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Effect of sintering temperature on aging resistance and mechanical properties of 3Y-TZP dental


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Effect of sintering temperature on aging resistance and mechanical properties of 3Y-TZP dental ceramics
ZHANG Jingchao1,2, LIAO Yunmao1,2, LI Wei1,2,*, JIANG Li1,2, ZHAO Yongqi1,2, YUN Xiaofei1,2
(1. West China College of Stomatology, Sichuan University, Chengdu, 610041, China; 2. State Key Laboratory of Oral Diseases, Sichuan University, Chengdu, 610041, China)

Abstract:In order to investigate the effect of sintering temperature on aging properties and mechanical properties of 3Y-TZP dental ceramics in a simulated oral environment, 3Y-TZP nanopowder compacts were prepared by pressureless sintering at 1350℃, 1400℃, 1450℃, 1500℃, respectively, then treated by soaking in artificial saliva (65℃,pH=7) for two months. The treated specimens sintered at 1350℃showed no phase transformation but significantly improved strength and toughness(P<0.05), while those sintered at 1400℃-1500℃ revealed a small amount of phase transformation and insignificant mechanical reinforcement (P>0.05). No micro-cracks were detected but increase in lattice volume was found in all specimens. Lowering sintering temperature favors aging resistance and mechanical reinforcement of 3Y-TZP in simulated oral environment. Key words:3Y-TZP; dental ceramic; sintering temperature; simulated oral environment; low temperature degradation

1 Introduction
All-ceramic dental restorations have become more and more important due to their favorable esthetics and outstanding biological compatibility when compared with metal and resin restorations. In particular, 3mol% yttria-stabilized tetragonal zirconia polycrystal (3Y-TZP) attracts the most attention due to exceptionally high strength and toughness, which provides the base for applications in multi-unit fixed partial dentures and dental implant abutments. The superior mechanical properties of 3Y-TZP are related to the stress-induced phase transformation of tetragonal zirconia into monoclinic symmetry (t-ZrO2→m-ZrO2) accompanied by an volume expansion of ~4% that opposes the crack propagation, which is called transformation toughening [1] . Unfortunately, Y-TZPs are susceptible to various environment such as humid air, water vapor and other fluids over a temperature range of 65℃-500℃. This phenomena, is called low-temperature degradation (LTD), which is related to the metastability of the t-ZrO2 at low temperature and is responsible for the formation of micro-cracks and deterioration of mechanical properties [2,3]. There are many mechanisms that have been put forward to explain LTD of Y-TZP ceramics. It is worth mentioning the mechanism proposed by Lange et al. and confirmed later by Li et al. using X-ray photoelectron spectroscopy (XPS) analysis [4,5]. These authors proposed that there was a reaction between water molecules and Y2O3 at the surface of Y-TZP resulting in the formation of small crystallites of Y(OH)3 of nanometric size (20–50nm). Thus, a tetragonal surface grain is destabilized and transforms into the monoclinic structure when water vapor draws out a sufficient amount of yttrium. After this, growth of monoclinic sites can continue spontaneously without further yttrium diffusion. Oral cavity is a moist environment in which temperature varies frequently with the food-intake. Although the above mechanism is usually very

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slow at oral temperatures, it may lead to a significant decrease in the strength of dental restorations over periods of several decades [6]. Thus, determining the influence of LTD to mechanical properties in simulated oral environment is prerequisite to predicting the sustainable success of Y-TZP-based restorations in dentistry. Though LTD of densely-sintered 3Y-TZP is well documented in the literature [7], the LTD of 3Y-TZP with compromised density and its effect on mechanical properties are still not yet well known. In fact, most 3Y-TZP restorations in dental clinics are not densely sintered due to limitations in time, technique, equipment, etc during fabrication process. Moreover, prefabricated 3Y-TZP bulks manufactured by different companies show different degree of densification. In this study, 3Y-TZP sintered at 1350℃, 1400℃, 1450℃, 1500℃, respectively, were exposed to simulated oral environment (artificial saliva, 65℃,pH=7)in vitro for 2 months. The phase composition, microstructure and mechanical properties of the treated specimens were examined.

2 Experiments
2.1 Preparation of 3Y-TZP specimens The fabrication of bulk ceramics was established with a commercially available biomedical-grade 3Y-TZP nanopowder (TZ-3YB-E, Tosoh, Japan) containing 3mol% yttria, 0.25wt% alumina and 3wt% organic binder. The powder was dual phase including 23wt% monoclinic phase and 77wt% tetragonal phase. Green compacts were fabricated by mold-pressing under 80MPa following cold isostatic pressing under 200MPa. Four sets of bar compacts (n=40 in each set) were randomly grouped then sintered in air at 1350℃, 1400℃, 1450℃ and 1500℃, respectively, for 1.5 h at a heating rate of 5℃/min, in order to obtain 3Y-TZP specimens with different microstructural features (grain size and relative density). All sintered specimens were ground with sand paper in a stepwise process. Finally, the standard specimens with final dimensions of 18mm× 4mm×2mm were polished with a diamond paste of 1μm particles, then washed and annealed at 920℃ for 1 h. 2.2 Aging treatment Artificial saliva was prepared according to the recipe in ISO/TR 10271 [8]. The pH of the solution was adjusted to 7 with 0.5 mol/L NaOH and 30% acetic acid. The specimens sintered at each temperature were randomly divided into 2 groups of 20 each and subjected to different process: (a) control group, the specimens were analyzed without any treatment; (b) experimental group, the specimens were subjected to aging treatment by soaking in artificial saliva at 65℃ for 2months, then washed in deionized water, dried and analyzed. 2.3 Characterization The bulk density of the specimens in control group was measured by the Archimedes method in distilled water at 20℃ at standard atmospheric pressure(ρwater=0.998g/cm3), and the relative density was determined correlating the bulk density with theoretical density (DT = 6.1 g/cm3). Average grain size of the specimens was measured by the planimetric method using scanning electron microscopy (SEM) (Model S-4500, Hitachi, Tokyo, Japan). The phase composition of all specimens was determined using X-ray diffraction (XRD) (Philips X’Pert System, Almelo, Overijssel, Netherlands) using Cu-Kα radiation in the range of 15° (2θ), with a step width of 0.02° 3 s of exposure time per position. The volume fraction –70° and of the monoclinic phase on the surface of the specimens, X m, was calculated using the following equation [16]:

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_

Xm=

I (1 11) m + I (1 1 1) m
_

I (1 11) m + I (1 1 1) m + I (1 1 1) t

(1)

_

where I (1 1 1) m , I (1 11) m and
_

I (1 1 1) t

were the integrated intensity from the monoclinic (1 1 1) , peaks, respectively. The predicted microcracks

monoclinic (1 1 1 ) and the tetragonal

(1 1 1)

resulted from phase transformation in experimental group were detected by atomic force microscope (AFM) (Model CH-3, Shimadzu Corporation, Kyoto, Japan). 2.4 Mechanical test Mechanical test was carried out using a universal testing machine (Fast Track 8874, Instron, UK) at room temperature. The bending strength of all specimens (10 for each subgroup) was measured by three-point bending with a span of 15 mm and a crosshead rate of 0.5 mm/min. Their fracture toughness (10 specimens for each subgroup) was measured by the single-edge-notched-beam method (SENB) with a notch thickness of 0.2 mm, a span of 15 mm, and crosshead rate of 0.05 mm/min. The data were evaluated by statistical analysis (SPSS 11.0) using One-way ANOVA and LSD test to compare the strength and toughness values among the groups. P<0.05 was considered as significant.

3 Results and Discussion
3.1 Characterization Table 1 summarizes the results of relative density and average grain size of 3Y-TZP specimens in control group. Below 1450℃, the density increased gradually with the sintering temperature rising and almost fully dense specimens with a relative density of 99.64% were obtained at 1450℃. Afterwards, remarkable decrease in density was revealed in specimens sintered at 1500℃, indicating rapid grain growth and recrystallization at high temperature.
Table 1 The relative density and average grain size of as-sintered 3Y-TZP against sintering temperature Sintering temperature (℃) Relative density (%) Average grain size (nm) 1350 95.95 198.4 1400 98.96 248.3 1450 99.64 295.3 1500 99.07 423.1

As shown in Fig.1, XRD analysis revealed monophase t-ZrO2 in all as-sintered specimens regardless of sintering temperature. After treatment, both t-ZrO2 and transformation-produced m-ZrO2 were found in all specimens except those sintered at 1350℃, suggesting a critical grain size larger than the 198.4nm in simulated oral environment. Table 2 presented the quantitative results of the degree of phase transformation. With sintering temperature rising, the amount of monoclinic phase increased and the most intensive phase transformation was found in specimens sintered at 1500℃ due to the largest grain size. Besides, the cell volume of tetragonal grains in each subgroup decreased after treatment regardless of sintering temperature as shown in Table 3. According to the research by Lange et. al., whether t-ZrO2 transforms to monoclinic structure or not, grain boundary segregation of yttrium ions (yttrium diffusion) is inevitable for Y-TZPs exposed to aqueous solution [4]. 3Y-TZP is substitutional solutes and the ionic radii of Y3+ (0.089nm)is larger than that of Zr4+(0.079nm), yttrium diffusion undoubtedly lead to the cell volume reduction of t-ZrO2, which was confirmed in this study.

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Fig.1 XRD spectrum of 3Y-TZP specimens sintered at the temperature range 1350℃-1500℃. (a) Control group; (b) Experimental group。▲tetragonal zirconia,▼monoclinic zirconia Table 2 The amount of monoclinic phase on the surface of 3Y-TZP specimens after aging process Sintering temperature (℃) Xm (Vol%) 1350 0 1400 8.41 1450 10.27 1500 15.35

Table 3 Lattice parameters of tetragonal zirconia in 3Y-TZP specimens before and after aging treatment Sintering temperature (℃) Control group Cell parameter (a=b) (A° ) Cell parameter (c) (A° ) Cell volume (a2· c) Experimental group Cell parameter (a=b) (A° ) Cell parameter (c) (A° ) Cell volume (a · c)
2

1350 3.60409 5.17108 67.17 3.60107 5.16272 66.95

1400 3.59685 5.16222 66.79 3.59584 5.16257 66.75

1450 3.59519 5.16352 66.74 3.59371 5.15346 66.56

1500 3.59524 5.16542 66.77 3.59196 5.15339 66.49

No microcrack was detected by AFM observation on the transformed specimen surface, but a small amount of newly-formed particles was found anchoring in grain boundaries and residual pores as shown in Fig.2, no remarkable difference was found among specimens sintered at different temperatures. In fact, only t-ZrO2 with grain size larger than a critical value can generate microcracks when they transforms into m-ZrO2. The accumulative volume expansion in small t-ZrO2 grains is not enough to induce tensile stress up to the breaking load of the surrounding ceramic matrix. Thus, there is a critical grain size which makes the production of transformation accompanied micro-cracks possible. Tetragonal zirconia with grain size smaller than the critical value leads to simply residual stress when they are transformed into monoclinic structure. The absence of microcracks in this study indicated that the grain size of t-ZrO2 was not large enough or the degree of t→m phase transformation was too low. Both microcracks and residual stress contribute to toughness of zirconia ceramics, but the presence of microcracks degrades the

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strength of zirconia [9,10].

49.94 [nm]

0.00 500.00 nm
4-rxh

1.00 x 1.00 um

Fig.2 AFM image of the 3Y-TZP specimens (1450℃ sintered) surface after aging treatment

3.2 Mechanical test In control group, bending strength and fracture toughness of 3Y-TZP specimens showed a tendency of increase first and then descend as sintering temperature rising, which reached a maximum in 1450℃ group (1141.56± 117.73MPa,9.18± 0.54 MPa· 0.5). The strength and m toughness of specimens in 1450℃ group were significantly higher than those in the other three groups (P<0.05). After aging treatment, the specimens sintered at 1350℃ showed significant increase in both strength and toughness (P<0.05), while the strength and toughness of specimens sintered at 1400℃-1500℃ increased gently (P>0.05). For 3Y-TZP specimens sintered at the temperature range 1400℃-1500℃, the mechanical reinforcement tendency was in accordance with that of spontaneous phase transformation.

Fig.3 Mechanical properties of 3Y-TZP specimens before and after aging treatment as a function of sintering temperature.﹡P<0.05

Generally, the spontaneous t→m phase transformation and the accompanied volume expansion can generate microcracks in the transformed surface, thereby degrading the strength and surface properties of the material. In this study, however, though phase transformation was found in specimens sintered in the temperature range 1400℃-1500℃ after aging treatment, their mechanical properties were gently reinforced instead. Because their mechanical reinforcement tendency was in accordance with that of spontaneous phase transformation and the harmful microcracks was absent in the transformed specimen surface, it can be inferred that the mechanical reinforcement was related to the compressive stress (residual stress) resulted from

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t→m spontaneous phase transformation accompanied by volume expansion. Similar mechanical reinforcement of Y-TZP after aging treatment was found by Wang J. et. al. in 1989[11]. The compressive stress help reduce the stress concentration at crack tips and resist crack propagation[12,13], thus improve the mechanical strength and toughness of 3Y-TZP with a certain amount of spontaneous t→m phase transformation. As to 3Y-TZP specimens sintered at 1350℃, no spontaneous t→m phase transformation was found, but their strength and toughness of the specimens was significantly improved (~10%) after aging treatment. It’s widely known that the superior mechanical properties of 3Y-TZP are related to transformation toughening and the phase stability, strength and toughness of 3Y-TZP ceramics depend strongly on critical grain size [14]. Tetragonal zirconia grains with grain size larger than the critical value (>~0.6μm) are destabilized and prone to transform spontaneously which cause mechanical degradation. On the other hand, too small a tetragonal grain size (< ~0.2μm) would be over-stabilized and not transformable which also result in decreased mechanical properties. In this study, the average grain size of 3Y-TZP specimens sintered at 1350℃ was about 0.2μm, thus, a great amount of tetragonal grains in the specimens were over-stabilized to some extent and not transformable. This was another reason for the relatively poor mechanical properties of 3Y-TZP specimens sintered at 1350℃ in control group besides their compromised density. As mentioned above, the exposure of 3Y-TZP to simulated oral environment resulted in grain boundary segregation of yttrium ions. For over-stabilized tetragonal zirconia grains, a small amount of yttrium ions segregation helps activate the grains and promotes stress-induced phase transformation. Stress-induced phase transformation contributes to both strength and toughness [15], which explained the phenomena that both strength and toughness of 3Y-TZP specimens sintered at 1350℃ was remarkably reinforced after aging treatment.

4 Conclusions
Based on the present results, it can be concluded that (1) Lowering sintering temperature favors the aging resistance of 3Y-TZP; (2) For 3Y-TZP with transformable tetragonal zirconia grains, a small amount of spontaneous t→m phase transformation result in favorable compressive stress and improved strength and toughness. Though the strength and toughness of 3Y-TZP specimens sintered at 1350℃ increased by ~10% after aging treatment for 2 months due to grain boundaries segregation of yttrium ions, the long-term behavior of 3Y-TZP ceramic in simulated oral environment need further investigation. Acknowledgements This project was financially supported by a grant from the National High Technology Research and Development Program of China (No. 2006AA03Z440).

References
[1] Garvie RC, Hannink RH, Pascoe RT. Ceramic steel[J]? Nature, 1975, 258: 703–704. [2] Kim YS, Jung CH, Park JY. Low temperature degradation of yttria-stabilized tetragonal zirconia polycrystals under aqueous solutions[J]. J. Nucl. Mater., 1994, 209: 326-331. [3] Guo X. On the degradation of zirconia ceramics during low-temperature annealing in water or water vapor[J]. J. Physics. Chem. Solids, 1999, 60: 539-546. [4] Lange FF, Dunlop GL, Davis BL. Degradation during ageing of transformation- toughened ZrO2-Y2O3 materials at 250° C[J]. J. Am. Ceram. Soc., 1986, 69: 237-240. [5] Li JF, Watanabe R, Zhang BP, et al. X-ray Photoelectron Spectroscopy Investigation on the Low-Temperature

7 Degradation of 2 mol% Y2O3-ZrO2 Ceramics[J]. J. Am. Ceram. Soc., 1996, 79: 3109 – 3112. [6] Papanagiotou HP, Morgano SM, Giordano RA, et al. In vitro evaluation of low-temperature aging effects and finishing procedures on the flexural strength and structural stability of Y-TZP dental ceramics[J]. J. Prosthet. Dent., 2006, 96: 154-164. [7] Fukatsu K, Pezzotti G, Hayaishi Y, et al. Evaluation of Phase Stability in Zirconia Femoral Heads From Different Manufacturers After In Vitro Testing or In Vivo Retrieval[J]. The Journal of Arthroplasty, 2009, 24 (8): 1225-1230. [8] Sun J. Biological Evaluation of Biomaterials and Medical Devices[J]. CHINESE JOURNAL OF MEDICAL INSTRUMENT, 2003, 27(1):1-4. (in Chinese) [9] Rice RW. Microcracking in toughening of zirconia ceramics[J]. Ceram Eng Sci Proc, 1981, 2 (4):661-668. [10] Evans AG, Faber KT. Toughening of ceramics by circumferential micro-cracking[J]. J. Am. Ceram. Soc., 1981, 64 (7):394-399. [11] Wang J, Stevens R. Surface toughening of TZP ceramics by low temperature ageing[J]. Cer. Int., 1989, 15: 15-21. [12] Tanaka T, Isono Y, Ueda S. Influences of surface roughness and phase transformations induced by grinding on the strength of ZrO2-Y2O3[J]. Precision Engineering, 1995, 17 (2): 117-123. [13] Kosmac T, Oblak C, Jevnikar P, et al. The effect of surface grinding and sandblasting on flexural strength and reliability of Y-TZP zirconia ceramic[J]. Dent. Mater., 1999, 15 (6): 426-433. [14] Li SX, Zhu DG. Effects of the Mechanical Properties of 3Y-TZP Ceramics on Different Sintering Methods[J]. FOSHAN CERAMICS, 2007, 9:1-4. (in Chinese) [15] Gogotsi GA. Fracture behaviour of Mg-PSZ ceramics: Comparative estimates[J]. Cer. Int., 2009, 35 (7): 2735-2740.


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