Leucite and cooling rate effect on porcelain–zirconia mechanical behavior

doi:10.1016/j.dental.2016.09.018

Dental Materials

Volume 32, Issue 12, December 2016, Pages e382-e388

https://doi.org/10.1016/j.dental.2016.09.018 Get rights and content

Highlights

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Zirconia veneered by porcelain containing or not leucite present similar mechanical performance.
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The presence of leucite had no effect on the probability of failure of the bilayer specimens.
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The combination of materials, at recommended thickness of 1 mm, are less sensitive to cooling rate.

Abstract

Objective

This study investigated the influence of the cooling protocol on the mechanical behavior of Y-TZP veneered with porcelain with different compositions. The tested hypotheses were: (1) Y-TZP infrastructures veneered with porcelain containing leucite in its composition presents higher flexural strength (σ) and reliability (m), and (2) slow cooling protocol results in greater σ and m.

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 were divided into four groups (n = 30) according to the porcelain composition (containing or not leucite) and cooling protocol. Fast cooling was performed by opening the furnace chamber at sintering temperature. For the slow cooling, the chamber was maintained closed until it reached the room temperature. Specimens were tested in three-point bending with the porcelain surface under tension using a universal testing machine, in 37 °C water, at 0.5 mm/min crosshead speed. Data were analyzed by two-way ANOVA, Tukey post-hoc test (α = 0.05) and Weibull.

Results

Y-TZP veneered with porcelains with different microstructural composition presented similar σ and m values (p = 0.718). The cooling protocol had no influence on the σ 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.

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], [2], [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], [5], [6], [7], [8], [9], [10], [11], [12], [13].

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], [17], [18]; porcelain–zirconia bond strength [18], [19], [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], [25], [26], [27], [28], [29], [30], [31], [32], [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], [25], [26], [27], [28], [29], [30], [31], [32], [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.

Section snippets

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).

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 (σ₀) 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).

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 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

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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To evaluate the effect of firing temperature and heating rate on the volumetric shrinkage, translucency, flexural strength, hardness, and fracture toughness of a zirconia veneering ceramic.
Zirconia veneering ceramic specimens (N = 45) with varying final temperatures (730 °C, 750 °C, and 770 °C) and heating rates (70 °C/min, 55 °C/min, and 40 °C/min) were fabricated (n = 5). Each specimen's shrinkage, translucency, flexural strength, hardness, and fracture toughness were determined. Two-way analysis of variance, Scheffé test, and Pearson's correlation analysis were used to evaluate data (α = 0.05).
The shrinkage (44.9 ± 3.1–47.5 ± 1.6 vol%) and flexural strength (74.1 ± 17.4–107.0 ± 27.1 MPa) were not affected by tested parameters (P ≥ 0.288). The interaction between the main factors affected the translucency, hardness, and fracture toughness of the specimens (P ≤ 0.007). Specimens with 770 °C final temperature and 70 °C/min heating rate had the lowest (21.8 ± 3.2 %) translucency (P ≤ 0.039). The hardness ranged between 4.98 ± 0.51 GPa (730 °C; 70 °C/min) and 5.60 ± 0.37 GPa (770 °C; 70 °C/min). Fracture toughness ranged between 0.54 ± 0.04 MPa√m and 0.67 ± 0.08 MPa√m with the highest values for specimens fired at 730 °C with 70 °C/min (P ≤ 0.001). There was a positive correlation between translucency and hardness (r = 0.335, P = 0.012), and a negative correlation between fracture toughness and all parameters other than shrinkage (translucency: r = −0.693/P < 0.001, flexural strength: r = −0.258/P = 0.046, hardness: r = −0.457/P < 0.001).
Heating rate and final temperature should be considered while fabricating veneered zirconia restorations with tested ceramic as they affected the translucency, hardness, and fracture toughness.
Effect of processing methods on the chipping resistance of veneered zirconia
2022, Journal of the Mechanical Behavior of Biomedical Materials
To evaluate the edge chipping resistance (R_eA) and the fracture toughness (K_C) of 3Y-TZP bilayers produced with the following materials/processing combinations: fluorapatite glass-ceramic applied on zirconia using the traditional layering and hot-pressing (press-on) techniques; feldspathic porcelain using rapid layer technology (RLT); and lithium disilicate glass-ceramic using CAD-on method. The influence of the cooling rate (slow and fast) was analyzed for layering and hot-pressing.
Bilayer bars (25x4x2 mm) were made following manufacturers' instructions. The edge chipping test was performed in an universal testing machine, using a coupled Vickers indenter. R_eA was calculated dividing the critical load at fracture by the edge distance. Fracture toughness was calculated by a regression fit with a fixed slope of 1.5 correlating the critical chipping load regarding edge distance and also with indentation fracture (IF) method. Data were statistically analyzed using ANOVA and Tukey's test (α = 5%).
R_eA and K_C was significantly higher for the CAD-on bilayers. RLT showed intermediate R_eA means, and layering and hot-pressing techniques showed the lowest R_eA values. For both processing methods there was no effect of the cooling protocol on the R_eA and fracture toughness.
There is a significant effect of the material/processing association on the edge chipping resistance and fracture toughness of the bilayers. There was no effect of the cooling protocol on the edge chipping resistance and fracture toughness for the specimens processed by both the layering and hot-pressing techniques.
On the behaviour of zirconia-based dental materials: A review
2021, Journal of the Mechanical Behavior of Biomedical Materials
Zirconia-based dental materials are extensively used in clinical practice due to their tooth-like appearance, biofunctionality, biocompatibility, and affordability. However, premature clinical failures of veneering porcelains raise a concern about their integrity. Extensive studies have been performed over a decade to resolve this issue, but it is challenging to reference all information effectively. A single source identifying the significance of potential parameters on material performance has not previously been available. An evidence-based meta-narrative review technique was used to review the characteristic parameters that can affect the overall behaviour of zirconia-based materials. Keywords were chosen to assess manuscripts based on scientific coherence with this paper's research objective. Online keyword searches were carried out on ScienceDirect, PubMed, and SAGE databases for relevant published manuscripts from year 1985–2020.261 out of 3170 identified manuscripts were included. A total of 10 parameters were identified and classified into the material, manufacturing, and geometric aspects. The effect of every parameter was reviewed on the performance of the material. A discrepancy in findings was observed and is attributed to the fact that there is no standard methodology. This review acts as a single source that summarizes various parameters' contribution to zirconia-based dental materials' performance. This review facilitates manufacturing improvements by accounting for every parameter's effect on overall performance.
Effect of cooling protocol on mechanical properties and microstructure of dental veneering ceramics
2019, Dental Materials
Citation Excerpt :
Increasing the crystal content is well known to increase the CTE of feldspathic porcelains [16]. A positive mismatch is desirable [30] to achieve compressive stresses in the veneer. For metal frameworks, the metal CTE is quite similar to the VM13 porcelain, (αmetal ∼14.0 ppm/°C verses αVM13 ∼ 13.3 ppm/°C) resulting in a positive mismatch of Δα ≅ 0.7 ppm/°C.
Understand how cooling protocols control the microstructure and mechanical properties of veneering porcelains.
Two porcelain powders were selected, one used to veneer metallic frameworks (VM13) and one for zirconia frameworks (VM9). After the last firing cycle, the monolithic specimens were subjected to two cooling protocols: slow and fast. Flexural strength (FS) was evaluated by three-point beam bending and fracture toughness (K_IC) was evaluated by the single-edge V-notch beam (SEVNB) method. Scanning electron microscopy (SEM) was performed to determine the leucite crystal volume fraction (%), particle size, and matrix microcrack density. The results were compared by analysis of variances (ANOVA) and Tukey’s multiple comparison test.
The mechanical properties were significantly (p < 0.05) higher for the VM13 porcelain (FS = 111.0 MPa, K_IC = 1.01 MPa.√m) compared to VM9 (FS = 79.6 MPa, K_IC =0.87 MPa.√m) regardless of cooling protocol due to ∼250% higher volume fraction of leucite crystals. The slow cooled VM13 and fast cooled VM9 resulted in the highest and lowest mechanical properties, respectively, while the VM9 slow cooled properties were similar to the VM13 fast cooled. The SEM revealed that the slow cooling significantly increased the volume fraction of leucite crystals by 33–41 %. Across both porcelains, a significant linear correlation between both mechanical properties (strength and toughness) and leucite crystal content was found. Slow cooling was also associated with increased crystal growth resulting in more matrix microcracking.
Controlled crystallization using slow cooling can be applied as a means of strengthening dental porcelains. However, the benefits of slow cooling may be partially offset by increasing the microcrack density in the glass matrix. To achieve the maximum benefit of slow cooling, it is recommending to develop heat treatments to produce porcelain with fine-grained and homogenously dispersed leucite crystals to achieve minimal glass matrix microcracking.
Do thermal treatments affect the mechanical behavior of porcelain-veneered zirconia? A systematic review and meta-analysis
2019, Dental Materials
Citation Excerpt :
In contrary, when slow cooling is used, the restoration stays under controlled temperature reduction, thus, the whole structure of the restoration is expected to cool uniformly, effectively reducing residual thermal stresses [21]. However, not all previous studies have shown difference in the strength of bilayer systems after fast or slow cooling [22–24]. Various other thermal treatments have been proposed to overcome the chipping susceptibility and improve the fracture resistance of veneer ceramic, including glaze [25], number of veneer firings [26,27], and additional firing at different temperatures [28].
A systematic review of in vitro studies was conducted to assess the effect of thermal treatments on flexural strength or critical load to failure of porcelain-veneered zirconia (PVZ).
Literature searches were performed up to June 2018 in PubMed/MEDLINE, Scopus and Web of Science databases, with no publication year or language limits.
From 393 relevant studies, 21 were selected for full-text analysis, from which 7 failed to meet the inclusion criteria. The 14 remaining papers were included in the systematic review: 8 for meta-analysis and 6 restricted to descriptive analyses. Hand searching of reference lists resulted in no additional papers.
In vitro studies using PVZ specimens testing the influence of thermal treatments on the fracture resistance to monotonic or cyclic loading. Papers evaluating cooling rate were divided into those applying fast cooling from above the porcelain glass transition temperature ( $T_{g}$ ), or from below it. Meta-analyses were performed separately for flexural strength and critical load to failure, using random effects at a 5% significance level.
Delaying furnace opening at a temperature below the porcelain $T_{g}$ is advised for PVZ restorations, in order to improve their fracture resistance. Additional information is required to confirm the apparent beneficial effect of self-glaze and repeated veneer firings on the mechanical properties of these restorations. Finally, in order to obtain conclusive and relevant evidence regarding thermal treatments and the fracture resistance of PVZs, future studies should concentrate on anatomically-correct crown specimens.
Influence of residual thermal stresses on the edge chipping resistance of PFM and veneered zirconia structures: Experimental and FEA study
2019, Dental Materials
Chipping fractures of the veneering porcelain are frequently reported for veneered all-ceramic crowns. In the present study, the edge chipping test is used to measure the toughness and the edge chipping resistance of veneered zirconia and porcelain-fused-to-metal (PFM). The aim is to describe an edge chipping method developed with the use of a universal testing machine and to verify the accuracy of this method to determine the influence of residual thermal stresses on the chipping fracture resistance of veneering porcelain. A finite element analysis (FEA) was used to study the residual stress profiles within the veneering porcelain.
Veneered zirconia and PFM bar specimens were subjected to either a fast or a slow cooling protocol. The chipping resistances were measured using the edge chipping method. The load was applied in two different directions, in which the Vickers indenter was placed in the veneering porcelain either parallel or perpendicular to the veneer/framework interface. The mean edge chipping resistance (R_eA) and fracture toughness (K_C) values were analysed. R_eA was calculated by dividing the critical force to cause the chip by the edge distance. K_C was given by a fracture analysis that correlates the critical chipping load (F_C) regarding edge distance (d) and material toughness via K_C = F_C/(βd^1.5).
The R_eA revealed similar values (p > 0.005) of chipping resistance for loads applied in the parallel direction regardless of framework material and cooling protocol. For loads applied in the perpendicular direction to the veneer/framework interface, the most chip resistant materials were slow cooled veneered zirconia (251.0 N/mm) and the PFM fast cooled (190.1 N/mm). K_C values are similar to that for monolithic porcelain (0.9 MPa.√m), with slightly higher values (1.2 MPa.√m) for thermally stressed PFM fast cooled and veneered zirconia slow cooled groups.
The developed and reported edge chipping method allows for the precise alignment of the indenter in any predetermined distance from the edge. The edge chipping method could be useful in determining the different states of residual thermal stresses on the veneering porcelain.

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Leucite and cooling rate effect on porcelain–zirconia mechanical behavior

Highlights

Abstract

Objective

Methods

Results

Significance

Introduction

Section snippets

Materials and methods

Results

Discussion

Conclusion

Acknowledgments

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