Crystal Structures of Two Orthorhombic Zirconias

The crystal structures of orthorhombic zirconias formed by cooling magnesia-partially-stabilized zirconia (Mg-PSZ) (space group Pbc 2₁) and by quenching zirconia powder from 600°C and 6 GPa (space group Pbca ) are compared and contrasted. It is demonstrated that the two structures are easily distinguished by the neutron powder diffraction techniques used to establish them. The occurrence of two distinct phases is hence proved. Structural relationships between these two phases and also with the in situ high-pressure structure proposed from X-ray diffraction (XRD) are discussed. The three structures are virtually indistinguishable by XRD and so the structure of the high-pressure form "in situ" is considered to remain unknown.     

Abrasive Wear Behavior of Heat-Treated ABC-Silicon Carbide

Hot-pressed silicon carbide, containing aluminum, boron, and carbon additives (ABC-SiC), was subjected to three-body and two-body wear testing using diamond abrasives over a range of sizes. In general, the wear resistance of ABC-SiC, with suitable heat treatment, was superior to that of commercial SiC. When the fine-scale (3 μm) diamond abrasives were used, it was found that thermal annealing at 1300°C increased the resistance to three-body wear by a factor of almost three, and two-body wear by a factor of almost two, compared with as-hot-pressed samples. Higher annealing temperatures, however, led to a decline in wear resistance from its highest value. Similar behavior was seen for 1300°C-annealed samples subjected to 15 μm diamond abrasive, although higher-temperature annealing at 1500°–1600°C enhanced the wear resistance again. When coarse abrasives (72 μm) were used, the wear resistance progressively increased with increased annealing temperature from ∼1000° to 1600°C. Corresponding transmission and scanning electron microscopy studies indicated that, whereas transgranular, conchoidal cracking was dominant in the mild abrasive wear with fine-scale (3 μm) abrasives, intergranular cracking and subsequent grain pullout was far more predominant in the more severe abrasive wear with coarse abrasives. Because the hardness and indentation toughness were barely altered during thermal treatment, the observed wear behavior was attributed mainly to the thermally induced microstructural changes, including the crystallization of glassy grain-boundary films, the possible strengthening of the boundaries due to the enhancement of the aluminum, and the formation of aluminum-rich, coherent nanoscale precipitates in the matrix grains above 1300°C.     

Pro- and Subeutectoid Behavior of the Tetragonal Phase in Magnesia-Partially-Stabilized Zirconia

Magnesia-partially-stabilized zirconia (Mg-PSZ) is industrially important because of transformable metastable tetragonal precipitates which interact with and arrest cracks. This work addresses the precipitation of tetragonal phases at one composition, 9.5 mol% MgO, throughout a range of temperatures. High-purity zirconia samples were sintered at 1700°C and rapidly quenched to heat-treatment temperatures ranging from 1600° to 1100°C, then quenched to room temperature. Cooling rates through the tetragonal + MgO and the monoclinic + MgO two-phase regions were found to affect the phase content. The kinetics of nonequilibrium phase transformation for high-purity Mg-PSZ are presented in terms of time-temperature-transformation diagrams.     

Cyclic Fatigue Life and Crack-Growth Behavior of Microstructurally Small Cracks in Magnesia-Partially-Stabilized Zirconia Ceramics

Cyclic fatigue stress/life ( S / N ) and crack-growth properties are investigated in magnesia-partially-stabilized zirconia (Mg-PSZ), with particular reference to the role of crack size. The material studied is subeutectoid aged to vary the steady-state fracture toughness, K_c , from ∼3 to 16 MPa · m^1/2· S / N data from unnotched specimens show markedly lower lives under tension—compression compared with tension—tension loading; "fatigue limits"(at 10⁸ cycles) for the former case approach 50% of the tensile strength. Under tension—tension loading, cyclic crack-growth rates of "long"(> 3 mm) cracks are found to be power-law dependent on the stress-intensity range, Δ K , with a fatigue threshold, Δ K _TH, of order 50% of K_c . Conversely, naturally occurring "small"(1 to 100 μm) surface cracks are observed to grow at Δ K levels 2 to 3 times smaller than Δ K _TH, similar to behavior widely reported for metallic materials. The observed small-crack behavior is rationalized in terms of the restricted role of crack-tip shielding (in PSZ from transformation toughening) with cracks of limited wake, analogous to the reduced role of crack closure with small fatigue cracks in metals. The implications of such data for structural design with ceramics are briefly discussed.     

Fracture Mechanics of High-Toughness Magnesia-Partially-Stabilized Zirconia

Crack growth resistance in MgO-partially-stabilized ZrO₂ (Mg-PSZ) has been studied using notched double-cantilever beams (DCB's). High-toughness Mg-PSZ exhibited nonlinear mechanical behavior in the form of residual displacements, related to the transformation of tetragonal ( t ) ZrO₂ precipitates to monoclinic ( m ) symmetry. The influence of this residual displacement on crack resistance behavior (" R -curve" behavior) was analyzed using several different fracture mechanics approaches. Specifically, the "global" resistance W _R (Δ a ), a J -integral type parameter W _J (Δ a ), were determined as a function of crack extension (Δ a ). Some of these parameters displayed a geometry dependence; their form depended on the initial notch depth and the size of the unbroken ligament. The early stages of crack growth were best described by W _J (δ a ). The residual strains building up in the wake during crack growth and their effect on specimen displacement made the W _R curves (and to some extent the W _J curves) dependent on the ratio between initial notch depth and crack extension. The only curves independent of geometry were the G (δ a ) curves, but only in a restricted range of geometry. However, the material resistance of Mg-PSZ is clearly under-estimated with such a linear elastic approach.     

Texture from Domain Switching of Tetragonal Zirconias

Although crystallographic textures are most often attributed to slip or twinning, textures from compression, tension, or grinding of tetragonal zirconias have been ascribed to ferroelastic domain switching. If consideration of this phenomenon is limited to the effect of switching, each deformation mode should have a particular textural result. In this paper a simple geometric model for texture from domain switching is proposed.     

Precipitation During Controlled Cooling of Magnesia-Partially-Stabilized Zirconia

A study has been made of the precipitation and growth processes which occur during cooling from solution treatment and under isothermal hold conditions in a magnesia-partially-stabilized zirconia alloy. Three types of precipitate have been identified which develop during cooling or during isothermal hold treatments just above and below the eutectoid temperature. These precipitate forms are termed (i) primary, (ii) large random, and (iii) secondary. Further precipitation, slow growth of existing precipitates, and subeutectoid decomposition result when an additional 1100°C aging treatment is given to the previously cooled material. It is shown that type (iii) secondary precipitates form rapidly within the temperature range of 1300° to 1375°C. The secondary precipitates are largely responsible for the improved room-temperature strength properties of the heat-treated samples. The formation and effects on mechanical properties of each type of precipitate are discussed.     

Perturbed-Angular-Correlation Study of Zirconias Produced by the Sol-Gel Method

Zirconia powders, prepared by a sol-gel method using zirconium n -propoxide (ZNP) and different H₂O/ZNP and alcohol/ZNP ratios, were investigated by the perturbed-angular-correlation method, along with X-ray diffraction (XRD), Raman spectroscopy, differential thermal analysis (DTA), and thermogravimetric analysis (TGA) techniques. The hyperfine interaction was measured after annealing the samples at increasing temperatures up to 1080°C. Two disordered structures, giving tetragonal patterns in XRD and Raman analysis, were identified in as-produced powders, which consisted of chains of fully hydrolyzed species and chains containing residual organic groups. As the annealing temperature was increased, crystallization occurred, producing the tetragonal and monoclinic phases of zirconia. All powders were essentially monoclinic at the highest temperatures. Those representing low H₂O/ZNP ratios retained a large amount of tetragonal phase over a wide temperature range and transformed to monoclinic with a higher activation energy.     

Annealing of Test Specimens of High-Toughness Magnesia-Partially-Stabilized Zirconia

The peak toughness of a commercial magnesia-partially-stabilized zirconia was strongly dependent on a postprocessing, postspecimen preparation, and modest annealing (1000°C for 20 min), increasing it from ∼9 to almost ∼18 MP a·m^1/2. Further study of this surprising phenomenon revealed that increases in toughness occurred for annealing temperatures as low as 275°C. The low toughness of the as-received materials is due to a post-manufacture room-temperature aging.     

Pore Structure of Heat-Treated Nonwoven Materials

The pore structure of heat-treated nonwoven materials is determined by the conditions of heating and cooling them. The effect of the shrinkage properties of a bicomponent fibre on the pore structure of the materials is manifested at a treatment temperature above the melting point of polypropylene. __________ Translated from Khimicheskie Volokna, No. 3, pp. 41–43, May–June, 2005.     

Stress–Strain Behavior of Alumina, Magnesia-Partially-Stabilized Zirconia, and Duplex Ceramics and Its Relevance for Flaw Resistance, KR-Curve Behavior, and Thermal Shock Behavior

The stress–strain behavior for Al₂O₃ of different grain size, for three different Mg-PSZ grades, and for various differently composed duplex structures is investigated and compared with their flaw resistance, K^R -curve behavior, and thermal shock behavior measured in previous works. The experimental results seem to reveal that, for most materials, quasi ductility increases with increasing flaw resistance, increasingly pronounced K^R -curve behavior, and increasing thermal shock retained strength. However, brittle ceramics can exhibit rising K^R -curves, whereas pronounced quasiductile materials can exhibit flat K^R -curves. An explanation for the apparent pseudo relationship between quasi ductility and K^R -curve behavior may be that, apart from genuine transformation ductility, most quasi-ductile effects such as microcracking have only a minor contribution to rising R -curve behavior, but require the existence of strong residual stresses, which are, on the other hand, responsible for the occurrence of most toughening mechanisms. Also discussed is the influence of microcracking on flaw resistance and thermal shock strength degradation.     

High-Resolution Transmission Electron Microscopy of Transformed Magnesia-Partially-Stabilized Zirconia Precipitates

Transformed Mg-PSZ precipitates consist of monoclinic bands that are twin-related across either (100)_m or (001)_mplanes in an arrangement that leads to accommodation of the shear component of the transformation shape strain. High-resolution images of these variants reveal these twin relationships quite clearly. It is shown that complete compensation of shear does not always occur. This paper also shows observations of monoclinic variants related by a rotation of 90° around the normal to (001)_m, in addition to those related by a 180° rotation for the twin-related variants. These findings conform to the predictions of martensite theory and their implications are discussed in terms of shear compensation and reduction in overall strain energy.     

Surface Morphology of Heat-Treated Ceramic Thin Films

The morphology of surfaces of several ceramic materials has been examined using transmission electron microscopy. The approach used was to prepare a sample for examination in the microscope, carefully clean it, and then heat-treat it. In the case of the oxides studied (alumina and spinel) the samples were heated in air; the non-oxides (α-SiC and β -SiC) were annealed under vacuum. The morphology in all but one case was such that the surface faceted parallel to the nearest low-index plane to give well-defined terraces; these were separated by ledges which also tended to facet parallel to the traces of low-index planes. The exception was the {1100} alumina surface, which appears to be unstable in air at temperatures close to 1400°C. A computer program using a multislice approach was used to estimate the height of the steps on the (0001) surface; the step heights appear to be multiples of the c lattice parameter. A reconstruction of this surface as a result of this heat treatment is also proposed.     

Coarsening Kinetics of the Tetragonal Phase in a Magnesia-Partially-Stabilized Zirconia

The coarsening kinetics of the tetragonal phase in a magnesia-partiallystabilized zirconia at 1420°C was examined using transmission electron microscopy and found to be controlled by volume diffusion. The rate constant was larger and the particle-size distribution was broader than predicted by the Lifshitz–Slyozov–Wagner theory of particle coarsening. A volume fraction modified coarsenng theory was found which better fits the experimental rate constant and particle-size distribution.     

Experimental Design Applied to the Chemical Durability of Sol–Gel-Derived Zirconias

A statistical design was used to investigate the effects of various processing conditions on the chemical durability of sol–gel-derived zirconias. Eight processing variables were investigated: water:alkoxide molar ratio, alcohol:alkoxide molar ratio, nitric acid:alkoxide molar ratio, mixing temperature during hydrolysis, firing temperature, heating rate, soak at firing temperature, and firing atmosphere. Processing variables were set at high and low limits in a main-effects statistical design. The design identified four critical processing variables (listed in descending order of significance): firing temperature, firing atmosphere, water:alkoxide ratio, and nitric acid:alkoxide ratio. Sol–gel-derived zirconias were characterized by differential scanning calorimetry, X-ray diffractometry, and mercury intrusion. Suggestions are offered that may help to explain why these particular processing conditions influenced the chemical durability of sol–gel-derived zirconias.     

Structural Analysis of Heat-Treated Chromium Oxycarbide Films

Chromium oxycarbide (Cr₂CO) films were deposited on stainless steel and copper plates via plasma-assisted chemical vapor deposition. The films were heated to 1073 K under high-vacuum conditions, to examine the high-temperature stability of the films. X-ray diffractometry revealed that the films decomposed to a mixed structure of Cr₂O₃ and Cr₃C₂. Thermogravimetric analysis and differential thermal analysis revealed that the decomposition occurred in the temperature range of 858-910 K.     

Contributions of Transformation and Microcracking to the Plastic Deformation of Magnesia-Partially-Stabilized Zirconia

The measured plastic deformation of Mg-PSZ under stress contains contributions from both transformation and microcracking. A method is described to separate the contributions from transformation and microcracking for samples deformed in uniaxial tension. Such a separation enables the ratio of the longitudinal to transverse strains arising from the transformation to be estimated. The results are compared to the predictions of the shear–dilatation model of Chen and Reyes-Morel. Serious discrepancies are found between the experimental results and the predictions of the model and the possible reason for this is discussed.     

Spatially Resolved Raman Spectroscopy Study of Transformed Zones in Magnesia-Partially-Stabilized Zirconia

Raman vibrational spectroscopy provides an effective phase characterization technique in materials systems containing particle dispersions of the tetragonal and monoclinic polymorphs of zirconia, each of which yields a unique Raman spectrum. An investigation is reported to assess a novel, spatially resolved Raman spectroscopy system in the study of transformed zones surrounding cracks in partially stabilized MgO–ZrO₂ (PSZ). The experimental arrangement uses an imaging (two-dimensional) photomultiplier tube to produce a one-dimensional Raman profile of phase compositions along a slitlike laser beam without translation of either the sample or the laser beam and without scanning the spectrometer. For the present optical configuration, the spatial resolution is estimated to be equivalent to the detector resolution of 28 pm and does not appear to be reduced because of secondary scattering events in the PSZ. Results from phase characterization studies of the size, frontal morphology, and extent of transformation of transformation zones surrounding cracks produced under monotonic and cyclic loading conditions are presented. Particularly large zones are observed in the peak-toughened material, extending 1300 μm ahead of the crack tip with widths of up to 3000 μm. Good agreement is found with similar results determined using optical interference microscopy.