Influence of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, Gamma_BN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence Gamma_BN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in Gamma_BN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m², remaining constant through 950°C. A sharp increase in Gamma_BN, to 60 J/m², was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in Gamma_BN to 39 J/m². The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m². A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.     

Influence of Cu2O and CuAlO2 Interphases on Crack Propagation at Cu/α-Al2O3 Interfaces

In-situ crack-propagation experiments, in conjunction with thermochemical experiments, have been used to examine the role of discontinuous interphases on the fracture behavior of solid-state diffusion-bonded Cu/α-Al₂O₃ couples. Clean, interphase-free interfaces exhibit crack extension by brittle decohesion at the crack tip at an initiation fracture energy of 125 J/m². Crack propagation is characterized by an increase in the fracture energy with increases in the crack length ( R -curve behavior). When interfacial chemical reaction products are present, the crack growth is altered, depending on the characteristics of the interphase. The presence of Cu₂O needles results in preferential debonding along the Cu₂O/Al₂O₃ interface. On the other hand, finer CuAlo₂ needles visibly impede crack propagation and result in a higher interface initiation fracture energy (}190 J/m²) than that of the interphase-free interface. The effects of the Cu₂O and CuAlo₂ phases on the fracture energy are discussed.     

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.     

Effects of Moisture on Grain-Boundary Strength, Fracture, and Fatigue Properties of Alumina

The role of moisture in affecting both intrinsic and extrinsic aspects of the fracture and fatigue-crack growth resistance of a polycrystalline alumina (99.5% pure, 25 μm grain size) has been examined in both moist and dry environments at ambient temperature. The intrinsic (crack-tip) toughness, deduced from measured crack-opening profiles, is found to be less than for a single crystal and is 30% lower (∼0.6 MPa·m^1/2) in moist air versus in dry N₂, implying that the grain-boundary theoretical strength is higher in a dry environment. Despite this, in dry atmospheres, the R -curves (which derive from crack deflection and grain bridging) initially rose more steeply and nominal fatigue-crack growth thresholds for short crack sizes (20–60 μm) were more than 1.3 MPa·m^1/2 higher. Owing to this quicker crack bridging development, strengths for natural flaws could be more than doubled in dry atmospheres, a difference that well exceeds the effect solely due to the intrinsic toughness change. After ∼2 mm of crack growth, however, the R -curve and steady-state fatigue behavior appeared similar in both environments, although altering the atmosphere for such fatigue cracks in situ induced large, abrupt changes in transient growth rates. The environment influences the nature of the bridging zones, with uncracked-ligament bridges playing a larger role in dry atmospheres, while frictional bridges are predominant in moist air. Evidently, to achieve optimal toughness in bridging ceramics, the window for the requisite grain-boundary strength may be small; whereas weak boundaries are required to induce the necessary intergranular fracture, if too weak, shallower R -curves, less strengthening, and poorer fatigue resistance all follow.     

Processing and Characterization of SiC-Whisker-Reinforced Alumina-Matrix Composites

Brittle monolithic alumina can be reinforced with highstrength single-crystal SiC whiskers with the effect of increasing fracture toughness. In this study, well-mixed and nearly fully dense SiC whisker/alumina composites were fabricated by wet-blending the constituents and uniaxially hot-pressing the resulting powder. The alumina-matrix grain size depended on whisker volume fraction, whisker surface-oxygen content, and hot-pressing environment. Fracture toughness, measured by an indentation-fracture method, increased from 3.0 MPa·m^1/2 for the hot-pressed unreinforced alumina to 10.7 MPa·m^1/2 for the composite containing 25 vol% SiC whiskers. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers. The observed increase in fracture toughness of alumina due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms.     

Fracture Mechanisms of a Coarse-Grained, Transparent MgAl2O4 at Elevated Temperatures

The fracture of a transparent, large-grain-sized MgAl₂O₄ spinel has been studied through temperatures of 1400°C. Fracture toughness values, falling between about 1.3 and 2.3 MPa.m^12;1/2, behaved sigmoidally with temperature, with a lower shelf transition appearing near 800°C. Rising R -curves, displaying a run-arrest character, were found at both the lowest and the highest temperatures, while only minimal values of d K _R/dΔ a were produced at intermediate temperatures of 800°C. This fracture character is ascribed largely to finite concentrations of a residual LiF pressing aid identified on the fracture surface, while additional influences associated with the lower shelf region at the highest temperatures may include the increased fraction of intergranular crack path and the onset of plasticity. This cubic monolithic ceramic displayed strong nonlinear fracture behavior in both temperature regimes. In both cases the fracture character is linked directly to an active toughening mechanism in the wake region, which depends upon crack face bridging.     

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.     

Mechanical Properties of Tape-Cast Alumina-Glass Dental Composites

Alumina-glass dental composites were prepared by tape casting and sintering at 1120°C, followed by glass infiltration at 1100°C. Flexural strength and fracture toughness of the composites were investigated in terms of the influence of tape constituents, namely, alumina powder, binder, and plasticizer, on the mechanical properties. Both strength and toughness increased with increasing alumina fraction in tapes and decreased with increasing binder content in binder/plasticizer mixtures. These observations were consistent with the influence of the constituents on mean alumina particle distance in tapes, suggesting that the high strength of glass-infiltrated alumina composites is related to toughening by crack bowing. The strength and fracture toughness of the tape-cast composites, optimized for forming dental crowns, were 508 MPa and 3.1 MPa·m^1/2, respectively, obtained from biaxial tests. Shrinkage of the composites decreased with increasing thermocompression pressure, applied to the tapes prior to sintering, and heating rate to the sintering temperature.     

Cyclic Fatigue-Crack Propagation in Magnesia-Partially-Stabilized Zirconia Ceramics

The subcritical growth of fatigue cracks under (tension-tension) cyclic loading is demonstrated for ceramic materials, based on experiments using compact C(T) specimens of a MgO-partially-stabilized zirconia (PSZ), heat-treated to vary the fracture toughness K _c from ∼3 to 16 MPa·m^1/2 and tested in inert and moist environments. Analogous to behavior in metals, cyclic fatigue-crack rates (over the range 10⁻¹¹ to 10⁻⁵ m/cycle) are found to be a function of the stress-intensity range, environment, fracture toughness, and load ratio, and to show evidence of fatigue crack closure. Unlike toughness behavior, growth rates are not dependent on through0-thickness constraint. Under variable-amplitude cyclic loading, crack-growth rates show transient accelerations following low-high block overloads and transient retardations following high-low block overloads or single tensile overloads, again analogous to behavior commonly observed in ductile metals. Cyclic crack-growth rates are observed at stress intensities as low as 50% of K _c , and are typically some 7 orders of magnitude faster than corresponding stress-corrosion crack-growth rates under sustained-loading conditions. Possible mechanisms for cyclic crack advance in ceramic materials are examined, and the practical implications of such "ceramic fatigue" are briefly discussed.     

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.     

ZrO2-Transformation-Toughened Glass-Ceramics Prepared by the Sol-Gel Process from Metal Alkoxides

Glasses of composition 3ZrO₂O · 2SiO₂ were prepared by the sol-gel process from metal alkoxides. Tetragonal ZrO₂ was precipitated by appropriate heat treatment at 1000° to 1200°C. The fracture toughness of these glass-ceramics increased with increasing crystallite size of the tetragonal ZrO₂, reaching ∼5.0 MN/m^3/2 at a size of ∼40 nm. The higher fracture toughness was attributed to tetragonal → monoclinic ZrO₂ transformation toughening.     

Microcracking in B4C-TiB2 Composites

Boron carbide/titanium diboride composites with 20 and 40 vol% particulate TiB₂ and various amounts of free carbon were investigated with respect to microcrack toughening. In agreement with previous work, the mere addition of TiB₂ was found to raise the toughness from 2.2 MPa·m^1/2 up to 3.0 and 3.5 MPa·m^1/2, respectively. A further and very significant increase of composite toughness up to 6.0 MPa·m^1/2 was discovered upon the incorporation of free carbon. SEM and TEM observations reveal that this toughening is associated with microcracking at B₄C-TiB₂ phase boundaries. Microcracking is triggered by thin carbon interlayers, which are located at hetero interfaces and supply a weak fracture path.     

Fracture Toughness of High-Pressure-Sintered Diamond/Silicon Nitride Composites

Diamond particles were dispersed in silicon nitride, sintered at 6 GPa and 1600°C for 30 min, and heat-treated under vacuum at 1300°C to induce graphitization of the diamond. Improved fracture toughness values up to 7.5 MN/m³¹² were achieved for these composites. Stress-induced microcrack toughening is considered to be a plausible toughening mechanism. Comparisons with diamondlalumina ceramics were made.     

Microstructure and Mechanical Properties of Hot-Pressed α-Al2O3-Seeded γ-Alumina Ceramics

High-quality alumina ceramics were fabricated by a hot pressing with MgO and SiO₂ as additives using α-Al₂O₃-seeded nanocrystalline γ-Al₂O₃ powders as the raw material. Densification behavior, microstructure evolution, and mechanical properties of alumina were investigated from 1250°C to 1450°C. The seeded γ-Al₂O₃ sintered to 98% relative density at 1300°C. Obvious grain growth was observed at 1400°C and plate-like grains formed at 1450°C. For the 1350°C hot-pressed alumina ceramics, the grain boundary regions were generally clean. Spinel and mullite formed in the triple-grain junction regions. The bending strength and fracture toughness were 565 MPa and 4.5 MPa·m^1/2, respectively. For the 1300°C sintered alumina ceramics, the corresponding values were 492 MPa and 4.9 MPa·m^1/2.     

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.     

Mechanical Behavior at 20° and 1200°C of Nicalon-Silicon-Carbide-Fiber-Reinforced Alumina-Matrix Composites

Tensile and fracture tests were conducted at 20° and 1200°C on a ceramic-matrix composite that was composed of an alumina (Al₂O₃) matrix that was bidirectionally reinforced with 37 vol% silicon carbide (SiC) Nicalon fibers. The composite presented nonlinear behavior at both temperatures; however, the strength and toughness were significantly reduced at 1200°C. In accordance with this behavior, matrix cracks were usually stopped or deflected at the fiber/matrix interface, and fiber pullout was observed on the fracture surfaces at 20° and 1200°C. The interfacial sliding resistance at ambient and elevated temperatures was estimated from quantitative microscopy analyses of the saturation crack spacing in the matrix. The in situ fiber strength was determined both from the defect morphology on the fibers and from the size of the mirror region on the fiber fracture surfaces. It was shown that composite degradation at elevated temperature was due to the growth of defects on the fiber surface during high-temperature exposure.     

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.     

Mechanical Properties and Fracture Toughness of α-Al2O3-Platelet-Reinforced Y-PSZ Composites at Room and High Temperatures

YPSZ/Al₂O₃-platelet composites were fabricated by conventional and tape-casting techniques followed by sintering and HIPing. The room-temperature fracture toughness increased, from 4.9 MPa·m^1/2 for YPSZ, to 7.9 MPa·m^1/2 (by the ISB method) for 25 mol% Al₂O₃ platelets with aspect ratio = 12. The room-temperature fiexural strength decreased 21% and 30% (from 935 MPa for YPSZ) for platelet contents of 25 vol% and 40 vol%, respectively. Al₂O₃ platelets improved the high-temperature strength (by 110% over YPSZ with 25 vol% platelets at 800°C and by 40% with 40 vol% platelets at 1300°C) and fracture toughness (by 90% at 800°C and 61% at 1300°C with 40 vol% platelets). An amorphous phase at the Al₂O₃-platelet/YPSZ interface limited mechanical property improvement at 1300°C. The influence of platelet alignment was examined by tape casting and laminating the composites. Platelet alignment improved the sintered density by >1% d _th , high-temperature strength by 11% at 800°C and 16% at 1300°C, and fracture toughness by 33% at 1300°C, over random platelet orientation.     

Control of Composition and Structure in Laminated Silicon Nitride/Boron Nitride Composites

Based on a biomimetic design, Si₃N₄/BN composites with laminated structures have been prepared and investigated through composition control and structure design. To further improve the mechanical properties of the composites, Si₃N₄ matrix layers were reinforced by SiC whiskers and BN separating layers were modified by adding Si₃N₄ or Al₂O₃. The results showed that the addition of SiC whiskers in the Si₃N₄ matrix layers could greatly improve the apparent fracture toughness (reaching 28.1 MPa·m^1/2), at the same time keeping the higher bending strength (reaching 651.5 MPa) of the composites. Additions of 50 wt% Al₂O₃ or 10 wt% Si₃N₄ to BN interfacial layers had a beneficial effect on the strength and toughness of the laminated Si₃N₄/BN composites. Through observation of microstructure by SEM, multilevel toughening mechanisms contributing to high toughness of the laminated Si₃N₄/BN composites were present as the first-level toughening mechanisms from BN interfacial layers as crack deflection, bifurcation, and pull-out of matrix sheets, and the secondary toughening mechanism from whiskers in matrix layers.     

Cyclic Fatigue-Crack Growth and Fracture Properties in Ti3SiC2 Ceramics at Elevated Temperatures

The cyclic fatigue and fracture toughness behavior of reactive hot-pressed Ti₃SiC₂ ceramics was examined at temperatures from ambient to 1200°C with the objective of characterizing the high-temperature mechanisms controlling crack growth. Comparisons were made of two monolithic Ti₃SiC₂ materials with fine- (3–10 μm) and coarse-grained (70–300 μm) microstructures. Results indicate that fracture toughness values, derived from rising resistance-curve behavior, were significantly higher in the coarser-grained microstructure at both low and high temperatures; comparative behavior was seen under cyclic fatigue loading. In each microstructure, Δ K _th fatigue thresholds were found to be essentially unchanged between 25° and 1100°C; however, there was a sharp decrease in Δ K _th at 1200°C (above the plastic-to-brittle transition temperature), where significant high-temperature deformation and damage are first apparent. The substantially higher cyclic-crack growth resistance of the coarse-grained Ti₃SiC₂ microstructure was associated with extensive crack bridging behind the crack tip and a consequent tortuous crack path. The crack-tip shielding was found to result from both the bridging of entire grains and from deformation kinking and bridging of microlamellae within grains, the latter forming by delamination along the basal planes.