An Unusual Twin Structure in Transformed Precipitates in Y-PSZ Single Crystals

Mosaic-like monoclinic ZrO₂ particles form in TEM thin foils in 3.4-mol%-Y₂O₃-partially-stabiIized ZrO₂ (Y-PSZ) single crystals aged for 150 h at 1600°C. These particles transformed martensitically during TEM foil preparation; the parent phase existed as internally twinned tetragonal ZrO₂ precipitates which had formed during aging. These monoclinic particles contain (100)_m and {110}_m transformation twins, and the twin variants have uniform thickness. The origin of this transformation microstructure, the lattice correspondences, and the applicability of the invariant plane strain model of martensitic transformations are discussed.     

Fracture Toughness of Al2O3 with an Unstabilized ZrO2Dispersed Phase

The fracture toughness of Al₂O₃ is considerably increased by the incorporation of fine monoclinic ZrO₂ particles. Hot-pressed composites containing 15 vol % ZrO₂ yield K_lcvalues of ∼ 10 MN/m^3/2, twice that of the A1₂O₃ matrix. It is hypothesized that this increase results from a high density of small matrix microcracks absorbing energy by slow propagation. The microcracks are formed by the expansion of ZrO₂during the tetragonal → monoclinic transformation. Since extremely high tensile stresses develop in the matrix, very small ZrO2 particles can act as crack formers, thus limiting the critical flaw size to small values.     

Critical Microstructures for Microcracking in Al2O3-ZrO2 Composites

The internal strains asSociated with the martensitic phase transformation of zirconia were used to introduce microcracks into Al₂O₃/ZrO₂ composites. The degree of transformation was found to be dependent on the volume fraction of ZrO₂ and its size, the latter of which could be controlled by suitable heat treatments. The microstructural changes that occurred during the heat treatments were studied using quantitative microscopy and X-ray diffraction. For materials containing more than 7.5 vol% Zr0₂, the ZrO₂ particles were found to pin the Al₂O₃ grain boundaries, thus limiting the Al₂O₃ grain growth. The limiting grain size was found to be dependent on size and volume fraction of ZrO₂. Heat treatments for the higher volume fraction materials (>7.5 vol% ZrO₂) caused micro-structural changes which resulted in increased amounts of monoclinic ZrO₂ at room temperature; elastic modulus measurements indicated that this was occurring concurrently with microcracking. By combining the ZrO₂ grain-size distributions with the X-ray analysis it was possible to calculate the critical ZrO₂ size required for the transformation. The critical size was found to decrease with increasing amounts of ZrO₂. Hardness and indentation fracture toughness were measured on the composites. Grain fragmentation was observed at the edge of the indentations and microcracks were observed directly, using an AgNO₃ decoration technique, near the indentations.     

Growth of Monoclinic ZrO2 Thin, Flaky Crystals by Hydrothermal Decomposition of Zirconium Oxide Sulfate Crystals

Thin platelike hexagonal crystals of zirconium oxide sulfate (ZOS) with a chemical composition of Zr₃O₅SO₄. n H₂O were previously synthesized by the hydrolysis of an aqueous solution of 2 mol/L ZrOSO₄ at 240°C. The ZOS particles obtained were hydrothermally treated again in dilute sulfuric acid solution (<0.15 mol/L) at 240°C. The decomposition of ZOS and the nucleation and growth morphology of the monoclinic ZrO₂ crystals were automatically controlled with increasing sulfuric acid released by the decomposition of ZOS, and were influenced by the morphology of ZOS as precursors. Anisotropic monoclinic ZrO₂ single crystals, elongated thin and flaky particles (length: >2 (Am; width: about 0.1 μm; thickness:«0.01 μm), could be obtained under some conditions without the coexistence of another type of anisotropic ZrO₂ fibrous twin crystal.     

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.     

Microcrack Nucleation in Ceramics Subject to a Phase Transformation

The nucleation of microcracks from twins formed in mono-clinic ZrO₂ was analyzed. The twin formation was shown to result in large tensile stress concentrations capable of spontaneous microcrack nucleation. The role of interface dislocations in microcrack nucleation was also considered.     

Twin Boundaries in Monoclinic ZrO2 Particles Confined in a Mullite Matrix

Internal defects and twin structures exist in thermally transformed monoclinic ( m )-ZrO₂ particles confined in a mullite matrix. The defects were analyzed by transmission electron microscopy. Twin boundaries on (100), (110), (001), and (011) were found. Frequently, (100) and (110) twins formed complicated mosaiclike structures. Some particles could be divided into two or more twinned sections with well-defined mutual orientation relations. Domains of closure were observed at the particle-matrix interface. The observations are discussed in terms of a simple model which considered (i) the anisotropic volume increase of ZrO₂ during the t→m transformation, (ii) the anisotropic thermal expansion coefficient of ZrO₂, and (iii) the rather isotropic thermal expansion coefficient of the mullite matrix.     

Formation of Acicular Monoclinic Zirconia Particles under Hydrothermal Conditions

Acicular monoclinic ZrO₂, particles were prepared by hydrothermal treatmentat 250°C using sulfuric acid solutions containing zirconium ions. The formation process and morphology of the ZrO₂, particles were investigated. Two types of acicular monoclinic Zr0₂, particle morphologies were obtained, both elongated in the 〈001〉 direction, and the range of acicular ZrO₂, particle sizes changedfrom 0.3 to 1.3 μ m with hydrothermal conditions. Addition of MgSO₄, to the starting solution promoted the crystallization of the monoclinic ZrO₂, particles.     

Strength-Toughness Relations in Sintered and Isostatically Hot-Pressed ZrO2-Toughened Al2O3

The fracture toughness of fine-grained undoped ZrO₂-toughened Al₂O₃ (ZTA) was essentially unchanged by postsintering hot isostatic pressing and increased monotonically with ZrO₂ additions up to 25 wt%. The strength of ZTA with 5 to 15 wt% tetragonal ZrO₂, which depended monotonically on the amount of ZrO₂ present before hot isostatic pressing, was increased by pressing but became almost constant between 5 and 15 wt% ZrO₂ addition. The strength appeared to be controlled by pores before pressing and by surface flaws after pressing; the size of flaws after pressing increased with ZrO₂ content. The strength of ZTA containing mostly monoclinic ZrO₂ (20 to 25 wt%) remained almost constant despite the noticeable density increase upon hot isostatic pressing because the strength was controlled by preexisting microcracks whose extent did not change on postsintering pressing. These strength-toughness relations in sintered and isostatically hot-pressed ZTA are explained on the basis of R -curve behavior. The importance of the contribution of microcracks to the toughness of ZTA is emphasized.     

Infrared Absorption Spectroscopy in Zirconia Research

Infrared absorption spectra are shown for monoclinic ZrO₂ and for cubic stabilized ZrO₂. Nine bands are reported in monoclinic ZrO₂ in the region 800 to 200 cm^−l, whereas only one broad band is observed in cubic ZrO₂ over the same frequency range.     

Slow Growth of Microcracks: Evidence for One Type of ZrO2 Toughening

Alumina containing 15 vol% monoclinic ZrO₂ dispersed at the grain boundaries exhibited very high room-temperature fracture toughness (∼11 MPa·m^1/2) on cooling from 1275°C when microcrack precursors nucleated at Ts. With increasing time (up to ∼12 h) at room temperature, K_Ic and Young's modulus decreased when dilational and thermal-expansion strains subcritically propagated inter granular microcracks. Thus, transformation toughening of ceramics with inter crystalline ZrO₂ dispersions is to a great extent caused by microcrack nucleation and extension.     

Microstructural Studies in Alkoxide-Derived Mullite/Zirconia/Silicon Carbide-Whisker Composites

Mullite composites toughened with ZrO₂ (with or without a MgO or Y₂O₃ stabilizer) and/or SiC whiskers (SiC( w )) were fabricated by hot-pressing powders prepared from Al, Si, Zr, and Mg(Y) alkoxide precursors by a sol–gel process. Micro-structures were studied by using XRD. SEM, and analytical STEM. Pure mullite samples contained prismatic, preferentially oriented mullite grains. However, the addition of ZrO₂, as well as the hot-pressing temperature, affected the morphology and grain size in the composites; a fine, uniform, equiaxed microstructure was obtained. The effect of SiC( W ) was less pronounced than that of ZrO₂. Glassy phases were present in mullite and mullite/SiC( W ) composites, but were rarely observed in Al₂O₃-rich or ZrO₂-containing samples. The formation of zircon due to the reaction between ZrO₂ and SiO₂ and the considerable solid solution of SiO₂ in ZrO₂ prevented the formation of the glassy phase, whereas the reaction between Al₂O₃ and MgO in MgO-containing samples formed a spinel phase and also deprived the ZrO₂ phase of the stabilizer. Intergranular ZrO₂ particles were either monoclinic or tetragonal, depending on size and stabilizer content; small intragranular ZrO₂ inclusions were usually tetragonal in structure.     

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 (1 2) _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.     

Effect of YO1.5 Dopant on Unit-Cell Parameters of ZrO2 at Low Contents of YO1.5

The effect of YO_1.5 dopant on unit-cell parameters of ZrO₂ (YO_1.5=0 to 14.6 mol%) were examined by the X-ray whole-powder-pattern decom-position technique. The unit cell of monoclinic ZrO₂ has the largest expansion along the direction perpen-dicular to (100). The rate of increase of the unit-cell volume of monoclinic ZrO₂ with YO_1.5 content is greater than that of tetragonal ZrO₂ and comparable to that of cubic ZrO₂.     

Toughening by Monoclinic Zirconia

The toughening induced by monoclinic ZrO₂ in the absence of microcracking was investigated, using ZnO as the host material. Toughness levels K_c in excess of the host toughness K_c^M were achieved, attaining a peak toughness K_c/K_c^M ∼1.7, at monoclinic ZrO₂ volume concentrations 0.2. This toughening is attributed to crack/particle interactions, associated with the deflection and bowing of the crack by the residual strain field around the monoclinic ZrO₂ particles.     

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.     

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.     

Particle Interactions in Zirconia-Toughened Alumina

In order to determine the effect of the coarse tail of the ZrO₂ size distribution of the monoclinic ZrO₂ content of zirconia-toughened alumina, quantitative hot-stage X-ray measurements were made on (1) A1₂O₃-10% Zr0₂ (submicrometer with a coarse tail >1 μm), (2) A1₂O₃-10% ZrO₂ with the coarse tail of the ZrO₂ carefully removed by centrifugation, and (3) Al₂O₃-10% ZrO₂ in which 10% of Zr0₂(1) and 90% of Zr0₂(2) were used. The monoclinic content of (3) was compared with the weighted average of(1) and (2). A disproportionate amount of monoclinic ZrO₂ was found in (3) and it was concluded that transformation of the coarse particles promotes transformation in finer particles. Microstructural examination supports this conclusion. Results were interpreted as being due both to autocatalytic transformation and to constraint relief from microcracking.     

Effects of Oxygen Partial Pressure on the Nucleation Behavior and Morphology of Chemically-Vapor-Deposited Zirconia on Hi-Nicalon Fiber and Si

A ZrO₂ coating was prepared on Hi-Nicalon fiber and single-crystal Si by chemical vapor deposition (CVD) using ZrCl₄, CO₂, and H₂ as precursors at 1050°C. The effects of oxygen partial pressure on the nucleation behavior of the CVD-ZrO₂ coating were systematically studied by intentionally varying the controlled amount of O₂ into the CVD chamber. Characterization results suggested that the number density of tetragonal ZrO₂ nuclei apparently decreased with increasing the oxygen partial pressure from 4 × 10⁻³ to 1.6 Pa. Also, the coating layer became more columnar and contained larger monoclinic ZrO₂ grains. The observed relationships between the oxygen partial pressure and the nucleation and morphologic characteristics of the ZrO₂ coating were attributed to the grain size and oxygen deficiency effects, which have been previously reported to cause the stabilization of the tetragonal ZrO₂ phase in bulk ZrO₂ specimens.     

Morphological Evolution and Weak Interface Development within Chemical-Vapor-Deposited Zirconia Coating Deposited on Hi-Nicalon™ Fiber

The phase contents and morphology of a ZrO₂ fiber coating deposited at 1050°C on Hi-Nicalon^TM by chemical vapor deposition (CVD) were examined as a function of deposition time from 5–120 min. The morphological evolution in the ZrO₂ coating was correlated to the development of delamination within the ZrO₂ coating. The delamination appears to occur as a result of: (i) continuous formation of tetragonal ZrO₂ nuclei on the deposition surface; (ii) martensitic transformation of the tetragonal phase to a monoclinic phase on reaching a critical grain size; and (iii) development of significant compressive hoop stresses because of the volume dilation associated with the transformation. Our observations suggest that it will be of critical importance to further understand and eventually control the nucleation and grain growth behavior of CVD ZrO₂ and its phase transformation behavior for its potential applications for composites.