Effect of glazing application side and mechanical cycling on the biaxial flexural strength and Weibull characteristics of a Y-TZP ceramic

Abstract Glaze application on monolithic zirconia (Y-TZP) can be a practical approach to improve the mechanical properties of this material. Objective Our study evaluated the effect of glazing side and mechanical cycling on the biaxial flexure strength (BFS) of a Y-TZP. Methodology Eighty sintered Y-TZP discs (Ø:12 mm; thickness: 1.2 mm - ISO 6872) were produced and randomly assigned into eight groups (n=10), according to the factors “glazing side” (control – no glazing; GT – glaze on tensile side; GC – glaze on compression side; GTC – glaze on both sides) and “mechanical aging” (non-aged and aged, A – mechanical cycling: 1.2×106, 84 N, 3 Hz, under water at 37°C). Specimens were subjected to BFS test (1 mm/min; 1,000 Kgf load cell) and fractured surfaces were analyzed by stereomicroscopy and SEM. Hsueh’s rigorous solutions were used to estimate the stress at failure of glazed specimens. Two-way ANOVA, Tukey’s test (5%), and Weibull analysis were performed. Results The “glazing side”, “mechanical aging” and the interaction of the factors were significant (p<0.05). Groups GC (1157.9±146.9 MPa), GT (1156.1±195.3 MPa), GTC (986.0±187.4 MPa) and GTC-A (1131.9±128.9 MPa) presented higher BFS than control groups (Tukey, 5%). Hsueh’s rigorous solutions showed that the maximum tensile stress was presented in the bottom of zirconia layer, at the zirconia/glaze interface. Weibull characteristic strength (σo) of the GC was higher than all groups (p<0.05), except to GT, GTC-A and GTC, which were similar among them. The fractography showed initiation of failures from zirconia the tensile side regardless of the side of glaze application and fatigue. Conclusion Glazing zirconia applied on both tensile and compression sides improves the flexural strength of Y-TZP, regardless the mechanical aging.


Introduction
Recent studies have shown that zirconium oxide ceramics present higher mechanical properties, 1,2 biocompatibility and low bacterial adhesion characteristics. 3,4 However, considering that the conventional zirconia, first generation of yttriastabilized tetragonal zirconia polycrystals (Y-TZP), has very low translucence, it requires a glass ceramic veneering application, which can favor the chipping and fracture of veneering ceramic, 5 mainly due to the low thermal conductivity of zirconia. 6 Chipping is one of the most predominant failures in bilayer zirconia restorations. 5,7,8 Therefore, the use of monolithic (translucent) zirconia restorations have increased, [9][10][11] on which surfaces only polishing 12 or glazing 13,14 have been recommended. Glazing is preferred because it prevents surface damages that may lead to phase transformation from tetragonal to monoclinic and, eventually, to low temperature degradation. 15 The amount of glaze is negligible compared to that of glass ceramic on a bilayer zirconia restoration.
Thus, the residual stresses at the interfaces is probably also very lower. So far, there is evidence that the application of a glass layer with low elastic modulus to zirconia promotes better stress distribution, because the maximum tensile stress is directed to the highmodulus zirconia, 16,17 improving the mechanical properties of the material. 18 Glazing has also been applied in the internal surfaces of zirconia crowns to improve bonding to resins. 19,20 Furthermore, considering that one of the main areas subjected to tensile stress is the internal surface of the crowns, 21 glaze application in this area can decrease tensile stress of the zirconia. 16   to the factor "glazing side" (4 levels) and "mechanical aging" (2 levels).

Glazing application
The zirconia discs were ultrasonically cleaned in isopropyl alcohol 9% for 5 min and dried in an oil-free air stream at room temperature. The low-fusing glaze ceramic (Vita Akzent, Vita Zahnfabrik, Bad Säckingen, Germany) was applied to the zirconia discs, according to the following groups: -Control: no glazing.
-Glaze on tensile side (GT): a low-fusing glaze ceramic layer was applied to one side of the zirconia discs.
-Glaze on compression side (GC): a low-fusing glaze ceramic layer was applied to one side of the specimen.
-Glaze on tensile and compression sides (GTC): a low-fusing glaze ceramic layer was applied on both sides of the discs.

Mechanical aging in water
Half of the specimens were stored in water at 37°C for 24 h and the other half were subjected to 1.2 ×  The fractured surfaces showed that the glaze and zirconia layers were bonded without discontinuities.
Porosity was observed in the glaze layer. However, fracture origins were observed at the lowest zirconia surface, where surface flaws were typically present.

Discussion
In an attempt to increase zirconia strength with a procedure that can be easily achieved in a prosthesis laboratory, the zirconia surface was glazed on the tensile and/or compression side to simulate the application of glaze on the inner and occlusal surface, respectively. The first hypothesis, which states that the glaze on tensile and/or compression increase BFS of Y-TZP was partially accepted, since only glaze applied on both side showed higher BFS than control groups for non-aged and aged conditions. Glaze on compression side and tensile side separately increased the BFS of Y-TZP only in non-aged condition. In fact, the glazing of the porcelain tends to create superficial compressive stresses, 24 which may be similar to those in zirconiaglazed specimens, justifying the high strength values across the glaze layers. This is particularly important because there was no chipping or delamination of the glaze, which is commonly seen in veneered specimens as a result of thermal stresses caused by coefficient of thermal expansion mismatch and zirconia's poor thermal conductivity. 25 Higher BFS of most glazed groups is probably due to more favorable distribution of stresses 23 throughout the tough zirconia layer, as described for graded zirconia. 16 In this case, the reduced modulus in the near-surface regions caused the transfer of the majority of stresses to the inner core material, which is stiffer, 26 with hardly any in the glaze layer, despite the pores found therein. In our study, the analysis of the stresses across the glaze and zirconia layers for the glazed groups showed that the maximum tensile stress was presented at the bottom surface of the zirconia (tensile side), considering the zirconia/glaze interface. In general, the glaze layer survived cyclic fatigue loading, and fracture occurred in the bottom surface of the zirconia layer, as demonstrated by the fracture origins, probably because it is much stiffer than the glaze layer. 23 Few information about the effect of glazing side were found in the literature. 27 Hjerppe, et al. 27 (2010) reported that the application of glaze on tensile side decreased the BFS, whereas glaze on compression side was similar to control (no glazing), in non-aged condition. These results differ from our study, since none of the glazed groups showed BFS lower than control. The authors discussed that the residual stress caused by the cooling rate of the glaze that  values. 16 The current study performed this analysis with a sample size smaller than the conventional

Clinical relevance
Glazing of tensile and compression areas seems to be a promising approach to improve zirconia ceramic mechanical properties and long-term.

Conflicts of interest
The authors report no conflicts of interest Authors' contributions