Subcritical Crack Propagation in 3Y-TZP Ceramics: Static and Cyclic Fatigue

A detailed analysis of crack propagation in tetragonal zirconia polycrystals doped with 3 mol% of Y₂O₃ (3Y-TZP) ceramics is presented. Crack propagation tests have been conducted for crack velocities of 10^-12-10^-3 m/s in several environments, including air, water (in the temperature range of 3°-85°C), secondary vacuum (10^-5 mbar), and silicon oil. Analysis of the experimental results-three propagating regimes that are dependent on the environment and a marked threshold below which no propagation occurs-shows that stress corrosion by water molecules is the key mechanism for crack propagation. The effect of grain size on the crack velocity is quantified and analyzed in terms of transformation toughening. Experiments under cyclic loading have been conducted to quantify the effects of cyclic fatigue. Crack velocities are higher under cyclic loading than that predicted by stress corrosion alone, and the threshold is lower. Experiments that have been conducted at two different frequencies (0.1 and 1 Hz) and static-fatigue/cyclic-fatigue sequences show that both stress corrosion by water and pure cyclic-fatigue effects are operative under alternative stresses.     

Sintering Behavior of 0.8 mol%-CuO-Doped 3Y-TZP Ceramics

In recent years, 3 mol% yttria-stabilized tetragonal zirconia polycrystals (3Y-TZP) doped with copper oxide has obtained increasing interest due to its enhanced superplastisity and good potential in tribological applications. In this work, the effect of addition of small amounts (0.8 mol%) of copper oxide on the sintering behavior of 3Y-TZP was studied using a dilatometer and high-temperature X-ray diffraction (XRD). A qualitative sintering model was established based on several reactions during sintering as indicated by thermal analysis and XRD. Some of these reactions remarkably retard densification and consequently result in low final density (86%) of the sample sintered at 1400°C in air. The reaction between molten Cu₂O and yttria as segregated to the Y-TZP grain boundaries at around 1180°C leads to the depletion of yttria from Y-TZP grains, which results in the formation of monoclinic phase during cooling. A relatively higher oxygen partial pressure can inhibit the dissociation of CuO to Cu₂O. This inhibition in dissociation is one of the reasons why a dense (>96%) 0.8 mol% CuO-doped 3Y-TZP ceramic can be obtained after sintering at 1400°C in flowing oxygen.     

Cyclic Fatigue Crack Propagation Behavior of 9Ce-TZP Ceramics with Different Grain Size

Cyclic fatigue crack growth behavior has been investigated in 9 mol% Ce-TZP ceramics with grain sizes varying from 1.1 to 3.0 μm. To ascertain the interaction between crack resistance curve behavior and cyclic fatigue crack growth, cyclic fatigue tests were conducted with short double-cantilever-beam specimens in two conditions: (a) with a sharp precrack without preexisting t - m transformation and (b) with a sharp crack after R -curve measurements, i.e., with preformed t - m transformation in the crack region. Fatigue crack propagation occurs at applied stress intensity factor values as low as about 40% of the K _I,∞ values measured in the R -curves. The size and shape of the t - m transformation zones are found to be different for specimens obtained in monotonic loading R -curve measurements and in cyclic fatigue tests. For the specimens without preexisting t - m transformation the overall crack growth behavior can be described by the Paris power law relation: d a /d N = AδK^m _I with m values of 15 for the 1.1-μm grain size and between 8 and 9 for the material with larger grain sizes. For the specimens with the preformed transformation zone, a "V"shape d a /d N versus Δ K _I relation is obtained. Explanations for these different results in the two conditions are discussed in terms of crack tip shielding effects.     

Crack Healing Behavior of Silicon Carbide Ceramics

This study focuses on the crack healing behavior of three kinds of commercial SiC ceramics. Specimens with and without cracks were subjected to thermal treatment at different temperatures, and their strengths were measured by a three-point bending test in accordance with JIS standards. The tests were performed in air at both room temperature and elevated temperatures between 600° and 1500°C. The healed specimens showed a complete recovery of strength at room temperature for the investigated crack sizes of 2 c ≅ 100 μm and 2 c ≅ 200 μm, and their strength increased in accordance with the healing temperature. The behavior of the healed specimens at elevated temperatures was influenced by the material used and the test temperature. Generally, the strength decreased at a high temperature, but the degree of strength reduction was determined by the kind of ceramic. The most important difference between the healed and smooth specimens was exhibited in material A. It was observed that at 1400°C, the bending strength of the healed specimens made from this ceramic was about 37% of the value for specimens in an as-received state. Static fatigue tests were also performed for ceramic B at 900° and 1000°C. The experiment demonstrated that the static fatigue limit of a healed specimen is about 75% of the monotonic bending strength at the same temperature.     

Low-Temperature Aging of Y-TZP Ceramics

The isothermal tetragonal-to-monoclinic transformation of a 3Y-TZP ceramic is investigated from 70° to 130°C in water and in steam by X-ray diffraction and optical interferometer techniques. Aging kinetics followed by X-ray diffraction are fitted by the Mehl-Avrami-Johnson law, suggesting nucleation and growth to be the key mechanisms for transformation. Optical interferometer observations of highly polished samples effectively reveal a nucleation and growth micromechanism for tetragonal-to-monoclinic transformation. A model based on surface change analysis is developed that fits closely to the X-ray diffraction results.     

Detection and Analysis of Crack Propagation in PZT Multilayer Ceramics

The flaw propagation in Lead zirconate titanate (PZT) multilayer ceramics under mechanical load was examined using impedance spectroscopy and three‐point bending studies. Initial flaws were generated by applying a positive sinusoidal electric field to the specimens. The cracks were sequentially propagated and after the release of the external mechanical load, impedance spectroscopy was conducted. The shift in the resonance frequencies and the subresonance height of the impedance spectroscopy were used as a measure of flaw extension. A functional dependence of the resonance frequency and the phase shift on the crack length was found. The crack propagation was studied on flaws starting at the positive and negative electrode, respectively. The maximum fracture strength, as well as the crack path, depends on the electrode potential. The variation in the fracture strength was caused by different observed fracture mode: interface cracking, matrix‐cracking, or a combination of both. The morphology of the fracture surfaces was ascribed to a textured microstructure, which is created by the sample processing, for example, by the poling process. A modified poling procedure with a lower poling temperature was analyzed, which yielded a reduction of the anisotropy of the electrode strength. Impedance spectroscopy was found to be a reliable measurement tool for automated flaw detection in PZT multilayer ceramics.     

Development of Nanocrystalline Wear-Resistant Y-TZP Ceramics

The present contribution reports some interesting and new results obtained while developing yttria-stabilized tetragonal zirconia (Y-TZP) using spark plasma sintering (SPS). The experimental results clearly showed that ZrO₂-nanoceramics with high hardness(∼14.5 GPa) can be processed at a lower sintering temperature of 1200°C in a short time (5 min). Another important result is that the newly developed Y-TZP nanoceramics, compared with the conventional sintered TZP, exhibit better fretting wear resistance against bearing steel. The intergranular fracture and the grain pullout were observed as the major wear mechanisms of the zirconia nanoceramics.     

Crack Deflection and Propagation in Layered Silicon Nitride/Boron Nitride Ceramics

Crack deflection and the subsequent growth of delamination cracks can be a potent source of energy dissipation during the fracture of layered ceramics. In this study, multilayered ceramics that consist of silicon nitride (Si₃N₄) layers separated by boron nitride/silicon nitride (BN/Si₃N₄) interphases have been manufactured and tested. Flexural tests reveal that the crack path is dependent on the composition of the interphase between the Si₃N₄ layers. Experimental measurements of interfacial fracture resistance and frictional sliding resistance show that both quantities increase as the Si₃N₄ content in the interphase increases. However, contrary to existing theories, high energy-absorption capacity has not been realized in materials that exhibit crack deflection but also have moderately high interfacial fracture resistance. Significant energy absorption has been measured only in materials with very low interfacial fracture resistance values. A method of predicting the critical value of the interfacial fracture resistance necessary to ensure a high energy-absorption capacity is presented.     

Unified Theory of Thermal Shock Fracture Initiation and Crack Propagation in Brittle Ceramics

A fracture-mechanical theory is presented for crack propagation in brittle ceramics subjected to thermal shock. The criteria of crack stability are derived for a brittle solid uniformly cooled with triaxially constrained external boundaries. Thermal stress crack instability occurs between two values of critical crack length. For short initial crack length, crack propagation occurs kinetically, with the total area of crack propagation proportional to the factor S_t² (1-2 v )/EG, where S_t is tensile strength, v is Poisson's ratio, E is Young's modulus, and G is surface fracture energy. Under these conditions the newly formed crack is subcritical and requires a finite increase in temperature difference before propagation will proceed. For long initial crack length, crack propagation occurs in a quasi-static manner and can be minimized by maximizing the thermal stress crack stability parameter R_st= [G/α² E ]^1/2, where α is the coefficient of thermal expansion. For heterogeneous brittle solids, such as porous refractories, the concept of an "effective flaw length" is introduced and illustrated on the basis of experimental data in the literature. The relative change in strength of a brittle solid as a function of increasing severity of thermal shock is estimated. Good qualitative agreement with literature data is found.     

Crack Propagation Behavior of Polycrystalline Alumina under Static and Cyclic Load

Various causes for cyclic-loading fatigue in ceramics have been proposed. Degradation in the grain-bridging effect is the most important cause for cyclic-loading fatigue in nontransforming ceramics. Cyclic- and static-loading crack propagation behavior in terms of crack propagation rate, load—strain curve, and R -curve in compact tension specimens of polycrysalline aluminas with two types of average grain size is reported. Significant bridging is observed in coarse-grained alumina. The results are consistent with the proposal that grain bridging increases with grain size and that degradation in grain bridging is the most important cause for cyclic-loading fatigue in alumina ceramics.     

Optimized Nanosecond Pulsed Laser Micromachining of Y-TZP Ceramics

Owing to the inherent difficulty of mechanically machining hard, ytrria-stabilized tetragonal zirconia polycrstal (Y-TZP) ceramics, laser processing has recently been investigated as an alternative. However, owing to the vast range of variables such as wavelength, focus position, pulse width, power, and repetition rate, finding the optimum processing conditions can be difficult. In this report, we examine the parameters for a nanosecond pulsed (60 ns) Nd:YAG laser. It was found that at room temperature, the absorption of Y-TZP at 1064 and 532 nm is very low; however, it becomes highly absorbing above a machining threshold with a penetration depth of 1.6 μm above the threshold. In this condition, we have found the combination of processing variables that provides the optimum material removal rate (up to ∼2 mm³/min). Additionally, it was found that the pulse overlap has a significant influence on the efficiency of the processing and therefore the combination of scan speed and repetition rate must be carefully considered.     

Cyclic Fatigue–Crack Propagation Behavior in Silicon Carbide: Long- and Small-Crack Behavior

Cyclic fatigue properties of high-toughness SiC with additives of Al₂O₃ and Y₂O₃ were examined, with a focus on differences between long- (>3 mm) and small-crack (<200 μm) behavior. Small cracks were initiated with Vickers indents placed on the tensile surfaces of beams, and crack extension was monitored optically under cyclic load. For small cracks, high growth rates which exhibited a negative dependence on the far-field driving force were observed. Such behavior was explained by both indent-induced residual stresses and the relative size of cracks compared with bridging zone lengths.     

Characterization of Grain Boundaries in Superplastically Deformed Y-TZP Ceramics

The effects of compressive deformation on the grain boundary characteristics of fine-grained Y-TZP have been investigated using surface spectroscopy, impedance analysis, and transmission electron microscopy. After sintering at low temperature (1150°C), the grain boundaries are covered by an ultrathin (1nm) yttrium-rich amorphous film. After deformation at 1200°–1300°C under low stress, some grain boundaries are no longer covered by the amorphous film. Yttrium segregation seems to occur only at wetted grain boundaries. Evidence has been found that the extent of dewetting increases with increasing applied stress.     

The Strength and Hydrothermal Stability of Y-TZP Ceramics for Dental Applications

Effects of dental grinding, sandblasting, fatigue, and aging on the biaxial flexural strength of yttria partially stabilized tetragonal zirconia ceramics were evaluated. Grinding lowers the mean strength and reliability under static and cyclic loading. In contrast, sandblasting provides a powerful tool for surface strengthening; it also hinders the propagation of the diffusion-controlled transformation during exposure to aqueous environments. By introducing moderate porosity, a reasonable compromise between the mechanical strength and the elastic modulus has been achieved. Such biscuit-sintered specimens also exhibited full hydrothermal stability. Novel experimental dental root posts were designed that exhibit certain advantages over the zirconia posts already in clinical use.     

Measurement of Rising R-Curve Behavior in Toughened Silicon Nitride by Stable Crack Propagation in Bending

A simple procedure for measuring the R -curve properties of ceramics by a stable fracture test in three-point bending is described. As a typical case, data are displayed for a Si₃N₄ material toughened by the presence of acicular grains in situ grown during the sintering process. The fracture mechanics specimen was a single-edge double-notched beam (SEDNB), whose notch was sharpened to a radius of <10 μm in order to reduce the amount of elastic energy stored at its root prior to crack extension. Furthermore, a stabilizer, specially designed for the bending geometry, was used to control crack stability. During stable extension, the crack could be easily arrested at selected locations of the load-displacement curve, the load quickly released, and the stable crack extension directly measured by the die-penetration technique. The crack resistance, K _R, of the material was calculated from the measured crack extent and the onset load value before unloading. This method enabled us to precisely monitor the critical load value at which the load-displacement curve deviated from linear behavior, as well as crack extensions from a few tens of micrometers to about 1 mm. As an application of this method, the fracture resistance of a Si₃N₄ material with rising R -curve behavior was measured and found to increase from about 5.5 to 9.0 MPaμm^1/2 within a 0.8-mm extension.     

建陶制品裂纹釉的研制

本文简要介绍了裂纹釉的形成机理、组成及其影响因素,为在陶瓷制品上施以裂纹釉提供参考。     

Brittleness Dependence of Crack Branching in Ceramics

A linear correlation between the degree of overloading on the crack tip and brittleness was observed. The intercept, which was √2, is identified with a minimum condition of overloading where branching occurs at twice the fracture energy. The slope was identified with the degree of plastic deformation at the crack tip necessary to nucleate the branching crack. A total crack tip strain criterion for crack branching in ceramics was investigated. This criterion is supported by the fact that average total elastic strain at the boundary of the yield zone is approximately equal to the square of the slope of the curve.     

Microstructure, Flaw Tolerance, and Reliability of Ce-TZP and Y-TZP Ceramics

Ce-TZP and Y-TZP ceramics were heat-treated for various times and temperatures in order to vary the microstructure. Flaw tolerance was investigated using the indentation–strength test. Reliability was quantified using conventional two-parameter Weibull statistics. Some Ce-TZP specimens were indented at slightly elevated temperatures where no transformation was observed. Results indicated that the Ce-TZP specimens were extremely flaw tolerant, and showed a relatively high Weibull modulus that scaled with both R -curve behavior and flaw tolerance. Y-TZP, on the other hand, with very little if any R -curve behavior or flaw tolerance, had a low Weibull modulus. The results also show that flaw history, i.e., whether or not a transformation zone exists along the wake of the crack, has a significant influence on strength. Strength was much less dependent on initial crack size when the crack had an associated transformation zone, whereas strength was highly dependent on cracks typical of natural processing defects. It is argued that the improvement in reliability, flaw tolerance, and dependence on flaw history are all ramifications of pronounced R -curve behavior.