Thermal Shock Behavior of Duplex Ceramics

The thermal-stress fracture behavior of duplex ceramics is investigated by quenching in water and in oil. Comparison with the matrix materials shows that the critical quenching temperature difference, Δ T _c , is not or is only slightly reduced, even for duplex ceramics of significantly reduced strength. In sintered composites, thermal-stress-induced microcracking within pressure zones and crack initiation at pressure zone–matrix interfacial defects develop before Δ T _c is reached. The effects are accompanied by a gradual reduction in strength. At Δ T _c , critical crack propagation occurs. The retained strength after thermal shock of duplex ceramics is significantly improved compared with the respective matrix materials. This behavior can be related reasonably well with the K ^R -curve behavior.     

Transformation Toughening in ZrO2-Containing Ceramics

Transformation toughening in ZrO₂-containing ceramics is discussed. Specifically, microstuctures of the three distinct types of ZrO₂-toughened ceramics are presented, after which microstructural evolution in MgO-partially-stabilized ZrO₂ Mg-PSZ) is reviewed. The mechanical properties of such transformation-toughened ceramics are dominated by "resistance-curve" ( R -curve) behavior, wherein the crack resistance increases during the course of crack propagation. Ceramics subject to R -curve behavior require a more detailed failure criterion than those subject to the usual linear elastic fracture mechanics criterion involving a critical stress intensity factor, K_IC.R -curve-controlled fracture in ceramics provides a degree of very desirable flaw insensitivity, but can lead to counterintuitive relationships concerning strength, toughness, and initial flaw size. Examples of R curves of Mg-PSZ with different thermal histories are given.     

R-Curve and Compliance Change upon Renotching

A novel approach is addressed in the application of the renotching technique to the consideration of toughening processes and mechanisms of ceramic materials. Crackbridging stresses and the contribution of other mechanisms which enhance crack-face friction are stepwise removed during renotching. This technique enables one to determine the amount and distribution of bridging stresses behind the crack tip, as well as to separate the contributions of crack-face, crack-wake, and crack-tip processes and mechanisms. Five ceramic materials with different microstructures are used for experimentally determining their conventional K ^R - and "renotched" ^R - curves in addition to measuring the compliance changes during extending as well as renotching the crack. A simple theoretical approach is presented and applied to the experimental results, which enables one to easily determine the magnitude and the distribution of crack-face bridging stresses. The contributions of various toughening processes to the conventional and the renotched R -curve behavior are discussed.     

Indentation Studies on Y2O3-Stabilized ZrO2: II, Toughness Determination from Stable Growth of Indentation-Induced Cracks

Stable indentation cracks were grown in four-point bend tests to study the fracture toughness of two Y₂O₃-stabilized ZrO₂ ceramics containing 3 and 4 mol% Y₂O₃. By combining microscopic in situ stable crack growth observations at discrete stresses with crack profile measurements, the dependence of toughness on crack extension was determined from crack extension plots, which graphically separate the crack driving residual stress intensity and applied stress intensity factors. Both materials exhibit steeply rising R -curves, with a plateau toughness of 4.5 and 3.1 Mpa·m^1/2 for the 3- and 4-mol% materials, respectively. The magnitude of the plateau toughness reflects the fraction of tetragonal grains contributing to transformation toughening.     

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.     

Fiber Debonding in Residually Stressed Brittle Matrix Composites

The competition between initial fiber debonding versus fiber failure marks a crucial event of the microstructural failure process in fiber-reinforced brittle matrix composites. In this study, the role of a thermal residual stress field on the debonding conditions is examined theoretically and analytically. The analysis is based on two critical observations, the first being that the mechanics at the tip of a kink crack are driven only by the singularity at the main crack tip. Following from the first is the second observation that any thermal stress effects on the debonding criteria should enter only through the phase angle ψ ^T of the total stress intensity factor at the main crack tip. In general, this stress intensity factor has a thermal as well as a mechanical load contribution. It is shown that when the thermal and mechanical stress intensities, K ^R and K ^t , respectively, are in phase , i.e., ψ ^R =ψ ^t , the existing debonding conditions are universal and can be used even in the presence of thermal loads. On the contrary, when K ^R and K ^t are out of phase , i.e., ψ ^R ≠ψ ^t , events such as the delamination of thick films or debonding of inclined aligned fibers in brittle matrix composites become sensitive to the presence of the thermal stresses.     

Slow Crack Growth Behavior in Transformation-Toughened Al2O3-ZrO2(Y2O3) Ceramics

Significant increases in the critical fracture toughness (K _IC ) over that of alumina are obtained by the stress-induced phase transformation in partially stabilized ZrO₂ particles which are dispersed in alumina. More importantly, improved slow crack growth resistance is observed in the alumina ceramics containing partially stabilized ZrO₂ particles when the stress-induced phase transformation occurs. Thus, increasing the contribution of the ZrO₂ phase transformation by tailoring the Y₂O₃ stabilizer content not only increases the critical fracture toughness (K_IC) but also the K _Ia to initiate slow crack growth. For example, crack velocities ( v )≥10^–9 m/s are obtained only at K _Ia≥5 MPa.m^1/2 in transformation-toughened ( K _IC=8.5 MPa.m^1/2) composites vs K _Ia≥2.7 MPa.m^1/2 for comparable velocities in composites where the transformation does not occur ( K _IC=4.5 MPa.m^1/2). This behavior is a result of crack-tip shielding by the dissipation of strain energy in the transformation zone surrounding the crack. The stress corrosion parameter n is lower and A greater in these fine-grained composite materials than in fine-grained aluminas. This is a result of the residual tensile stresses associated with larger (≥1 μm) monoclinic ZrO₂ particles which reside along the intergranular crack path.     

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.     

Crack Initiation and Arrest in a SiC Whisker/A12O3 Matrix Composite

The fracture initiation and arrest stress intensity factors were determined for a SiC-whisker-reinforced AI₂O₃ matrix composite. A chevron-notched, three-point-bend specimen was used to genera e the load/displacement curve, which exhibited repeated crack initiation, followed by crack arrest behavior. Corresponding stress intensity factors were determined for both situations using the compliance technique. Calculated crack arrest positions were in agreement with fractographic observations. Both the crack arrest and the crack initiation stress intensity factors exhibited a rising R -curve with increasing crack length, suggesting the presence of wake toughening effects on the crack growth resistance.     

Effects of Fast Neutron Irradiation on Thermal Conductivity of Li2O and LiAlO2

Li₂O and LiAlO₂ are two candidates for solid breeder materials in the United States' Fusion Power Program. Critical to breeder design efforts are thermophysical data, the bulk of which have only recently become available, for un-irradiated lithium ceramics. This paper expands the current limited data base by presenting thermal conductivity data between 373 and 1173 K for both materials following fast neutron irradiation. Samples were irradiated at 773 to 1173 K to lithium burnups ≤11.5 × 10²⁰ captures/cm³. Comparisons are made between these data and those from unirradiated archive samples of these same materials.     

Effects of Damage on Thermal Shock Strength Behavior of Ceramics

When subjected to severe thermal shock, ceramics suffer strength degradation due to the damage caused by the shock. A fracture-damage analysis is presented to study the effects of damage on the thermal shock behavior of ceramics. It is assumed that a narrow strip damage zone is developed at the tip of a preexisting crack after a critical thermal shock and the damage behavior can be described by a linear strain-softening constitutive relation. Damage growth and strength degradation are determined based on fracture and damage mechanics. Numerical calculations are carried out for two ceramic materials, and the strength degradation agrees quite well with experimental results. The effects of bridging/damage stress, the fracture energy of the bridging/damage zone, and specimen size on thermal shock strength behavior are studied. A higher fracture energy can enhance the residual strength of thermally shocked ceramics and, for a given fracture energy, a higher bridging stress is needed to reduce the strength degradation. It is also shown that the thermal shock strength behavior is size-dependent, and a high value of ( K _IC/O_b)², where K _IC is the intrinsic fracture toughness and O_b is the bending strength, can improve significantly the residual strength.     

Subcritical Crack Growth of Macrocracks in Alumina with R-Curve Behavior

Subcritical crack growth of macroscopic cracks in two Al₂O₃ ceramics is investigated with single-edge-notched bending specimens under constant load. The resulting v - K _I-curves are in complete contrast to the behavior of natural cracks. In spite of the monotonic increase of the externally applied stress intensity factor due to crack extension, the crack growth rates first decrease. This behavior is caused by crack shielding due to crack border interaction and can be described by a rising crack growth resistance. Two methods are applied to determine the R -curve under subcritical crack growth conditions.     

Fracture Resistance of SiC-Fiber-Reinforced Si3N4 Composites at Ambient and Elevated Temperatures

The crack growth behavior in unidirectional SiC-fiber-rein-forced Si₃N₄-matrix composites fabricated in our laboratories was investigated as a function of fiber volume fraction and temperature. Both the stress-intensity factor and an energy approach were adopted in the characterization of the crack growth behavior. Crack resistance increased with crack extension ( R -curve or T -curve) as a result of bridging effects associated with the intact fibers. Large-scale bridging was observed, and was considered in the determination of the R -curves. Temperature and fiber volume fraction affected the crack propagation behavior. At room temperature a single crack was initiated at the notch tip; it then branched and delaminated upon further loading. In contrast, at 1200°C, little crack branching was observed. Increasing fiber volume fraction increased the degree of crack branching. Temperature and fiber volume fraction also affected the R -curve behavior. Raising the temperature to 1200°C did not significantly degrade the room-temperature R -curve effect. Increasing the fiber volume fraction from 14% to 29% substantially enhanced the toughening effect and the R -curve behavior.     

Preparation of High-Strength Calcium Phosphate Ceramics with Low Modulus of Elasticity Containing β-Ca(PO3)2 Fibers

Novel calcium phosphate ceramics were fabricated by hot-pressing fibrous products extracted from crystallized products of calcium ultraphosphate glasses by aqueous leaching. The ceramics were dense materials with a relative density of >95%; these ceramics were composite materials that consisted of β-Ca(PO₃)₂ fibrous crystals with CaO–P₂O₅ glass, which was formed during hot pressing, as the matrix phase. These ceramics showed a high bending strength of 150–220 MPa and a low Young's modulus of 30–60 GPa. The high toughness contributed to the high strength, with fiber pull-out and crack deflection observed as the primary toughening mechanism.     

Fracture Behavior of Multilayer Silicon Nitride/Boron Nitride Ceramics

The fracture behavior of multilayer Si₃N₄/BN ceramics in bending has been studied. The materials were prepared by a process of tape casting, coating, laminating, and hot pressing. The Si₃N₄ layers were separated by thin, weak BN interlayers. Crack patterns in bending bars were examined with a scanning electron microscope. The weak layers deflected cracks in bending and thus prevented catastrophic failure. In one well-aligned multilayer ceramic A, a main crack propagated through the specimen although along a zigzag path. A second multilayer ceramic B was made to simulate a wood grain structure. Its failure was dominated by shear cracking along the weak BN layers. Besides crack deflection, interlock bridging between toothlike layers in the wake of the main crack appeared also to contribute to toughening.     

Si3N4/SiC-Whisker Composites without Sintering Aids: II, Fracture Behavior

The fracture behavior of an Si₃N₄/SiC-whisker composite fabricated without sintering aids is investigated using a double approach based on the examination of R -curve behavior and a statistical analysis of crack propagation. In the composite with 20 vol% whisker, a 30% increase in toughness over the matrix value can be attributed to crack-tip phenomena. Strong interfacial bonding prevents any contribution to toughening by mechanisms operating in the wake region of the crack. Based on experimental observations of microfracture in both SiC whiskers and Si₃N₄ grains, toughening caused by crack-tip phenomena is quantitatively discussed in terms of fracture energy and whisker-distribution parameters.     

Evidence for Bulk Residual Stress Strengthening in Al2O3/SiC Nanocomposites

The fracture behavior of Al₂O₃/SiC nanocomposites has been studied as a function of the SiC volume fraction and compared to that of the pure Al₂O₃ matrix. A pronounced strengthening effect was only observed for materials with low SiC content (i.e., ≤10 vol%) although no evidence of concurrent toughening was found. Assessment of near-tip crack opening displacement (COD) could not experimentally substantiate significant occurrence of an elastic crack-bridging mechanism, in contrast with a recently proposed literature model. Quantitative fractography analysis indicated that transgranular crack propagation in Al₂O₃/SiC nanocomposites depends on the location of the SiC dispersoids within the matrix texture; the higher the fraction of transgranularly located dispersoids, the more transgranular the fracture mode. Experimental evidence of remarkably high residual stresses arising from thermal dilatation mismatch (upon cooling) between Al₂O₃ and SiC phases were obtained by fluorescence and Raman spectroscopy. A strengthening mechanism is invoked which merely arises from residual stress through strengthening of Al₂O₃ grain boundaries.     

Elevated-Temperature R-Curve Behavior of a Polycrystalline Alumina

The fracture behavior of a polycrystalline alumina was examined at temperatures ranging from ambient through 1400°C, using three-point bend bar test specimens. R -curves were determined at all temperatures studied, and when accompanied by renotching procedures, a wake removal technique, conclusive evidence was provided to support the existence of a following wake region in this monolithic ceramic material. The crack closure stresses identified in this region are responsible for all toughening with crack extension observed in this study. Room-temperature " K _IC" fracture toughness values of 4.5 MPa · m^1/2 for the chevron-notch specimen and 3.9 MPa · m^1/2 for the straight-notch configuration were obtained. The critical stress intensity factor of the renotched chevron-notch specimen compared very closely with that of the straight-notch specimen. These findings further confirm the toughening role of the microstructural features found in the following wake region. This paper considers, in detail, these observations in terms of the microstructure and its role in the toughening mechanism.     

Predictability of the Mechanical Properties of Inclusion-Containing Ceramics

The incorporation of inclusions into a brittle matrix can change its properties significantly. The quantitative change of strength, toughness, and retained strength after severe thermal shock and Vickers indentation damage in so-called "duplex ceramics" can be reasonably well predicted by relating these properties to the empirically derived internal stress-intensity factor K ₁. In the present paper the relationship between K ₁ and three other recently measured properties—flaw resistance behavior, K _R-curve behavior, and Stress–Strain behavior—is investigated. In addition, the strength-degrading influence of pressurized inclusions observed in "duplex ceramics" is compared to the strength reduction and enhancement reported by other authors for composites containing a glass matrix and glass and alumina inclusions. An attempt is made to identify the important parameters which control the strength development in duplex structures.     

Toughness Anisotropy in Textured Ceramic Composites

In this investigation, a model predicting toughness anisotropy in textured ceramics containing elongated grains and in composites reinforced with rod-shaped particles is presented. The model predictions are based on the assumption that crack deflection is the only toughening mechanism. In the model, toughness anisotropy is calculated as a function of texture degree. For composite materials, the volume fraction of the reinforcement phase is also an input parameter. Correspondence between model and experiment was established by comparing measured toughness anisotropies in β-Si₃N₄ and Al₂O₃/SiC whisker composites to model predictions. In these model predictions, measured orientation distributions from hot-pressed and hot-forged specimens were employed. The potential for relating other toughening mechanisms in a similar format is also addressed, since the model and experimental measurements give different results. The crack deflection model simultaneously overpredicts the toughening enhancement and underpredicts the toughening anisotropy observed in the experiments.