Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

Transformation and Microcrack Toughening as Complementary Processes in ZrO2-Toughened Al2O3

Both tetragonal ( t ) and monoclinic ( m ) ZrO₂ particles in ZrO₂-toughened Al₂O₃ can give rise to toughening. In the stress field of propagating cracks, the t -ZrO₂ particles can undergo the stress-induced t → m transformation, and the residual stresses around already-transformed m -ZrO₂ particles can cause microcracking. The t -ZrO₂ particles transformed in crack tip stress fields do not, however, also cause appreciable microcracking. The toughening increments via these distinct mechanisms are comparable. It appears that optimally fabricated Zr0₂-toughened Al₂O₃'s should contain a mixture of t - and m -ZrO₂.     

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.     

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.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Toughening Behavior Involving Multiple Mechanisms: Whisker Reinforcement and Zirconia Toughening

Since the contribution of transformation toughening increases with the loca crack resistance (which is proportional to the toughness of the matrix), ZrO₂ particles were added to a toughened, whisker-reinforced ceramic matrix. Analysis revealed that the combination of these multiple toughening agents should result in ceramic composites tougher than (1) that achieved by either mechanism by itself or (2) the sum of the two processes. The toughness of mullite could be increased 1.8-and 2.4-fold with a 20 vol% addition of ZrO₂ particles or SiC whiskers, respectively. However, when 20 vol% of both ZrO₂ particles and SiC whiskers were added, the toughness was increased at least 3-fold with monoclinic m -ZrO₂ and by >5-fold with tetragonal t -ZrO₂. The differences in the toughening achieved when t -ZrO₂ vs m -ZrO₂ is present in the SiC-whisker-reinforced mullite are attributed to differences in their interdependencies upon the whisker reinforcement.     

Zirconium Oxide Structure Prepared by the Sol–Gel Route: I, The Role of the Alcoholic Solvent

Sol–gel-derived powder samples of zirconia (ZrO₂) prepared via the dissolution of zirconium n -propoxide in methanol, ethanol, and 2-propanol have been characterized mainly using perturbed angular correlations spectroscopy, as a function of temperature. Results indicate that the nanostructures and subsequent thermal evolution are alcohol dependent: the shorter the alcohol chain, the more hydrolyzed the product. ZrO₂ powder that has been obtained using ethanol as the solvent is the product that exhibits the better stabilization of the metastable tetragonal phase ( t -phase). It undergoes a clear and detailed t ₁-form → t ₂-form → monoclinic ZrO₂ thermal transformation and shows the highest activation energy against the transformation to the monoclinic phase.     

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.     

Homogeneous Fabrication and Densification of Cordierite–Zirconia Composites by a Mixed Colloidal Processing Route

Based on the electrokinetic properties of aqueous silica, boehmite, and ZrO₂ dispersions, cordierite-ZrO₂ composites were fabricated by a mixed colloidal processing route. The fabricated composite was characterized by a dense and homogeneous microstructure and by a uniform spatial distribution of submicrometer-sized tetragonal ZrO₂ particles throughout the matrix. Increasing ZrO₂ content enhanced densification and resulted in a full density composite at 20 wt% ZrO₂. Fracture toughness was also increased with increasing ZrO₂ content. The enhanced toughening was partly attributed to the martensitic transformation of the dispersed tetragonal ZrO₂ particles in a cordierite matrix. The formation of zircon was suppressed by suitably adjusting the heating schedule during sintering.     

Experimental Evaluation of Toughening Mechanisms in Alumina-Zirconia Composites

The contributions of transformation toughening and microcrack toughening in an 85Al₂O₃15ZrO₂ (vol%) composite were quantitatively evaluated. Three types of hot-pressed samples with similar size (∼0.5 µm) and size distribution of ZrO₂ grains, but with different contents (vol%) of monoclinic- ( m -) ZrO₂ (0 (AZS), 45 (AZT), and 67 (AZM)) were prepared using Y₂O₃ and MoO₂ dopants. Therefore, the possible effect of ZrO₂ grain size on each toughening mechanism was eliminated. When the measured fracture toughnesses of m -ZrO₂ volume fractions before and after fracture were compared, the transformation-toughening constant was estimated as 3.0 MPam^1/2 and the microcrack-toughening constant 0.2 MPam^1/2 for a 100% monoclinic transformation of ZrO₂ grains. This result indicates that transformation toughening was the dominant toughening mechanism in the studied composite.     

Microstructural Evolution in a ZrO2 Wt% Y2O3 Ceramic

Several unusual microstructural features, i.e., 90° tetragonal ZrO₂ twins containing antiphase domain boundaries, tetragonal ZrO₂ precipitates in a colony morphology, and precipitate-free zones at the perimeter of cubic ZrO₂ grains containing fine tetragonal ZrO₂ precipitates, were observed in a single ZrO₂-12 wt% Y₂O₂ ceramic annealed at 1550°, 1400°, and 1250°C, respectively. The type of phase transformation responsible for each microstructural feature is described.     

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.     

Measurement of the Crystallographically Transformed Zone Produced by Fracture in Ceramics Containing Tetragonal Zirconia

During fracture of ceramics containing tetragonal zirconia particles, a volume of zirconia material on either side of the crack irreversibly transforms to the monoclinic crystal structure. Transformation zone sizes, measured using Raman microprobe spectroscopy, are presented for three sintered ceramics. In a single-phase ZrO₂−3.5 mol% Y₂O₃ material, an upper bound measurement of 5 μm is obtained for the zone size. In the Al₂O₃/ZrO₂ composites studied, the zone size is deduced to correspond to ∼1 grain in diameter. On the basis of the monoclinic concentrations derived from the Raman spectra it is further concluded that only a fraction of the ZrO₂ grains within the transformation zone transform, providing indirect evidence for the effect of particle size on the propensity for transformation.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

Phase Stability and Physical Properties of Cubic and Tetragonal ZrO2 in the System ZrO2–Y2O3–Ta2O5

The cubic ( c -ZrO₂) and tetragonal zirconia ( t -ZrO₂) phase stability regions in the system ZrO₂–Y₂O₃–Ta₂O₅ were delineated. The c -ZrO₂ solid solutions are formed with the fluorite structure. The t -ZrO₂ solid solutions having a c/a axial ratio (tetragonality) smaller than 1.0203 display high fracture toughness (5 to 14 MPa · m^1/2), and their instability/transformability to monoclinic zirconia ( m -ZrO₂) increases with increasing tetragonality. On the other hand, the t -ZrO₂ solid solutions stabilized at room temperature with tetragonality greater than 1.0203 have low toughness values (2 to 5 MPa · m^1/2), and their transformability is not related to the tetragonality.     

Internal Stresses and the Martensite Start Temperature in Alumina-Zirconia Composites: Effects of Composition and Microstructure

n alumina-based composites containing ceria-stabilized tetragonal zirconia, the martensite start temperature ( M_s ) of the tetragonal-to-monoclinic zirconia phase transformation exhibits a grain size dependence that becomes increasingly pronounced as the zirconia content decreases. Neutron diffraction experiments confirm earlier dilatometry measurements of M _s in composites containing ≥20 vol% ZrO₂ and were instrumental in obtaining M _s values in lower zirconia content (i.e., 10 vol%) composites. The dependence of M _s on zirconia content is related to the internal stresses that arise from differences in thermal expansion coefficients between the two phases. Neutron diffraction measurements show that the internal tensile stresses in the zirconia grains increase with decreasing zirconia content. The measured internal stresses are in quantitative agreement with predictions based on models assuming isolated ZrO₂ particles at low zirconia contents and a continuous ZrO₂"matrix" phase at higher zirconia contents. This assumption is consistent with the observed microstructural development in which the low zirconia contents result in isolated zirconia grains, whereas higher zirconia contents result in more interconnected zirconia grains.     

Crystallization and Phase Transformation Process in Zirconia: An in situ High-Temperature X-ray Diffraction Study

Amorphous zirconia precursors were made by the precipitation of a zirconium tetrachloride solution with either slow (8 h) or rapid additions of ammonium hydroxide at a pH of 10.5. Following calcination at 500°C for 4 h, the rapidly precipitated precursor exhibited predominantly monoclinic ZrO₂ phase, while the slowly precipitated precursor produced the tetragonal ZrO₂ phase. The crystallization and phase transformations were followed by in situ high-temperature X-ray diffraction (HTXRD) for both specimens in helium and in air. Each amorphous precursor first crystallizes as the tetragonal phase at about 450°C. A tetragonal-to-monoclinic phase transformation of the rapidly precipitated material was observed on cooling at about 275°C. Surface impregnation of sulfate ions following precipitation inhibited the tetragonal-to-monoclinic transformation for the rapidly precipitated ZrO₂ sample. The crystallite size for the t -ZrO₂ of all samples, irrespective of whether they transform to monoclinic, was approximately 11 nm, indicating that the t → m transformation in these materials is not controlled by differences in crystallite size. It is therefore suggested that anionic vacancies control the tetragonal-to-monoclinic phase transformation on cooling, and that oxygen adsorption triggers this phase transformation.     

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.