Effects of porcelain thickness on the flexural strength and crack propagation in a bilayered zirconia system

Abstract Objective: This study evaluated the influence of porcelain (VM9, VITA Zahnfabrik, Germany) thickness on the flexural strength and crack propagation in bilayered zirconia systems (YZ, VITA Zahnfabrik, Germany). Material and Methods: Thirty zirconia bars (20.0x4.0x1.0 mm) and six zirconia blocks (12.0x7.5x1.2 mm) were prepared and veneered with porcelain with different thickness: 1 mm, 2 mm, or 3 mm. The bars of each experimental group (n=10) were subjected to four-point flexural strength testing. In each ceramic block, a Vickers indentation was created under a load of 10 kgf for 10 seconds, for the propagation of cracks. Results: The results of flexural strength were evaluated by One-way ANOVA and Tukey's test, with a significance level of 5%. The factor “thickness of the porcelain” was statistically significant (p=0.001) and the l-mm group presented the highest values of flexural strength. The cracks were predominant among the bending specimens with 1 and 2 mm of porcelain, and catastrophic failures were found in 50% of 3-mm-thick porcelain. After the indentation of blocks, the most severe defects were observed in blocks with 3-mm-thick porcelain. Conclusion: The smallest (1 mm) thickness of porcelain on the zirconia infrastructure presented higher values of flexural strength. Better resistance to defect propagation was observed near the porcelain/ zirconia interface for all groups. Higher flexural strength was found for a thinner porcelain layer in a bilayered zirconia system. The damage caused by a Vickers indentation near and far the interface with the zirconia shows that the stress profiles are different.


Introduction
Ceramic materials are increasingly used in dental restorations because of the excellent combination of esthetic, biological, and mechanical properties, such as wear resistance and rigidity 20 , making them a material of choice for oral rehabilitation 11,12 . Dental ceramics can be used for framework, e.g., yttrium-oxide partially and fracture toughness, but limited translucency 20,26 .
Excellent esthetic properties are achieved by applying a veneering ceramic (feldspathic glass or porcelain) to the framework 23 .
The most common clinical complications associated with the infrastructure of zirconia restorations are loss of retention, the need for endodontic treatment, veneering ceramic fractures and bleeding on probing 12 .
Mechanical complications such as porcelain fracture tend to be more prevalent in veneered zirconia crowns than in those with metal coping 18,24 . Clinical reasons for the failure of these ceramic restorations include mechanical stress for high occlusal loads, which causes crack propagation and later results in "chipping" 20,28 and location in the dental arch 21 . From a biomechanical materials perspective, residual stresses in compression and tension generated along the porcelain layer play a critical role in the failure of porcelain veneered zirconia crowns 27 .
behavior of ceramic crowns and when combined with functional stresses may lead to restoration failure 17 .
Among the factors that control the amount of residual stresses are the geometry of the infrastructure 6,8,16,19 , the thickness of the ceramic overlay and framework 16 , and the cooling rate at temperatures above the glass transition temperature 10,15 . The nature of stresses, either compressive or tensile, also affects ceramics strength, the former increases strength and the latter increases crack propagation under occlusal loadings. This variation in thickness is due to the dental crown complex shape, which has regions, such as cusps and axial walls built up with varying porcelain thicknesses, promoting different stress magnitudes 27 . Few studies different thicknesses of porcelain-zirconia 25 . Therefore, it is necessary to know the effects of mechanics and crack propagation at different thicknesses of porcelainzirconia infrastructure performance.
The methodologies used to analyze the cracks in ceramics include microtomography 19 , transillumination 3 , and Scanning Electron Microscopy (SEM). However, the literature does not report crack behavior of ceramic bilayer system with regard to its depth, volume, and the extent of dependence on porcelain thickness. This propagation of a bilayered system by means of confocal microscopy analysis. The hypotheses to be tested were differences in the propagation of defects (indicative of residual stresses), depending on veneering ceramic thicknesses.

Material and Methods
The materials used in this study are shown in  were obtained after a sintering process in a Zyrcomat furnace (Vita Zahnfabrik, Bad Sackingen, Germany).
Specimens were cleaned in an ultrasonic bath (Vitasonic, Vita Zahnfabrik, Bad Sackingen, Germany) for 5 min in 10% isopropyl alcohol to remove any residue from previous steps and dried at room temperature.
They were then randomized into experimental groups, with 10 bars and 2 blocks for each group. The groups followed the porcelain thickness, accordingly: Group 1: Bars and blocks with 1 mm of zirconia and 1 mm of porcelain.

Analysis by stereomicroscopy
The failures of the bar-shaped specimens were observed with a stereo microscope (Discovery z-20,       Table 3-Mean value of depth, extension, and volume close to and far from zirconia    Table 3. Propagation of defects showed higher depth, extension, and volume close to zirconia, while all these parameters were lower when far from zirconia, except for depth in the 1 mm group. Figures 5 and 6 show defects generated in the block specimens.

Discussion
Investigating the failures in porcelain-zirconia infrastructures has become a major focus of research in the science of dental materials. In particular, understanding the role of residual stresses in these failures remains a challenge for researchers. In the present study, we showed that thin porcelain layers indentation (defect) progression did not depend on porcelain thickness, but was more severe near the zirconia substrate than on the porcelain surface.
These results agree with Lima, et al. 15  In the present study, we were not able to quantify spatially residual stresses in the porcelain layers using Vickers indentations, although this has been done before with the same method on curved surfaces 1 .

Conclusions
The thinnest (1 mm) porcelain/zirconia specimens close to the porcelain/zirconia interface grew faster than defects close to the porcelain surface for all thicknesses, suggesting that the occurrence of residual tensile stresses is higher at this site.