Superplasticity-like Deformation of Nanocrystalline Monoclinic Zirconia at Elevated Temperatures

The deformation behavior of nanocrystalline monoclinic ZrO₂ polycrystals (nanocrystalline MZP) was studied at 1273–1373 K in compression tests. The deformation of nanocrystalline MZP was characterized by stress exponent ( n = 2.5), grain-size exponent ( p = 2.5), and apparent activation energy ( Q = 350 kJ/mol). The values of n and p were similar to the superplasticity of high-purity Zn-22% aluminum alloy. The strain rate of nanocrystalline MZP was faster than that of Y₂O₃-stabilized tetragonal ZrO₂ (Y-TZP) at temperatures lower than the monoclinic-tetragonal transition temperature. The strain rate of MZP gradually approached to that of Y-TZP as the temperature increased to the transition temperature. The comparison of present data with published data suggested that trace amount of impurities affected the deformation behavior of MZP.     

Transformation of Yttria-Doped Tetragonal ZrO2 Polycrystals by Annealing in Water

Phase changes and the microstructure resulting from low-temperature annealing of yttria-doped tetragonal ZrO₂ polycrystals in water were investigated at 65° to 120°C. Tetragonal ZrO₂ on the surface of the sintered body transformed to the monoclinic phase, accompanied by microcracking. The transformation rate in water, which was much greater than that in air, was first order with respect to surface concentration of tetragonal ZrO₂. Nonaqueous solvents with a molecular structure containing a lone-pair electron orbital opposite a proton donor site also greatly enhanced the transformation.     

Transmission Electron Microscopic Observation of the Martensitic Phase Transformation in Tetragonal ZrO2

The present work involves the observation of phase transformation in Y₂O₃-containing tetragonal ZrO₂ polycrystals (Y-TZP) using electron microscopy. The observations indicate that the martensitic phase transformation ususally starts from grain boundaries. Transformation was also observed at the tip of a microcrack and its adjacent region. Our observations also indicate that the tetragonal ZrO₂ grains do not all transform at the same time at the fracture surfaces.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Stress Corrosion Cracking of Single-Crystal Tetragonal ZrO2(Er2O3)

The flexure strength of partially-stabilized tetragonal ZrO₂(Er₂O₃) single-crystal monofilaments manufactured by the laser-heated floating zone method was measured as a function of the environment (air versus water) and temperature (from 25° to 800°C) at loading rates spanning three orders of magnitude to ascertain their susceptibility to the environmental conditions. These mechanical tests were completed with parallel tests on fully annealed monofilaments (to relieve the thermal residual stresses induced during growth) and by detailed analysis of the fracture surfaces using scanning electron microscopy and micro-Raman spectroscopy. While environmental susceptibility of ZrO₂(Y₂O₃) in previous investigations was always associated with the destabilization of the tetragonal phase, monoclinic phase was not detected on the fracture surfaces of the ZrO₂(Er₂O₃) monofilaments and it was concluded that slow crack growth in this material at high temperature or immersed in water was due to stress corrosion cracking.     

Stability of Tetragonal ZrO2 Particles in Ceramic Matrices

The stability of tetragonal ZrO₂ particles in ceramic matrices was considered, with particular reference to Al₂O₃-ZrO₂ composites and to partially stabilized ZrO₂. In both systems, particles above a "critical" size transform martensitically to monoclinic symmetry on cooling to room temperature. The critical factors that could affect the size dependence of the transformation temperature—surface and strain energy effects, the chemical free energy driving force, and the difficulty of nucleating the martensitic transformation—were considered. Nucleation arguments are probably the most important.     

Stability of Ultrafine Tetragonal ZrO2 Coprecipitated with Al2O3 by the Spray-ICP Technique

The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO_2–Al₂O₃, prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO₂ ( mt- ZrO₂) and γ-Al₂O₃. On heating, the mt- ZrO₂ (or tetragonal ZrO₂, t -ZrO₂) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO₂ ( m -ZrO₂) occurred and the amount of the m -ZrO₂ decreased with the increase in Al₂O₃ content, thus indicating the stabilization of the t -ZrO₂ by the Al₂O₃, which seems to be explained in terms of the retardation of grain growth.     

Effect of Ta2O5, Nb2O5, and HfO2 Alloying on the Transformability of Y2O3-Stabilized Tetragonal ZrO2

The addition of Ta₂O₅, Nb₂O₅, and HfO₂ enhanced the transformability of Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (Y-TZP), which was indicated by an increase in phase transformation temperatures and fracture toughness of Y-TZP. Comparison of the alloying effects of these oxides on the transformability and crystal structure of Y-TZP suggested that an alloying oxide which increases the c/a axial ratio (tetragonality) of TZP also increases the transformability. Empirical equations to predict the tetragonality are proposed. Calculated tetragonalities showed good agreement with measured values in the systems ZrO₂-Y₂O₃-Ta₂O₅, -Nb₂O₅, and -HfO₂.     

Transformation Toughening in Large-Grain-Size CeO2-Doped ZrO2 Polycrystals

The fracture and transformation behavior of tetragonal polycrystalline ZrO₂ alloys containing 18 mol% CeO₂ (Ce-TZP) was investigated. In the absence of applied stress the tetragonal phase was found to be stable in large-grained (>30 μm) samples at room temperature. The monoclinic phase was detected in regions of high residual stress near hardness indentations although no evidence of a wake of monoclinic phase along the fracture surface was observed. The fracture toughness increased from 4 to 7 MPa · m^1/2 as density and/or grain size increased. It is proposed that the relatively high toughness of these materials is due to the occurrence of stress-driven tetragonal-to-monoclinic transformation near the crack tip, which reverses when the crack has passed.     

High-Strain-Rate Superplasticity in Y2O3-Stabilized Tetragonal ZrO2 Dispersed with 30 vol% MgAl2O4 Spinel

High-strain-rate superplasticity is attained in a 3-mol%-Y₂O₃-stabilized tetragonal ZrO₂ polycrystal (3Y-TZP) dispersed with 30 vol% MgAl₂O₄ spinel: tensile elongation at 1823 K reached >300% at strain rates of 1.7 × 10⁻²– 3.3 × 10⁻¹ s⁻¹. The flow behavior and the microstructure of this material indicate that the MgAl₂O₄ dispersion should enhance accommodation processes necessary for grain boundary sliding. Such an effect is assumed to arise from an enhancement of the cation diffusion by the dissolution of Al and Mg ions into the ZrO₂ matrix and from stress relaxation due to the dispersed MgAl₂O₄ grains.     

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

Up to 50 vol% of TiB₂, TiC_0.5N_0.5, 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·m^1/2), excellent bending strength (up to 1237 MPa), and increased hardness, with respect to ZrO₂ (Vickers hardness up to 1450 kg/mm²), were obtained.     

Chemical Interactions Promoting the ZrO2 Tetragonal Stabilization in ZrO2–SiO2 Binary Oxides

Metastable tetragonal ZrO₂ phase has been observed in ZrO₂–SiO₂ binary oxides prepared by the sol–gel method. There are many studies concerning the causes of ZrO₂ tetragonal stabilization in binary oxides such as Y₃O₂–ZrO₂, MgO–ZrO₂, or CaO–ZrO₂. In these binary oxides, oxygen vacancies cause changes or defects in the ZrO₂ lattice parameters, which are responsible for tetragonal stabilization. Since oxygen vacancies are not expected in ZrO₂–SiO₂ binary oxides, tetragonal stabilization should just be due to the difficulty of zirconia particles growing in the silica matrix. Furthermore, changes in the tetragonal ZrO₂ crystalline lattice parameters of these binary oxides have recently been reported in a previous paper. The changes of the zirconia crystalline lattice parameters must result from the chemical interactions at the silica–zirconia interface (e.g., formation of Si–O–Zr bonds or Si–O⁻ groups). In this paper, FT-IR and ²⁹Si NMR spectroscopy have been used to elucidate whether the presence of Si–O–Zr or Si–O⁻ is responsible for tetragonal phase stabilization. Moreover, X-ray diffraction, Raman spectroscopy, and transmission electron microscopy have also been used to study the crystalline characteristics of the samples.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: I, Transformation in Tetragonal ZrO2 Grains

The martensitic transformation in tetragonal ZrO₂ grains in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied using in situ observations in the transmission electron microscope. Transformation occurred by the nucleation and growth of monoclinic laths; thermoelastic equilibrium can be maintained at different extents of transformation by continuously varying the applied stress. The product phase was always twinned, but two twinning mechanisms were found-sequential formation of twin-related variants and posttransformation deformation twinning. In one example, a (401)/(410) pair of martensite habit planes led to a (100) conjunction twinning plane.     

Stabilization of Tetragonal ZrO2 in ZrO2–SiO2 Binary Oxides

Gel-glasses of various compositions in the x ZrO₂^.(10 – x )SiO₂system were fabricated by the sol–gel process. Precipitation due to the different reactivities between tetraethyl orthosilicate (TEOS) and zirconium(IV) n -propoxide has been eliminated through the use of 2-methoxyethanol as a chelating agent. Thermal treatment of these gels produced crystalline ZrO₂particles. While monoclinic is the stable crystalline phase of zirconia at low temperatures, the metastable tetragonal phase is usually the first crystalline phase formed on heat treatment. However, stability of the tetragonal phase is low, and it transforms to the monoclinic phase on further heat treatment. In this study, it has been found that the transformation temperature increases as the SiO₂content in the ZrO₂–SiO₂ binary oxide increases. The most significant results were from samples containing only 2 mol% SiO₂, where the metastable tetragonal phase formed at low temperatures and remained stable over a broad temperature range. X-ray diffraction, transmission electron microscopy, and Fourier transform infrared spectroscopy were used to elucidate the structure of these binary oxides as a function of temperature.     

Transformation of Yttria-Doped Tetragonal ZrO2 Poly crystals by Annealing under Controlled Humidity Conditions

Phase changes on the surface resulting from low-temperature annealing of yttria-doped tetragonal ZrO₂ polycrystals under controlled humidity conditions were investigated at 0.1 to 7350 Pa of water vapor pressure and 100° to 600°C for 1 to 50 h. The tetragonal-to-monoclinic phase transformation was greatly accelerated by increasing water vapor pressure; the transformation rate was first order with respect to the concentration of tetragonal ZrO₂ on the surface of the sample.     

Transformation Plasticity of CeO2-Stabilized Tetragonal Zirconia Polycrystals: I, Stress Assistance and Autocatalysis

Transformation plasticity in CeO₂-stabilized tetragonal zirconia polycrystals due to the tetragonal-to-monoclinic transformation was studied by inducing volumetric and shear deformation under compression and bending between the burst temperature of martensite (monoclinic) formation ( M_b ) and the burst temperature of austenite (tetragonal) formation ( A_b ). The stress-strain curve features a load drop, a perfect plastic regime, and an extended strain-hardening regime before the exhaustion of transformation. Macroscopic shear bands formed in the perfect plastic regime. The yield stress has a strong, positive pressure and temperature sensitivity but is strain-rate sensitive only in the last stage of deformation. These results are rationalized in terms of stress assistance to the transformation which, in a homogeneous tetragonal polycrystal, may propagate autocatalytically. Autocatalysis can be impeded by a second phase, such as monoclinic ZrO₂ or Al₂O₃, and is suppressed at higher temperature. Flow localization is found to precede and precipitate crack formation. As a result, the actual fracture energy is much less than the total plastic work. The implications of stress-assisted, autocatalytic transformation on strength and toughness are explored.     

Sintering and Mechanical Properties of Yttria-Doped Tetragonal ZrO2 Polycrystal/Mullite Composites

Yttria-doped tetragonal ZrO₂ polycrystal (Y-TZP)lmullite composites were sintered at 1450° to 1500°C in air to disperse rodlike mullite grains at the grain boundary of Y-TZP and the mechanical and thermal properties were investigated. The aspect ratios of mullite grain were >2. High fracture strength of 1000 MPa and fracture toughness of 12 MPa·m^1/2 were obtained by dispersing <20 vol% of mullite into Y-TZP. The thermal expansion coefficient of Y-TZP/mullite composites decreased with increasing mullite content. The thermal shock resistance of Y-TZP was greatly improved by dispersion of rodlike mullite grains.     

Degradation During Aging of Transformation-Toughened ZrO2-Y2O3 Materials at 250°C

The detrimental aging phenomenon observed in ZrO₂-Y₂O₃ materials, which causes tetragonal ZrO₂ to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y₂O₃ and SiO₂ content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO₂ to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)₃. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y₂O₃) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.     

Characterization and Stabilization of Metastable Tetragonal ZrO2

Fine single-domain and polydomain particles of tetragonal ZrO₂ were prepared by hydrothermal and heat treatment of ZrO₂· n H₂O. The particles were characterized by X-ray diffraction, electron microscopy, NMR, mass spectrometry, and ir spectroscopy. The main impurity in the samples was 1 to 2 wt% OH⁻ ions, most of which were concentrated on the particle surfaces or at domain boundaries; some were also distributed in the lattice. Fine single-domain tetragonal particles were strain-free, but polydomain particles had large strains. The single-domain tetragonal particles were transformed much more easily than the polydomain particles by mechanical treatment. The stablization of metastable tetragonal ZrO₂ cannot be explained adequately by the surface-energy theory. An explanation based on the concept of a martensitic transformation may be more reasonable.     

Structural Changes by Compressive Stresses of 2.0-mol%-Yttria-Stabilized Tetragonal Zirconia Polycrystals

Reorientation of the tetragonal (002) peak in tetragonal ZrO₂ polycrystals (TZP) (so-called domain switching) was studied by XRD and residual stress measurement using TZP specimens containing 2.0 mol% Y₂O₃ that had undergone mechanical and thermal treatments and compressive stress. The observed domain switching was due to a preferred transformation of the tetragonal phase caused by compressive stress above 70 MPa leading to a remnant c-axis orientation normal to the compressive direction. Domain switching did not depend on thermal stress but arose directly from the tetragonal phase with little relation to monoclinic phase.