ZrO2 Single Crystals Research Articles

High-temperature plastic deformation of Y2O3-stabilized ZrO2single crystals. I. The origin of the yield drop and associated glide polygonization

Abstract Plastic deformation in Y2O3-stabilized ZrO2 single crystals, oriented for single glide on the {001} system, has been studied at 1400[ddot]C. Stress-strain curves exhibit a yield drop which is due to dislocation multiplication but involves activation of secondary {111} slip systems. X-ray topography revealed that dislocation walls perpendicular to the active slip planes formed at small strains and persisted throughout the deformation.

Microhardness and Fracture Toughness Anisotropy in Cubic Zirconium Oxide Single Crystals

Vickers and Knoop indentation tests have been used to study the fracture and deformation characteristics of 9.4‐mol%‐Y2O3‐stabilized ZrO2 single crystals. Kc is anisotropic, with values of 1.9 and 1.1 MPa·m1/2 for radial cracks propagating along (100) and (110), respectively. The toughness for these two orientations was also determined using the single‐edge notched‐beam geometry, and yielded values of 1.9 and 1.5 MPa·m1/2.

Thermal Diffusivity of Partially and Fully Stabilized (Yttria) Zirconia Single Crystals

Laser flash measurements of thermal diffusivity of ZrO2 single crystals partially and fully stabilized with Y2O3 were compared with measurements for polycrystalline cubic ZrO2, and single crystals and polycrystals of Al2O3 and MgAl2O4. In general, the thermal diffusivities of the ZrO2 materials examined initially decrease with increasing temperature, although significantly less than for the Al2O3 and MgAl2O4 materials. The diffusivity subsequently rises with increasing temperature for the single crystals of ZrO2 but not for polycrystalline cubic ZrO2, with this increase for ZrO2 crystals occurring at much lower temperatures than for Al2O3 and MgAl2O4 crystals. As the ZrO2 materials went from fully stabilized with 20 wt% Y2O3 to partially stabilized with 5 wt% Y2O3, the room‐temperature diffusivity increased from 0.70x10‐6 to 0.97x10‐6 m2/s. This indicated that the lattice defects, which increase with Y2O3 content, are more important sources of phonon scattering than are the precipitates which appear at lower Y2O3 compositions. On the other hand, at 1000°C the diffusivity of the fully cubic ZrO2 crystals with 20 wt% Y2O3 was 1.10x10‐6 m2/s while that of the partially stabilized ZrO2 crystals with 5 wt% Y2O3 was 1.05x10‐6 m2/s. The increased diffusivity at higher temperatures can be attributed to radiative heat transfer, with the lower relative diffusivity of the partially stabilized crystals resulting from photon scattering by the precipitate structures present.

Toughening mechanisms in duplex alumina-zirconia ceramics

A series of Al2O3-ZrO2 ceramics has been fabricated using both conventional sintering and a hot-pressing route, which results in various microstructures including (i) Al2O3 with well-dispersed ZrO2 single crystals; (ii) Al2O3 With TZP (tetragonal zirconia polycrystals) agglomerates (20 to 50μm); and (iii) Al2O3-ZrO2 duplex structures, in which both well-dispersed ZrO2 single crystals and TZP agglomerates are dispersed. The fracture strength of the composites has been measured by means of three-point bending and the fracture toughness by means of the micro-indentation technique. The microstructural characterization was carried out using scanning and transmission electron microscopy, and phase analysis of the zirconia by means of X-ray diffraction. The high toughness values of ≈ 12 M Pa m1/2 measured for the duplex structure have been correlated with the toughening mechanisms operative and the fracture strength with the matrix grain size and with larger defects present in the structure. A combined toughening process is proposed to account for the improved properties, including transformation toughening, microcrack toughening and crack deflection, which are discussed in context with the property measurements and the microstructural observations.

Non-destructive phase transformation of ZrO2 single crystal at high pressure

Non-destructive phase transformation of ZrO2 single crystal at high pressure

Characterization of different (Me,Zr)O2 single crystals grown by the “skull-melting” technique

Single crystals of cubic ZrO 2 stabilized by various amounts of MgO, CaO, Y 2 O 3 , CeO 2 and MnO were grown by the “skull-melting” technique. ZrO 2 melts containing up to 25 mol% Nb 2 O 5 and Ta 2 O 5 formed a monoclinic ZrO 2 compound. When grown with TiO 2 a titanate was obtained. Crystallographic data are presented for all grown crystals. Chemical composition was checked by microprobe analysis. Single-crystal XRD patterns show oriented phase transformations. Optical constants were determined for the first time by the prism method and various reflection measurements. Refractive indices correspond with experimental density measurements and were calculated. Vickers microhardness data are given for all grown stabilized ZrO 2 crystals.

Plastic Deformation and Solid‐Solution Hardening of Y2O3‐Stabilized ZrO2

Fully stabilized cubic ZrO2 single crystals containing various Y2O3 concentrations were deformed at 1400°C in air. The orientation chosen favored (001)(110) slip, which was confirmed by slip‐trace analysis and transmission electron microscopy; (111)(110) slip could also be activated. The yield and flow stresses increased with increasing Y2O3 concentration, and stress‐strain curves were dominated by a region of zero work hardening from very small strains for all compositions.

High-Temperature Precipitation Hardening of Two-Phase Y2O3-Partially-Stabilized ZrO2 Single Crystals: A First Report

Precipitation hardening was observed in two-phase (cubic plus tetragonal) Y2O3-partia1ly-stabilized ZrO2 single crystals deformed at 1400°C. Slip was activated on (001) 〈110〉, primarily in Luders bands.

ChemInform Abstract: CRYSTALLIZATION AND TRANSFORMATION OF ZIRCONIA UNDER HYDROTHERMAL CONDITIONS

AbstractWährend gleichmäßigen Au?ieizens auf 350°C in konz. NaOH‐Lösungen kristallisiert hydratisiertes amorphes ZrO2 kubisch.

Room-Temperature Strength and Fracture of ZrO2-Y2O3 Single Crystals

Strength and fracture toughness results are presented for ZrO2 single crystals stabilized with Y2O3. The crystals (2 cm in diameter by 5 cm long) were prepared by skull melting. The partially stabilized compositions with 4 to 6 wt% Y2O3 showed a dramatic improvement in mechanical properties over the fully stabilized samples containing 20 wt% Y2O3, i.e. a strength exceeding 1000 MPa and a fracture toughness of 8 Mpa,.m 1/2 were achieved compared to 200 MPa and 2 Mpa.m1/2, respectively, for fully stabilized ZrO2 single crystals.

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.

Thermoluminescence from X-Ray-Irradiated Stabilized ZrO2 Single Crystals

Thermoluminescence is investigated from yttria-stabilized ZrO2 single crystals irradiated at 100 K to high doses of X-rays. Three main peaks are observed and their activation energies, computed by a number of methods, give consistent values averaging to 0.13, 0.37, and 0.67 eV. Determination of the spectral composition of the three peaks shows separate contribution in each from wavelengths around 4880 and 5680 A which may be attributed to luminogen centres formed by Ti4+ and another impurity ion. Oxygen vacancies (in complexes) already present before irradiation are the likely electron traps. Analysis of the data indicates that the process can be explained in terms of second order kinetics.

Optical and X‐Ray Single Crystal Studies of the Monoclinic ⇌ Tetragonal Transition in ZrO2

The monoclinic ⇌ tetragonal phase transition in ZrO2 single crystals was studied at temperature by transmission optical microscopy and X‐ray diffraction techniques. A series of timelapse photographs illustrated the relations between the events that occur during the transition. The events themselves were recognized by direct observation using a high‐temperature microscope stage and by scrutiny of several high‐temperature Laue photographs. During heating the monoclinic phase transforms to the tetragonal by the motion of an interface parallel to the (100)m plane; simultaneous twinning also occurs behind the advancing interface. The tetragonal phase is usually twinned on the (12)bct or (12)bct plane, and the extent of twinning is influenced by the heating rate. Cooling transforms the untwinned tetragonal form into a twinned monoclinic form with the orientation of the monoclinic twins parallel to the trace of the (001)m plane when observations are made in the (100)m plane. Transformation of a twinned tetragonal crystal results in twins on the {110}m and {001}m planes. Orientation relations in the ZrO2 transformation are: (100)m‖(110)bct, [010]m‖[001]bct, and by the virtue of twinning, (100)m‖(110)bct, [001]m‖[001]bct. During cooling the same topotaxial relations are maintained.

The Growth of Monoclinic ZrO2 Single Crystals by Na3AlF6 Flux Method

Monoclinic ZrO2 single crysta1s were grown from the fluxed melt of 22 wt fe 5 ZrO2 and 78 wt%o Na3AIFe by slow cooling under the evaporation of the flux components.A starting mixt e was placed in a 100ml platinum crucible. After maintaining it in the range of temperatures from 1010 to 1080eC, it was cooled at a constant rate from 1 to 10eC/hr, ZrO2 single crystals were fouhd in the growth at a higher te pe: rature and a faster cooling rate. When ai r-coQled from 1024 C at the rate of about 2000C/hr, only small ZrO2 single crystals were obtaiped.Many of monoclinic ZrO2 single ctystals grown were of tabular form which was generally bounded by {100}, {110} and. {111} faces and their maximum size was 6.7x4.0x1.4 mmS, Some of them were twins on par allel growths. Crystals were shown to be very pure by spectroscopic analyses, and also had few cavities.The amount, of evaporqt. ion from the fuxed melt was about 7, 5g/day (10500C), depending upon a sectional area of the crucible.Many crystals showed growth hills similar to an acorn, a rhomb and a circle in shape on {100}, {OIO} and {111} surfaces, respectively.Etching of ZrO2 crystals was made with a hot 46% HF etchant. The etch pits resulting from a several hours etch were, slender hexagonal and these from a long hours etch were rhombic on {100} surfaces.