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 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.     

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.     

Zirconia-Related Phases in the Zirconia/Titanium Diffusion Couple After Annealing at 1100°–1550°C

A diffusion couple of 3 mol% Y₂O₃–ZrO₂ and titanium was isothermally annealed in argon at temperatures between 1100° and 1550°C. The phases and microstructure in the ceramic side were investigated using scanning electron microscopy and transmission electron microscopy, both attached to an energy-dispersive spectrometer. After annealing at 1100°C/6 h, zirconia grains did not grow conspicuously and evolved only traces of oxygen, resulting in t -ZrO_{2− x} but not α-Zr. At temperatures above 1300°C, a significant amount of oxygen evolved from zirconia, reducing the O/Zr ratio, such that α-Zr was excluded from t -ZrO_{2− x} during cooling, yielding a higher O/Zr ratio (≈2). When held at 1550°C/6 h, zirconia grains grew rapidly. The α-Zr was segregated on grain boundaries during cooling by the exsolution of zirconium from ZrO_{2− x} , while twinned t '-ZrO_{2− x} or lenticular t -ZrO_{2− x} , which was embedded in ordered c- ZrO_{2− x} , was found. The ordered c -ZrO_{2− x} was identified by the     {113} superlattice reflections of its electron diffraction patterns.     

Measurement of Phase Abundance in Magnesia-Partially-Stabilized Zirconia by Rietveld Analysis of X-ray Diffraction Data

The relative abundance of the cubic ( c ), tetragonal ( t ), monoclinic ( m ), and orthorhombic ( o ) polymorphs of ZrO₂, and the δ phase, Mg₂Zr₅O₁₂, present in samples of 3.4-wt%-magnesia-partially-stabilized zirconia have been determined by Rietveld analysis of X-ray powder diffraction data. The samples studied correspond to the as-fired (AF), and subetectoid-aged maximum-strength (MS) and thermalshock (TS) states, with their surfaces in the ground or polished condition. The polymorph abundances of the bulk and near-surface regions are discussed in relation to the type of surface treatment. Grinding produces significant quantities of both m - and o -ZrO₂ in the near-surface regions of all samples. The m content increases from about 5 wt% in the bulk, to 10, 24, and 33 wt% in AF, MS, and TS material, respectively, while the o content increases from trace amounts to about 11 wt% in all samples. The m and o phases both increase at the expense of t -ZrO₂, and the transformation is accompanied by significant lattice distortion and/or crystal size reduction. Thus, measurement of only the 'ground-surface-monoclinic' content does not give an accurate indication of the total amount of transformable t -ZrO₂ in ceramics of this kind. Polishing removes some of the ground-surface m -ZrO₂ in MS and TS, and all of the m -ZrO₂ in AF material. The o -ZrO₂ produced by grinding also declines substantially in AF and MS, but is not removed by polishing of TS. As a result, the bulk composition cannot be guaranteed, in the general case, to be accessible by X-ray analysis of polished surfaces.     

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.     

Metastable Phase Selection and Partitioning for Zr(1−x)AlxO(2−x/2) Materials Synthesized with Liquid Precursors

Aqueous solutions of zirconium acetate and aluminum nitrate were spray pyrolyzed at 250°C and upquenched to different temperatures to yield metastable solid solutions of composition Zr_{(1− x )}Al_xO_{(2− x /2)}. An amorphous oxide forms first during pyrolysis which subsequently crystallizes as a single phase for x ≤ 0.57 (≤40 mol% Al₂O₃). The crystallization temperature increased with Al₂O₃ content. Electron diffraction, supported by Raman spectroscopy, indicates that the initial phase is tetragonal. At higher temperatures, the initial solid solation partitions to other metastable phases, viz., t -ZrO₂+γ-Al₂O₃, prior to achieving their equilibrium phase assemblage, m -ZrO₂+α-Al₂O₃. Partitioning yields a nanocomposite microstructure with grain sizes of 20–100 nm, compared to the 3 to 5 nm in the initial, single phase. Compositions containing 45 to 50 mol% Al₂O₃ concurrently crystallize and partition. The structure selected during crystallization and the partitioning phenomena are discussed in terms of diffusional constraints during crystallization, which are conceptually similar to those operating during rapid solidification.     

Reaction Between Titanium and Zirconia Powders During Sintering at 1500°C

Zirconia–titanium (ZrO₂–Ti) composites have been considered potential thermal barrier graded materials for applications in the aerospace industry. Powder mixtures of Ti and 3 mol% Y₂O₃ partially stabilized ZrO₂ in various ratios were sintered at 1500°C for 1 h in argon. The microstructures of the as-sintered composites were characterized by X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy. Ti reacted with and was mutually soluble in ZrO₂, resulting in the formation of α-Ti(O, Zr), Ti₂ZrO, and/or TiO. These oxygen-containing phases extracted oxygen ions from ZrO₂, whereby oxygen-deficient ZrO₂ was generated. For relatively small Ti/ZrO₂ ratios, specimens with ≤30 mol% Ti, TiO were formed as oxygen could be sufficiently supplied by excess ZrO₂. For the specimens with ≥50 mol% Ti, lamellar Ti₂ZrO was precipitated in α-Ti(Zr, O), with no TiO being found. Both m -ZrO_{2− x} and t -ZrO_{2− x} were found in specimens with ≤50 mol% Ti; however, only c -ZrO_{2− x} was formed in the specimen with 70 mol% Ti. As ZrO₂ was gradually dissolved into Ti, yttria was retained in ZrO₂ because of the very limited solubility of yttria in α-Ti(O, Zr) or TiO. The concentration of retained yttria and the degree of oxygen deficiency in ZrO₂ increased with the Ti content. The complete dissolution of ZrO₂ into Ti was followed by the precipitation of Y₂Ti₂O₇ in the specimen with 90 mol% Ti.     

In Situ Martensitic Transformation in a Ternary MgO-Y2O3-ZrO2 Alloy: II, Transformation in Tetragonal ZrO2 Precipitates

The stress-induced martensitic transformation of t -ZrO₂ precipitates in a ternary MgO-Y₂O₃-ZrO₂ alloy has been studied in situ in the transmission electron microscope. The transformation occurs autocatalytically and takes place by piecewise growth of two twin-related m -ZrO₂ variants. Unloading causes retransformation of partially transformed precipitates, but this reverse ( m → t ) transformation of fully transformed precipitates only occurs on heating. The martensitic transformation in this system is clearly thermoelastic .     

Microstructural Characterization and Fracture Toughness of Cordierite-ZrO2 Glass-Ceramics

Crystallization of an MgO-Al₂O₃-SiO₂-ZrO₂ sintered glass frit was studied. Heat treatment at 850° or 900°C caused initial crystallization of μ-cordierite and tetragonal ( t ) ZrO₂. The t -ZrO₂ crystallized with an irregular dendritic morphology and could be transformed to monoclinic ( m ) symmetry under certain conditions; the cordierite underwent the μ→α a transformation with extended annealing. Heat treatments at 1000°C caused crystallization of t -ZrO₂ rods and spheroids in an α-cordierite matrix; these ZrO₂ crystals, however, are resistant to transformation to m -ZrO₂. The beneficial effects of ZrO₂ on the fracture toughness of cordierite-based glass-ceramics are described.     

Surface Strengthening of Alumina-Zirconia Composite by Addition of Molybdenum Dioxide

MoO₂ is found to stabilize m -ZrO₂ Because of the addition of MoO₂ into ZrO₂ at the surface region of an Al₂O₃-15 vol% ZrO₂composite, the volume fraction of m -ZrO₂ at the surface increases from 0.3 to 0.5–0.6 without changing the size of the ZrO₂grains. This enhanced transfomation results in an increase in flexural strength of up to 30% because of compressive stresses built in at the surface. The increase in flexural strength is proportional to the difference in volume fraction of m -ZrO₂ between the surface layer and the bulk matrix, in agreement with a previous analysis.     

Fracture Strength–Fracture Resistance Response and Damage Resistance of Sintered Al2O3–ZrO2 (12 mol% CeO2) Composites

The fracture strengths of sintered Al₂O₃ containing 20 and 40 vol% ZrO₂(12 mol% CeO₂)—zirconia-toughened alumina (ZTA)—composites along with the fracture resistance can be increased (e.g., to ∼900 MPa and >12 Mpa·m^1/2, respectively), by increasing the mean grain size of the t -ZrO₂ (and the Al₂O₃) from ∼0.5 μm to ∼3 μm. At these lower t -ZrO₂ contents, the fracture strength-fracture resistance curves show a continuous rise as opposed to the strength maxima observed in polycrystalline t -ZrO₂(12 mol% CeO₂), CeTZP, and ZrO₂(12 mol% CeO₂) ceramics containing ≤20 vol% Al₂O₃. The toughened composites also exhibit excellent damage resistance with fracture strengths of 500 MPa retained with surfaces containing ∼150- N Vickers indentations which produce cracks of ∼160-μm radius. Greater damage resistance correlates with an increase in the apparent R -curve response of these composites.     

Crystallization of MgO-Al2O3-SiO2-ZrO2 Glasses

The crystallization of MgO-Al₂O₃-SiO₂-ZrO₂ glasses at 1000°C was studied. Isothermal heat treatments of a cordierite-based glass (2MgO.2Al₂O₃.5SiO₂= Mg₂Al₄Si₅O₁₈) with 7 wt% ZrO₂ produced surface crystallization of α-cordierite and tetragonal ZrO₂ ( t -ZrO₂). These phases advanced into the glass by cocrystallization of t -ZrO₂ rods in an α-cordierite matrix with a well-defined orientation relation. The t -ZrO₂ rods were unstable with respect to diffusional breakup (a Rayleigh instability) and decomposed into rows of aligned ellipsoidal and spheroidal particles. The t -ZrO₂ was very resistant to transformation to monoclinic symmetry. With a similar glass containing 15 wt% ZrO₂, surface crystallization of α-cordierite and t -ZrO₂ was accompanied by internal crystallization of t -ZrO₂ dendrites. Transformation of the dendrites to mono-clinic symmetry was observed under some conditions.     

Modeling and Fabrication of Fine-Grain Alumina-Zirconia Composites Produced from Nanocrystalline Precursors

Fully dense fine-grained 32.6-vol%-zirconia-toughened alumina composites have been fabricated from nanocrystalline rapidly solidified material. A model considering the thermodynamics of the constrained t -ZrO₂ m -ZrO₂ phase transformation was developed for this percolated two-phase material. This analysis indicated that the grain size at which this phase transformation is thermodynamically favorable was 1.26 µm in a composite that contained 32.6 vol% ZrO₂ and was stabilized with 1.50 mol% Y₂O₃. These results of the model compared favorably with experimental results, showing that grains of this size could be retained after heating to temperatures of as high as 1600°C. The rapidly solidified precursor was ball-milled into submicrometer powder and centrifugally cast into green specimens that were pressureless sintered to full density at temperatures as low as 1500°C. A composite containing nearly 100% t -ZrO₂ was produced by pressureless sintering at 1500°C and a composite containing 45 vol% t -ZrO₂/55 vol% m -ZrO₂ was obtained by sintering at 1600°C. The resulting two-phase microstructures contained uniformly distributed, micrometer-size grains whose sizes are consistent with the facilitation of transformation and microcrack toughening.     

Phase Equilibration in ZrO2-Y2O3 Alloys by Liquid-Film Migration

The tetragonal ( t ) and cubic ( c ) ZrO₂ solid solutions in two-phase ZrO₂-8 wt% Y₂O₃ ceramics have low and high solute content, respectively. Annealing samples sintered at 1600°C between 700° and 1400°C requires a change in the volume fraction of the coexisting phases, as well as their equilibrium Y₂O₃ content. The enrichment in Y₂O₃ content of the c -ZrO₂ grains is accomplished by liquid-film migration involving the ubiquitous silicate grain-boundary phase, while the volume fraction of t -ZrO₂ increases by the nucleation and growth of cap-shaped t -ZrO₂ lenses. The interfaces between the c -ZrO₂ matrix and the growing t -ZrO₂ lenses are semicoherent.     

Thin-Foil Phase Transformations of Tetragonal ZrO2 in a ZrO2-8 wt% Y2O3 Alloy

Tetragonal zirconia ( t -ZrO₂) grains in an annealed ZrO₂ 8 wt% Y₂O₃ alloy transformed to orthorhombic ( o ) or monoclinic ( m ) symmetry by stresses induced by localized electron beam heating in the transmission electron microscope. Different transformation mechanisms were observed, depending on foil thickness and orientation of individual grains. In thicker grains (≥150 nm), the transformation proceeded by a burst-like growth of m laths, and this is believed to approximate bulk behavior. In thinner grains near the edge of the foil, usually those with a [100], orientation perpendicular to the thin-foil surface, "continuous" growth of an o or m phase with an antiphase-boundary-containing microstructure was observed. The o phase is believed to be a high-pressure poly-morph of ZrO₂, which forms (paradoxically) as a thin-foil artifact because it is less dense than t -ZrO₂, but more dense than m -ZrO₂. In some very thin grains, the t → m transformation was thermoelastic. Furthermore, a mottled structure often occurred just before the t → m or t → o transformation, which is attributed to surface transformation. Aside from the lath formation, the observed transformation modes are a result of the reduced constraints in thin foils.     

Diffusionless Tetragonal–Cubic Transformation Temperature in Zirconia–Ceria Solid Solutions

Diffusionless tetragonal ( t' )  cubic ( c' ) phase transformation in 65-mol%-CeO₂-ZrO₂ was investigated around the c'-t' equilibrium temperature T₀ ^c'-t' using powder X-ray diffraction, where c' was defined as a tetragonal or cubic phase with an axial ratio of unity. The {400} peak profile of t' -ZrO₂ broadened at 860° and 900°C. This indicated that the t' -ZrO₂ transformed partially into c' -ZrO₂. The {400} peak profile of c' -ZrO₂ also broadened at 860° and 900°C, which indicated that the c' -ZrO₂ partially transformed to t' -ZrO₂. The finish point of t'→c' transformation T_f^t'→^c' was investigated by annealing the t' -ZrO₂ containing 30–65 mol% CeO₂ at various temperatures. The T'_f^t'→c' line existed in the vicinity of the cubic solubility limit within the t + c two-phase region. This T_f^t'→c' location, which should have been located in the vicinity of T₀^c'-t' could not be explained by a simple regular solution model. However, it was described successfully by a thermodynamic model based on Landau's phenomenologic theory.     

Formation of Zirconia Solid Solutions Containing Alumina Prepared by New Preparation Method

In the system ZrO₂–Al₂O₃, a new method for preparing ZrO₂ solid solutions from ZrCl₄ and AlCl₃ using hydrazine monohydrate is investigated. c -ZrO₂ solid solutions containing up to ∼40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials. The formation mechanism is discussed from IR spectral data. The values of the lattice parameter α increase linearly from 0.5072 to 0.5105 nm with increasing Al₂O₃ content. At higher temperatures, transformation of the solid solutions proceeds as follows: c ( SS ) → t ( ss ) → t ( ss ) +α-Al₂O₃→ m +α-Al₂O₃. m -ZrO₂–α-Al₂O₃ composite ceramics are fabricated by hot isostatic pressing for 2 h at 1250°C and 196 MPa. Microstructures and mechanical properties are examined, in connection with increasing Al₂O₃ content.     

Processing and Microstructure of Mullite-Zirconia Composites Prepared from Sol-Gel Powders

A high-purity stoichiometric mullite precursor was obtained by hydrolysis of the alkoxides Al(OC₃H₇)₃ and Si(OC₂H₂)₄. Fully sintered mullite ceramics can be prepared from sol-gel powders by sintering them at 1600°C for 4 h in air with the addition of 15 to 20 Vol% ZrO₂ or 1 to 3 mol% Y₂O₃ or both. Introduction of 1 to 3 mol% Y₂O₃ aids the retention of tetragonal ZrO₂; the volume fraction of t -ZrO₂ retained increases with increasing Y₂O₃ content. The maximum t -ZrO₂ retained reaches 34% in a matrix of synthetic mullite with 3 mol% Y₂O₃, but most of this t -ZrO₂ does not undergo stress-induced transformation during grinding.