Extended glaze improves the resistance to crack initiation and propagation of PVZ.
Glaze and extended glaze do not lead to perceptible changes in color and translucency.
The effect of extended glaze had no interaction with the cooling protocol applied.
This study evaluated the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of self-glaze firings on the mechanical (flexural strength and crack propagation) and optical (color and translucency) properties of a porcelain-veneered zirconia system.
Bilayer disc-shaped samples were prepared (Vita VM9 + In-Ceram YZ) and divided according to the final thermal treatment: glaze firing followed by slow cooling (furnace opening at 200 °C) (G-S) or fast cooling (furnace opening at 600 °C) (G-F, manufacturer-recommended protocol), extended glaze firing (15 min of dwell time) followed by slow cooling (EG-S) or fast cooling (EG-F), or no thermal treatment (CTRL). Porcelain roughness (Ra and Rz) was measured before and after glaze firings. Color (ΔE 00 ) and translucency (TP 00 ) alteration were also evaluated. Flexural strength was measured with the piston-on-three-ball test and crack propagation analysis was performed after Vickers indentations. Complementary analyzes of crystalline phase and scanning electron microscopy were carried out.
Significant effect of dwell time was observed, with extended glaze leading to higher flexural strength and shorter crack lengths. Cracks of EG groups were observed to end in clusters of crystals. Color and translucency changed below perceptibility thresholds. All treatments led to a smoother surface and EG groups reached the lowest Rz values. An extra SiO 2 peak was revealed in control and EG groups. No effect of cooling protocol was found.
Extended glaze firing was able to improve the resistance to crack initiation and propagation of porcelain-veneered zirconia without clinically perceptible changes in optical properties.
Porcelain-veneered zirconia (PVZ) system is one of the best options for anterior and posterior single or multiple unit restorations, especially when enhanced esthetics is required. It combines both the zirconia’s mechanical properties and porcelain’s optical properties [ ], which make these restorations popular among dental professionals.
High fracture rates of the porcelain veneer layer were reported in clinical studies of PVZ crowns with up to 10-year follow-up [ ] and in a systematic review [ ]. Thereby, in the last few years, laboratorial procedures have been refined by adopting new processing protocols, such as slow cooling after the last firing and anatomically corrected frameworks. Rinke et al., [ , ] clinically followed PVZ crowns and metal-ceramic fixed dental prostheses (FDPs) fabricated using slow cooling and anatomic-shaped frameworks. The authors observed no difference in survival rates between the bilayer systems (metal-ceramic and PVZ). However, they observed less veneer chipping events on the metal-free restorations when compared to findings from previous studies [ ]. Despite that, papers still report ceramic fractures as a current problem, particularly due to minor veneer chippings [ ].
Although a minor chipping does not lead to the replacement of the restoration, it still causes discomfort to patients and requires clinical intervention, such as polishing or filling. Porcelain’s susceptibility to fracture is related to multiple factors: overly thick or non-homogeneous porcelain layer thickness [ , ], inadequate framework design (non-anatomic, which leads to a lack of support to the veneer) [ ], low strength and toughness of the porcelain [ ], inadequate ceramic firing [ ], defects caused by occlusal adjustment [ ], and incompatibility between framework and veneer thermal expansion coefficients [ ]. In addition, fast cooling the restorations after the firing cycle also contributes to the development of residual thermal stresses and, consequently, increased fracture susceptibility [ , ].
Annealing procedures are capable of relaxing residual stresses in glasses [ ]. This procedure consists of maintaining the material at high constant temperatures – close to its glass transition temperature – during the time necessary to decrease its viscosity and allow molecular structural rearrangement, which results in stresses relaxation [ , ]. Previous studies investigated the effect of annealing protocols [ , ] or glaze firings [ ] on mechanical properties of glass ceramics; most of them reported an increase in strength values [ , ]. However, some of these proposed treatments seem unfeasible for dental laboratories, due to the long time required. In addition, metal oxides, responsible for the material color, may become unstable depending on the firing protocol performed [ , ].
In this context, this study aimed to evaluate the effect of dwell time (conventional or extended) and cooling protocol (fast or slow) of glaze firings (self-glaze) on the flexural strength and resistance to crack propagation of a porcelain-veneered zirconia (PVZ) system. We designed definitive tests to determine which parameter dominants these mechanical properties by characterizing microstructural and surface changes. In addition, the optical properties were also tested to evaluate the esthetics feasibility of the treatments. The hypotheses tested were that 1) the extended glaze firing would improve the resistance to crack initiation and propagation, 2) as well as a slow cooling protocol, and 3) color and translucency would not be affected by glaze firing or cooling protocols.
Materials and methods
This in vitro study evaluated two factors: dwell time (glaze or extended glaze), and cooling protocol (fast or slow). The response variables analyzed were flexural strength, crack length, roughness, color difference, and translucency. The materials used in this study are described in Table 1 .
|Material||Commercial brand||Composition a|
|Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP)||Vita In Ceram YZ (Vita Zahnfabrik, Bad Säckingen, Germany)||Zr 2 O 3 (90−95%), Y 2 O 3 (4−6%), HfO 2 (1−3%), Al 2 O 3 (0−1%), Pigments (0−1%)|
|Feldspathic ceramic||Vita VM9 (Vita Zahnfabrik, Bad Säckingen, Germany)||SiO 2 (60–64%), Al 2 O 3 (13–15%), K 2 O (7–10%), Na 2 O (4–6%), TiO 2 (<0.5%), CeO 2 (<0.5%), ZrO 2 (0–1%), CaO (1–2%), B 2 O 3 (3–5%), BaO (1–3%), SnO 2 (<0.5%), Mg, Fe, and P oxides (<0.1%)|
|Modelling liquid||Vita VM Modelling Liquid (Vita Zahnfabrik, Bad Säckingen, Germany)||Ethanol and sodium hydroxide|
Pre-sintered blocks of a 3 mol% yttria-stabilized tetragonal zirconia (3Y-TZP) ceramic (In-Ceram YZ, Vita Zahnfabrik, Bad Säckingen, Germany) with dimensions of 20 mm × 19 mm × 15.5 mm were ground to Ø 18 mm cylinders with a #400 grit SiC paper under water cooling. The cylinders were cut into 1.2 mm thick discs (n = 88) using a low speed diamond saw (Isomet 1000, Buehler, Lake Buff, USA). The surfaces to be porcelain-veneered were ground with #220 SiC paper for 20 s on each axis (x and y) by a single operator. The opposite surface was polished with a #1200 grit SiC paper to 0.9 mm thick. The zirconia discs were ultrasonically cleaned in ethanol, dried, and sintered in a high-temperature furnace (Vita Zyrcomat 6000MS, Vita Zahnfabrik, Bad Säckingen, Germany) at 1530 °C (heating rate: 200 °C/h) as suggested by the manufacturer. The after sintering dimensions of the zirconia discs were Ø 15 × 0.7 mm.
The discs were placed in a metallic mold for applying a 1.5 mm thick veneer layer. The feldspathic ceramic powder (VM9, Vita Zahnfabrik, Bad Säckingen, Germany) was mixed with the building liquid (Vita VM Modelling Liquid, Vita Zahnfabrik, Bad Säckingen, Germany) to form a slurry (1:1 proportion) that was poured into the metallic mold. The porcelain was condensed with manual vibration and the excess of liquid was removed with absorbent paper. The bilayer discs were removed from the mold and placed in a ceramic furnace (Vita Vacumat 6000MP, Vita Zahnfabrik, Bad Säckingen, Germany) for porcelain sintering, according to manufacturer’s instructions: pre-heating to 500 °C per 6 min, heating up to 910 °C at 55 °C /min, vacuum during 7 min, cooling down to 800 °C with furnace closed, and to 600 °C with 25% of the furnace opened before totally open the furnace. Three firings were performed to all samples in order to compensate the porcelain shrinkage. After that, the porcelain surface was polished with #400, #600, and #1200 grit SiC papers under water cooling. The final thickness was 1.4 mm (porcelain veneer: 0.7 ± 0.02 mm, zirconia: 0.7 ± 0.02 mm).
After all polishing procedures, the samples were ultrasonically cleaned with ethanol, dried and randomly divided into four groups ( n = 22) using a numeric sequence generated by the website random.org. Two groups were subjected to a glaze firing followed by fast or slow cooling, and two groups were subjected to an extended glaze firing followed by fast or slow cooling. One group (control) was not subjected to any firing after polishing. The four firing cycles for self-glaze are described in Table 2 . The samples were removed from the firing base immediately after the furnace opening.
|Group||Thermal treatment||Initial temperature (ºC)||Temperature increase rate (ºC/min)||Final temperature (ºC)||Heating bath dwell time (min)||Cooling|
|EG-F||Extended glaze firing||500||80||900||15||Fast a|
|G-F c||Glaze firing||500||80||900||1||Fast a|
|CTRL||No extra firing cycle|
Color difference and translucency measurements
The samples were analyzed with a spectrophotometer (SP-60, X-Rite, Grand Rapids, USA) using the CIE L*a*b* color system ( Commission Internationale de l’Éclairage ). Each sample was measured three times over white ( L* = 93.07, a* = −1.28, b* = 5.25) (LENETA Card – model 12H – Cor&Aparência, São Paulo, Brazil), black ( L* = 27.94, a* = −0.01, b* = 0,03) (LENETA Card – model 12H – Cor&Aparência, São Paulo, Brazil), and gray (CIE L* = 50.30, a* = −1.41, b* = −2.37) (Mennon gray cards, Mennon photographic and technical Co., Beijing, China) backgrounds, before and after glazing, to obtain the CIE L*a*b* values ( L* = lightness axis, a* = green-red axis, and b* = blue-yellow axis). The average of the three measurements of each sample over each background was used for the calculations. A drop of a coupling agent (glycerol C 3 H 8 O 3 , refractive index 1.47) (Vetec Química Fina Ltda, Rio de Janeiro, Brazil) was used between the sample and the background to minimize light scattering [ ] during the measurements.
The values obtained from gray background, measured before and after glazing, were used to calculate the samples’ color difference (ΔE 00 ) with the CIEDE2000 formula (Eq. 1). The perceptibility (ΔE 00 > 0.81) and unacceptability (ΔE 00 > 1.80) thresholds described by Paravina et al., [ ] were considered for clinical inference.
where ΔL′, ΔC′, and ΔH′ are the differences in lightness, chroma, and hue, respectively, for a pair of measurements (before and after thermal treatments). The rotation function R T accounts for the interaction between chroma and hue differences in the blue region. Weighting functions S L , S H , and S C adjust the total color difference for variation in the location of the color difference pair in L′, a′, b′ coordinates. The parametric factors K L , K C , and K H are correction terms for deviation from reference experimental conditions. In this study, these parametric factors of the CIEDE2000 formula were set as 1.
The translucency parameter (TP 00 ) was also calculated with the CIEDE2000 formula (Eq. 1). However, the pair of measurements used were the CIE L*a*b* parameters obtained from each sample over the white and black backgrounds, separately before and after the glaze firings. We considered the findings obtained by Salas et al., [ ] as clinical inference: a difference between two TP 00 values (before and after glaze – ΔTP 00 ) is perceptible when it reaches 0.62, and clinically unacceptable when it reaches 2.62.
The average roughness (Ra) and ten-point-mean roughness (Rz) were evaluated before and after the glaze firings with a SJ-410 roughness tester (Mitutoyo, Takatsu-Ku, Japan). The tests were performed with a 0.80 mm cutoff, 0.0001 μm resolution (8 μm range), 0.5 mm/s speed, and total length of 4 mm, according to ISO 4287/1997 [ ]. Three measurements of the x and y axes were performed for each sample, and the average was used in the statistical analysis.
Two samples of each experimental group were subjected to X-ray diffraction analysis (XRD) to characterize its crystalline structure. The analyzes were carried out in an X-ray diffractometer (D8 Advance XRD, Bruker AXS GmbH, Germany) using a wavelength of 1.5416 Å (CuK α ), scan range of 20° to 35°, 0.01° step size, and 0.35 s per step. Peaks were identified from the values available for powder diffraction patterns (International Centre for Diffraction Data/Joint Committee for Powder Diffraction Studies).
In addition, representative samples of glazed groups (n = 2) had the porcelain layer etched with 4.5% hydrofluoric acid for 10 s and were analyzed in a Scanning Electron Microscope (SEM) (EVO 50, Carl Zeiss, Gottingen, Germany) using secondary electrons to evaluate surface microstructural differences.
Flexural strength analysis
The resistance to crack initiation was studied through a flexural strength analysis. The piston-on-three-ball test was carried out in a universal testing machine (DL-1000 Emic, Brazil), according to ISO 6872/2015 [ ]. The porcelain layer was placed over the three steel spheres (2.5 mm of diameter, 120° apart forming a 10 mm diameter circle) and the load was applied on the zirconia layer (perpendicular to the center) at a 0.5 mm/min rate using a flat cylindrical steel piston (1.4 mm diameter). A polyethylene strip was placed between the supporting balls and the samples, and between the sample and the piston to uniformly distribute the contact stress [ ]. The compressive load was applied until complete fracture of the sample. The flexural strength (MPa) was obtained using the Eqs. (2) , (3) , (4) , and (5) [ ]. The fractured surfaces of representative samples were analyzed in Scanning Electron Microscope (SEM) to determine the failure mode based on the fracture origin through fractographic principles.