surface grains in a humid atmosphere at low temperatures which later progresses towards the bulk of the material while the water penetrates in the material [ ]. At the same time the Al _{2} O _{3} grain size was decreased and repositioned to the zirconia grain boundaries [ , ]. The desire for a highly translucent zirconia was fulfilled with the development of the third-generation zirconia (5Y- TZP) zirconia in 2015 to please the aesthetics. Likewise, as first two generations, 5Y-TZP is also a partially stabilized zirconia but contains in contrast, approximately 50% of cubic phase in proportion to the tetragonal phase [ , ]. The cubic phase is achieved due to a higher content of yttrium oxide in zirconia material which makes the zirconia more stable and more resistant to hydrothermal aging, thus less susceptible to low temperature degradation [ , , , ]. The main reason for the increased translucency is the isotropic property of the cubic phase, that prohibits birefringence of the remaining tetragonal phase [ ] which reduces the optical scattering coefficient [ , , ]. The translucency is also influenced by the microstructure, translucency increases with a decrease in the number of grain boundaries and thus correlates to the grain size [ ]. The higher the sintering temperature and the yttrium content (and with that more cubic phase) is, the bigger the fraction of cubic crystalline phase inside the zirconium oxide ceramic [ ]. One disadvantage of this zirconia is the decrease in flexural strength and a decrease in fracture toughness due to the stable cubic lattice and its lower or no ability of stress-induced transformation toughening [ , , , ]. Because the third generation of zirconia did not fulfill the mechanical requirements for long span dental restorations, a new, fourth generation (4Y-TZP ^{1} ) with approximately 30 % of cubic phase in proportion to the tetragonal phase was developed as a compromise between the second and the third zirconia generation [ ].

The large selection of zirconia materials makes it difficult for the user to evaluate them and pick the most suitable generation for the specific treatment case. Unfortunately, the scientific data on the third and fourth zirconia generation is very limited, particularly after aging. Therefore, zirconia materials need to be evaluated and compared with each other to determine whether the material performs similar to previous generations in terms of translucency, Martens parameters, flexural strength and fracture toughness. The aim of this study was to compare the optical and mechanical properties of initial and aged zirconia materials among one and another.

The scientific hypotheses of this in vitro study are that the choice of zirconia material (3Y-TZP with 0.25 and 0.05 wt.% Al _{2} O _{3} , 4Y-TZP and 5Y- TZP), as well as the aging affect the translucency, Martens hardness, indentation modulus and the flexural strength. The second scientific hypothesis was that the choice of the zirconia material has an influence on the fracture toughness and that there are differences between the zirconia materials for the tested properties.

## 2

## Materials and methods

In this study, 320 specimens of four different non-shaded zirconia materials were tested ( Table 1 ). In summary, 240 platelet specimens with a 14 mm diameter and 1.45 mm ± 0.04 mm thickness (N = 60/initial – without aging, N = 180/aged) and 80 rectangular specimens with w=(4.0 ± 0.2) mm, b=(3.0 ± 0.2) mm and l = 45.0 mm dimensions were designed and milled using a 5-axis milling machine (Ceramill Motion 2, Amann Girrbach, Koblach, Austria). The same platelet specimens were used for translucency, Martens hardness and biaxial flexural strength measurements. The specimens were ground with SiC abrasive paper P4000 (Struers, Ballerup, Denmark; 3-times per side). For the rectangular specimens the SiC P1200 and P2500 (Buehler, Illinois, USA) abrasive papers were used to achieve the desired specimen dimensions. All specimens were sintered in a sintering furnace according to manufacturer’s recommended firing program: from room temperature to 1450 °C at 10 °C/min, holding time of 2 h at firing temperature and cooling to room temperature at 10 °C/min (LHT 02/16, Nabertherm, Lilienthal/Bremen, Germany). The 180 specimens for long term investigation were divided into three aging categories: i. thermo-mechanical aging with 1,200,000 chewing cycles, ii. thermo-mechanical aging with 2,400,000 chewing cycles and iii. hydrothermal aging in an autoclave. Chewing simulation included a changing of the distilled water ambient temperature between 5 °C and 55 °C for 6000 and 12,000 cycles, while mechanical loading was performed with a steel antagonist of 4 mm diameter at 10 N. The platelet specimens were mounted in a for the chewing simulator designed holder with the plane side of the cylindrical specimen facing upwards and were loaded centrally (SD Mechatronik, Feldkirchen, Germany) (n = 15/zirconia material and cycle number) [ ] and hydrothermal aging for 5 h at 134 ± 2 °C and 0.2 MPa in an autoclave (Euroklav 29-S, Schembera, Munich, Germany) (n = 15/zirconia material).

Zirconia | Material | Abbreviations | Manufacturer | Shade | Y _{2 }O _{3 }Content (wt%) |
Al _{2 }O _{3 }Content (wt%) |
Batch number |
---|---|---|---|---|---|---|---|

3Y-TZP | Opaque | O | pritidenta GmbH | White | 4.65−5.95% | <0.25 | X473 20 |

W894 18 | |||||||

W274 18 | |||||||

W294 20 | |||||||

W617 14 | |||||||

W405 14 | |||||||

W618 14 | |||||||

W407 12 | |||||||

W618 14 | |||||||

W406 14 | |||||||

Z120 107 | |||||||

3Y-TZP ^{a } |
Translucent | T | pritidenta GmbH | White | 4.65−5.95% | <0.05 | W340 18T |

W341 14T | |||||||

W255 14T | |||||||

W383 12T | |||||||

X439 14T | |||||||

W380 14T | |||||||

Z111 18T | |||||||

W255 14T | |||||||

W341 14T | |||||||

X632 14T | |||||||

4Y-TZP | Extra Translucent | ET | pritidenta GmbH | White | 6.65−7.95% | <0.01 | W035 16ET |

W753 16ET | |||||||

W021 12ET | |||||||

W021 10ET | |||||||

5Y-TZP | High Translucent | HT | pritidenta GmbH | White | 8.55−10.11% | <0.01 | Y189 10HT |

W078 12HT | |||||||

WO85 14HT | |||||||

Y189 10HT |

## 2.1

## Translucency measurements

For the translucency evaluation the thickness (1.45 mm ± 0.04 mm) of each specimen was measured (n = 15/zirconia material) with electronic micrometer (Holex 421490, 0−25 mm, ± 0.001 mm, Hoffmann, Munich, Germany). The translucency measurement was conducted by using a UV–vis-Spectrophotometer (LAMBDA 35, PerkinElmer LAS, Rodgau, Germany). Before the measurement, the specimens were cleaned with 96 % alcohol (Ethanol 96 % (V/V), Otto Fischar, Saarbrücken, Germany). Translucency of each specimen was measured at wavelengths <SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax_Error style="POSITION: relative" data-mathml='λ’>[Math Processing Error]λ

λ

between 400 nm and 700 nm with 1 nm intervals. The transmission coefficient was calculated using the following formula:

tcλ=Lspecimenλ/Lsourceλ×100%

where: <SPAN role=presentation tabIndex=0 id=MathJax-Element-3-Frame class=MathJax_Error style="POSITION: relative" data-mathml='tc(λ)’>[Math Processing Error]tc(λ)

t c ( λ )

: transmission coefficient (%); <SPAN role=presentation tabIndex=0 id=MathJax-Element-4-Frame class=MathJax_Error style="POSITION: relative" data-mathml='Lspecimen(λ):’>[Math Processing Error]Lspecimen(λ):

L s p e c i m e n ( λ ) :

light transmitted through the specimen; <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax_Error style="POSITION: relative" data-mathml='Lsource(λ)’>[Math Processing Error]Lsource(λ)

L s o u r c e ( λ )

: reference intensity of the monochromatic light source.

The overall transmission T was calculated as the integration t _{c} (λ) dλ.

## 2.2

## Martens parameters

In the measurements for Martens parameter the Martens hardness (HM) and the indentation modulus (E _{IT} ) were determined (n = 15/zirconia material). For the measurement, the specimen was loaded with 9.81 N for 10 s by means of Vickers diamond indenter (α = 136°). Martens hardness is defined through the indentation force and includes plastic as well as elastic deformation. The Martens hardness and indentation hardness were calculated within the software (testX-pert V12.3, Master, Zwick, Ulm, Germany) using the following formulas from ISO 14577-1:2002:

HM=F/ASh

where <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax_Error style="POSITION: relative" data-mathml='HM’>[Math Processing Error]HM

H M

: Martens hardness (N/mm ^{2} ); <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax_Error style="POSITION: relative" data-mathml='F’>[Math Processing Error]F

F

: test force (N); <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax_Error style="POSITION: relative" data-mathml='ASh’>[Math Processing Error]ASh

A S h

: surface area of indenter at distance <SPAN role=presentation tabIndex=0 id=MathJax-Element-10-Frame class=MathJax_Error style="POSITION: relative" data-mathml='h’>[Math Processing Error]h

h

from the tip.

where <SPAN role=presentation tabIndex=0 id=MathJax-Element-11-Frame class=MathJax_Error style="POSITION: relative" data-mathml='Fmax’>[Math Processing Error]Fmax

F m a x

: maximum applied force; <SPAN role=presentation tabIndex=0 id=MathJax-Element-12-Frame class=MathJax_Error style="POSITION: relative" data-mathml='AP’>[Math Processing Error]AP

A P

is the cross-sectional area of contact between indenter

The indentation modulus <SPAN role=presentation tabIndex=0 id=MathJax-Element-13-Frame class=MathJax_Error style="POSITION: relative" data-mathml='EIT’>[Math Processing Error]EIT

E I T

was calculated using the following formula:

EIT=1-vS22APhC/πS-1-vi2/Ei-1

where <SPAN role=presentation tabIndex=0 id=MathJax-Element-15-Frame class=MathJax_Error style="POSITION: relative" data-mathml='APhC’>[Math Processing Error]APhC

A P h C

: projected contact area of the intender under load (mm ^{2} ); <SPAN role=presentation tabIndex=0 id=MathJax-Element-16-Frame class=MathJax_Error style="POSITION: relative" data-mathml='S’>[Math Processing Error]S

S

: contact stiffness evaluated from the force removal curve; <SPAN role=presentation tabIndex=0 id=MathJax-Element-17-Frame class=MathJax_Error style="POSITION: relative" data-mathml='Ei’>[Math Processing Error]Ei

E i

: indentation modulus of the Vickers pyramid indenter (kN/mm ^{2} ) and Poisson’s ratio of the specimen <SPAN role=presentation tabIndex=0 id=MathJax-Element-18-Frame class=MathJax_Error style="POSITION: relative" data-mathml='vS’>[Math Processing Error]vS

v S

= 0.25 [ ] and of the indenter <SPAN role=presentation tabIndex=0 id=MathJax-Element-19-Frame class=MathJax_Error style="POSITION: relative" data-mathml='vi’>[Math Processing Error]vi

v i

= 0.07 [ ]. This modulus is comparable with the Young’s modulus of the material.

## 2.3

## Flexural strength

Biaxial flexural strength was measured in a universal testing machine via the piston-on-three balls method (1445 Zwick/Roell, Ulm, Germany) on n = 15/subgroup. The three balls are made from tempered steel with a diameter of 3.2 mm. The steel balls lay on a metallic platform positioned in an equidistant triangle and a radius of 5.1 mm. The testing specimens were positioned concentrically between the support circle and the piston. The force was applied centrally on the specimen with a crosshead speed of 1 mm/min by a piston with 0.8 mm radius until fracture. The fracture load of each specimen was recorded, and the flexural strength was calculated using the following formula (DIN ISO 6872:2019-01):

σ=-0.2387P(X-Y)/d2

where: <SPAN role=presentation tabIndex=0 id=MathJax-Element-21-Frame class=MathJax_Error style="POSITION: relative" data-mathml='σ’>[Math Processing Error]σ

σ

: flexural strength (MPa); <SPAN role=presentation tabIndex=0 id=MathJax-Element-22-Frame class=MathJax_Error style="POSITION: relative" data-mathml='P’>[Math Processing Error]P

P

: fracture load (N); <SPAN role=presentation tabIndex=0 id=MathJax-Element-23-Frame class=MathJax_Error style="POSITION: relative" data-mathml='d’>[Math Processing Error]d

d

: specimen thickness (mm). The coefficients X and Y were determined as followed:

X=1+vlnr2/r32+1-v/2∙r2/r32