Metallographic Observation of the Monoclinic-Tetragonal Phase Transformation in ZrO2

The monoclinic-tetragonal phase transformation of ZrO₂ was examined with a vacuum hot-stage microscope. Polished sections of vacuum-sintered 99.7% pure ZrO₂ were observed from room temperature to 1300°C. The rapid formation of platelet substructure within ZrO₂ grains in the temperature range 1050° to 1150°C was associated with the heating transformation. Photomicrographs and motion pictures were taken of the specimen as the transformation progressed. The surface deformation was irreversible, preventing observation of the phase inversion on cooling. From these hot-stage studies, supplementary DTA, X-ray, and thermal expansion data, and other existing information, it is concluded that the transformation is of the diffusionless, athermal type, characteristic of Fe-Ni martensitic transformations.     

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.     

Eutectoid Decomposition of MgO-Partially-Stabilized ZrO2

Euctectoid decomposition of cubic ( c ) ZrO₂ in MgO-partially-stabilized ZrO₂ (Mg-PSZ) has been studied using optical, scanning electron, and transmission electron microscopy. Alloys containing from 8.1 to 18.6 mol% MgO were decomposed by annealing between 1100° and 1300°C for times up to 16 h. The eutectoid products nucleated heterogeneously at the grain boundaries and advanced into the adjoining two-phase grains. Decomposition proceeded as a "cellular" reaction involving the cooperative growth of MgO and a low-solute ZrO₂ phase with either monoclinic ( m ) or tetragonal ( t ) symmetry. The MgO morphology is rodlike and exhibits a well-defined orientation relationship to m -ZrO₂. The rate of eutectoid decomposition was a maximum at 1200°C and was greater in the solute-rich materials at all temperatures. At 1100°C, SrO doping decreased the nucleation rate of the eutectoid product in a 9.7 mol% alloy, thus strongly suppressing the decomposition rates; at the higher decomposition temperatures, the SrO was less effective.     

Grain Growth in Two-Phase Ceramics: Al2O3 Inclusions in ZrO

The effect of Al₂O₃ inclusions with a greater average size (0.6 μm) than the average particle size of the major phase powder (<0.1 μm) on grain gowth was examined by sintering ZrO₂/Al₂O₃ composites (0,3,5,10, and 20 vol%) at 1400°C and then heat-treating at temperatures up to 1700°C. Normal grain growth was observed for all conditions. The inclusions appeared to have no effect on grain growth until the ZrO₂ grain size was ∼1.5 times the average inclusion size. Grain growth inhibition increased with volume fraction of the Al₂O₃ inclusion phase. At temperatures 1600°C, the inclusions were relatively immobile and most were located within the ZrO₂ grains for volume fractions <0.20; at higher temperatures, the inclusions could move with the grain boundary to coalesce. Grain growth was less inhilited when the inclusions could move with the boundaries, resulting in a larger increase in grain size than observed at lower temperatures. Analogies between mobile voids, entrapped within grain at lower temperature due to abnormal grain growth during the last state of sintering, and the observations concerning the mobile inclusions are made suggesting that grain-boundary movement can "sweep" voids to grain boundaries and eventually of four-grain junctions, where they are more likely to disappear by mass transport.     

Effect of MgO Addition on the Microstructure Development of 3 mol% Y2O3—ZrO2

MgO addition to 3 mol% Y₂O₃–ZrO₂ resulted in enhanced densification at 1350°C by a liquid-phase sintering mechanism. This liquid phase resulted from reaction of MgO with trace impurities of CaO and SiO₂ in the starting powder. The bimodal grain structure thus obtained was characterized by large cubic ZrO₂ grains with tetragonal ZrO₂ precipitates, which were surrounded by either small tetragonal grains or monoclinic grains, depending on the heat-treatment schedule.     

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.     

Hindrance of Grain Growth in Al2O3 by ZrO2 Inclusions.

Alumina and Al₂O₃/ZrO₂ (1 to 10 vol%) composite powders were mixed and consolidated by a colloidal method, sintered to >98% theoretical density at 1550°C, and subsequently heat-treated at temperatures up to 1700°C for grain-size measurements. Within the temperature range studied, the ZrO₂ inclusions exhibited sufficient self-diffusion to move with the Al₂O₃ 4-grain junctions during grain growth. Growth of the ZrO₂, inclusions occurred by coalescence. The inclusions exerted a dragging force at the 4-grain junctions to limit grain growth. Abnormal grain growth occurred when the inclusion distribution was not sufficiently uniform to hinder the growth of all Al₂O₃ grains. This condition was observed for compositions containing ≤2.5 vol% ZrO₂, where the inclusions did not fill all 4-grain junctions. Exaggerated grains consumed both neighboring grains and ZrO₂, inclusions. Grain-growth control (no abnormal grain growth) was achieved when a majority (or all) 4-grain junctions contained a ZrO₂ inclusion, viz., for compositions containing ≥5 vol% ZrO₂. For this condition, the grain size was inversely proportional to the volume fraction of the inclusions. Since the ZrO₂ inclusions mimic voids in all ways except that they do not disappear, it is hypothesized that abnormal grain growth in single-phase materials is a result of a nonuniform distribution of voids during the last stage of sintering.     

Solubility of TiO2 in ZrO2

The solubility of TiO₂ in tetragonal ZrO₂ is 13.8±0.3 mol% ui 1300°C, 14.9±0.2 mol% at 1400°C, and 16.1±0.2 mol% at 1500°C. These solid solutions transform to metastable monoclinic solid solutions without compositional change on cooling to room temperature.     

Phase Transformation in Y2O3-Partially-Stabilized ZrO2 Polycrystals of Various Grain Sizes during Low-Temperature Aging in Water

ZrO₂-2 mol% Y₂O₃ crystals with average grain sizes from 0.51 to 0.96 µm were prepared by sintering in air at 1400°C for 2 to 100 h. The tetragonal-to-monoclinic phase transformation associated with the low-temperature degradation was investigated to clarify how the presence of water directly affects the influence of grain size on transformation. The specimens were exposed to water at 80–120°C, a temperature range in which transformation by thermal activation is difficult in the absence of water. Contrary to expectations, this type of low-temperature transformation did not accelerate monotonously with increasing grain size. Instead, the amount of phase transformation first decreased, reaching a constant value, and then increased with increasing grain size. Such interesting results can be explained satisfactorily by the combined influences of grain size on the nucleation process, because of preferential dissolution of yttrium at the grain boundaries, and the intrinsic transformability of Y₂O₃-doped tetragonal ZrO₂ grains.     

Low-Temperature Equilibria Among ZrO2, Tho2, and UO2

Studies made on low-hafnium-content ZrO₂, show that the monoclinic-tetragonal inversion temperature is 1170°C., and it is raised to approximately 1190°C. in the "natural" ZrO², which contains approximately 2% HfO₂. No explanation could be found for the knownmarked hysteresis during cooling, when the reverse polymorphic transformation takes dace at 1040°C. In the system ZrO₂-ThO₂ the monoclinic-tetragonal ZrO₂, inversion temperature is lowered to 1000°C., although the maximum solid solution extent of ZrO₂, in Thon and vice versa is approximately only 2% at this temperature. Below about 400°C. under hydrothermal conditions it was possible to prepare a continuous, although metastable series of solid solutions with the fluorite structurewith compositions varying from ThO₂, to nearly pure ZrO₂. Contrary to earlier work only 8 mole ZrO₂, dissolves in UO₂ and less than 4 mole of UO, in ZrO₂ at temperatures up to 13OO0C. A continuous series of solid solutions could be made between Th₂ and UO₂ at 13OO°C., and extensive defect fluorite solid solutions could be prepared between Tho₂ and U₃O₈; there is some evidence for exsolution into uranium-rich and thorium-rich members at low temperatures.     

Transformation Kinetics of Ba2Ti9O20 with ZrO2 Additive

Pure Ba₂Ti₉O₂₀ (BT29) was synthesized by a solid-state reaction in one step with various amounts of ZrO₂ powder additive. The transformation kinetics of BT29 were investigated by quantitative X-ray diffractometry (XRD). The results show that stoichiometric powder mixtures transform to the BT29 phase by nucleation and growth mechanism between 1200° and 1300°C with 1.0 mol% ZrO_2. The activation energy of the transformation was found to be 620±60 kJ/mol, but decreases to 515±30 kJ/mol when doped with 1.0 mol% ZrO₂. The addition of ZrO₂ possibly changes the phase transformation mechanism of BT29 from diffusion controlled to interface controlled.     

Processing and Mechanical Behavior of CrN/ZrO2(2Y) Composites

CrN powder consisting of granular particles of ∼3 μm has been prepared by self-propagating high-temperature synthesis under a nitrogen pressure of 12 MPa using Cr metal. Dense pure CrN ceramics and CrN/ZrO₂(2Y) composites in the CrN-rich region have been fabricated by hot isostatic pressing for 2 h at 1300°C and 196 MPa. The former ceramics have a fracture toughness ( K _IC) of 3.3 MPa ·m^1/2 and a bending strength (σ_b) of 400 MPa. In the latter materials almost all of the ZrO₂(2Y) grains (0.36–0.41 μm) are located in the grain boundaries of CrN (∼4.6 μm). The values of K _IC (6.1 MPa · m^1/2) and σ_b (1070 MPa) are obtained in the composites containing 50 vol% ZrO₂(2Y).     

Formation Reaction of ZrO2 Scatterers in ZrF4-Based Fluoride Fibers

This paper clarifies the formation reaction of ZrO₂ crystals which appear as extrinsic scatterers in fluoride fibers. EPMA analysis indicates that BaO exists at grain boundaries of BaF₂ purified by sublimation. BaO reacts with ZrF₄ to form ZrO₂ at 600°C during a glass-melting process. The ZrO₂ formation reaction is influenced by H₂O. Ba(OH)₂, which is formed by the reaction between BaO and water vapor, melts at 370° to 420°C and reacts with ZrF₄ to form ZrO₂ at 450° to 520°C. When low-oxide-content BaF₂ is used for fiber preparation, scatterers significantly decrease.     

Subsolidus Phase Relations in the Pseudoternary System ZrO2-YO1.5-CrO1.5 in Air

The pseudoternary system ZrO₂-YO_1.5CrO_1.5 was studied between 1300° and 1600C in air by °a quenching method. No ordered phase of the type ZrY₆O₁₁ was detected, but an ordered Zr₃Y₄O₁₂ phase at 1300°C and YCrO₃ were observed as intermediate compounds. Solid solutions ofZrO₂ and YO_1.5 coexisting with CrO_1.5 and/or YCrO₃ formed; the apex occurred between 26.5 and 27.5 wt% YO_1.5 for the cubic ZrO₂+CrO_1.5+YCrO₃, three-phase region; CrO_1.5 is slightly soluble in ZrO₂(ss).     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

Mechanical Properties of CoAl Materials with the Combined Additions of ZrO2(3Y) and Al2O3

Intermetallic CoAl powder has been prepared via self-propagating high-temperature synthesis (SHS). Dense CoAl materials (99.6% of theoretical) with the combined additions of ZrO₂(3Y) and Al₂O₃ have been fabricated via spark plasma sintering (SPS) for 10 min at 1300°C and 30 MPa. The microstructures are such that tetragonal ZrO₂ (0.3 μm) and Al₂O₃ (0.5 μm) particles are located at the grain boundaries of the CoAl (8.5 μm) matrix. Improved mechanical properties are obtained; especially the fracture toughness and the bending strength of the materials with ZrO₂(3Y)/Al₂O₃= 16/4 mol% are 3.87 MPa·m^1/2 and 1080 MPa, respectively, and high strength (>600 MPa) can be retained up to 1000°C.     

Effect of ZrO2 Inclusions on the Sinterability of Al2O3

The sinterability of Al₂O₃/ZrO₂ composite powder compacts containing 2 and 10 vol% ZrO₂ was compared to the sinterability of their single-phase constituents through constant-heating-rate experiments. The ZrO₂ inclusion phase delayed the initiation of bulk shrinkage and the temperature of maximum strain rate by 100°C. The ZrO₂ inclusion phase also significantly inhibited grain growth. These results, discussed with regard to the thermodynamics of pore disappearance, suggest that phenomena inhibiting grain growth may also inhibit densification.     

Coupled Grain Growth Effects in Al2O3/10 vol% ZrO2

Coupled crystallization has been observed in the Al₃O₃/10%-ZrO₂ system by heating an amorphous precursor Al /Zr copolymerized alkoxide network structure. A finely divided two-phase material results which stabilizes tetragonal ZrO₂ to 1700°C and exhibits an unprecedented microstructure. During crystallization, the grain growth of ZrO₂ is coupled to the γ→α phase transformation of Al₂O₃.     

Transformation-Toughened ZrO2: Correlations between Grain Size Control and Composition in the System ZrO2-Y2O3

The average grain size of ZrO₂(+Y, o,) materials sintered at 1400°C was observed to depend significantly on the Y₂O₃ content. The average grain size decreased by a factor of 4 to 5 for Y₂O₃ contents between 0.8 and 1.4 mol% and increased at Y₂O₃ contents of 6.6 mol%. Grain growth control by a second phase is the concept used to interpret these data; compositions with a small grain size lie within the two-phase tetragonal + cubic phase field, and the size of the tetragonal grains is believed to be controlled by the cubic grains. This interpretation suggests that the Y₂O₃-rich boundary of the two-phase field lies between 0.8 and 1.4 mol% Y₂O₃. Transformation toughened materials fabricated in this binary system must have a composition that lies within the two-phase field to obtain the small grain size required, in part, to retain the tetragonal toughening agent.     

Low-Temperature Sintering of Seeded Sol—Gel-Derived, ZrO2-Toughened Al2O3 Composites

Seeding a mixture of boehmite (AIOOH) and colloidal ZrO₂ with α-alumina particles and sintering at 1400°C for 100 min results in 98% density. The low sintering temperature, relative to conventional powder processing, is a result of the small alumina particle size (∼0.3 μm) obtained during the θ-to α-alumina transformation, homogeneous mixing, and the uniform structure of the sol-gel system. Complete retention of pure ZrO₂ in the tetragonal phase was obtained to 14 vol% ZTA because of the low-temperature sintering. The critical grain size for tetragonal ZrO₂ was determined to be ∼0.4 μm for the 14 vol% ZrO₂—Al₂O₃ composite. From these results it is proposed that seeded boehmite gels offer significant advantages for process control and alumina matrix composite fabrication.