To determine the effects of different aging methods on the degradation and flexural strength of yttria-stabilized tetragonal zirconia (Y-TZP)
Sixty disc-shaped specimens ( ∅ , 12 mm; thickness, 1.6 mm) of zirconia (Vita InCeram 2000 YZ Cubes, VITA Zahnfabrik) were prepared (ISO 6872) and randomly divided into five groups, according to the aging procedures ( n = 10): (C) control; (M) mechanical cycling (15,000,000 cycles/3.8 Hz/200 N); (T) thermal cycling (6,000 cycles/5–55 °C/30 s); (TM) thermomechanical cycling (1,200,000 cycles/3.8 Hz/200 N with temperature range from 5 °C to 55 °C for 60 s each); (AUT) 12 h in autoclave at 134 °C/2 bars; and (STO) storage in distilled water (37 °C/400 days). After the aging procedures, the monoclinic phase percentages were evaluated by X-ray diffraction (XRD), and topographic surface analysis was performed by 3D profilometry. The specimens were then subjected to biaxial flexure testing (1 mm/min, load 100 kgf, in water). The biaxial flexural strength data (MPa) were analyzed by 1-way ANOVA and Tukey’s test ( α = 0.05). The data for monoclinic phase percentage and profilometry (Ra) were analyzed by Kruskal–Wallis and Dunn’s tests.
ANOVA revealed that flexural strength was affected by the aging procedures ( p = 0.002). The M (781.6 MPa) and TM (771.3 MPa) groups presented lower values of flexural strength than did C (955 MPa), AUT (955.8 MPa), T (960.8 MPa) and STO (910.4 MPa). The monoclinic phase percentage was significantly higher only for STO (12.22%) and AUT (29.97%) when compared with that of the control group (Kruskal–Wallis test, p = 0.004). In addition, the surface roughnesses were similar among the groups ( p = 0.165).
Water storage for 400 days and autoclave aging procedures induced higher phase transformation from tetragonal to monoclinic; however, they did not affect the flexural strength of Y-TZP ceramic, which decreased only after mechanical and thermomechanical cycling.
The quest for esthetics in dentistry has led to the search for metal-free restorative materials, such as ceramics . The use of stabilized zirconium oxide (ZrO 2 ) as an esthetic material in ceramic restorations has emerged as meeting the needs of most resistant prostheses. Zirconium oxide is a polymorphic crystal found in three crystallographic forms: monoclinic, tetragonal, and cubic. The addition of 3% mol of yttrium oxide to zirconium oxide improves its molecular stability, allowing for the production of zirconia stabilized by yttria (Y-TZP). Thus, zirconium oxide is kept in its tetragonal phase at room temperature and has improved mechanical properties .
Improvement of the mechanical behavior of Y-TZP is attributed to the following: when a fracture in this material is initiated, the grains of tetragonal-phase zirconia are transformed in monoclinic phase at the crack tip. In this process an increase of grains volume caused by the phase transformation induces to compressive stress difficulting the fracture propagation .
However, problems with the degradation of zirconia containing yttria as a stabilizer have been reported in femoral prostheses . Tetragonal-to-monoclinic transformation was observed on the ceramic surfaces maintained at a temperature of 100–500 °C in contact with water or body fluids at 37 °C, and this is called low-temperature degradation (LTD). Degradation of Y-TZP is a process dependent on the time of exposure , and the tetragonal-to-monoclinic transformation is accompanied by a 4% volume increase of zirconia grains under the surface in contact with moisture . The pressure generated in the grains around the monoclinic zirconia results in stress in this area, inducing phase transformation in the surrounding grains. Monoclinic grain detachment occurs, creating a roughened surface with microcracks . The uncontrolled transformation at the surface is accompanied by decreased mechanical performance .
Studies on the degradation of dental zirconia have been conducted to determine whether degradation has an impact on the survival of this material . It is known that stress can affect the tetragonal-to-monoclinic transformation. When tetragonal zirconia undergoes tension, the stress is minimized through the increase in its dimensions when it becomes monoclinic. This expansion reduces the initially applied tractional tension . This stress can be caused by laboratory tests, with cyclic loading in water, which significantly reduces the mechanical properties of the material and can then affect their long-term performance . In contrast, according to Borchers et al. , the decreased load-support capacity after thermal and mechanical cycling and after long-term storage in water at 37 °C did not appear to be caused by degradation of zirconia, perhaps because the transformation zone did not extend to a sufficient depth; thus, additional studies on this topic are needed.
Y-TZP has been the material of choice for infrastructure in metal-free restorations. Thus, better understanding is needed of aging mechanisms and their relationship with Y-TZP strength and microstructure. The effects of zirconia aging, specifically in the oral environment, and its relation to phase transformation from tetragonal to monoclinic have not yet been elucidated.
Since zirconia restorations are exposed to wet environments with cyclic loading in the oral cavity, an evaluation of this material under similar conditions is essential to its successful use in dentistry . Therefore, the aim of this study was to evaluate the influence of aging procedures on the topographic surface, structural stability, and flexural strength of a ZrO 2 -based dental ceramic. The hypotheses investigated were that: (1) the aging procedures would decrease the flexural strength of the ceramic, (2) the monoclinic phase percentage would be higher for all experimental groups, and (3) the aging procedures would increase the roughness values for ZrO 2 ceramic.
Materials and methods
Preparation of specimens
Zirconia pre-sintered blocks (Vita InCeram 2000 YZ Cubes, VITA Zahnfabrik, Bad Säckingen, Germany) were ground and cut under irrigation with a diamond cutting disc (Extec, Enfield, CT, USA) in a precision cutter (Isomet ® 1000 Precision Saw, Buehler, Lake Bluff, IL, USA). Sixty disc-shaped zirconia specimens were obtained, with initial dimensions of 15 mm diameter and 2 mm thickness. Before sintering, the discs were polished with 400-, 600-, and 1200-grit sandpaper under water cooling for 5 min each. They were then immersed in an isopropyl alcohol ultrasonic bath for 15 min and sintered in a Zyrcomat T furnace (VITA Zahnfabrik) at a temperature of 1530 °C for 120 min. The disc final dimensions were 12 mm diameter and 1.6 mm thickness.
The discs were randomly divided into 6 groups ( n = 10), according to the factor “aging procedure”:
C: Control – no aging treatment was performed.
M: mechanical cycling – a load of 200 N was applied with a 3.8 Hz frequency for 15,000,000 cycles in 37 °C distilled water.
T: thermal cycling – 5 °C/55 °C water thermal variation (30 s each) for 6,000 cycles.
TM: thermomechanical cycling – a load of 200 N was applied with a 4 Hz frequency for 1,200,000 cycles with water thermal variation (5 °C/55 °C, 60 s each).
AUT: autoclave – 12 h in an autoclave at 134 °C and 2 bars.
STO: storage in 37 °C distilled water for 400 days.
For mechanical and thermomechanical cycling, the specimens were subjected to mechanical cycling (ER-11000, Technical and Scientific Equipment, ERIOS Ltda, São Paulo, Brazil) with a load of 200 N at a frequency of 3.8 Hz with a sphere-shaped piston made with stainless steel (1.6 mm curvature radius). Throughout all mechanical and thermomechanical procedures, the specimens were immersed in water and the temperature was controlled by a thermostat contained in the mechanical cycling device. The discs were placed in a metal device whose upper face contained three balls ( Ø = 3.2 mm), fixed equidistant between their centers forming a plane (ISO 6872). The load was applied at the center of the sample.
Relative amount of transformed monoclinic zirconia (FM)
Before and after aging procedures, crystallographic characterization by X-ray diffraction (XRD) analysis was performed. The determination of phase transformation percentage (tetragonal to monoclinic) induced by aging was evaluated by X-ray diffractometry (Model X’pert Powder, PANalytical, Almelo, Netherlands), Cu-Kα, λ = 1, 54060 Åm, 45 kV, and 40 mA, 20 and 80°, angular pitch of 0.02, integration time of 10 s. In mechanically aged groups (M and TM), the analysis was performed on the load application surface.
The relative amount of transformed monoclinic zirconia (%) on the treated surfaces was determined from the integral intensities of the monoclinic (−111)M and (111)M, and the tetragonal (101)T peaks obtained after XRD, according to the method described by Toraya et al. and with the equations (A and B) below:
F M = 1.311 × X M 1 + 0.311 × X M
X M = − 111 M + 111 M − 111 M + 111 M + 101 T
where: (−111) M , 2 θ = 28 ο ; (111) M , 2 θ = 31.20 ο ; and (101) T , 2 θ = 30 ο represent the integrated intensities of the peaks diffracted in the monoclinic planes (−111) M and (111) M and in the tetragonal plane (101) T . Two measurements were carried out on each sample to obtain the mean values of FM.
For topographic analysis and to obtain the roughness values (Ra – arithmetic mean) of the ceramic surfaces after different methods of aging, we analyzed two samples from each group using a digital optical profiler (Wyko NT 1100, Veeco, Plainview, NY, USA), in five areas per sample. Wyko Vision 32 software (Veeco) was used for data analysis. The 3D measurement parameters were performed with 200× magnification in an area of 301.3 × 229.2 μm.
Biaxial flexural strength test
The samples were subjected to flexure test (1 mm/min, 100-kgf load, in water) in a testing machine (Emic DL-1000, Emic, São José dos Pinhais, PR, Brazil) until fracture occurred.
Using Eqs. (1–3) , according to the guidelines of ISO 6872, we calculated the biaxial flexural strength (MPa):
S = − 0.2387 P ( X − Y ) d 2
where S is the maximum tensile stress in Pascals, P is the total load causing fracture in Newtons, and d is the specimen thickness at the origin of the fracture, in millimeters. X and Y were determined as follows:
X = 1 + v ln r 2 r 3 2 + 1 − v 2 r 2 r 3 2
Y = ( 1 + v ) 1 + ln r 1 r 3 2 + ( 1 − v ) r 1 r 3 2