Degradation During Aging of Transformation-Toughened ZrO2-Y2O3 Materials at 250°C

The detrimental aging phenomenon observed in ZrO₂-Y₂O₃ materials, which causes tetragonal ZrO₂ to transform to its monoclinic structure at temperatures between 150 and 400°C, was investigated with respect to the gaseous aging environment and the Y₂O₃ and SiO₂ content of the material. It is shown that the aging phenomenon is caused by water vapor and that inter-granular silicate glassy phases play no significant role. Transmission electron microscopy of thin foils, before and after aging, showed that the water vapor reacted with yttrium in the ZrO₂ to produce clusters of small (20 to 50 nm) crystallites of α-Y(OH)₃. It is hypothesized that this reaction produces a monoclinic nucleus (depleted of Y₂O₃) on the surface of an exposed tetragonal grain. Monoclinic nuclei greater than a critical size grow spontaneously to transform the tetragonal grain. If the transformed grain is greater than a critical size, it produces a microcrack which exposes subsurface tetragonal grains to the aging phenomenon and results in catastrophic degradation. Degradation can be avoided if the grain size is less than the critical size required for microcracking.     

Measurement of Fracture Toughness of Plasma-Sprayed Al2O3 Coatings Using a Tapered Double Cantilever Beam Method

The fracture toughness of plasma-sprayed Al₂O₃ coatings in terms of the critical strain energy release rate G _Ic was measured using a tapered double cantilever beam (TDCB) approach. The fracture surfaces were examined using a scanning electron microscope (SEM). The measurement yielded the mean G _Ic values from 13 to 27 J/m² for the sprayed Al₂O₃ coatings at different spray distances. These values agree well with those obtained by the conventional double cantilever beam approach. The dependence of the observed G _Ic on spray distance is consistent with that for the lamellar bonding ratio. These results suggest that the TDCB test is a reliable approach for measuring the G _Ic of thermal-spray coatings without the crack-length measurement.     

Relation between Strength, Microstructure, and Grain-Bridging Characteristics in In Situ Reinforced Silicon Nitride

The grain size of in situ Si₃N₄ is varied, and its effects on strength-flaw size relations are related to the behavior of a bridging zone behind the crack tip. The bridging-zone properties are calculated from a Dugdale model assuming that the bridging zone has a constant bridging stress ( p *) and length ( D _b) at the moment of the critical fracture. The results show that as grain size increases, p * decreases while D _b and the critical bridging zone opening ( u *) first increase and then decrease, resulting in a maximum for short-crack fracture toughness at an intermediate grain size. The initial increase of u * and D _b with grain size is attributed to an increase in debonding length, while the decrease of p * is attributed to a decrease in strength for bridging grains due to a statistical effect which also causes D _b and u * to drop in the large-grain regime. Implications on microstructure design are discussed.     

Effect of Fiber Volume Fraction on the Off-Crack-Plane Fracture Energy in Strain -Hardening Engineered Cementitious Composites

In this paper, the results of an experimental study on the effect of fiber volume fraction on the off-crack-plane fracture energy in a strain-hardening engineered cementitious composite (ECC) are presented. Unlike the well-known quasi-brittle behavior of fiber reinforced concrete, ECC exhibits quasi-ductile response by developing a large damage zone prior to fracture localization. In the damage zone, the material is microcracked but continues to strain-harden locally. The areal dimension of the damage zone has been observed to be on the order of 1000 cm² in double cantilever beam specimens. The energy absorption of the off-crack-plane inelastic deformation process has been measured to be more than 50% of the total fracture energy of up to 34 kJ/m². This magnitude of fracture energy is the highest ever reported for a fiber cementitious composite.     

Factor Analysis of Fracture-Toughness Test Parameters for Al2O3

The double-cantilever-beam technique was used to determine the effects of varying 6 factors related to specimen preparation, size, and testing conditions on the fracture toughness of polycrystalline Al₂O₃. Experimental design and statistical factor analysis techniques were used to investigate each factor at two levels. Direct fracture surface replication and electron microscopy provided supporting information about the fracture mode and fracture surface features for each test condition. The fracture toughness of Al₂O₃ was higher for 30-μm grain size than for 10-μm grain size. Pretest annealing (900°C) and specimen width were both significant factors for 10-μm Al₂O₃. The effects of variations of beam width, beam height, and test machine speed were masked by data scatter and are being studied further. The ratio of specimen width to fracture web width caused no effect in the range studied. The sensitivity of the test results to sample dimensions and surface finish is small enough that special care in cutting and measuring of samples is not required.     

Fracture Toughness of a Partially Stabilized ZrO2 in the System CaO-ZrO2

The room-temperature fracture behavior of partially stabilized ZrO₃ (PSZ) in the system CaO-ZrO₂ was investigated. Fracture energy was measured using standard single-edge-crack and work-of-fracture techniques. Attempts were made to relate the fracture toughness parameters to the microstructure of the material. Stable crack propagation was always observed; a model is proposed to explain these observations on the basis of the formation of a microcrack zone at the tip of a propagating crack. The occurrence of initial stable crack propagation is explained in terms of an increase in microcrack zone size. The possibility that crack stability results from testing geometry superimposed on the microcracking stability is also discussed.     

Effect of Microcracking on the Thermal Diffusivity of Fe2TiO5

The effect of microcracking on the thermal diffusivity of polycrystalline Fe₂TiO₅ subjected to a range of annealing treatments was investigated. At fine grain size (∼1 μm), the thermal diffusivity exhibited the decrease with increasing temperature common for dielectrics. Extensive microcracking in the larger-grain-sized materials significantly decreased their thermal diffusivity. On heating, the microcracked materials exhibited increased thermal diffusivity at elevated temperatures which can be attributed primarily to microcrack closure and healing; on cooling, they exhibited a pronounced hysteresis, attributable to irreversible crack opening and closing. Thermal cycling closed the hysteresis curves, which suggests permanent changes in microcrack morphology. It appears that microcracking is a promising technique for tailoring ceramic materials to a combination of high thermal shock resistance and good insulating capability.     

Quantifying Local Microcrack Density in Ceramics: A Comparison of Instrumented Indentation and Thermal Wave Techniques

Instrumented indentation and thermal wave techniques are used as local, quantitative probes of microcrack density in a silicon nitride that has been subjected to severe Hertzian contact stress. The techniques act as local probes, sampling volumes of material <10⁻³ mm³. Microcracked regions are created using a tungsten carbide spherical indenter, as a result of high shear stress in the compressive zone beneath the indenter. The microcracked zones, studied in cross section using a "bonded interface" technique, increased both in size and in degree of damage with increasing Hertzian load. Within the zones, Young's modulus is measured using instrumented indentation, and the thermal diffusivity using a thermal wave technique; both quantities are lower in the damaged regions than in the undamaged regions. The shifts in modulus and diffusivity are used independently to calculate microcrack densities using related models of the effect of microcracks on each property. The two techniques show the same functional relationship between Hertzian contact load and microcrack density, and yield densities that agree to within a factor of approximately 2.     

Measurement of the Crystallographically Transformed Zone Produced by Fracture in Ceramics Containing Tetragonal Zirconia

During fracture of ceramics containing tetragonal zirconia particles, a volume of zirconia material on either side of the crack irreversibly transforms to the monoclinic crystal structure. Transformation zone sizes, measured using Raman microprobe spectroscopy, are presented for three sintered ceramics. In a single-phase ZrO₂−3.5 mol% Y₂O₃ material, an upper bound measurement of 5 μm is obtained for the zone size. In the Al₂O₃/ZrO₂ composites studied, the zone size is deduced to correspond to ∼1 grain in diameter. On the basis of the monoclinic concentrations derived from the Raman spectra it is further concluded that only a fraction of the ZrO₂ grains within the transformation zone transform, providing indirect evidence for the effect of particle size on the propensity for transformation.     

Electric-Field-Induced Crack Growth Behavior in PZT/Al2O3 Composites

Hard lead zirconate titanate (PZT) and PZT/Al₂O₃ composites were prepared and the alternating-electric-field-induced crack growth behavior of a precrack above the coercive field was evaluated via optical and scanning electron microscopy. The crack extension in the 1.0 vol% Al₂O₃ composite was significantly smaller than that in monolithic PZT and the 0.5 vol% Al₂O₃ composite. Secondary-phase Al₂O₃ dispersoids were found both at grain boundaries and within grains in the composites. A large number of dispersoids were observed at the grain boundaries in the 1.0 vol% Al₂O₃ composite. It appears that the Al₂O₃ dispersoids reinforce the grain boundaries of the PZT matrix as well as act as effective pins against microcrack propagation.     

Role of Microstructure in Dynamic Fatigue of Glass-Ceramics after Contact with Spheres

Dynamic fatigue data are reported for fine- and coarse-grained micaceous glass-ceramics after contact damage with spheres. The strengths of indented specimens are measured at stressing rates from ∼10⁻² to 10⁴ MPa·s⁻¹ in water. The strength degradation is substantially faster in the coarse-grained structure, and is accelerated further by multicycle contact loading. Failures originate from contact sites in all cases but undergo a progressive transition from classical cone cracks to quasi-plastic microcrack zones with increases in the grain size and the number of contact cycles. The results highlight the particularly deleterious effect of quasi-plastic damage accumulation on lifetime.     

Experimental Evaluation of Toughening Mechanisms in Alumina-Zirconia Composites

The contributions of transformation toughening and microcrack toughening in an 85Al₂O₃15ZrO₂ (vol%) composite were quantitatively evaluated. Three types of hot-pressed samples with similar size (∼0.5 µm) and size distribution of ZrO₂ grains, but with different contents (vol%) of monoclinic- ( m -) ZrO₂ (0 (AZS), 45 (AZT), and 67 (AZM)) were prepared using Y₂O₃ and MoO₂ dopants. Therefore, the possible effect of ZrO₂ grain size on each toughening mechanism was eliminated. When the measured fracture toughnesses of m -ZrO₂ volume fractions before and after fracture were compared, the transformation-toughening constant was estimated as 3.0 MPam^1/2 and the microcrack-toughening constant 0.2 MPam^1/2 for a 100% monoclinic transformation of ZrO₂ grains. This result indicates that transformation toughening was the dominant toughening mechanism in the studied composite.     

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.     

Influence of ZrO2 Grain Size and Content on the Transformation Response in the Al2O3─ZrO2 (12 mol% CeO2) System

Based on experimental and modeling studies, the rate of increase in the martensite start temperature M _s for the tetragonal-to-monoclinic transformation with increase in zirconia grain size is found to rise with decrease in ZrO₂ content in the zirconia-toughened alumina ZTA system. The observed grain size dependence of M _s can be related to the thermal expansion mismatch tensile (internal) stresses which increase with decrease in zirconia content. The result is that finer zirconia grain sizes are required to retain the tetragonal phase as less zirconia is incorporated into the alumina, in agreement with the experimental observations. At the same time, both the predicted and observed applied stress required to induce the transformation are reduced with increase in the ZrO₂ grain size. In addition, the transformation-toughening contribution at temperature T increases with increase in the M _s temperature brought about by the increase in the ZrO₂ grain size, when T > M _s. In alumina containing 20 vol% ZrO₂ (12 mol% CeO₂), a toughness of ∼10 MPa. √m can be achieved for a ZrO₂ grain size of ∼2 μm ( M _s∼ 225 K). However, at a grain size of ∼2 μm, the alumina–40 vol% ZrO₂ (12 mol% CeO₂) has a toughness of only 8.5 MPa. √m ( M _s∼ 150 K) but reaches 12.3 MPa. ∼m ( M _s∼ 260 K) at a grain size of ∼3 μm. These findings show that composition (and matrix properties) play critical roles in determining the ZrO₂ grain size to optimize the transformation toughening in ZrO₂-toughened ceramics.     

Modeling and Fabrication of Fine-Grain Alumina-Zirconia Composites Produced from Nanocrystalline Precursors

Fully dense fine-grained 32.6-vol%-zirconia-toughened alumina composites have been fabricated from nanocrystalline rapidly solidified material. A model considering the thermodynamics of the constrained t -ZrO₂ m -ZrO₂ phase transformation was developed for this percolated two-phase material. This analysis indicated that the grain size at which this phase transformation is thermodynamically favorable was 1.26 µm in a composite that contained 32.6 vol% ZrO₂ and was stabilized with 1.50 mol% Y₂O₃. These results of the model compared favorably with experimental results, showing that grains of this size could be retained after heating to temperatures of as high as 1600°C. The rapidly solidified precursor was ball-milled into submicrometer powder and centrifugally cast into green specimens that were pressureless sintered to full density at temperatures as low as 1500°C. A composite containing nearly 100% t -ZrO₂ was produced by pressureless sintering at 1500°C and a composite containing 45 vol% t -ZrO₂/55 vol% m -ZrO₂ was obtained by sintering at 1600°C. The resulting two-phase microstructures contained uniformly distributed, micrometer-size grains whose sizes are consistent with the facilitation of transformation and microcrack toughening.     

Elastic Properties and Microcracking Behavior of Particulate Titanium Diboride–Silicon Carbide Composites

The spontaneous microcracking of particulate TiB₂–SiC composites is studied as a function of TiB₂ volume fraction. The degree of microcracking was examined by measuring elastic properties from room temperature to 1300°C. The results showed that only one composition contains microcracks. All other compositions did not microcrack regardless of TiB₂ volume fraction. This was attributed to the difference in the sintering aids. In particular, the Al₂O₃ sintering aid needed in these compositions had reacted with SiO₂ to form an amorphous grain boundary phase that allowed residual stresses to relax by viscous flow at moderate to high temperatures. The existence of this amorphous grain boundary phase was directly observed by transmission electron microscopy.     

Strain-Rate-Dependent Transformation Behavior of a 9 mol% Ce-TZP Ceramic

The strain rate dependence of the stress-induced t–m transformation behavior of a Ce-TZP ceramic with a grain size of 1.1 μm has been investigated both in four-point bend and Charpy impact tests. The crosshead speed was varied from 8.3 × 10⁻⁸ m/s to 8.3 × 10⁻³ m/s in the four-point bend tests. In the impact tests, the striking velocity was 2.5 m/s. Double notches ( a/w = 0.5) were made on one side of the specimen to measure the stress-induced t–m transformation zone size at the nonpropagated notch tip. It was found that a transition of the transformation zone size with crosshead speed occurred at about 10⁻⁶ to 10⁻⁵ m/s. Above this critical speed, the transformation zone was at least an order of magnitude larger in size. This new strain-rate-dependent transformation behavior has not been previously reported.     

Computer Simulation of Grain Growth and Ostwald Ripening in Alumina-Zirconia Two-Phase Composites

The kinetics of grain growth and Ostwald ripening in Al₂O₃–ZrO₂ two-phase composites was systematically investigated using two-dimensional (2-D) computer simulations, based on a diffuse-interface field model. Using average values for the experimentally measured ratios of the grain boundary energies to the interphase boundary energy as the input, the predicted 2-D microstructural features and their evolution are in excellent qualitative agreement with experimental observations on 2-D cross sections of 3-D Al₂O₃–ZrO₂ two-phase composite microstructures. It was found that the coupled grain growth in Al₂O₃–ZrO₂ composites is controlled by long-range diffusion and the average size ( Rt ) as a function of time ( t ) follows the power-growth law, R ^m _t − R ^m ₀= kt with m = 3, which is independent of the initial microstructures and volume fractions of the two phases. The predicted variation of the kinetic coefficient ( k ) on the volume fraction follows a trend similar to that experimentally measured through the entire range of volume fractions. The scaling of grain size distributions is observed at a given volume fraction, i.e., they are time-invariant in the steady state. However, the characteristics of size distributions vary with the initial microstructures and the volume fractions. The relationship between matrix grain size and second-phase grain size is discussed.     

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.     

High-Temperature Failure of an Alumina-Silicon Carbide Composite under Cyclic loads: Mechanisms of Fatigue Crack-Tip Damage

Experimental results are presented on the mechanisms of tensile cyclic fatigue crack growth in an A1₂0₃-33-vol%-SiC-whisker composite at 1400°C. The ceramic composite exhibits subcritical fatigue crack propagation at stress-intensity-fator values far below the fracture toughness. The fatigue characterized by the stressintensity-factor range, ΔK, and crack propagation rates are found to be strongly sensitive to the mean stress (load ratio) and the frequency of the fatigue cycle. Detailed transmission electron microscopy of the fatigue crack-tip region, in conjunction with optical microscopy, reveals that the principal mechanism of permanent damage ahead of the advancing crack is the nucleation and growth of interfacial flaws. The oxidation of Sic whiskers in the crack-tip region leads to the formation of a silica-glass phase in the 1400°C air environment. The viscous flow of glass causes debonding of the whisker-matrix interface; the nucleation, growth, and coalescence of interfacial cavities aids in developing a diffuse microcrack zone at the fatigue crack tip. The shielding effect and periodic crack branching promoted by the microcracks result in an apparently benefcial fatigue crack-growth resistance in the A1₂0₃—SiC composite, as compared with the unreinforced alumina with a comparable grain size. A comparison of static and cyclic load crack velocities is provided to gain insight into the mechanisms of elevated temperature fatigue in ceramic composites.