Zirconia veneered by porcelain containing or not leucite present similar mechanical performance.
The presence of leucite had no effect on the probability of failure of the bilayer specimens.
The combination of materials, at recommended thickness of 1 mm, are less sensitive to cooling rate.
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 .
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.
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.
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.
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 . 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) .
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 ; ceramics thermal mismatch ; porcelain–zirconia bond strength ; wetting of zirconia ; low mechanical properties of porcelain ; transient and residual stresses developed inside the porcelain, especially related to thick layers and fast cooling rates ; different techniques of porcelain veneering ; and inadequate dental preparations with insufficient axial reduction .
Higher cooling rates are associated with the development of temperature gradients within the ceramic body . Thermal contraction (change of volume and density) and a non-uniform solidification are induced by these temperature gradients, resulting in the development of stresses . 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 . Therefore, slow cooling is a recommended practice for materials containing glass matrix to prevent residual stresses.
Alternatively, a study published by Christensen and Ploeger 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 .
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.
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).
|Ceramics||Brand name||Manufacturer||Chemical composition a||CTE||Tg|
|×10 −6 °C||°C|
|Zirconia||Vita In-Ceram YZ||VITA-Zahnfabrik, Germany||ZrO 2 (95%); Y 2 0 3 (<5%); <3% Al 2 O 3 ; <1% SiO 2||10.5||–|
|Porcelain with leucite||Vita VM9||VITA-Zahnfabrik, Germany||SiO 2 (60–64%); Al 2 O 3 (13–15%); K 2 O (7–10%); Na 2 O (4–6%); B 2 O 3 (3–5%)||9.1||510|
|Porcelain without leucite||Ceramco PFZ||DENTSPLY, USA||SiO 2 (60%); K 2 O (15%); Al 2 O 3 (10%); Na 2 O (4–5%); BaO (3–4%); Tb 2 O 3 (3–4%).||9.4||560|
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 . 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).
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 modeling 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 . 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.
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 . 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.
Flexural strength test
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) :
σ f = 3 E t LP ( E c t c 2 + 2 E c t c t t + E t t t 2 ) 2 w ( E c 2 t c 4 + 4 E c E t t c 3 t t + 6 E c E t t c 2 t t 2 + 4 E c E t t c t t 3 + E t 2 t t 4 )