Leucite and cooling rate effect on porcelain–zirconia mechanical behavior

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Leucite and cooling rate effect on porcelain–zirconia mechanical behavior P.D. Meirelles, Y.O. Spigolon, M. Borba, P. Benetti ∗ University of Passo Fundo, Post-Graduate Program in Dentistry, Passo Fundo, RS, Brazil

a r t i c l e

i n f o

a b s t r a c t

Article history:

Objective. This study investigated the influence of the cooling protocol on the mechani-

Received 15 July 2015

cal behavior of Y-TZP veneered with porcelain with different compositions. The tested

Received in revised form

hypotheses were: (1) Y-TZP infrastructures veneered with porcelain containing leucite in

12 May 2016

its composition presents higher flexural strength () and reliability (m), and (2) slow cooling

Accepted 3 September 2016

protocol results in greater  and m.

Available online xxx

Methods. A total of 120 bilayer porcelain-Y-TZP bar-shaped specimens were prepared with the dimensions of 1.8 mm (0.8 mm Y-TZP ± 1.0 mm porcelain) × 4.0 mm × 16.0 mm. Specimens

Keywords:

were divided into four groups (n = 30) according to the porcelain composition (containing

Zirconia

or not leucite) and cooling protocol. Fast cooling was performed by opening the furnace

Porcelain

chamber at sintering temperature. For the slow cooling, the chamber was maintained closed

Leucite

until it reached the room temperature. Specimens were tested in three-point bending with

Residual stress

the porcelain surface under tension using a universal testing machine, in 37 ◦ C water, at

Fracture strength

0.5 mm/min crosshead speed. Data were analyzed by two-way ANOVA, Tukey post-hoc test (˛ = 0.05) and Weibull. Results. Y-TZP veneered with porcelains with different microstructural composition presented similar  and m values (p = 0.718). The cooling protocol had no influence on the  and m values of the experimental groups (p = 0.718). Cracking represented 95% of failures, whereas the initial flaw propagated from the porcelain surface towards the interface. Significance. Y-TZP veneered with porcelain containing or not leucite present similar mechanical behavior and, at 1-mm thickness, is not sensitive to the cooling protocol. © 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Zirconia-based all ceramic restorations combine high mechanical properties of a polycrystalline ceramic infrastructure with good optical characteristics (aesthetic) of a glass veneer. Yttria-partially stabilized tetragonal zirconia (Y-TZP) show higher fracture strength and toughness than other dental ceramics and provide a more natural appearance to

the restoration than metallic infrastructures [1–3]. However, veneering the Y-TZP infrastructure with a glass-ceramic, such as porcelain, is recommended due to its opacity, resulting in a bilayer restoration. Therefore, despite the good mechanical behavior of Y-TZP, porcelain fractures and chipping are a frequently found technical complication in clinical studies, occurring more often than with other types of all-ceramics and metal-ceramics restorations (15–62% over 3–5 years) [4–13].

∗

Corresponding author at: BR 285, São José, Passo Fundo, RS CEP: 99052-900, Brazil. E-mail addresses: [email protected], [email protected] (P. Benetti). http://dx.doi.org/10.1016/j.dental.2016.09.018 0109-5641/© 2016 The Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.

Please cite this article in press as: Meirelles PD, et al. Leucite and cooling rate effect on porcelain–zirconia mechanical behavior. Dent Mater (2016), http://dx.doi.org/10.1016/j.dental.2016.09.018

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Table 1 – Description of the ceramics used in this study. Ceramics

Brand name

Chemical compositiona

Manufacturer

CTE ×10−6 ◦ C

Tg

◦C

Zirconia

Vita In-Ceram YZ #36940

VITA-Zahnfabrik, Germany

ZrO2 (95%); Y2 03 (<5%); <3% Al2 O3 ; <1% SiO2

10.5

–

Porcelain with leucite

Vita VM9 #35770

VITA-Zahnfabrik, Germany

SiO2 (60–64%); Al2 O3 (13–15%); K2 O (7–10%); Na2 O (4–6%); B2 O3 (3–5%)

9.1

510

Porcelain without leucite

Ceramco PFZ #14001599

DENTSPLY, USA

SiO2 (60%); K2 O (15%); Al2 O3 (10%); Na2 O (4–5%); BaO (3–4%); Tb2 O3 (3–4%).

9.4

560

a

Information obtained by the authors using EDS.

The literature suggests several factors that are potentially related to the high porcelain susceptibility to fractures in zirconia-based all-ceramic restorations, such as insufficient support of porcelain by the infrastructure [4,14,15]; ceramics thermal mismatch [5,16–18]; porcelain–zirconia bond strength [18–20]; wetting of zirconia [18,21]; low mechanical properties of porcelain [22,23]; transient and residual stresses developed inside the porcelain, especially related to thick layers and fast cooling rates [6,16,24–33]; different techniques of porcelain veneering [34]; and inadequate dental preparations with insufficient axial reduction [35]. Higher cooling rates are associated with the development of temperature gradients within the ceramic body [6,31,36]. Thermal contraction (change of volume and density) and a non-uniform solidification are induced by these temperature gradients, resulting in the development of stresses [30,37]. On the other hand, when the porcelain is slowly cooled, the glass structure is provided with time and sufficient energy to rearrange/reorganize its molecules, resulting in a different behavior at the temperatures around the glass transition—Tg [30]. Therefore, slow cooling is a recommended practice for materials containing glass matrix to prevent residual stresses. Alternatively, a study published by Christensen and Ploeger [7] raised a new factor that could be associated with these high chipping rates: the presence of leucite crystals in the porcelain composition. The authors observed, clinically, that zirconia-based all-ceramic restorations veneered with porcelain containing leucite had a lower frequency of porcelain fractures (less than 30% of chipping and major fractures in 2 years) in comparison to restorations veneered with porcelain with no leucite in its composition (60% in 2 years). The leucite crystals that are present in the porcelain composition aim to match the coefficient of thermal expansion (CTE) of the veneer and the infrastructure ceramics (Y-TZP). Leucite can also increase porcelain viscosity, resulting in a flow reduction during firing. Thus, the structural relaxation of the porcelain containing leucite can generate lower magnitude transient and residual stresses during cooling than porcelain without leucite. Previous studies have demonstrated that stresses generated during cooling are associated with the nucleation and crack propagation from pre-existing defects in the porcelain [6,24–33]. There is a lack of information in the literature regarding the effect of the presence of leucite on the mechanical behavior of porcelain-YTZP structures and its association with different cooling protocols. Therefore, the first study objective was

to evaluate the influence of the porcelain composition on the fracture strength and reliability of the porcelain-YTZP bilayer structures, testing the hypothesis that specimens veneered with porcelain containing leucite have superior mechanical properties. The second objective was to investigate the effect of the cooling protocol on the mechanical behavior of porcelain-YTZP bilayer structures, considering the tested hypothesis was that the slow cooling protocol results in higher strength and reliability.

2.

Materials and methods

The materials used in the study are presented in Table 1. Porcelain surface characteristics can be observed in Fig. 1. One hundred and twenty bilayer bar-shaped specimens of Y-TZP infrastructure veneered with porcelain were produced with the final dimensions of 1.8 mm (0.8 mm Y-TZP ± 1.0 mm porcelain) × 4.0 mm × 16.0 mm. Specimens were divided into four groups (n = 30) according to the veneering porcelain (VM9 or PFZ) and cooling protocol (F—fast or S—slow).

2.1.

Y-TZP infrastructures

Partially-sintered zirconia CAD-CAM blocks were cut into bars (n = 120) in a cutting machine (Miniton Struers, Copenhagen, Denmark) using a diamond disk at 250 rpm under water cooling. The dimension of the infrastructure was 22% larger than the required dimension for final specimen, in order to compensate for the ceramic sintering shrinkage. After cutting, the infrastructures were polished using metallographic paper (#800 and #1000) in a polishing machine (Abramin, Struers, Copenhagen, Denmark) at 250 rpm under water cooling. Two external longitudinal edges of the samples were chamfered, following ISO 6872 standard [38]. The infrastructures were sintered (Zyrcomat, Vita Zahnfabrik, Germany) according to the manufacturer’s recommendations and the final dimensions (0.8 mm × 4.0 mm × 16.0 mm) were measured with a digital caliper (Mitutoyo Corporation, Tokyo, Japan).

2.2.

Porcelain veneering

A thin layer of porcelain (≤0.1 mm) was applied on the Y-TZP specimens and submitted to wash firing, as recommended by the manufacturer. The Y-TZP infrastructure was placed inside a silicon mold (Optosil-Comfort Putty, Heraeus, Germany) for porcelain veneering. Porcelain powder was mixed with mod-

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Fig. 1 – Porcelain surfaces after treatment with 2% fluoridric acid for 15 s: (A) VM9, showing particles agglomerations (white arrows), probably leucite crystals embedded in a glass matrix; (B) Ceramco PFZ, smother surface, with no particles observed.

eling liquid (Liquid Modeling Vita, Vita-Zahnfabrik, Germany) and applied directly to the infrastructure (unchamfered surface), as recommended by the manufacturer. The ceramic powder was compacted into the mold by vibration and the excess liquid was removed using absorbent paper. Specimens were removed from the mold and placed in the internal chamber of a ceramic furnace (MP 6000, Vacumat, Vita, Germany) to perform the sintering cycle. Two veneering applications were necessary to obtain a final 1-mm thick uniform layer of porcelain. Half of the Y-TZP infrastructures were veneered with porcelain with leucite and the other half with porcelain without leucite (n = 60). No surface treatment was performed in the Y-TZP infrastructures prior to veneering. The porcelain surface was also polished using metallographic paper (#600, #800, #1000 and #1200) in a polishing machine (Abramin, Struers, Copenhagen, Denmark) at 250 rpm under water cooling. External longitudinal edges of veneering porcelain were chamfered and chamfer width was standardized at 0.1 mm [38]. Final dimensions were measured using a digital caliper in three different areas of the bar-shaped specimen (middle, right and left margins) in order to ensure parallelism. After final polishing, specimens were divided into subgroups for final sintering, performed with fast or slow cooling protocol, as described below.

2.4.

Flexural strength test was performed using a universal testing machine (EMIC DL 2000, São José dos Pinhais, PR, Brazil) with a three-point bending device. Specimens were immersed in water at 37 ◦ C during the test. The porcelain surface was placed on the top of two support rollers. The compressive load was applied in the Y-TZP surface by a third roller, at 0.5 mm/min crosshead speed, until the first sign of fracture was detected (sound emission and/or drop of load observed in the stress–strain plot). Load at fracture (N) was recorded and used to calculate the flexural strength according to Eq. (1) [3,39]:



f =

Cooling protocols

For the fast cooling protocol, the firing chamber of the furnace was opened immediately after reaching the recommended firing temperature (final time and temperature recommended by the manufacturer) and the furnace was turned off. The slow cooling protocol was determined by maintaining the chamber closed until the temperature reached 50 ◦ C below the porcelain glass transition temperature (Tg), with a cooling rate of 10 ◦ C/min [31,36]. Based on the Tg value reported by the manufacturers for VM9 (510◦ C) and PFZ (560◦ C), the furnace was opened at 460 ◦ C for VM9 and at 510 ◦ C for PFZ.



3Et LP Ec t2c + 2Ec tc tt + Et t2t



2w E2c t4c + 4Ec Et t3c tt + 6Ec Et t2c t2t + 4Ec Et tc t3t + E2t t4t



(1)

where P is the maximum load at fracture (N); L is the distance between the support rollers (12 mm); Et is the elastic modulus of the material under tension (VM9—64 GPa and PFZ 75—GPa); Tt is the thickness of the material under tension (mm); Ec is the elastic modulus of the material under compression (YZ—205 GPa); Tc is the thickness of the material under compression (mm) and w is the width of the specimen (mm).

2.5. 2.3.

Flexural strength test

Statistical analysis

Flexural strength data passed the Shapiro–Wilk Normality Test (p = 0.299) and the Equal Variance Test (p = 0.514). Data were analyzed with two-way ANOVA and Tukey (˛ = 0.05). Weibull analysis was performed to obtain the characteristic strength ( 0 ) and the Weibull modulus (m) parameters.

2.6.

Fracture analysis

After testing, specimens were analyzed in a stereomicroscope (ZTX, Ningbo, ProWay) at magnification of 40× and transillumination to confirm the presence, the extension and propagation path of the initial crack. The failure

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Table 2 – Mean flexural strength ( f ) and standard deviation (SD), characteristic strength ( 0 ) and Weibull modulus (m) followed by the confidence intervals (CI), for each experimental group. Groups

 f (MPa)*

SD (MPa)

VM9L VM9F PFZL PFZF

61.1 62.9 67.1 67.2

13.8 14.2 16.9 16.3

∗

 0 * (MPa) 66.5 68.4 735 73.5

CI␴ 61.8–71.6 63.1–74.2 67.6–80.0 65.6–77.3

m*

ICm

5 5 4 5

4–7 4–6 3–6 3–6

There was no statistical difference between the groups (p > 0.05).

Fig. 2 – Probability of failure for the tested groups.

mode was classified according to the fracture extension: (1) cracking—crack propagating from the porcelain surface to the porcelain-Y-TZP interface; (2) porcelain chipping—fracture of the porcelain layer without exposure of Y-TZP infrastructure; (3) delamination—fracture of the porcelain layer with exposure of Y-TZP infrastructure; (4) catastrophic failure—fracture of the specimens involving both porcelain and Y-TZP layers.

3.

Results

The results are shown in Table 2. The fracture strength of VM9 groups were statistically similar to PFZ groups (p = 0.066). The cooling protocol had no influence on the flexural strength of the experimental groups (p = 0.718). The Weibull modulus (m) and characteristic strength ( 0 ) were also similar among groups (Fig. 2). Regarding the failure mode, 95% of failures were classified as cracking, in which the initial flaw propagated from the porcelain surface towards the interface (Fig. 3). Catastrophic failure occurred in 5% of the specimens. It can be observed that the VM9R group had a higher number of catastrophic fail-

Fig. 3 – Fractured specimen showing the crack in the veneering porcelain extending to the zirconia interface.

ures (n = 4, 13%), while in PFZR group this type of fracture did not occur.

4.

Discussion

The mechanical evaluation of ceramics using multilayered structures significantly contributes to predict the clinical performance of all-ceramic restorations. It provides data regarding the origin and mode of failure, quality of the interface, and distribution of residual or transient stresses, which

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are difficult to be obtained when the materials are evaluated separately [3,30]. In the present study, porcelain-YTZP bilayer structures were evaluated and no effect of the porcelain composition on its flexural strength and reliability was found, rejecting the first study hypothesis. Fracture strength of porcelains highly depends on the quality of the glass matrix [40]. However, the literature reports a significant improvement of porcelain fracture toughness by increasing on the leucite content, especially above 22% [41]. Studies showed that the crack deflects when finding a leucite crystal and changes the direction of propagation, which reduces the stress intensity factor at the crack tip. Crack deflection occurs because of the action of tensile and compressive stress fields on the glass matrix and the leucite interface induced during cooling, resulting from the CTE differences between these two phases [41,42]. The similar flexural strength and Weibull modulus values found for VM9 and PFZ demonstrated that the low concentration of leucite in the VM9 porcelain (7.5%) is not significant to improve the mechanical behavior of the material, which was also confirmed in previous studies [3,41]. The leucite content in the porcelain is influenced by the thermal history, such as the time of the sintering cycle, the number of firings and the cooling rate [43,44]. Taskonak et al. [25] studied the crystallinity of a feldspathic porcelain, and observed the predominance of amorphous glass phase, but an increase of the crystallinity index after slow cooling. Therefore, thermal changes can result in the formation of a leucite crystal phase in feldspathic porcelain (increase from 8.5 to 55.8%), especially in slow cooling [25,45,46]. In this study, the porcelain surface were analyzed by scanning electron microscopy (SEM) to evaluate the presence and amount of crystalline phase. Crystals were observed on the VM9 porcelain surface with no signs of cracks in the surrounding area for either slow or fast cooling (low residual stresses). Crystals were not found in the PFZ porcelain, suggesting that the different cooling protocols were not able to produce a significant amount of leucite crystals in this material. The mechanical behavior of multilayered structures is also strongly related to the CTE (˛) compatibility of materials. Considering that a perfect combination of Y-TZP and porcelain CTE is unlikely to occur, a small positive difference (+˛ × 10−6 /◦ C) is recommended to avoid residual stresses at the interface and spontaneous fracture [4,17,26]. The difference in the CTE (˛) between Y-TZP and VM9 porcelain (1.4 × 10−6 /◦ C) is similar to the ˛ between Y-TZP and PFZ (1.1 × 10−6 /◦ C). Therefore, the magnitude of stresses generated by the difference in thermal contraction is expected to be comparable, which could also explain the similarity among the flexural strength results for these two porcelains. The thermal compatibility is not the exclusive predictive parameter for distribution of residual stresses. Changes in the cooling rate have been shown to modify the stress gradients through the porcelain [47]. However, in the present study there was a negligible effect of the cooling protocol on the flexural strength and reliability of porcelain-Y-TZP bilayer structures, which agrees with Swain [6], who found low magnitude thermal stresses in thin zirconia-porcelain structures. The rapid cooling protocol could result in lower reliability of veneered zirconia. When submitted to fast cooling the

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expanded structure of the porcelain relaxes to lower volume (length) than during slow cooling, when the glass structure can rearrange [43,48]. Removing leucite from the porcelain results in a ceramic with higher content of glass-modifying oxides (K2 O, Al2 O3 , Na2 O, BaO, Tb2 O3 ) which are added to the material to match the CTE to the Y-TZP infrastructure. The addition of modifying oxides also decreases the viscosity of the porcelain at temperatures above the Tg, allowing adequate wetting into the irregularities of the zirconia surface. The decrease of the viscosity (increase of viscoelasticity) results from the porcelain structural relaxation, which occurs at elevated temperatures and is accompanied by greater expansion of the whole structure during the heating, and higher contraction of porcelain during cooling [48–51]. Therefore, PFZ porcelain, without leucite, could be even more sensitive to the rapid cooling protocol [29]. However, this was not confirmed by this study, probably because of the reduced porcelain thickness (1 mm). Given that the porcelain and zirconia compatibility is based on CTE similarities at temperatures below the Tg, knowledge on the compatibility of volumetric expansion at temperatures around or above the Tg is scarce. The CTE of the porcelain is linear at temperatures below the Tg, and nonlinear above the Tg because the viscoelastic structure is expanded. The study of the thermal behavior of the porcelain at temperatures around the Tg is important because of the changes in CTE and heat capacity, especially in ceramic restorations with non-homogeneous thickness of porcelain (as often occurs in crowns and bridges with anatomic details) [6,17,30,37]. The porcelain without leucite allows a greater relaxation (thermal expansion) of the structure at temperatures around the Tg, especially during slow cooling (when there are adequate time and temperature for molecular rearrangements), which could reduce the compatibility with zirconia. However, the addition of leucite is responsible not only for adjusting the CTE of porcelain, but also for increasing its viscosity, resulting in lower structural relaxation (flow) during firing [47,51]. The decrease of structural relaxation in the porcelain containing leucite could generate stresses of lower magnitude during cooling than in the porcelain without leucite (not found in the present study). In the present study, a homogeneous thickness of 1 mm of porcelain was used to veneer the Y-TZP infrastructures. The use of a small volume of porcelain is recommended [6,14,26,28] because it allows the inner and outer portions of the porcelain to cool homogeneously at temperatures below the Tg, thus preventing the development of thermal gradients, and consequently, transient and residual stresses originated from the differences in volume and viscosity of the layers with different temperatures [6,30,31,36]. Therefore, when a 1 mm-thick layer of porcelain is used, the presence/absence of leucite and the cooling protocol have negligible influence on the mechanical behavior of bilayer porcelain-Y-TZP structures. Following this recommendations, both systems may be considered safe and mechanically reliable for clinical use. On the other hand, when a greater thickness of porcelain is necessary, cooling the system slowly is important to prevent transient and residual stresses, which could nucleate and propagate cracks inside the porcelain until its complete fracture [6,30].

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All fractures originate from the porcelain surface, whereas the predominant failure mode is cracking (95%), without fracture of the porcelain layer. Catastrophic failures were found for a few specimens in the group veneered with porcelain containing leucite, which could indicate high interface quality between porcelain and Y-TZP [3,28,52,53]. This supports the observations of Choi et al. [53], who compared veneering porcelains with zirconia, where porcelain with leucite had better interfacial adhesion than porcelain without leucite. The fracture strength values obtained in catastrophic failures were not included in the subsequent statistical analyses because the values exceeded the porcelain strength, mainly representing the strength of Y-TZP (not representative of the main clinical problem studied: chipping). The bilayer structures were tested with the porcelain under tension to simulate the stress situation to which the pontic and connectors of a fixed partial denture are subjected in the mouth [3,28,53]. In addition, the variables investigated in the present study are related to the porcelain layer (composition and cooling protocol), rather than the infrastructure. Therefore, the failure load related to the porcelain fracture was registered using a methodology described in previous studies [3,28], in which the test is interrupted after detecting the initial flaw sound emission and the porcelain is further analyzed by transillumination using a light microscope to confirm the presence of cracks (Fig. 3). The chosen test configuration (specimens with simplified design and basic equipment for tests) is recommended to determine the influence of the microstructure of the porcelain and fabrication technique on strength and probability of failure of a material or combination of materials (using Weibull statistics) [38,39]. Choi et al. [53,54] investigated the surface residual stress of the four different porcelains using the indentation fracture toughness technique in monolithic diskshaped specimens. Significantly higher compressive residual stresses were found on the surface of the structures submitted to fast cooling in the last firing cycle [54]. These surface compressive stresses could also minimize the effect of transient or residual tensile stresses formed through the bilayer structure during cooling and contributed for the similarity in strengths found in the groups of the present study. Until the present moment, there are no internationally accepted or standardized parameters to reproduce the clinical stress state of ceramic restorations on laboratorial crown-like specimens. Therefore, future studies should investigate protocols to test crown-shaped specimens in laboratory, which is still a challenging methodology.

5.

Conclusion

The hypotheses of the study were rejected as the presence of leucite and different cooling protocols have no effect on the fracture strength and reliability of porcelain–zirconia combinations. Based on the parameters investigated, zirconia restorations produced with both types of porcelains and cooling protocols could present similar mechanical performance.

Acknowledgments This investigation was partially supported by Capes (#1188699), research grant CNPq (#460094/2014-9), FAPERGS/CNPq 16/2551-0000193-6, Vita-Zahnfabrik and Dentisply. The authors thank to Coral Dental Laboratory (Passo Fundo, RS, Brazil), particularly to Mr. Ireno de Brito.

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Please cite this article in press as: Meirelles PD, et al. Leucite and cooling rate effect on porcelain–zirconia mechanical behavior. Dent Mater (2016), http://dx.doi.org/10.1016/j.dental.2016.09.018