Statement of problem
Fully stabilized monolithic zirconia (FSZ) has been developed as an alternative to zirconia veneered with porcelain. However, how sintering conditions might affect its microstructure and optical and mechanical properties is unclear.
The purpose of this in vitro study was to determine the effect of different sintering temperatures on the microstructure and optical and mechanical properties of FSZ.
Material and methods
Bar-shaped FSZ specimens were prepared and divided into 2 groups (n=15) according to final sintering temperatures (1450 °C and 1600 °C). The average reflectance, opacity, translucency parameter, and sum of light absorption-scattering values were obtained by using a spectrophotometer, and ΔE 00 was calculated. The 3-point bend test was performed in a universal testing machine. Scanning electron microscopy (SEM) was conducted for microstructure analysis. Crystalline phase quantification was obtained by X-ray diffraction (XRD). Data were analyzed by using D'Agostino-Pearson and Student t tests (α=.05).
A significant difference was detected in the reflectance and sum of light absorption-scattering values between the 2 groups. The translucency parameter, opacity, and flexural strength showed no statistical differences. ΔE 00 was 0.98. XRD indicated cubic (47.41% for 1450 °C; 46.04% for 1600 °C) and tetragonal content (52.59% for 1450 °C; 53.96% for 1600 °C). No monoclinic content was found. SEM images showed more definite grain boundaries in the 1600-°C group. Mean grain size was 0.49 μm for the 1450-°C group and 1.99 μm for the 1600-°C group.
Higher sintering temperatures increased the grain size but did not change the crystal phase concentration. A significant difference was found in the reflectance and sum of light absorption-scattering, but no differences were found among the translucency parameter, opacity, or flexural strength.
Processing fully stabilized monolithic zirconia with an increased final sintering that does not reduce its mechanical properties may improve the esthetics of the material.
The pursuit of esthetic excellence in fixed dental prostheses (FDPs) has been guided by the evolution of dental ceramics. An ideal ceramic should cost low and have biocompatibility, optimal mechanical properties, and esthetics that replicate those of natural teeth. Yttria-stabilized tetragonal zirconia polycrystal (Y-TZP) is a widely used ceramic biomaterial for FDPs because of its excellent mechanical performance. However, owing to its high opacity, conventional zirconia was veneered with feldspathic porcelain, although chipping has been a problem with these FDPs.
To solve the problem of chipping, more translucent zirconias were developed for use in anatomic contour prostheses. The opacity and translucence of zirconia are influenced by size, crystal isotropy, and thickness. When light reaches a solid such as zirconia, after initial reflection and some absorption, it can be lost through refraction and scattering (diffuse reflection), through birefringence, and through pores and impurities in the material. Y-TZP is partially stabilized in the tetragonal phase at an ambient temperature by metal oxides (especially yttria). Tetragonal crystals improve the mechanical resistance of Y-TZP because of phase transformation toughening when a tetragonal to monoclinic transformation is induced mechanically around a crack. However, tetragonal crystals are optically anisotropic and reduce its translucency because of differences in the refractive index (birefringence). According to the Rayleigh scattering model described by Apetz and van Bruggen, when light rays are incident on a birefringent material, the smaller the particle dispersed for a given wavelength, the larger the light transmission at the grain boundaries. Tetragonal grain sizes range from 0.2 μm to 0.8 μm, which is greater than the wavelength range of visible light (400 nm to 700 nm). Therefore, the use of nanometric tetragonal crystals should minimize the birefringence effect and improve light transmission.
In contrast, FSZ exhibits higher translucence with increasing yttria concentration, which stabilizes higher cubic phase content. Cubic grains have an isotropic orientation, which means that there is less interference with light transmission among the grains. Another advantage is that cubic grains are larger than tetragonal grains, and this reduces the number of grain boundaries, which are sources of light scattering. However, increased cubic phase may reduce mechanical properties as the transformation-toughening effect is diminished because cubic grains do not undergo the tetragonal to monoclinic phase transformation.
The sintering temperature of zirconia affects the ceramic's mean grain size, which results in a change in its optical properties. Furthermore, pore diameter, microstructure, mechanical properties, and low-temperature degradation behavior could be affected by the sintering conditions.
When it was first introduced, the zirconia used in this study had a final manufacturer-recommended sintering temperature of 1600 °C. However, the sintering protocol has since changed to a final sintering temperature of 1450 °C. The purpose of this study was to determine the effect of different sintering temperatures on the microstructure and optical and mechanical properties of a fully stabilized zirconia. The null hypothesis was that the microstructure, flexural strength, and optical properties would not be affected by changes in sintering temperature.
Material and methods
Bars (25×5×2.1 mm) were cut from presintered fully stabilized zirconia blocks (Prettau Anterior; Zirkonzahn) with a diamond wheel (Diamond Cup Grinding Wheel; Buehler) and ground by using abrasive papers (P1200, P1500, and P2500; Imperial Wetordry; 3M) to a thickness of 1.5 mm. According to the manufacturer information, the composition of the zirconia was 8% to 12% Y 2 O 3 , 0% to 1% Al 2 O 3 , 0.02% SiO 2 , 0.01% Fe 2 O 3 , and 0.04% Na 2 O. The bars were divided into 2 groups (n=15) by simple randomization and sintered at 2 different temperatures: 1450 °C and 1600 °C. The sintering protocol was started at room temperature, with an 8 °C per minute heating rate up to the maximum temperature (1450 °C or 1600 °C). After a 2-hour step time, the temperature was decreased to room temperature at an 8 °C per minute cooling rate. After sintering, the bars had dimensions of approximately 20×4×1.2 mm ±0.01 mm, according to the ISO6872: 2015 standard.
To assess the optical properties, spectral reflectance was measured at wavelengths of 400 nm to 740 nm at 10-nm intervals by using a computerized spectrophotometer (CM 2600d; Konica Minolta Sensing Inc). The conditions included ambient room temperature (24 ±3 °C), standard primary illuminant D65 (daylight, 6504 K), UV secondary illuminant, observer at 2 degrees, and the Commission Internationale de l'Eclairage L*a*b* (CIELab) system. Measurements were made at the center of the specimens, limiting the surface area to a diameter of 3 mm by means of a black mask (CM-A147; Konica Minolta Sensing Inc) coupled to the spectrophotometer optical port. The instrument was calibrated with a white plate (CM-A145; Konica Minolta Sensing Inc). The reflectance data acquired were processed by means of a computer program (Spectra-Magic version 3.61; Konica Minolta Sensing Inc) that provided lightness (L*) and chromaticity (a* and b*) values. The optical properties of average reflectance, opacity, translucency parameter, and absorption-scattering sum of light were calculated according to the equations of Shiraishi et al :
Average reflectance: average of reflectance percentage in each wavelength.
Opacity percentage was calculated by using the following equation:
, where R bl is the average reflectance at each wavelength over black—CIE L*=25.6976, a*=-0.09, and b*=-0.81—and R wh is the average reflectance at each wavelength over white backgrounds—CIE L*=99.45, a*=-0.11, and b*=-0.13.
The translucency parameter was obtained by using the following equation:
, where bl is measurements against a black background—CIE L*=25.6976, a*=-0.09, and b*=-0.81—and wh is measurements against a white background—CIE L*=99.45, a*=-0.11, and b*=-0.13.
The percentage of absorption-scattering sum of light (S/A) was obtained by subtracting the sum of scattered/absorbed light in each wavelength divided by 100. This parameter was automatically calculated by the spectrophotometer program with the data collection.
Additionally, the color difference calculated according to CIEDE2000 (ΔE 00 ) is based on the equation described by Ghinea et al :
, where ΔL, ΔC, and ΔH represent the differences in lightness, chroma, and hue in CIEDE2000; RT is a function that accounts for the interaction between chroma and hue differences in the blue region. S L , S C , and S H are weighting functions that adjust the total color difference for variation in the location of color difference in L′, a′, b′ coordinates. K L , K C , and K H are parametric factors to correct terms for experimental conditions. In the present study, the parametric factors of the CIEDE2000 color difference formula were set to 1.
X-ray diffraction analysis was performed (DRX-Rint 2000; Rigaku) (n=1) to identify the crystalline phases present after each sintering protocol. Scanning conditions were as follows: Cu-Kα radiation (40 kV, 70 mA) and scan range from 20 degrees to 90 degrees (2θ) with a step size of 0.02 degrees for 3 seconds. In addition, the percentage crystalline content was analyzed by the Rietveld refinements.
Scanning electron microscopy was conducted (MIRA3; TESCAN) to observe microstructural changes and characterize the surface. The average grain size was calculated directly from the SEM images according to the linear intercept method based on at least 90 grains by using a computer software program (Motic Images Plus 2.0 ML; Motic). The specimens were given a gold coating (40 seconds, 18.7 mA) to improve electron scattering.
The 3-point flexural strength test was conducted according to the ISO 6872: 2015 standard. Fifteen specimens were tested. The test was conducted under dry conditions and performed in a universal testing machine (MTS 810; Materials Testing System) at a crosshead speed of 1 mm/min until failure. Flexural strength was calculated from the following equation: σ=3Nl/(2bd2), where σ is the flexural strength, N is fracture load, l is distance between supports (mm), b is width of the specimen (mm), and d is thickness of the specimen (mm).
Optical properties and the 3-point flexural strength data were tested for distribution (D'Agostino & Pearson Omnibus, α=.05) and showed a normal distribution. For comparison between groups, an unpaired Student t test was used (α=.05).