Effect of Glazing on Flexural Strength of Full-Contour Zirconia

Objective The purpose of this study was to evaluate the effect of glazing on flexural strength of highly translucent zirconia materials. Materials and Methods Specimens of three brands of zirconia bars (Prettau Zirconia, Zirkonzahn; inCoris TZI, Sirona; and Zirlux FC, Pentron Ceramics) were prepared and polished according to manufacturers' instructions. Final specimen dimensions were 20 × 4 × 2 mm. The specimens from each brand were divided into 3 groups (N = 10): control, heat-treated, and glazed. Heat-treated specimens were fired without the application of the glaze material. The glaze material was applied to the glazed specimens before being fired. A three-point bending test (15 mm span) was performed in an Instron universal testing machine (ISO 6872). Data were analyzed by ANOVA and Tukey's HSD post hoc test (α = 0.05). Results Two-way ANOVA showed a significant influence of surface treatments on flexural strength of zirconia materials (P ≤ 0.05). There was no significant difference in flexural strength among the different brands of highly translucent zirconia (P ≥ 0.05). Tukey's HSD post hoc test showed that specimens in the “glazed” group had significantly lower flexural strength than the control and heat-treated groups (P ≤ 0.05). Conclusion Within the limitations of the study, external glazing decreased the flexural strength of highly translucent zirconia.


Introduction
e glazing of porcelain dental restorations is a routine procedure designed to provide esthetic and hygienic glasscoated surfaces on the nished restoration [1,2]. Applied overglaze is a low-fusing clear porcelain that is painted onto the surface of the restoration and red at a fusing temperature much lower than that of the dentin and enamel porcelain [3]. Glazing for the purpose of strengthening brittle ceramics can be considered the production of a surface layer of lower thermal expansion glass that serves many functions when cooled. It places the surface in a compressive state. e thin layer of glass also reduces the depth and width of surface aws and could theoretically strengthen the material [4]. Glazed surfaces result in less plaque accumulation. In addition, glazed porcelain can imitate the gloss and characterization of the natural tooth [5]. It decreases the exposure of the dental restoration to the oral cavity and provides the necessary smoothness [2].
However, many studies have shown that glazing does not increase exural strength [1,4,6]. It has also been shown that autoglazing does not cause a di erence in the exural strength of porcelain specimens [1,4,6]. Although glazing reduces the wear of opposing enamel, it was found that glazing causes cracks in the porcelain and thus decreases exural strength [4]. Yener et al. [7] compared the biaxial exural strength of three di erent brands of zirconia (Zirkonzahn, Cercon, and Ceramill) with and without glazing. eir results showed that glazing signi cantly decreased the exural strength for all systems. e introduction of highly translucent yttrium oxidestabilized tetragonal zirconia polycrystals (Y-TZPs) enabled the esthetics of monolithic zirconia restorations to be suciently improved. e high translucency of the Y-TZP was achievable by decreased alumina content [8], which is normally incorporated into zirconia materials to increase their stability during aging yet causes light scattering due to a refractive index di erent from that of zirconia. Reduction or elimination of alumina content in zirconia materials makes them vulnerable to problems with stability during aging [9].
Although many clinicians prefer to glaze monolithic zirconia restorations, data regarding the e ect of glazing on the mechanical properties of these materials are still lacking. e purpose of this study was to evaluate the e ect of glazing on the exural strength of highly translucent zirconia materials. e study examined the exural strength of three di erent brands of translucent zirconia with di erent surface treatments.

Materials and Methods
ree di erent brands of zirconia and two di erent glaze materials (Table 1) were tested in this study. In total, 10 rectangular specimens of zirconia for each group were prepared (dimensions, 2.5 × 5 × 25 mm) by means of a 15LC diamond-wafering blade mounted on an Isomet 2000 precision saw (Buehler, Lake Blu , Illinois, USA). e cuts were made at 800 rpm with a 300-gram load, with cooling provided by a dual-nozzle water irrigation system. Specimen dimensions were veri ed after they were sectioned by means of a Mitutoyo absolute IP-67 digital vernier caliper (Mitutoyo, Kawasaki, Japan).
Each specimen was polished and beveled (45°, 0.15 mm edge chamfer) in a Buehler Ecomet 250 grinder/polisher (Buehler, Lake Blu , Illinois, USA). e polishing was then done with a 15-micron grit diamond polishing pad at 30 rpm with water irrigation for 90 s for each side, after which the specimen was rinsed thoroughly. For beveling, the specimen was placed at 45°on a 15-micron grit diamond polishing pad at 30 rpm with water irrigation. e specimen was inspected for the presence of chamfer edge after the beveling. All specimens were air-dried at room temperature for at least 24 h to minimize the possibility of water being trapped in the zirconia structure. e sectioned bars were sintered with the parameters as in Table 2.
After being sintered, the specimens were polished again in a Buehler Ecomet 250 grinder/polisher (Buehler, Lake Blu , Illinois, USA). e polishing was accomplished with a 15-micron grit diamond polishing pad, followed by a 6 μm polycrystalline diamond suspension with a polishing pad, respectively, at 30 rpm with water irrigation (90 s for each side), and then thoroughly rinsed. e dimensions of the bars after being sintered and polished were approximately 20 × 4 × 2 mm according to ISO 6872:2015.
Specimens were divided into di erent groups according to surface treatments: (a) no treatment, (b) heat-treated, and (c) glazed.

Heat-Treated Firing.
Zirconia specimens were air-dried and placed on a ring tray. ey were then red with the glaze ring cycle but without glaze materials. e ring parameters are shown in Table 3.

2.2.
Glazing. Zirconia specimens were air-dried. e overglaze pastes were mixed with glaze liquids and applied in a thin coat on the entire surface of each zirconia bar using a ceramic brush. For Prettau Zirconia and inCoris TZI specimens, Zirkonzahn glaze paste and Zirkonzahn ICE stain liquid were used. For Zirlux FC specimens, Zirlux FC glaze paste and universal liquid were used. e glazing group specimens were then placed on a ring tray. Glaze ring was done according to manufacturers' instructions ( Table 4).
All specimens were then stored in 37°C deionized water for 24 h before testing began. Specimens were tested for exural strength on a three-point bending test conducted on an Instron 5566A universal testing machine (Instron, Norwood, Massachusetts, USA) with a 1 kN load cell at a crosshead speed of 0.5 mm/min. Each specimen was loaded to failure and failure load data was obtained. e data were then calculated into exural strength according to the following formula: where F is the failure load (force) at the fracture point (N), L is the length of the support span, b is the width, and d is the thickness. Statistical analysis was performed by means of SPSS System 23 for Windows. e means of each group were analyzed by two-way ANOVA, with exural strength as the dependent variable and the zirconia systems and surface treatments as the independent factors. P values less than 0.05 were considered statistically signi cant in all tests. Data from all zirconia brands were pooled, and multiple comparisons between and among di erent surface treatments were evaluated by Tukey's HSD test.

Results
e exural strength data obtained from the three-point bending tests are presented in Table 5, Figures 1 and 2. e results of two-way ANOVA showed the signi cant in uence of surface treatments on exural strength (P < 0.05). e Tukey HSD post hoc test showed that specimens in the glazed group had signi cantly lower exural strength than did those in the control and heat-treated groups (P < 0.05).
ere was no signi cant di erence in exural strength between specimens in the control and heat-treated groups (P > 0.05).
ere was no signi cant in uence of zirconia brands on exural strength (P > 0.05).

Discussion
Glazing after grinding is believed to increase the strength of a ceramic restoration because it decreases the depths of surface cracks [4]. However, the strengthening e ect of glazing on porcelain is not clearly understood [4,10].
In this study, for investigation of the e ect of glaze on the exural strength of three di erent highly translucent zirconia systems, Prettau Zirconia, inCoris TZI, and Zirlux FC were 2 International Journal of Dentistry used. Glazes were applied according to the manufacturer's recommendations for each system. It has been reported that 0.05 mm glaze thickness is su cient to prolong its integrity [11]. erefore, a 0.05 mm thickness of glaze was applied to each surface (a total of 0.1 mm) of bar-shaped specimens. Statistical analysis showed that glazing decreased the exural strength of the highly translucent zirconia. is nding was supported by the results from another study [7], reporting that glazing decreased the biaxial exural strength of Zirkonzahn, Cercon, and Ceramill zirconia.
We conducted this study on heat-treated groups to determine whether the glaze-ring cycle had an e ect on the exural strength of zirconia. We found that there was no signi cant reduction in the exural strength of zirconia after being red with a glazing cycle without glazing materials. e only di erence between specimens in this group and      those in the glazed group was the absence of glaze materials (a mixture of glaze powder and liquid). erefore, the strength reduction in glazed zirconia found in this study might be due to the glazing.
Residual stresses played an important role in determining the strength of ceramic materials. In cases of residual tensile stress, preexisting stress will amplify the applied cycling stress and induce cracks in that region. Compressive global residual stresses within the ceramic surface can somehow strengthen the material; however, excessive compressive residual stresses can cause lateral cracks to grow and propagate on the surface, and these will eventually cause the material to fail. According to Swain [12], residual stresses can be introduced during the ring process due to thermal expansion mismatch and tempering stresses associated with temperature gradients during cooling.
Due to the metastability of tetragonal zirconia, stressgenerating surface treatments such as grinding or sandblasting are also capable of triggering the t→m transformation with the associated volume increase [13]. In this study, we polished all zirconia bars before performing any surface treatment. erefore, we expected that there would be some t→m transformation during this procedure. e coe cient of thermal expansion (CTE) of tetragonal zirconia is approximately 10.5 × 10 −6 ·K −1 , while the CTE of monoclinic zirconia is only 7.5 × 10 −6 ·K −1 . erefore, the CTE of polished zirconia might depend on the degree of phase transformation brought about by polishing. e CTE of porcelain was also believed to be changed during ring. Since glazed materials consist of porcelain powder (∼60%), the CTE change in porcelain might occur in glazed materials during porcelain ring.
Tempering stresses associated with temperature gradients during cooling have also been reported to cause residual stress on ceramics. e poor thermal conductivity of porcelain and zirconia, which is much lower than that of metal alloys, combined with poor thermal di usivity [14], results in a high temperature di erence through the specimens, resulting in high residual tempering stresses and thermal gradients. e composition of the highly translucent zirconia materials used in this study was di erent from that of the conventional zirconia designed for coping or a xed dental prosthesis (FDP) framework. e most notable di erence is the reduction or absence of alumina, which is reported to play a signi cant role in phase stabilization. e e ect of residual stresses on the mechanical properties of highly translucent zirconia can be more than that of conventional zirconia. Crown geometry, as is commonly understood, a ects residual stress. e scenario of crowns may never be captured in its entirety with the use of simple bar models as a result of their multifaceted geometric properties.
It would be particularly useful in understanding the stress mechanic of this study. Fractographic analysis can give useful information regarding failure origin, failure stress, fracture toughness, and residual stress. Further investigations including fractographic analysis and e ect of crown geometry are recommended to complement the present study.

Conclusions
Within the limitations of the study, the following conclusions can be drawn: (1) Glazing decreased the exural strength of the highly translucent zirconia tested (P < 0.05). (2) Glaze ring without the glaze material had no e ect on the exural strength of the highly translucent zirconia tested (P < 0.05). (3) ere was no signi cant di erence in the exural strengths of di erent brands of the highly translucent zirconia tested (P > 0.05).
Disclosure e study was presented at the 43rd Annual Meeting of the AADR, Charlotte, NC, in 2014.