Systematics of Phase Transition and Mixing Energetics in Rare Earth, Yttrium, and Scandium Stabilized Zirconia and Hafnia

Energetics of rare earth, yttrium, and scandium stabilized zirconia and hafnia have been systematically investigated by oxide melt solution calorimetry. The enthalpies of formation with respect to the oxide end members were simultaneously fit to a quadratic function to extract interaction parameters and enthalpies of transition of the oxide end members to the fluorite structure. ZrO₂–SmO_1.5 and HfO₂–SmO_1.5 show the most exothermic enthalpies of formation and interaction parameters, whereas ZrO₂–ScO_1.5 has the least exothermic enthalpy of formation and interaction parameter. This suggests that the ZrO₂–ScO_1.5 system shows the least short range order among all investigated systems, consistent with its high ionic conductivity. The extrapolated enthalpy of transition of the rare earth oxide end members to the cubic fluorite structure increase to more endothermic values with decreasing cation size. The γ-cubic fluorite phase transition in ZrO₂–ScO_1.5 was investigated by differential scanning calorimetry (DSC). The phase transition is reversible, occurs at 1000°–1200°C and shows hysteresis (∼100°C). The enthalpy of transition is endothermic on heating and increases from 1.7±0.1 kJ/mol (22 mol% ScO_1.5) to 2.9±0.2 kJ/mol (30 mol% ScO_1.5).     

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.     

Stabilization of Zirconia Sintered with Titanium

Electron and X-ray diffraction studies show that zirconia sintered with 5 to 15 mol% titanium under a vacuum of 10⁻¹ to 10⁻² torr (∼13 to 1.3 Pa) was partially stabilized as cubic and tetragonal phases, whose amounts increase with increasing Ti content. The stabilization of ZrO₂ is due to the dissolution of TiO which forms as a second phase in the sintered specimen. The grain size of ZrO₂ decreases with increase in Ti. The improvements in strength and thermal shock resistance of ZrO₂ sintered with Ti are attributed to the reduction in ZrO₂ grain size and the effect of partial stabilization.     

Metastable Alumina Structures in Melt-Extracted Alumina–25 wt% Zirconia and Alumina–42 wt% Zirconia Ceramics

Microstructures are examined of rapidly solidified hypoeutectic Al₂O₃–25 wt% ZrO₂ and eutectic Al₂O₃–42 wt% ZrO₂ ceramic alloys by using transmission electron microscopy and scanning transmission electron microscopy. Structures observed in the hypoeutectic alloy were dendritic. Three different types of dendrite morphologies were observed. These are believed to be the stable α- and metastable γ- and δ- modifications of alumina. The cores of the γ-alumina dendrites were somewhat richer in ZrO₂ than those of α-alumina dendrites; δ-alumina dendrites were substantially enriched in ZrO₂. The lamellar structure of the eutectic in the Al₂O₃–42 wt% ZrO₂ alloy became increasingly finer with increasing cooling rate and at the highest cooling rates was replaced by a fully amorphous structure (except in some instances a ZrO₂-rich needlelike phase identifiable as δ-alumina was found in the amorphous matrix). An interpretation is given of the results obtained, based on assumed metastable free energy curves and dendrite growth theory.     

Direct Calorimetric Measurement of Enthalpies of Phase Transitions at 2000°–2400°C in Yttria and Zirconia

High-temperature differential thermal analysis provided data on phase transitions in zirconia and yttria. The tetragonal form of ZrO₂ transforms to the cubic fluorite structure at 2311°±15°C with an enthalpy of 3.4±3.1 kJ/mol. Cubic C-type Y₂O₃ transforms, probably to the fluorite structure, at 2308°±15°C with Δ H =47.7±3.0 kJ/mol. This high-temperature polymorph melts at 2382°±15°C with an enthalpy of fusion of 35.6±3.0 kJ/mol.     

Phase Characterization of Partially Stabilized Zirconia by Raman Spectroscopy

Raman spectroscopy was used to characterize phase transformations and transition temperatures in partially stabilized zirconia containing ≤20 wt% Y₂O₃. The completeness of the martensitic transition and its thermal hysteresis was followed in samples with ≤4 wt% Y₂O₃. Between 5 and 12 wt% the spectra indicate a tetragonal modification precipitated in a disordered cubic fluorite matrix. Above 15 wt% a "density of states" spectrum prevails that is characteristic of the fully stabilized disordered cubic phase.     

Microstructural Characterization of ZrO2/O'-SiAION Composites

Zirconia has demonstrated a very moderate toughening effect in nitrogen-based ceramic composites because the reaction between tetragonal zirconia ( t -ZrO₂) and nitrogen results in additional zirconia stabilization to a nontrans-formable t ' or cubic structure. In O'-SiAlON matrices, the oxygen concentration increases and the oxygen-rich inter-granular glassy phase prevents zirconia from nitridation. As a result, tetragonal ZrO₂ is maintained and is transformable in the O'-SiAlON materials. The present study has provided transmission electron microscopy (TEM) evidence of the zirconia transformation and the associated toughening effect in a ZrO₂/O'-SiAlON composite. The implications and limitations of the transformation on toughening of the material are discussed.     

Structure and Thermomechanical Properties of Partially Stabilized Zirconia in the CaO-ZrO2 System

Partially stabilized zirconia (PSZ) ceramics in the system CaO-ZrO₂ were characterized. The microstructure, as revealed by optical microscopy, consisted of grains of pure ZrO₂ distributed in a matrix of fully stabilized material. Electron microscopy showed that the matrix grains have a complex substructure of 1000-Å domains of cubic and monoclinic ZrO₂. The grains appeared to fit Ubbelohde's concept of a hybrid single crystal. Evidence obtained indicated that the substructure provides an effective stress-relieving mechanism during thermal shock. It is proposed that initiation of phase inversion in pure ZrO₂ domains, even at subtransition temperatures (by thermal stresses), creates an extremely large microcrack density. On the basis of Hasselman's thermal-shock criterion, only quasi-static crack propagation occurs during thermal shock of PSZ; evidence is presented to support this concept.     

Subsolidus Phase Equilibria and Ordering in the System ZrO2-Y2O3

The subsolidus phase relations in the entire system ZrO₂-Y₂O₃ were established using DTA, expansion measurements, and room- and high-temperature X-ray diffraction. Three eutectoid reactions were found in the system: ( a ) tetragonal zirconia solid solution→monoclinic zirconia solid solution+cubic zirconia solid solution at 4.5 mol% Y₂O₃ and ∼490°C, ( b ) cubic zirconia solid solutiow→δ-phase Y₄Zr₃O₁₂+hexagonalphase Y₆ZrO₁₁ at 45 mol% Y₂O₃ and ∼1325°±25°C, and ( c ) yttria C -type solid solution→wcubic zirconia solid solution+ hexagonal phase Y₆ZrO₁₁ at ∼72 mol% Y₂O₃ and 1650°±50°C. Two ordered phases were also found in the system, one at 40 mol% Y₂O₃ with ideal formula Y₄Zr₃O₁₂, and another, a new hexagonal phase, at 75 mol% Y₂O₃ with formula Y₆ZrO₁₁. They decompose at 1375° and >1750°C into cubic zirconia solid solution and yttria C -type solid solution, respectively. The extent of the cubic zirconia and yttria C -type solid solution fields was also redetermined. By incorporating the known tetragonal-cubic zirconia transition temperature and the liquidus temperatures in the system, a new tentative phase diagram is given for the system ZrO₂-Y₂O₃.     

Development of WC–ZrO2 Nanocomposites by Spark Plasma Sintering

In the present work, we report the processing of ultrahard tungsten carbide (WC) nanocomposites with 6 wt% zirconia additions. The densification is conducted by the spark plasma sintering (SPS) technique in a vacuum. Fully dense materials are obtained after SPS at 1300°C for 5 min. The sinterability and mechanical properties of the WC–6 wt% ZrO₂ materials are compared with the conventional WC–6 wt% Co materials. Because of the high heating rate, lower sintering temperature, and short holding time involved in SPS, extremely fine zirconia particles (∼100 nm) and submicrometer WC grains are retained in the WC–ZrO₂ nanostructured composites. Independent of the processing route (SPS or pressureless sintering in a vacuum), superior hardness (21–24 GPa) is obtained with the newly developed WC–ZrO₂ materials compared with that of the WC–Co materials (15–17 GPa). This extremely high hardness of the novel WC–ZrO₂ composites is expected to lead to significantly higher abrasive-wear resistance.     

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.     

Precipitation in Partially Stabilized Zirconia

Transmission electron microscopy was used to study the sub-structure of partially stabilized ZrO₂ (PSZ) samples, i.e. 2-phase "alloys" containing both cubic and monoclinic modifications of zirconia, after various heat treatments. Monoclinic ZrO₂ exists as (1) isolated grains within the polycrystalline aggregate (a grain-boundary phase) and (2) small plate-like particles within cubic grains. These intragranular precipitates are believed to contribute to the useful properties of PSZ via a form of precipitation hardening. These precipitates initially form as tetragonal ZrO₂, with a habit plane parallel to the {100} matrix planes. The orientation relations between the tetragonal precipitates and the cubic matrix are
and      

Directional Solidification of the ZrO2-MgO Eutectic

The eutectic between ZrO₂ and MgO was located at 27 wt% MgO. Directional solidification of this composition yields MgO rods in a cubic ZrO₂ matrix. Plane front growth occurred at solidification rates of <2.0 cm/h. The interfiber spacing was proportional to the inverse square root of the solidification rate. In samples where cellular growth occurred, the colony size was not affected by the solidification rate. The room-temperature fracture strength, ∼23,000 psi, which was not affected by the MgO fiber size, was controlled by the colony size. Annealing for 24 h at 1200°C increased the fracture strength by 50% as a result of a compressive stress in the matrix caused by the cubic-to-monoclinic transformation in the ZrO₂.     

Phase Analysis in Zirconia Systems

Linear calibration curves were developed for determining the content of free ZrO₂ in partially stabilized zirconia ceramics by X-ray diffraction techniques. Two methods were studied. The matrix method, in which free ZrO₂ was considered to be distributed in a matrix (the cubic phase), gave approximately equal mass absorption coefficients for the monoclinic and cubic phases. The polymorph technique, in which the cubic phase was considered to be a polymorph of ZrO₂ and in which integrated intensities were used, gave the better results.     

Some Properties of Hafnium Oxide, Hafnium Silicate, Calcium Hafnate, and Hafnium Carbide

The behavior of hafnium oxide was studied particularly in the temperature range 1500° to 18OO°C. Properties of HfO₂ at these temperatures and its reactions with ZrO₂, SiO₂, and CaO are given in terms of lattice and other physical measurements, many of which are new. Mono-clinic hafnium oxide is stable to 1700°C., which is 600° higher than the corresponding inversion temperature of zirconia. Otherwise HfO₂ closely resembles ZrO₂ (a) in its lattice dimensions and sintering behavior, (b) in forming a high-temperature tetragonal phase closely resembling tetragonal ZrO₂, (c) in forming a continuous series of solid solutions with ZrOz, (d) in forming with silica a single compound (HfO₂.SiO₂) similar to zircon, (e) in forming a carbide, (f) in uniting with up to 40% CaO to form cubic solid solutions; thereafter a compound CaO.Hf O₂ appears which is very similar to the corresponding zirconia compound.     

Crystallization and Transformation of Zirconia Under Hydrothermal Conditions

During constant-rate heating to 350°C in concentrated NaOH solutions, cubic ZrO₂ crystallized at ∼120°C from hydrated amorphous ZrO₂; these One cubic ZrO₂ particles abruptly changed into needlelike monoclinic ZrO₂ single crystals at 300°C. Crystallization and phase transformation were studied by XRD, TEM, and EPMA. Cubic ZrO₂ appears to crystallize via collapse of the ZrO₂−nH₂O structure and subsequent slight rearrangement of the lattice. The abrupt formation of mono-clinic ZrO₂ was considered to result when the very fine cubic ZrO₂ particles coagulated in a highly oriented fashion.     

Thermodynamic Model of the Cubic → Tetragonal Transition in Nonstoichiometric Zirconia

Nonstoichiometric zirconia is described with a model recently developed for ZrO₂—Y₂O₃ alloys. It is thus possible to rationalize the experimental information on the cubic/tetragonal phase boundaries in zirconia.     

Twinning and Microcracking Associated with Monoclinic Zirconia in the Eutectic System Zirconia–Mullite

Crystallography and morphology of twins and microcracks in the eutectic system mullite–zirconia are discussed in view of the tetragonal-to-monoclinic transformation and associated toughening mechanisms. Specific twin relationships were observed in monoclinic ZrO₂. Highly symmetric and crystallographically well-defined microcracks were observed at the mullite–ZrO₂ interface. Microdiffraction revealed closely related crystallography of monnoclinic twins and microcracks. The number of twin variants depend on the monoclinic ZrO₂ particle size. A method to calculate twinning shear strain using microcrack morphology is suggested. This parameter is essential in several fracture-mechanics calculations.     

The Pseudobinary Ti-ZrO2

The vertical section Ti-ZrO₂ within the Ti-Zr-O system was investigated by metallographic, X-ray diffraction, electron probe, and melting point studies. Analyses were conducted using arcmelted specimens which had been equilibrated and quenched from temperatures of 600° to 1600°C. The Ti-ZrO₂ section is similar to the Zr-ZrO₂ system. At high temperatures, considerable amounts of Zr and O go into solid solution in Ti, stabilizing α-Ti to 30 wt% ZrO₂. From 30 to 98 wt% ZrO₂ an α-Ti+ZrO₂ region is defined, and at compositions above 98 wt% ZrO₂, single-phase ZrO₂( ss ) exists. At low temperatures an α-Ti+(Ti,Zr)₃O field exists from 22 to 32 wt% ZrO₂; this region decreases in size with increasing temperature until it disappears at 1200°C. Above 32 wt% ZrO₂, a three phase α-Ti+ (Ti,Zr)₃O+ZrO₂ field exists; its stability extends from 1200°C at 30 wt%      

New Method for the Preparation and Stabilization of Nanoparticulate t-ZrO2 by a Combined Sol–Gel and Solvothermal Process

Nonstabilized and silica-stabilized zirconia (ZrO₂) crystallites with sizes between 3 and 4 nm were synthesized by a novel combined sol–gel and solvothermal process. After adding zirconium n -propoxide to a solution of isobutanol, propionic acid, and water, a transparent nanoparticulate sol was synthesized by a sol–gel process. The average hydrodynamic diameter of the amorphous ZrO₂ nanoparticles was approximately 5 nm. A following solvothermal process led to a crystalline fraction of the powder consisting of 31 wt% tetragonal ( t ) phase ZrO₂. This fraction was doubled to 61 wt% by adding 10 mol% 3-methacryloxypropyl trimethoxysilane (MPTS) and increased to 96 wt% by a subsequent calcination at 800°C. The crystallite sizes were also confirmed by means of Brunauer–Emmett–Teller and high-resolution transmission electron microscopy.