Lowering Al 2 O 3 amount, increasing Y 2 O 3 content and adding 0.2 mol% La 2 O 3 were all effective to improve the translucency of 3Y-TZP ceramics.
A reduced alumina content however significantly lowered the aging stability.
Increasing Y 2 O 3 content had the best effect to enhance the translucency and aging stability, but sacrificing the toughness and strength.
Adding 0.2 mol% La 2 O 3 provided a promising combination of translucency, aging stability and mechanical properties.
The aim was to evaluate the optical properties, mechanical properties and aging stability of yttria-stabilized zirconia with different compositions, highlighting the influence of the alumina addition, Y 2 O 3 content and La 2 O 3 doping on the translucency.
Five different Y-TZP zirconia powders (3 commercially available and 2 experimentally modified) were sintered under the same conditions and characterized by X-ray diffraction with Rietveld analysis and scanning electron microscopy (SEM). Translucency (n = 6/group) was measured with a color meter, allowing to calculate the translucency parameter (TP) and the contrast ratio (CR). Mechanical properties were appraised with four-point bending strength (n = 10), single edge V-notched beam (SEVNB) fracture toughness (n = 8) and Vickers hardness (n = 10). The aging stability was evaluated by measuring the tetragonal to monoclinic transformation (n = 3) after accelerated hydrothermal aging in steam at 134 °C, and the transformation curves were fitted by the Mehl–Avrami–Johnson (MAJ) equation. Data were analyzed by one-way ANOVA, followed by Tukey’s HSD test ( α = 0.05).
Lowering the alumina content below 0.25 wt.% avoided the formation of alumina particles and therefore increased the translucency of 3Y-TZP ceramics, but the hydrothermal aging stability was reduced. A higher yttria content (5 mol%) introduced about 50% cubic zirconia phase and gave rise to the most translucent and aging-resistant Y-TZP ceramics, but the fracture toughness and strength were considerably sacrificed. 0.2 mol% La 2 O 3 doping of 3Y-TZP tailored the grain boundary chemistry and significantly improved the aging resistance and translucency. Although the translucency improvement by La 2 O 3 doping was less effective than for introducing a substantial amount of cubic zirconia, this strategy was able to maintain the mechanical properties of typical 3Y-TZP ceramics.
Three different approaches were compared to improve the translucency of 3Y-TZP ceramics.
Although porcelain-fused-to-metal (PFM) systems have represented the “gold standard” for many years to produce fixed dental prostheses (FDP) in restorative dentistry, the gray and opaque metal framework does not allow them to mimic the translucent dental tissues . All-ceramic restorations are becoming increasingly popular due to their aesthetic appearance, chemical inertness and biocompatibility . However, most dental ceramics are brittle, limiting their use to small anterior restorations . The excellent mechanical properties of zirconia ceramics, as the result of an inherent transformation toughening mechanism , has made them particularly attractive for fabricating diverse dental restorations . In particular, 3 mol% yttria-stabilized tetragonal zirconia polycrystalline (3Y-TZP) ceramics can be fully densified with fine-grained microstructure to combine a high wear resistance, fracture toughness (5–9 MPa m 1/2 ) and extraordinary bending strength (>1000 MPa) . Furthermore, 3Y-TZP is white, allows some light transmission and can be rather easily colored by adding trace amounts of rare-earth elements . Therefore, nowadays, 3Y-TZP has been widely accepted as a promising material for fabricating dental crowns and more importantly for larger all-ceramic FPDs in both anterior and stress-bearing posterior area .
The limited translucency however is a major drawback of 3Y-TZP restorations. 3Y-TZP was initially considered as an opaque material and zirconia-based dental restorations are obtained by porcelain veneering a zirconia core. Recently, different grades of 3Y-TZP showed some degree of light transmittance , but the translucency of zirconia-based ceramics is still lower than that of glass-ceramics, for which excellent aesthetic results are documented . The translucency of the framework, however, directly influences the appearance of dental restorations . In order to avoid the veneer chipping problem, monolithic zirconia using highly-translucent 3Y-TZP is the latest trend as a promising alternative to porcelain-fused-to-zirconia restorations . A higher translucency of ceramic frameworks can also improve the curing efficiency of light-cured cements .
Another potential risk of Y-TZP is low-temperature degradation, in which the tetragonal zirconia ( t -ZrO 2 ) phase spontaneously transforms to the monoclinic ( m -ZrO 2 ) phase in the presence of water or water vapor (hydrothermal aging) without applied stress . The consequences of hydrothermal aging are surface roughening, enhanced wear rates, loss of mechanical properties and even catastrophic failure .
Therefore, it is of utmost importance to improve the translucency of new dental restorative 3Y-TZP ceramics while ensuring and extending their aging stability without compromising the attractive mechanical properties. In principle, Y-TZP have the potential to gain optical translucency or even transparency, and the loss of optical transparency is mainly due to the light scattering by porosity, secondary phases, and grains with different crystallographic orientation (birefringence at the grain boundary) . It is known that a slight variation in the zirconia composition and minute differences in the microstructure can cause a considerable difference in properties. Different dental manufacturers eliminated/reduced the alumina addition or increased the yttria content to improve the translucency of dental restorative 3Y-TZP ceramics . Recently, we reported that the co-doping of 0.2 mol% La 2 O 3 and 0.1–0.25 wt.% Al 2 O 3 in 3Y-TZP resulted in a high translucency by manipulating the chemistry of the grain boundaries . In this work, different commercially available and experimental starting powder compositions were compared, aiming to seek the most desirable combination of translucency, hydrothermal stability and mechanical properties of the sintered 3Y-TZP.
Materials and methods
Five zirconia compositions were investigated. Three commercially available zirconia powders (TZ-3YE, Zpex ® and Zpex ® Smile (Tosoh, Japan)) and two lab-made zirconia powders were used. TZ-3YE contained 3 mol% (5.2 wt.%) Y 2 O 3 and 0.25 wt.% Al 2 O 3 , which was a widely used composition for dental restorations, and the resulting ceramic was referred to as 3Y-0.25Al throughout the text. Zpex ® and Zpex ® Smile were claimed to have a higher translucency by reducing alumina content and increased yttria content respectively. Zpex ® contained 3 mol% (5.2 wt.%) Y 2 O 3 and 0.05 wt.% Al 2 O 3 , referred to as 3Y-0.05Al. Zpex ® Smile contained 5 mol% (9.35 wt.%) Y 2 O 3 and 0.05 wt.% Al 2 O 3 , encoded as 5Y-0.05Al. The compositions of the two lab-made zirconia powders were 3 mol% (5.2 wt.%) Y 2 O 3 , 0.25 or 0.1 wt.% Al 2 O 3 and 0.2 mol% La 2 O 3 , further referred to as 3Y-0.25Al-0.2La and 3Y-0.1Al-0.2La. These two powders were prepared by mixing tetragonal zirconia powder (grade TZ-3Y, Tosoh, Japan) with La 2 O 3 (Chempur, purity of 99.99%) and Al 2 O 3 (TM-DAR, purity of 99.99%) on a multidirectional mixer (Turbula type T2C, Basel, Switzerland) for 24 h in ethanol using 5 mm Y-TZP milling balls, and on a bead mill (Dispermat SL, Germany) for 3 h at 5000 rpm using 1 mm ZrO 2 beads (grade TZ-3Y, Tosoh, Japan), followed by drying and sieving through a 250 μm screen. In order to avoid any influence of the Y 2 O 3 distribution, the lab-made powders were based on co-precipitated alumina-free 3Y-TZP powder which was also used as a reference material for the translucency measurement. More information on the preparation of these powders was provided elsewhere .
The X-ray diffraction (XRD) patterns of all starting powders are compared in Fig. S.1 of the Supplementary material. All starting powders contained a fraction of monoclinic phase zirconia. The starting powders of 3Y-0.25Al, 3Y-0.1Al-0.2La and 3Y-0.25Al-0.2La were similar, containing mainly tetragonal phase zirconia. The 3Y-0.05Al starting powder contained a higher amount of monoclinic zirconia. Diffraction peaks corresponding to cubic zirconia were clearly observed in the starting powder of 5Y-0.05Al. All powders were cold isostatically pressed at 250 MPa and pressureless sintered in air at 1500 °C for 2 h. The density of the sintered ceramics was measured according to the Archimedes principle in ethanol.
Phase identification of the sintered ceramics was done by X-ray diffraction (XRD, 3003-TT, Seifert, Ahrensburg, Germany), using Cu-Kα radiation at 40 kV and 40 mA from 20–90° (2 θ ) with a step size of 0.01 for 3 s. Rietveld analysis of the XRD pattern was performed with Topas academic software (BRUKER AXS, Karlsruhe, Germany). The phase structures were refined as: tetragonal zirconia (t) unit cell with space group P42/nmcZ, monoclinic zirconia (m) unit cell with space group P21/c, cubic zirconia (c) with space group Fm-3m. The quality of the Rietveld refinement was controlled with a low R value of <10%, and at least two specimens of each grade were checked for reproducibility. The Y 2 O 3 content in the tetragonal ZrO 2 phase was calculated based on the a and c unit cell parameters of the tetragonal ZrO 2 phase, according to :
Scanning electron microscopy (SEM, XL-30FEG, FEI, Eindhoven, The Netherlands) was used to characterize the microstructure on polished thermally etched (1250 °C for 25 min in air) and Pt-coated surfaces. The grain size was measured on SEM micrographs by counting at least 1000 grains, using IMAGE-PRO software according to the linear intercept method. The average results (±standard deviation) and the grain size distribution were reported without any correction.
The four-point bending strength was measured on 45 × 4 × 3 mm 3 test bars (n = 10/each grade). Bending bars were prepared according to ISO 13356 and ISO 6872 but without 45° edge chamfer, and loaded to failure at a crosshead speed of 1 mm/min to avoid subcritical crack growth during loading. The fracture surfaces obtained by four-point bending testing were examined by SEM to determine the mode of failure.
The hardness was measured by the indentation method using a Vickers microhardness tester (Model FV-700, Future-Tech Corp., Tokyo, Japan) with a load of 10 kg (n = 10/each grade) for 10 s.
The fracture toughness was measured by the single edge V-notch beam (SEVNB) method. 45 × 4 × 3 mm 3 bars (n = 8/each grade) were prepared according to ISO 6872 . The bending bars were notched (approximately 0.5 mm deep) with a diamond sawing blade with a thickness of ∼400 μm. The notch tip was sharpened by reciprocating sliding a razor blade in the notch while applying a 1 μm diamond paste. The notch tip of each specimen was examined by optical microscopy. A notch root radius <10 μm was obtained, as shown in Fig. S.2 of the Supplementary materials. The specimens with sharpened notch were loaded in a four point-bending test jig at a crosshead speed of 0.5 mm/min in air.
Disk-shaped zirconia specimens (n = 6/each grade, ∅ ≈ 15 mm) were prepared for translucency measurements. The discs were ground plan parallel to a thickness of about 0.5 mm and polished with 1 μm diamond paste. The final thickness was controlled during the polishing process with a digital micrometer (accuracy of 0.001 mm). After polishing, the surface roughness was measured with a surface profiler (Talysurf-120L, Taylor Hobson) to ensure an identical roughness level for all ceramic grades.
A spectrophotometer (SpectroShade™ MICRO, MHT Optic Research, Niederhasli, Switzerland) with a calibration plate was used to record the CIELAB coordinates (L*, a* and b*) of the ceramic discs. A thin layer of vaseline was put in-between the specimen and the background for better optical contact.
The translucency parameter (TP) was determined by calculating the color difference between the same specimen against black and white backgrounds, according to :
TP = ( L B * − L W * ) 2 + ( a B * − a W * ) 2 + ( b B * − b W * ) 2
where the subscripts B and W refer to the color coordinates over black and white backgrounds, respectively. A higher TP value indicates a higher translucency.
The contrast ratio, CR, was calculated from the spectral reflectance of light of the specimen (Y) over a black (Y b ) and white (Y w ) background, using the following equation:
CR = Y b Y w
The CR is 0.0 for a transparent material and 1.0 for a totally opaque material. The CR value linearly correlates with the TP value .
In vitro accelerated hydrothermal experiments were used to age the ceramics. The green CIPed disk-shaped zirconia specimens (∼3 mm thick) were smoothened with 4000 grade SiC grinding paper on both surfaces before sintering. The ceramics were aged in the as-sintered state, without additional surface treatment.
Specimens (n = 3/each grade) were autoclaved at 134 °C and 0.2 MPa in water vapor. The amount of tetragonal to monoclinic phase transformation was determined by X-ray diffraction (XRD, 3003-TT, Seifert, Ahrensburg, Germany) using Cu-Kα radiation at 40 kV and 40 mA. XRD patterns were recorded on both surfaces in the 27–33° (2 θ ) range in the θ –2 θ mode with a scan speed of 2 s/step and a scan size of 0.02° (the X-ray penetration depth was calculated to be 7.5 μm (Cu-Kα with 98% absorption)). The monoclinic phase content ( V m ) was calculated according to the formula of Garvie et al. and Toraya et al. , and the monoclinic phase content was plotted as a function of aging time. For each curve, at least 3 specimens (6 exposed surfaces) were tested and the average result was reported. The aging kinetics were obtained by fitting the transformation curves with the Mehl–Avrami–Johnson (MAJ) equation :
f = V m − V min V max − V min = 1 − exp ( − ( b t ) n )