To evaluate the effect of hydrothermal aging (H), mechanical cycling (M), and the combination of hydrothermal plus mechanical cycling (H + M) on biaxial flexural strength (BFS) and microstructure of two monolithic zirconias, indicated for anterior (AMZ) or posterior restorations (PMZ) and a conventional zirconia (IZr).
Disc specimens of each material (n = 12) were submitted to: i) H (8 h in autoclave at 134 °C); ii) M (10 6 cycles, at 40% of BFS); and iii) H + M. BFS was measured (ISO-6872) and Weibull modulus (m) and the characteristic strength (σ 0 ) were calculated. crystalline phase composition analyzed by XRD, and grain size measured by MEV analysis.
XRD analysis showed AMZ was not susceptible to monoclinic transformation in any treatment. Conventional zirconia (IZr) and PMZ had monoclinic transformation only after H and H + M. BFS of AMZ was lower than PMZ and IZr. Cubic phase was found in all conditions for AMZ and IZr, while it was identified in PMZ only after H and H + M. BFS of AMZ was affected by M and H + M. For IZr and PMZ the unique difference detected in BFS was in the comparison of H to M. H treatment induced lower Weibull modulus, but characteristic strength was compatible with the BFS results. AMZ grain size (μm 2 ) was 8.6 times larger than PMZ grains, and 13.6 times larger than IZr grains.
AMZ showed the largest mean grain size, had the lowest BFS values, and was affected when mechanical cycling was involved. Monoclinic transformation was not found in any treatment for AMZ, but was found in IZr and PMZ when hydrothermal aging was used alone or when combined with mechanical cycling. PMZ showed similar behavior to the IZr. H induced to higher fracture probability.
Translucent monolithic dental zirconia available on the market may behave differently under simulated oral aging. The relationship between composition and microstructure determines their properties presumably, and clinical performance.
Yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) have been widely used in dentistry for crowns and partial fixed prostheses. Given its high opacity, conventional zirconia demands veneering with feldspathic porcelain, but of significant concern is the relatively high rates of delamination or chipping of porcelain . In order to solve this problem, efforts have been made to develop a monolithic Y-TZP (also known as full-contour Y-TZP) with improved optical properties, which would dispense with the need for porcelain veneers . Nevertheless, monolithic Y-TZP is exposed directly to moisture, body temperature, and mechanical load from chewing, which supposedly makes it more susceptible to degradation.
Optimal mechanical performance of Y-TZP is due to a mechanism that enhances its fracture strength, known as phase transformation toughening . This mechanism, described by Garvie et al. , is triggered by superficial stress around microcracks, which induce crystalline phase transformation of metastable Y-TZP grains adjacent to the defect from the tetragonal-to-monoclinic crystal system. A volumetric expansion of the involved grains results from phase transformation, which induces compressive stress on the advancing crack, preventing or slowing down crack propagation . Nevertheless, tetragonal-to-monoclinic phase transformation can also occur spontaneously over time due to body temperature and humidity , described as low-temperature degradation (LTD). LTD is a thermodynamic and time-dependent phenomenon that can be triggered by water molecules at a wide temperature range (37–500 °C) by an energy barrier breaking after diffusion of water into the zirconia lattice . In addition to in-bulk stress augmentation originating from LTD , other major drawbacks might occur, such as the worsening of optical properties and an increase of wear rates as a consequence of expanded grains uplifting on the surface.
Monolithic zirconia development is addressed by diminishing the opacity of Y-TZP by means of different strategies to eliminate or decrease light-scattering sources such as sintering additives (typically alumina) , reduction of oxygen vacancies, pores and defects, controlled sintering environment (i.e. pressure and temperature) , and use of specific dopants . Taking into account that tetragonal zirconia crystal is optically anisotropic, refraction occurs in different crystallographic directions. Thus, the resultant birefringence may be contoured by the development of specific grain sizes . Translucency is dependent on the number of grain boundaries. Y-TZP with small grain size means a greater grain-boundary area, and consequent lower translucency. However, larger tetragonal grains are not desirable because their metastability is weaker, and consequently increases LTD susceptibility . Usually, Y-TZP has grain size in the range of 0.2–0.8 μm, which gives some translucency but only up to 1 mm thickness. On the other hand, it is possible to produce transparent to translucent Y-TZP using a nano-scale grain size (under 100 nm), which is markedly smaller than the wavelength range of visible light (400–700 nm) . Thus, a ray of visible light penetrating the material may not be reflected or refracted but transmitted . As result, the birefringent characteristic of Y-TZP given by the anisotropic nature of zirconia tetragonal crystals orientation is minimized.
Another strategy is the use of fully stabilized zirconia (FSZ), which has improved translucency due to the increase in cubic phase concentration. In contrast to tetragonal grain, scattering is reduced because cubic crystals have an isotropic refractive index. Cubic phase stabilization is achieved by adding yttria in concentrations higher than 8 mol%, while in partially stabilized zirconia (PSZ), tetragonal phase is stabilized with 2–5 mol% yttria content . However, a reduction in mechanical resistance is expected, once it has lost the toughening effect resultant from the tetragonal-to-monoclinic phase transformation.
Thus, different methods of producing translucent Y-TZP are obtained by opposite modifications in composition and microstructure. Monolithic zirconia is relatively recent, and there is little evidence based on clinical assays or in vitro long-term degradation studies, which would show how this different translucent ceramic would behave over time under LTD and mechanical challenges. Hence, the null hypothesis of this study is that the flexural strength and microstructure of two monolithic zirconias (for posterior teeth and for anterior teeth) are not affected by the following accelerated aging methods: mechanical cycling; simulation of LTD by hydrothermal aging; and the combination of mechanical cycling plus hydrothermal aging.
Materials and methods
The composition of each material is detailed in Table 1 . A monolithic zirconia for anterior restorations (AMZ) and one for posterior restorations (PMZ) were analyzed, a zirconia for infrastructure (IZr) was included as a control group.
|Material||Code||Composition a||Fabricant/batch number|
|High translucency anterior monolithic zirconia||AMZ||<12% Y 2 O 3 1% Al 2 O 3 , max. 0.02% SiO 2 ,
max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O
|Prettau Anterior, Zirkonzahn, Bruneck, Italy/ZB4221B|
|High translucency posterior monolithic zirconia||PMZ||4–6% Y 2 O 3 , <1% Al2O3, max. 0.02% SiO2,
max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O
|Prettau, Zirkonzahn, Bruneck, Italy/ZB3208A|
|Zirconium infrastructure ceramic (control)||IZr||4–6% Y 2 O 3 , <1% Al 2 O 3 , max. 0.02% SiO 2 ,
max. 0.01% Fe 2 O 3 , max. 0.04% Na 2 O
|ICE Zirkon, Zirkonzahn, Bruneck, Italy/ZB4131B|
Commercially available blocks were machined into cylindrical bars, which were sectioned (IsoMet 1000, Buehler, IL, USA/Extec High Concentration, Extec Enfield, CT, USA) under water cooling to obtain disc specimens. Prior to sintering process, specimens were smoothed on both sides using silicon carbide grinding paper (FEPA no. 4000 Struers LaboPol 21, Struers, Rodovre, Denmark) for 30 s using an automatic polisher (Aropol-2 V, Arotec, SP, Brazil) under distilled water cooling, at a rotational speed of 300 rotations per min and pressure of 0.98 N/cm 2 . The specimens were ultrasonically cleaned in distilled water for 15 min and dried for 2 h at 37 °C before sintering in the furnace, InFire HTC speed (Sirona, Salzburg, Austria). The heating and cooling rates were 8 °C/min and the materials were maintained at maximum temperatures for 2 h (anterior monolithic zirconia (AMZ) and posterior monolithic zirconia (PMZ) at 1600 °C, and IZr at 1500 °C). After sintering, the specimens’ final dimension (12.0 diameter and 1.2 mm thickness) was accomplished to ISO 6872:2015. Additionally, the specimens were polished using monocrystalline water-based diamond suspension (1 μm for 3 min, and 0.25 μm for 4 min at speed 300 RPM, 0.98 N/cm 2 load − MetaDi, Buehler, IL, USA) in a polisher (Aropol-2 V, Arotec).
Accelerated aging methods
Specimens were artificially aged according to the following treatment groups: I) hydrothermal degradation (H); II) mechanical cyclic load (M); III) mechanical cyclic plus hydrothermal degradation (H + M); IV) non-treated specimens (control group). Hydrothermal degradation was carried-out in an autoclave at 134 °C for 8 h under 0.2 MPa of pressure (AB-25, Phoenix Luferco, SP, Brazil) . Mechanical cyclic loading was applied to the specimens in 10 6 cycles of biaxial bending on a piston and three balls set-up (following ISO 6872:2015 configuration), using a universal cycling machine (Biocycle, Biopdi, SP, Brazil) under distillated water immersion at 37 °C. The cycling load was set at 40% of the mean static resistance, previously obtained from static biaxial flexure test of non-treated materials for each material (n = 12). Cycling load was set to 40% to ensure survival of the specimens. For AMZ the load was set at 350 N; for PMZ 450 N, and for IZr 420 N. Load was applied through stainless steel spherical indenters (8 mm diameter) at 125-ms intervals (4 Hz frequency), simultaneously in 10 specimens. During the cycling process, small pieces (10.0 mm diameter) of transparency acetate film for printing (transparency film CG5000, 3M, MN, USA), were lodged between the specimen surface and the indenter to avoid superficial crack induction. These acetate films were completely flexible to allow a complete force transmission.
Effect of aging processes on biaxial flexure resistance
Biaxial flexure tests were conducted according to ISO 6872:2015 (n = 12) in a universal test machine (DL, Emic, PR, Brazil), at a cross head speed of 1 mm/min until failure occurred. The load to failure was registered in N and then calculated in MPa according to the following equation (ISO: 6872:2015):
S = − 0.2387 P ( X − Y ) d 2
In which S represents the maximum tensile stress in MPa, P is the total load to fracture in Newtons and d is the specimen thickness (mm). X and Y were obtained as follows:
X = ( 1 + v ) I n ( r 2 r 3 ) 2 + [ ( 1 − v ) 2 ] ( r 2 r 3 ) 2 Y = ( 1 + v ) [ 1 + I n ( r 2 r 3 ) 2 ] + ( 1 − v ) ( r 1 r 3 ) 2