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.     

Effect of Crack-Interface Bridging on Subcritical Crack Growth in Ferrites

A transition between transgranular fracture and intergranular fracture in polycrystalline manganese zinc ferrites has been investigated by measuring subcritical crack growth in double-cantilever-beam specimens. Log velocity vs K ₁ plots are like those for other ceramic materials with three well-defined regions of growth. Unlike other ceramics, however, the K ₁ values at which crack growth became critical showed large variations with relative humidity. Crack velocities depended on prior history and, at K ₁ > 1.1 MPa · m^1/2 in dry N₂, crack growth was intermittent. All of these features were related to an increase in intergranular fracture with K ₁. A model is presented to explain the data in terms of the generation ad removal of crack-interface bridges which were produced by intergranular fracture.     

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.     

Relationship Between Microstructures and Material Properties of Novel Fibrous Al2O3–(m-ZrO2)/t-ZrO2 Composites

Novel fibrous Al₂O₃–(m-ZrO₂)/t-ZrO₂ (m, monoclinic; t, tetragonal) composites having a core/shell structure were fabricated by multi-extrusion, and their microstructures and material properties were investigated depending on the number of extrusions. The composites acquired a homogeneously fine fibrous structure as the number of extrusions increased. The bending strength and fracture toughness increased remarkably as the number of extrusions increased. In the fracture surface of the second passed composite, an Al₂O₃–(m-ZrO₂) core region appeared, flat type, although some local regions existed with an intergranular fracture. However, the fracture mode of the t-ZrO₂ region was of intergranular type having a sharp and rough surface. In the composite made by the fifth passed extrusion, the fracture strength and toughness values were high at about 665 MPa and 9.6 MPa·m^1/2, respectively. The main fracture mode was a typical intergranular mode having a rough fracture surface, and the main multi-toughening was because of mechanisms such as crack bridging, microcracking, and phase transformation.     

Subcritical Crack Growth in Y-TZP and Al2O3-Toughened Y-TZP

Crack velocity curves for Y-TZP and Al₂O₃-toughened Y-TZP were determined for long cracks in compact tension specimens with an in situ fracture device on the stage of an optical microscope. Indications for a crack velocity threshold were found for both materials. Above this threshold, at 2.6 MPa·m^1/2 for Y-TZP and 3.6 MPa·m^1/2 for Al₂O₃-toughened Y-TZP, chemically assisted subcritical crack growth occurs over an extended regime of applied stress intensity factors of width 2.1–2.8 MPa·m^1/2. It is recognized that the dependence of the shielding term on the crack-tip stress field renders transformation-toughened materials particularly susceptible to stress-corrosion cracking. This interrelation leads to the definition of a steady-state velocity at constant applied stress intensity factor. This velocity is obtained in the situation where the shielding term is fully defined by the present crack-tip stress field, not depending on prior loading history.     

Translucent Al2O3/LaAl11O18 Composite

A translucent alumina composite containing 1 vol% LaAl₁₁O₁₈, prepared by the hot isostatic pressing (HIP) method, displays both high translucency and high fracture toughness. Its total forward transmission at 600 nm is 75% (thickness 1 mm), and its bending strength and fracture toughness are estimated to be 574±15 MPa and 5.9±0.46 MPa·m^0.5, respectively. Its high translucency is due to the similarity of refractive index between the additive phase (LaAl₁₁O₁₈) and the matrix (alumina).     

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.     

Stoichiometry Effects on the Fracture of TiO2−x

Fracture characteristics of TiO_{2− x} were studied as a function of Stoichiometry. With increasing x , the fracture toughness K _{I e} and the fracture surface energy γ _f decrease and the amount of transgranular cleavage increases, corresponding to the increased concentration of planar defects within the grains. Increasing nonstoichiometry also shifted the ( K _I- V ) diagram to lower K _I values, commensurate with the K_{I e} decrease. Water accelerated stress corrosion by promoting intergranular failure during subcritical crack growth.     

Fracture Toughness and Spalling Behavior of High-Al2O3 Refractories

The fracture energies and spalling resistance of high-Al₂O₃ refractories were studied. The fracture energies, γ _WOF and γ _NBT , were measured by the work-of-fracture and the notched-beam-test methods, respectively. Spalling resistance, as measured by the relative strength retained in a water quench, correlated well with the thermal-stress resistance parameter applicable to stable crack propagation under conditions of thermal shock, (γ _WOF /α² E ₀). Many of the refractories exhibited high ratios of γ_WOF to γ_NBT; such high ratios were shown analytically to maximize the parameter ( R ¹¹¹¹= E _0γWOF/S₁²) which describes the resistance to catastrophic spalling. The increase of crack length with increasing quenching temperature difference (Δ T ) was somewhat less than that predicted theoretically; the discrepancy was attributed to an increase of crack density with Δ T . In general, the results show that fracture energy is important in establishing the spalling resistance of high-Al₂O₃ refractories.     

A ZrO2 (3Y) Matrix Composite Toughened with Fe3Al Intermetallic

Near fully dense ZrO₂(3Y)/Fe₃Al composites with significantly improved fracture toughness were synthesized by hot-press sintering at 1350°C. High fracture toughness and bending-strength values, 36 MPa·m^1/2 and 1321 MPa, respectively, were achieved in 40 vol% Fe₃Al composite ceramics, whereas those same values for ZrO₂(3Y) alone were 10 MPa·m^1/2 and 988 MPa, respectively. Microscopic observation of the crack path revealed that Fe₃Al particle uniformly dispersed in the matrix have obvious crack-bridging effect. Improved thermal-shock resistance was also obtained, which was attributed to higher toughness, thermal conductivity, and lower Young's modulus by adding of Fe₃Al particles.     

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.     

Slow Crack Growth from Controlled Surface Flaws in Hot-Pressed Si3N4

The elevated-temperature slow-crack-growth behavior of HS-130 Si₃N₄ was studied by extending "controlled" surface cracks in bars loaded in 4-point bending. Several such nonin-teracting cracks were produced on the tensile surfaces of bend bars by Knoop microhardness indentation. The stress and dimensions of the subcritically growing cracks were used to calculate the stress-intensity factor, K₁ , from fracture-mechanics formulas for semielliptical surface cracks in bending. The crack-growth velocity, v, was obtained by dividing crack extension by loading time interval. The data indicated very large scatter in measured velocities for given K₁ values, which was interpreted as due to the interaction of the small cracks with local material heterogeneities. No simple functional relation between K _I and v could be established for HS-130 Si₃N ₄ from the v − K ₁ data.     

Fracture Toughness Determination of A12O3 Using Four-Point-Bend Specimens with Straight-Through and Chevron Notches

Fracture toughness of a sintered A1₂O₃ was determined with four-point-bend specimens having either straight-through or chevron notches. For the straight-through notched specimens, measured K _Ic decreased with decreasing notch width. For the smallest notch width (66 μm) K _Ic= 3.42±0.13 MN m^−¾. For specimens with chevron notches, a crack initiates and extends from the tip of the notch under increasing load. K _Ic is calculated from the maximum load without measuring crack length, under the assumption that the derivative of the compliance is the same as that for a specimen with a straight-through crack. A refined calculation accounts for the truncated chevron crack shape at maximum load using Bluhm's slice model. For the chevronnotch configuration, a value of K _Ic= 3.49±0.11 MN m^−¾ was measured, which appears to be independent of the initial notch length a ₀ (distance from the crack mouth to the tip of the triangular notch). An effect of a ₁ (length of the chevron notch at the surface) on K _Ic was observed, independent of whether the calculation of K _Ic was based on the straight-through crack assumption or on the slice model.     

Control of Grain-Boundary Pinning in Al2O3/ZrO2 Composites with Ce3+/Ce4+ Doping

The control of the microstructure of Ce-doped Al₂O₃/ZrO₂ componsites by the valence change of cerium ion has been demonstrated. Two distinctively different types of microstructure, large Al₂O₃ grains with intragranular ZrO₂ particles and small Al₂O₃ grains with intergranular ZrO₂ particles, can be obtained under identical presintering processing conditions. At doping levels greater than ∼ 3 mol% with respect to ZrO₂, Ce³⁺ raises the alumina grain-boundary to zirconia particle mobility ratio. This causes the breakaway of grain boundary from particles and the first type of microstructure. On the other hand, Ce⁴⁺ causes no breakaway and produces a normal intergranular ZrO₂ distribution. The dramatic effect of Ce³⁺ on the relative mobility ratio is found to be associated with fluxing of the glassy boundary phase and is likewise observed for other large trivalent cation dopants. The ZrO₂ second phase acts as a scavenger for these trivalent cations, provided their solubility limit in ZrO₂ is not exceeded.     

Pulse Electric Current Sintering of Al2O3/3 vol% ZrO2 with Constrained Grains and High Strength

The pulse electric current sintering technique (PECS) was demonstrated to be effective in rapid densification of fine-grained Al₂O₃/3Y-ZrO₂ using available commercial powders. The composites attained full densification (>99% of TD) at 1450°C in less than 5 min. The composites sintered at a high heating rate had a fine microstructure. The incorporation of 3 vol% 3Y-ZrO₂ substantially increased the average fracture strength and the toughness of alumina to as high as 827 MPa and 6.1 MPa·m^1/2, respectively. A variation in the heating rate during the PECS process influenced grain size, microstructure, and strength, though there was little or no variation in the fracture toughness.     

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.     

High-Strength and High-Fracture-Toughness Ceramics in the Al2O3/LaAl11O18 Systems

Control of microstructure in the Al₂O₃/LaAl₁₁O₁₈ system was performed. Elongated alumina grains were formed by doping with small addition of silica, and 20 vol% lanthana- luminate was formed in situ by the reaction of LaAlO₃- A1₂O₃ in an alumina matrix. Strengths of over 600 MPa and a high fracture toughness (6 MPa.m^1/2) were achieved in the material with both elongated A1₂O₃ grains and LaAl₁₁O₁₈ platelets. Generally antagonistic properties such as strength and fracture toughness have been made compatible in the same ceramic system.     

R-Curve Behavior of In-Situ-Toughened Al2O3:CaAl12O19 Ceramic Composites

The mechanical behavior of reaction-sintered alumina: 30 vol% calcium hexaluminate (Al₂O₃:CaAl₁₂O_19,or A1₂O₃: CA₆) composites was evaluated using the indentation strength in bending technique. A composite in which the hexaluminate (CA₆) phase possessed a platelike morphology showed more-pronounced R -curve behavior than a composite in which the CA₆ phase consisted of equiaxed grains. Toughness curves derived from the indentation-strength data exhibited a "crossover," such that the platelet composite exhibited the lower toughness at small flaw sizes, but the higher toughness at large flaw sizes. Incorporation of the platelet CA₆ resulted in enhanced toughening, compared to single-phase alumina of comparable grain size, thus demonstrating the viability of the in-situ -toughening approach. A simple grain-pullout model was used to estimate the toughening increment due to bridging by the platelet grains; the value obtained was in good agreement with toughness curves derived from indentation-strength measurements. Finally, fabrication of trilayer specimens, whereby outer layers of equiaxed A1₂O₃:CA₆ composite were strongly bonded to the platelet A1₂O₃:CA₆ composite, demonstrated high strength over the range of tested flaw sizes.     

Microstructure and Mechanical Properties of In Situ Produced TiC/TiB2/MoSi2 Composites

A novel microstructure of in situ produced TiC/TiB₂/MoSi₂ composite and its mechanical properties were investigated. The results indicate that TiC/TiB₂/MoSi₂ composites can be fabricated by reactive hot pressing the mixed powders of MoSi₂, B₄C, and Ti. A novel microstructure consisting of hollow particles of TiC and TiB₂ grains in an MoSi₂ matrix was obtained. Grains of in situ produced TiC and TiB₂ were much finer, from 100 to 400 nm. During the fracture process, hollow particles relieved crack tip stress, encouraging crack branching and changing the original direction of the main crack. The highest bending strength of this composite achieved was 480 MPa, twice that of monolithic MoSi₂, and the greatest fracture toughness of the composite reached 5.2 MPa·m^1/2.     

R-Curve Behavior of Si3N4–BN Composites

R -curve behavior of Si₃N₄–BN composites and monolithic Si₃N₄ for comparison was investigated. Si₃N₄–BN composites showed a slowly rising R -curve behavior in contrast with a steep R -curve of monolithic Si₃N₄. BN platelets in the composites seem to decrease the crack bridging effects of rod-shaped Si₃N₄ grains for small cracks, but enhanced the toughness for long cracks as they increased the crack bridging scale. Therefore, fracture toughness of the composites was relatively low for the small cracks, but it increased significantly to ∼8 MPa·m^1/2 when the crack grew longer than 700 μm, becoming even higher than that of the monolithic Si₃N₄.