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

Effect of sintering temperature on aging resistance and mechanical properties of 3Y-TZP dental ceramicsZHANG Jingchao1,2, LIAO Yunmao1,2, LI Wei1,2,*, JIANG Li1,2, ZHAO Yongqi1,2, YUNXiaofei1,2(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 degradation1 IntroductionAll-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 calledlow-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 veryslow 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 of3Y-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 Experiments2.1 Preparation of 3Y-TZP specimensThe fabrication of bulk ceramics was established with a commercially availablebiomedical-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 treatmentArtificial 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 CharacterizationThe 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 (D T = 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 of15°–70°(2θ), with a step width of 0.02°and 3 s of exposure time per position. The volume fraction of the monoclinic phase on the surface of the specimens, X m, was calculated using the following equation [16]:where (111)mI ，_(111)m I and (111)t I were the integrated intensity from the monoclinic (111),monoclinic _(111) and the tetragonal (111) peaks, respectively. The predicted microcracks resulted from phase transformation in experimental group were detected by atomic forcemicroscope (AFM) (Model CH-3, Shimadzu Corporation, Kyoto, Japan).2.4 Mechanical testMechanical 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 Discussion3.1 CharacterizationTable 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 at1450℃. 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 (℃)1350 1400 1450 1500 Relative density (%)95.95 98.96 99.64 99.07 Average grain size (nm) 198.4 248.3 295.3 423.1 As shown in Fig.1, XRD analysis revealed monophase t-ZrO 2 in all as-sintered specimens regardless of sintering temperature. After treatment, both t-ZrO 2 and transformation-produced m-ZrO 2 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-ZrO 2 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 Y 3+（0.089nm ）is larger than that of Zr 4+（0.079nm ）, yttrium diffusion undoubtedly lead to the cell volume reduction of t-ZrO 2, which was confirmed in this study.__(111)+(111)=(111)+(111)+(111)m mm m m tI I X I I I (1)Fig.1 XRD spectrum of 3Y-TZP specimens sintered at the temperature range 1350℃-1500℃. (a) Control group；(b) Experimental group。