Yttria-stabilized Tetragonal Polycrystalline Research Articles

Comparing four different methods of measuring fracture toughness and its relationship with fractal dimension in a polycrystalline (3Y-TZP) dental ceramic.

Previous studies reported various methods of measuring fracture toughness of dental ceramics. The objectives of this study were to compare different methods and to validate fractal analysis to estimate fracture toughness for a polycrystalline dental ceramic. Bar-shaped specimens were prepared from 3 mol% yttria-stabilized tetragonal polycrystalline (3Y-TZP) ceramic. Each specimen was prepared with a series of indentations using a Vickers diamond at 150 N followed by loading in 4-point flexure until failure. Fracture toughness (Kc) was calculated using modified indentation (MI), quantitative fractography (QF), and surface crack in flexure with residual stresses (SCF-RS) techniques. We estimated fracture toughness using fractal analysis. Epoxy replicas of fracture surfaces were scanned using the atomic force microscope (AFM) followed by noise filtering. The FRACTALS software was used to determine fractal dimensional increment (D*) by the Minkowski cover algorithm. Fracture toughness was estimated using the relationship between Kc and square-root of D* for polycrystalline ceramics. Mean fracture toughness by MI, QF, and SCF-RS techniques were 4.56 ± 0.44 MPa·m1/2, 4.14 ± 0.82 MPa·m1/2, and 4.09 ± 0.93 MPa·m1/2, respectively. Mean fracture toughness estimated from D* values was 2.80 ± 0.13 MPa·m1/2. The fracture toughness of 3Y-TZP ceramic was calculated using different methods. Fractal dimension can be used to estimate the baseline fracture toughness for this material and can be paired with other methods to estimate the amount of transformation toughening.

Effect of Hydrothermal, Chemical, and Mechanical Degradation on Flexural Strength and Phase Transformation of Ground, Glazed, and Polished Zirconia.

Objectives: Evaluation of the effect of grinding on flexural strength of zirconia after low temperature degradation (LTD) and pH-cycling. Materials and Methods: Sixty-four bar-shaped specimens of yttria-stabilized tetragonal polycrystalline zirconia were milled, sintered, wet-polished, and divided into 8 groups (N=8). The four control groups were not aged while artificial aging was performed in the 4 experimental groups in three steps including LTD in steam for 40h, pH-cycling, and tooth brushing for artificial aging. All groups underwent surface preparations as follows: standard polishing without surface treatment (Sp), grinding with a blue-yellow band diamond instrument (Gr); grinding with a diamond rotary instrument (DRI) and then over-glazing (Gl); grinding with a DRI followed by two-step intraoral polishing (Po); standard polishing and aging (Sp-Ag); grinding and aging (Gr-Ag), grinding, over-glazing and aging (Gl-Ag); and grinding, polishing and aging (Po-Ag). Monoclinic content was assessed in one specimen of each group by X-ray diffraction (XRD). The 3-point flexural strength test was performed in a universal testing machine. The results were analyzed with two-way ANOVA and Tukey's test (α=0.05). Results: Mean flexural strength (Mpa) was significantly higher in groups Gr and Po compared to group Sp (both, P<0.0001) and group Gl (both, P<0.0001). In XRD analyses, the highest monoclinic phase before aging was observed in group Gr (12.6%), and after aging in group Gr-Ag (51.2%). Conclusion: Grinding and polishing increased the flexural strength, while glazing did not exhibit any significant effect on this parameter. Furthermore, aging did not adversely affect flexural strength.

Optical Characteristics of Monochrome and Multilayer Fully Stabilized Zirconia Upon Sintered Cooling Speed.

Firing protocols influence optical properties of dental ceramics. Effects of varying cooling rates of monochrome and multilayer 5 mol% yttria-stabilized tetragonal polycrystalline (5YTZP) on optical properties are subjected for investigation. Ninety specimens (width, length, thickness = 10 × 20 × 2 mm) were prepared from monochrome (Mo: Cercon xt) and multilayer (Mu: Cercon xt ML with cervical (C) and incisal (I) zoning) 5YTZP. Specimens were sintered and randomly treated with three cooling rates (n = 15/group): slow (S: 5°C/min), normal (N: 35°C/min), and fast (F: 70°C/min). Color appearance (∆EW), color appearance difference (∆E diff), translucency parameter (TP), contrast ratio (CR), and opalescence parameter (OP) were evaluated in CIEL*a*b* (Commission International de I'Eclairage) system. ∆E diff was achieved from the coordinate difference of specimen to VITA classic shade A2. Microstructures and compositions were evaluated by scanning electron microscope and energy dispersive spectroscopy. Monoclinic (m), tetragonal (t), and cubic (c) phases were investigated with X-ray diffraction. An analysis of variance and Bonferroni multiple comparisons were determined for significant differences (p < 0.05). ΔEW of MoF was highest (66.04 ± 1.86), while MuN-I was lowest (62.60 ± 0.86). TP and OP of MoS were highest at 2.85 ± 0.11, and 2.25 ± 0.10, while MuF-I was lowest at 2.16 ± 0.10 and 1.60 ± 0.12. CR of MuF-I was highest (0.948 ± 0.005), while MoS was lowest (0.936 ± 0.005). ΔEdiff of MoF was highest (3.83), while MuN-I was lowest (0.93). Limited grain growth and m-phase composition were indicated upon fast cooling. There were significant differences for all color parameters due to varied materials, cooling rates, and their interactions (p < 0.05) except for interaction in ∆EW and OP. Translucency of monochrome and multilayer 5YTZP were different, possibly due to colorant additives. Incisal layer of multilayer 5YTZP was perfectly matched with VITA shade. Increasing cooling speed resulted in smaller grain size, t-m transformation, and finally lower translucency and opalescence. Therefore, to achieve most favorable optical properties, slow cooling rate is recommended.

Annealing aged zirconia: Study of surface mechanical properties at the micrometric length scale

The annealing of hydrothermal aged zirconia has been studied by analysing the changes in microstructure and surface mechanical properties in terms of the annealing temperature. In this experimental work, a systematic micro- and nanomechanical study has been conducted in 3 mol% yttria-stabilized tetragonal polycrystalline zirconia (3Y-TZP) aged at 134 °C for 60 h and annealed at 600 °C and 850 °C for 1 h. Advance characterization techniques (micro-Raman, field emission scanning electron microscopy and focused ion beam) have been used to study the near surface microstructural changes induced by these treatments. The mechanical properties and resistance to damage of the near surface are determined by means of nanoindentation and nanoscratch testing. The observed behaviour is discussed in terms of the change in microstructure induced after ageing and annealing.

Assessment of low-temperature degradation of Y-TZP ceramics based on Raman-spectroscopic analysis and hardness indentation

Abstract This paper focuses on the analysis of hydrothermally induced phase transformation of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) and its influence on the hardness. Due to the hydrothermal exposure and the accompanied low-temperature degradation (LTD), a micro-cracked transformation zone is generated at the surface and progresses into the subjacent material. Raman-spectroscopic analysis of hydrothermally loaded and cross-sectioned samples revealed complete phase transformation within this zone. Its depth as well as its temperature-dependent growth rate was verified. Raman-spectroscopic measurements at the surfaces were correlated with the progression of the transformation zone. An efficient model, which assumes one extinction coefficient for tetragonal and monoclinic microstructure, enables to determine the depth of the transformation zone from the measured Raman signals. Furthermore, an exponentially decreasing Vickers hardness with increasing depth was determined. Finally, a differently sintered Y-TZP ceramic revealed enhanced resistance against LTD for the same hydrothermal loading conditions.

Stability of the bond between two resin cements and an yttria-stabilized zirconia ceramic after six months of aging in water

The behavior of the luting cement and the cementation protocol are essential in the clinical success of ceramic restorations. The purpose of this study was to evaluate the bond stability of 2 resin cements and a yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP) ceramic submitted to 2 surface treatments. Sixty plates of a Y-TZP ceramic were assigned to 3 groups according to the surface treatments: control, as sintered surface; methacryloxydecyl dihydrogen phosphate (MDP), coated with an MDP-based primer, and tribochemical silica-coating (TSC), coated with tribochemical silica. The plates of each group were further divided into 2 subgroups according to the resin cement as follows: RelyX adhesive resin cement (conventional) and RelyX Unicem (self-adhesive). Cylinders of resin cements (∅=0.75 mm× 0.5 mm in height) were built upon the ceramic surfaces, and the plates stored in distilled water at 37°C for either 24 hours or 6 months before being submitted to a microshear bond strength test. The data were submitted to 3-way ANOVA and the Tukey honestly significant difference test (α=.05). Three-way ANOVA showed statistical significance for the 3 independent factors: resin cement, surface treatment, andperiod of water immersion (P<.001). Unicem presented the highest microshear bond strength after 24 hours (MDP, 37.4±2.3and TSC, 36.2 ±2.1 MPa). Except for RelyX adhesive resin cement applied on ceramic surfaces treated with TSC, the microshear bond strength of all the other groups decreased after 6 months of aging in water. The microshear bond strength decreased most in the control groups (-81.5% for ARC and-93.1% for Unicem). In the group treated with TSC, the microshear bond strength for Unicem decreased by 54.8% and in that treated with MDP-based primer by-42.5%. Inthe group treated with MDP-based primer, the microshear bond strength for RelyX ARC decreased by-52.8%. Irrespective of surface treatments, self-adhesive resin cement was not able to maintain the bond to Y-TZP ceramic after 6 months of aging in water.

Effect of blasting pressure, abrasive particle size and grade on phase transformation and morphological change of dental zirconia surface

This study aimed to evaluate the effect of different blasting pressures and airborne particle composition and size on phase transformation and surface morphological change of yttria-stabilized tetragonal polycrystalline zirconia (Y-TZP). Specimens sintered at 1350°C for 2h were abraded with 50 and 110μm alumina and 30 and 110μm silica-coated alumina particle at pressures of 1, 1.5, 2, 2.5, 3 and 3.5bar. It was found that air abrasion changes the morphology of the Y-TZP ceramic surface. Flaws, microcracks, plastic deformations, pits, embedding of airborne particle on the ceramic surface, cluster of loosely nano-silica particle, melting of ceramic surface, coating of surface with non-uniform loosely nano-silica particle and t–m phase transformation were observed after air abrasion process. The extent of morphological change and t–m phase transformation of abraded surface depended on the pressures and size of abrasive particle. Higher pressure and larger size resulted in more extensive morphological change and more frequent t–m phase transformation. ConclusionThe extent of morphological change of ceramic surface depends on the blasting pressures and grain size of airborne particle. Nano-silica particle loosely covered the ceramic surface after air abrasion. The abrasion of the ceramic surface with 50 or 110μm alumina airborne particle at pressures of 2.5 or 1.5bar, respectively, was regarded as the optimum blasting condition.

Influence of CaTiO 3 impurity in SHS-TiN powder on the mechanical properties of ZrO 2–TiN composites

Yttria-stabilised tetragonal polycrystalline ZrO 2-based composites with 40 vol.% TiN were hot pressed at 1450 °C for 1 h using a jet-milled thermally synthesized and a self-propagating high-temperature synthesis (SHS) TiN powder. The ZrO 2 phase of the SHS-TiN powder-based composites was found to be substantially coarser than for the jet-milled TiN powder-based ceramics and prone to spontaneous transformation to m-ZrO 2 and microcracking, due to the CaTiO 3 impurity in the SHS-TiN starting powder. In order to prove this, a set of experiments was performed to investigate the effect of the addition of CaO and TiO 2 on an yttria-stabilised tetragonal ZrO 2 polycrystalline (Y-TZP). The addition of 0.2 mol% of CaO to a Y-TZP ceramic was found to destabilise the t-ZrO 2 phase, whereas the addition of 1 mol% TiO 2 results in significant grain growth and the formation of less transformable t-ZrO 2. The CaTiO 3 impurity could be removed from the SHS-TiN powder by hot hydrochloric acid leaching, allowing to obtain a similar microstructure and mechanical properties as with conventional TiN powder.

Grain boundary segregation in high-purity, yttria-stabilized tetragonal zirconia polycrystals (Y-TZP)

In this paper, we use high-resolution transmission electron microscopy and electron energy-loss spectroscopy to study the microstructure of grain boundaries and the segregation of yttrium, respectively, in yttria-stabilized tetragonal polycrystalline zirconia, sintered from different high-purity powders. No amorphous films were observed at the grain boundaries, and only the sample containing the highest amount of silicon impurity showed presence of an amorphous silicate phase in all triple grain junctions. A strong yttrium segregation to the grain boundaries is observed in all samples, despite different grain sizes and impurity levels.