Y2O3-stabilized Tetragonal ZrO2 Polycrystal Research Articles

Ultrahigh toughness zirconia ceramics

The attainment of both high strength and toughness is the ultimate goal for most structural materials. Although ceramic material has been considered for use as a structural material due to its high strength and good chemical stability, it suffers from the limitation of low toughness. For instance, although Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZPs) exhibit remarkable toughness among ceramics due to their phase transformation toughening mechanism, this toughness is still much weaker than that of metals. Here, we report Y-TZP-based ceramic materials with toughnesses exceeding 20 MPa m1/2, which is comparable to those of metals, while maintaining strengths over 1,200 MPa. The superior mechanical properties are realized by reducing the phase stability of tetragonal zirconia by tailoring the microstructure and chemistry of the Y-TZP. The proposed ceramic materials can further advance the design and application of ceramic-based structural materials.

Flash self‐joining of Y‐TZP ceramics assisted with an AC electric field

AbstractIn this study, flash joining experiments were conducted using an AC field on 3 mol% Y2O3‐stabilized tetragonal ZrO2 polycrystal (Y‐TZP) bodies. Furthermore, the necessary conditions to obtain an almost complete self‐joining of Y‐TZP bodies were clarified. The specimens were successfully joined by applying an AC field at 60 mA mm−2 for 80 s at a furnace temperature of 1000°C, thus resulting in a successfully joined specimen with 92% of the flexural strength of the as‐sintered Y‐TZP body. Almost complete self‐joining of Y‐TZP was achieved at current densities above 30 mA mm−2, and input energy densities of >24 J mm−3. Both the input energy density and electric current were critical factors for producing the reliable joining of ceramics.

Athermal effect of flash event on high-temperature plastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal

This study experimentally demonstrates the athermal effect of a flash event under a DC electric field on high-temperature plastic deformation in a dense, fine-grained 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (3Y-TZP). A high-temperature three-point flexural test in the conventionally sintered 3Y-TZP body with an average grain size of 0.36 µm was conducted under a flash event with a current density of 67 mA·mm−2 at a furnace temperature between 900 °C and 1300 °C. The flash event significantly enhanced the high-temperature plastic flow in the 3Y-TZP body, even though subtracting the effect of specimen temperature rise by Joule heating, while grain growth during the flexural tests was effectively suppressed under the examined conditions. The athermal effect on the plastic flow was attributed to the reduction in the activation energy for the high-temperature plastic flow.

Low-temperature degradation in yttria-stabilized tetragonal zirconia polycrystal: Effect of Y3+ distribution in grain interiors

Herein, the microstructure in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) at the early stage of the low-temperature degradation (LTD) was analyzed using scanning transmission electron microscopy (STEM)-nanoprobe and energy-dispersive spectroscopy (EDS) techniques. The effects of the grain size and Y3+ concentration within the grain interiors on the LTD behavior were experimentally investigated using Y-TZP bodies prepared through the isothermal sintering process. The tetragonal-to-monoclinic (T → M) phase transformation, which is the LTD origin, was accelerated by the Y3+ concentration decrease within the grain interiors, and was insensitive to the grain size. The STEM-nanoprobe EDS analysis revealed that the T → M phase transformation preferentially occurred from the tetragonal phase with low Y3+ concentration.

ファインセラミックス用ジルコニア粉末の製造方法

Y2O3-stabilized ZrO2 (YSZ) ceramics have been used for various engineering applications or as a solid electrolyte in oxygen sensors and fuel cells. The performance in YSZ strongly depends on the microstructure which is directly determined by the characteristic of raw powder. In this paper, the synthetic methods of YSZ powder and its starting material are first overviewed, and then the formation mechanism of YSZ powder is explained, focusing the hydrolysis process which is industrially adopted. Next, microstructural development during sintering of the hydrolytic YSZ powder is summarized, focusing Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) with excellent mechanical properties among YSZ. The author finally introduces that based on the hydrolytic process technique and microstructural knowledge, low temperature degradation that is fatal shortcoming of Y-TZP can be resolved by controlling the grain-boundary nanostructure and chemical composition distribution in Y-TZP.

Electrical performance of fine-grained Y-TZP/TiC composites obtained through a hydrothermal-assisted sol-gel process

Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) composites with various TiC contents were successfully synthesized via a modified hydrothermal-assisted sol-gel method. Fine precursor powders can be obtained with high crystallinity, nanoscale grain size and uniform morphology. SEM images of the ceramic products show that TiC particles are distributed homogeneously in the final Y-TZP matrix, and their average grain sizes are approximately 390–670 nm and 150–230 nm for the Y-TZP and TiC phases, respectively. A higher TiC volume fraction has a negative effect on the relative density and hardness but a significant positive influence on electrical conductivity. The electrical conductivity values are increased from 115 S/m to 1.23 × 105 S/m with TiC contents varying, demonstrating that the percolation threshold is approximately as low as 11.6 vol% for the samples, which is much lower than those of previous Y-TZP/TiC ceramics. The high electrical performance is probably due to the high D (the diameters of the insulating particles)/d (the diameters of conductive particles) ratio and submicron-sized grains.

Low temperature and high strain rate superplastic flow in structural ceramics induced by strong electric-field

High-strength structural ceramics with sub-micron grain sizes can exhibit superplasticity, but the superplasticity appears at high temperatures and low strain rates. Low temperature and high speed superplastic flow in structural ceramics was achieved by applying a strong electric field above a threshold value during deformation. The application of a direct-current field of 190V·cm−1 led to superplastic deformation in Y2O3-stabilized tetragonal ZrO2 polycrystal with a total tensile elongation of >150% at 800°C and an initial strain rate of 2×10−3s−1. The field-activated plasticity was attributed to highly-accelerated self-diffusion induced not only by temperature rise but also by a field effect.

Low-temperature degradation in yttria-stabilized tetragonal zirconia polycrystal doped with small amounts of alumina: Effect of grain-boundary energy

Low temperature degradation (LTD) behavior was examined in 3mol% Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) doped with 0–1.1mol% Al2O3 sintered at 1500°C. 0.12mol% Al2O3-doped Y-TZP exhibited monolithic microstructure without dispersion of Al2O3 grain, while second-phase Al2O3 particles were observed in the Y-TZP sintered bodies doped with 0.54–1.1mol% Al2O3. In the Y-TZP doped with 0.12mol% Al2O3, neither amorphous nor second phase was present along the grain boundaries, but Y3+ and Al3+ ions segregated in the vicinity of the grain boundaries. The tetragonal-to-monoclinic (T→M) phase-transformation rate during the accelerated LTD test significantly decreased at 0.12mol% Al2O3, but the phase-transformation rate slightly increased with the increasing doping amount of Al2O3. The suppressed T→M phase-transformation in Al2O3-doped Y-TZP is explained in terms of decrease in grain-boundary free energy of tetragonal phase owing to Al3+ ions’ grain-boundary segregation.

Nanocrystalline, ultra-degradation-resistant zirconia: its grain boundary nanostructure and nanochemistry.

Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) has been known to be an excellent structural material with high strength and toughness since the pioneering study by Garvie et al. in 1975. However, Y-TZP is not considered an environmental or biomedical material because it undergoes an inherent tetragonal-to-monoclinic (T→M) phase transformation in humid or aqueous environment, which leads to premature failure, so-called low-temperature degradation (LTD). In this study, we demonstrate for the first time that this fatal shortcoming of Y-TZP can be resolved by controlling the grain boundary nanostructure and chemical composition distribution in Y-TZP. Nanocrystalline Y-TZP doped with Al3+ and Ge4+ ions exhibits no LTD for more than 4 years in hot water at 140°C, whereas 70% of the tetragonal phase in conventional TZP transforms to the monoclinic phase within only 15 h. This innovative Y-TZP can be fabricated by pressureless sintering at 1200°C; far below the sintering temperature for conventional Y-TZP. The developed TZP ceramics will be useful in numerous environmental-proofing applications, particularly in the biomedical engineering field.

Kinetics and the role of off-stoichiometry in the environmentally driven phase transformation of commercially available zirconia femoral heads

The low-temperature polymorphic transformation behavior of two types of commercially available femoral head, both made of 3mol.% Y2O3-stabilized tetragonal ZrO2 polycrystals (3Y-TZP), was examined by in vitro experiments. Both materials contained a small amount (0.25wt.%) of Al2O3, but they differed slightly in their SiO2 impurity content, in the morphology and crystallinity of the dispersed Al2O3 phase, and in grain size. In vitro experiments were conducted in a water-vapor environment at temperatures in the range 90–134°C and for periods of time up to 500h. Despite the materials having the same nominal composition, quite different behaviors were found in the hydrothermal environment for the two types of femoral head investigated. A phenomenological description of the kinetics of monoclinic nuclei formation/growth led to the experimental determination of activation energy values for the environmentally driven polymorphic transformation. From the material physics viewpoint, cathodoluminescence spectroscopy enabled us to rationalize the role of surface stoichiometry on the mechanisms leading to polymorphic transformation. Spectroscopic experiments unveiled some new relevant aspects of surface off-stoichiometry, which lie behind the different phase transformation kinetics experienced by the investigated femoral heads.

Grain-Boundary Segregation and Phase-Separation Mechanism in Yttria-Stabilized Tetragonal Zirconia Polycrystal

Microstructure development during sintering in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) was systematically investigated in two sintering conditions: (a) 1100-1650°C for 2 h and (b) 1300°C for 0-50 h. In the sintering condition (a), the density and grain size in Y-TZP increased with the increasing sintering temperature. Scanning transmission electron microscopy (STEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, i.e., most of grains were the tetragonal phase, but cubic-phase regions with high Y3+ ion concentration were clearly formed in grain interiors adjacent to the grain boundaries at 1500°C. High-resolution transmission electron microscopy (HRTEM) and nanoprobe EDS measurements revealed that no amorphous or second phase is present along the grain-boundary faces, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries over a width below about 10 nm, respectively. These results indicate that the cubic-phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed this process a “grain boundary segregation-induced phase transformation (GBSIPT)” mechanism. In the sintering condition (b), the density was low and the grain-growth rate was much slow. In the specimen sintered at 1300°C for 50 h, the cubic-phase regions were clearly formed in the grain interiors adjacent to the grain boundaries. This behavior shows that the cubic-phase regions were formed without grain growth, which can be explained by the GBSIPT model.

Phase Separation and Hydrothermal Degradation of 3 mol% Y2O3-ZrO2 Ceramics

Phase separation and hydrothermal degradation in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (3Y-TZP) were studied by X-ray diffractometry. The Rietveld analysis showed that crystal structure of sintered body remarkably changes at high temperature. The sintered body is phase separated into high and low yttrium region with increasing sintering temperature. Both regions are assigned to the tetragonal phase. The high yttrium region can be analyzed using tetragonal phase model with c/a≒1. The amount of monoclinic phase induced by hydrothermal degradation increased with increasing degradation time. Aging kinetics of tetragonal to monoclinic phase transition was analyzed with Johnson-Mehl-Avrami equation. Fraction of transformable phase evaluated with the equation is in good agreement with the fraction of low yttrium region determined with the Rietveld analysis. The quantitative analyses clearly show that the low yttrium region selectively transforms into monoclinic phase under the hydrothermal condition, while the high yttrium region does not transform. Transformable fraction during the hydrothermal condition in 3Y-TZP ceramics is governed by the amount of low yttrium region. The formation of low yttrium region due to phase separation is responsible for hydrothermal degradation in sintered body.

Effect of 2‐Phosphonobutane‐1,2,4‐tricarboxylic Acid Adsorption on the Stability and Rheological Properties of Aqueous Nanosized 3‐mol%‐Yttria‐Stabilized Tetragonal‐Zirconia Polycrystal Suspensions

2‐phosphonobutane‐1,2,4‐tricarboxylic acid (PBTCA) was evaluated as a dispersant for nanosized 3‐mol%‐Y2O3‐stabilized tetragonal‐ZrO2 polycrystal (3Y‐TZP) suspensions. The adsorption of PBTCA was characterized using the decolorization method of ferric 5‐nitrosalicylate complexes. Maximum adsorption of the dispersant on the 3Y‐TZP powder was found to occur at pH 3.0. At pH >3.0, the adsorbed amount decreased with increased pH. Semiquantitative analysis using auger electron spectroscopy showed that PBTCA adsorbed irreversibly on the powder. The surface charge of the powder was evaluated by measuring the zeta‐potential in dilute powder suspensions. The suspension was most effectively stabilized at high pH by the high charge induced by the adsorption of PBTCA. Rheological properties of the suspension were evaluated as a function of dispersant amount and solids loading. The optimum amount of dispersant increased with increased solids loading for solids loading >20 vol%. A stable suspension of 35 nm 3Y‐TZP particles with a solids loading as high as 32 vol% was obtained using PBTCA as dispersant, in contrast to 28 vol% when using ammonium polyacrylate (NH4PAA). Theoretical calculations of the interaction between 3Y‐TZP particles showed that the stabilization of the suspensions was attributed to a combination of the electrostatic repulsion and a steric barrier caused by the adsorbed PBTCA. Induced coupling plasma analysis showed that PBTCA could be completely burned out during sintering, which confirmed its suitability as a dispersant for 3Y‐TZP.

Development and Characterization of Y2O3‐Stabilized ZrO2 (Y‐TZP) Composites with TiB2, TiN, TiC, and TiC0.5N0.5

Up to 50 vol% of TiB2, TiC0.5N0.5, TiN, or TiC was added to Y2O3‐stabilized tetragonal ZrO2 polycrystals (Y‐TZP) and hot pressed under vacuum. The influence of the type of secondary phase on the microstructure and mechanical properties was studied, as a function of the hot‐pressing temperature. The influence of the secondary‐phase content on the mechanical properties was studied by varying the TiB2 content up to 50 vol%. Fully dense Y‐TZP‐based composites with very high toughness (up to 10 MPa·m1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO2 (Vickers hardness up to 1450 kg/mm2), were obtained.

Compressive deformation properties and microstructures in the superplastic Y-TZP

The superplastic deformation properties of 3mol% Y2O3-stabilized tetragonal ZrO2 polycrystals (Y-TZP) were studied by uniaxial compression tests in air at temperatures up to 1500°C with strain rates ranging from 10-3 to 10-5s-1. The effect of microstructure on the superplasticity was also examined, Fine-grained Y-TZP with an average grain size of 0.3-0.4μm was deformed to the true strain of more than -1.5. The macroscopic strain rate (e) in superplasticity was expressed phenomenologically by the following equation, e=2×10-5exp(-380000/RT)σ2.1/d1.8, where T, σ, and d are temperature, stress, and grain size, respectively, and R is the gas constant. Although the microstructure of the deformed specimen showed slight grain growth and slight anisotropic deformation of each grain, the large strain was mainly caused by the grain boundary sliding. The cavitation was not observed in the compressed specimens. ZrO2 particles maintained tetragonal or cubic phase after the superplastic deformation. The deformation properties and the observed microstructure in the superplasticity of Y-TZP were explained by the Gifkins's model.

Mechanical Properties of Y2O3‐Stabilized Tetragonal ZrO2 Polycrystals After Ageing at High Temperature

Yttria‐partially‐stabilized zirconia (Y‐PSZ) materials containing 2.5, 3.0, and 5.0 mol% of Y2O3 prepared by hot pressing were subjected to ageing in air for >10 h at temperatures in the range of 800°C and 1200°C. The sintered materials were measured to determine the mechanical properties and microstructures and analyzed for trace elements. A sharp reduction in bending strength was observed after the ageing, the cause for which was suggested to be the formation of cavities produced by the oxidation of carbon.