Mechanical Behavior of Alumina Reinforced with Carbon-Coated Silicon Carbide Whiskers

SiC whiskers with 0, 20, and 50 Å carbon coatings were incorporated into an alumina matrix to modify residual thermal stress and interfacial bonding. Composites were characterized using triaxial X-ray diffraction for residual stress determination and electron microscopy to explore interfacial chemistry. Fracture toughness and R -curve behavior were examined for short and long crack lengths. Uncoated SiC whiskers optimized strength, fracture toughness, and R -curve behavior of these composites. A graphite interphase at the whisker/matrix interface decreased contributions to crack bridging without promoting additional toughening by whisker pullout.     

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 Toughness Anisotropy and Toughening Mechanisms of a Hot-pressed Alumina Reinforced with Silicon Carbide Whiskers

The fracture toughness of a hot-pressed alumina and that of a hot-pressed alumina/SiC-whisker composite containing 33 vol% SiC whiskers were measured by four-point bending on single-edge precracked bend bars having sharp precracks created by "bridge indentation." Two batches of the composite were examined, one exhibiting a greater degree of whisker clustering than the other. The fracture toughness of the alumina was around 4 MN·m^-3/2 whereas that of the composite varied between 5 and 8 MN·m^-3/2 depending on microstructural uniformity and crack-propagation direction. Crack deflection in combination with a change in fracture from intergranular to transgranular fracture is proposed as an explanation of the superior fracture toughness of SiC-whisker-reinforced alumina as compared to unreinforced alumina. The composite exhibited a variation in fracture toughness with the crack-propagation direction in identical crack planes. This effect could with good accuracy be described in terms of crack deflection for the composite with uniform whisker distribution. However, in the material with whisker clustering the variation of the fracture toughness with crack-growth direction was greater and could not entirely be explained by crack deflection.     

R-Curve Behavior of Long Cracks in Alumina

Coarse-grained alumina is among those monolithic ceramics which can exhibit an increase in crack resistance with crack extension. This R -curve behavior is most pronounced for intergranular fracture and does not depend exclusively on material properties. Crack and specimen geometries also influence the shape of the R -curves. The magnitude of the effect increases with increasing crack surface roughness, which is microstructure-dependent, and with crack-opening displacement, which is geometry-dependent. Based on experimental observations, a "dynamic" R -curve model is presented which relates the increasing resistance to an increasing crack tip shielding caused by crack surface bridging. Applying a J -integral approach, R -curves are calculated for two specimen geometries (short double cantilever beam and single-edged notched beam) and different grain sizes. The good agreement between calculation and experiment indicates that the R -curve behavior of long cracks in alumina can be predicted by a simple wake model.     

Correlation between Long and Short Crack R-curves in Alumina Using the Crack Opening Displacement and Fracture Mechanical Weight Function Approach

Both fine grain sized and coarse grain sized alumina has been used as a model material to correlate short crack R - curves by the surface-flaw-in-flexure method with long crack R -curves from a compact tension specimen. The analysis has been made using the crack opening displacement from long cracks combined with the appropriate weight function to compute the crack closure stresses by crack bridging as a function of crack opening displacement. Then the long crack R - curve, the short crack curve, as well as the crack opening displacement for short cracks are computed. The agreement with the experimental data is within an absolute value of about 0.3 MPa and attributed to uncertainties in determining the crack tip toughness and effects due to subcritical crack growth.     

R-Curve Behavior and Strength for In-Situ Reinforced Silicon Nitrides with Different Microstructures

R -curves for two in-situ reinforced silicon nitrides A and B of nominally the same composition are characterized using the Griffith equation and indentation fracture mechanics. These R -curves are calibrated against fine-grained silicon nitrides which have a known chevron-notch (long-crack) toughness and with a nearly flat R -curve behavior. Silicon nitride A, with its coarser microstructure and higher chevron-notch toughness, shows lower resitance to crack growth than silicon nitride B if the crack size is less than ∼200 μm. These results are consistent with the indentation–Strength measurements which show a crossover of strength between the two materials at an indentation load between 49 and 98 N, and below the crossover A has a lower strength. The toughening behavior is explained using an elastic-bridging model for the short crack, and a pullout model for the long crack. The effects of R -curve properties on design are discussed.     

Effect of Temperature on Toughness Curves in Alumina

The effect of temperature on fracture strength and toughness curves ( T -curves) in the short-crack region of a polycrystalline alumina was studied. The indentation-strength technique was used to measure strength and T -curve behavior in the temperature range of 25° to 1300°C. Grain-localized crack bridging resulted in improved flaw tolerance and rising T -curves in this alumina. Both strength and toughness were observed to decrease with increasing temperature. A theoretical grain-bridging model was used to calculate the T -curves and fit to the experimental data. This allowed the evaluation of the temperature dependence of important bridging parameters.     

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.     

Indenter Flaw Geometry and Fracture Toughness Estimates for a Glass-Ceramic

Shapes of cracks associated with Vickers indenter flaws in a glass-ceramic were assessed by stepwise polishing and measuring surface traces as a function of depth. The cracks were of the Palmqvist type even at200-N indentation load. The toad dependence of crack lengths and fracture toughness estimates were examined in terms of relations proposed for Palmqvist and half-penny cracks. Estimates based on the half-penny crack analogy were in closer agreement with bulk fracture toughness measurements despite the Palmqvist nature of the cracks.     

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.     

Toughness-Curve Behavior of an Alumina-Mullite Composite

Toughness-curve ( T - or R -curve) behavior of a composite of 30 vol%, polycrystalline, coarse-grained, spherical alumina agglomerates dispersed throughout a fine-grained, 50/50 vol% alumina-mullite matrix, and that of its microstructural end-members (100% matrix and 100% alumina), were studied using the indentation-strength-in-bending technique. T -curves were deconvoluted from indentation-strength data using an indentation fracture mechanics model. The monolithic matrix and alumina exhibited an invariant toughness and a moderate T -curve, respectively. In comparison, the composite exhibited a pronounced T -curve. The T -curve of the composite is best explained as deriving from the interaction of a propagating crack with the alumina agglomerates: crack propagation experiments revealing two possible toughening mechanisms-intra-agglomerate frictional grain bridging and elastic bridging ligaments in the matrix that appeared to be associated with alumina agglomerates. Rule-of-mixtures toughness calculations indicated that intra-agglomerate bridges could account for only a fraction of the toughening exhibited by the composite. It is suggested that the extra toughening arises from the elastic bridging ligaments.     

The Utility of R-Curves for Understanding Fracture Toughness-Strength Relations in Bridging Ceramics

The mechanical behavior of four rare earth (RE)-Mg-doped Si₃N₄ ceramics (RE=La, Lu, Y, Yb) with varying grain-boundary adhesion has been examined with emphasis on materials containing La and Lu (which represent the extremes of RE ionic radius). Fracture-resistance curves ( R -curves) for all ceramics rose very steeply initially, giving them exceptional strength and relative insensitivity to flaw size. The highest strength was seen in the Lu-doped material, which may be explained by its steeper initial R -curve; the highest "apparent" toughness (for fracture from millimeter-scale micronotches) was seen in the lowest strength La-doped material, which may be explained by its slowly rising R -curve at longer crack lengths. Excellent agreement was found between the predicted strengths from R -curves and the actual strengths for failures originating from natural flaws, a result attributed to careful estimation of the early part of the R -curve by deducing the intrinsic toughness, K ₀, and the fact that this portion of the R -curve is relatively insensitive to sample geometry. Finally, it was found that RE elements with relatively large ionic radius (e.g., La) tended to result in lower grain-boundary adhesion. This implies that there is a small window of optimal grain-boundary adhesion which can lead to the fastest rising R -curves (for short cracks) and the highest strengths. The importance of this work is that it reinforces the notion that factors which contribute to the early part of the R -curve are critical for the design of ceramic microstructures with both high strength and high toughness.     

Flat and Rising R-Curves for Elliptical Surface Cracks from Indentation and Superposed Flexure

Flat and rising R -curves, fracture resistance versus crack extension, were determined for a sintered 99%α-silicon carbide and for a hot-pressed composite of 25 wt% silicon carbide whiskers and alumina, respectively. The R -curves were evaluated from a combination of measured crack lengths, which were produced over a range of Vickers indentation loading, and of measured strengths, which were correlated either with the indentation flaws or with the most severe natural flaws on flexure specimens. A published analysis of the stress-intensity factor for a surface crack in flexure was interpreted to show that the crack front takes the form of a semiellipse where both the ratio of the minor to major radii and the configuration coefficient itself decrease with increasing crack extension. A power-law function of the indentation load was fitted to the product of an effective configuration coefficient and the flexural strength to evaluate the R -curves. When the configuration coefficient is assumed constant, a customary practice, the R-curves appear to have steeper rises. The assumed constancy of the coefficient of the indentation driving force may also have an effect on R-curves, but the effect would be much less.     

Development of SiC-Whisker-Reinforced Lithium Aluminosilicate Matrix Composites by a Mixed Colloidal Processing Route

Based on the surface properties of aqueous silica, boehmite, and SiC-whisker dispersions, SiC-whisker-reinforced lithium aluminosilicate (LAS) matrix composites were fabricated by a mixed colloidal processing route. The composites were characterized by a uniform spatial distribution of SiC whiskers throughout the matrix. The fracture toughness increased from 1.3 MPa.m^1/2 for the LAS specimen to 5.0 MPa.m^1/2 for the hot-pressed composite (950°C and 20 MPa for 20 min) containing 20 wt% SiC whisker. The increase in fracture toughness appears to result mainly from crack deflection and crack bridging by whiskers with some additional toughenings from load transfer and whisker pullout.     

Objective Evaluation of Short-Crack Toughness Curves Using Indentation Flaws: Case Study on Alumina-Based Ceramics

An objective methodology is developed for evaluating toughness curves ( T -curves) of ceramics using indentation flaws. Two experimental routes are considered: (i) conventional measurement of inert strength as a function of indentation load; (ii) in situ measurement of crack size as a function of applied stress. Central to the procedure is a proper calibration of the indentation coefficients that determine the K -field of indentation cracks in combined residual-contact and applied-stress loading, using data on an appropriate base material with single-valued toughness. Tests on a fine-grain alumina serve to demonstrate the approach. A key constraint in the coefficient evaluation is an observed satisfaction of the classical indentation strength–(load)^−1/3 relation for such materials, implying an essential geometrical similarity in the crack configurations at failure. T -curves for any alumina-based ceramic without single-valued toughness can then be generated objectively from inert-strength or in situ crack-size data. The methodology thereby circumvents the need for any preconceived model of toughening, or for any prescribed analytical representation of the T -curve function. Data on coarse-grained aluminas and alumina-matrix material with aluminum titanate second-phase particles are used in an illustrative case study.     

Crack-Shape Effects for Indentation Fracture Toughness Measurements

Various methods to measure fracture toughness using in-dentation precracks were compared using soda-lime glass as a test material. In situ measurements of crack size as a function of applied stress allow both the toughness K_c and the residual-stress factor χ to be independently determined. Analysis of the data showed that stress intensity factors based on classical half-penny crack shapes overestimate toughness values and produce an apparent R -curve effect. This is due to a constraint on crack shape imposed by primary lateral cracks in soda—lime glass. Models based on elliptical cracks were developed to account for the crack-shape effects.     

Controlled Crack Shapes for Indentation Fracture of Soda–Lime Glass

Radial cracks for indented soda–lime glass aged in distilled water were highly elliptical because of truncation by lateral cracks. Indentation in silicone oil minimized radial/lateral crack interaction but still produced cracks having nominally constant ellipticity during bend testing. Analysis of applied stress/indentation crack length data using stress intensity factors based on half-penny crack shape resulted in apparent R -curve behavior and/or overestimation of the fracture toughness. Incorporation of elliptical shape factors eliminated the R -curve behavior and reduced measured toughness to near the accepted value for soda–lime glass.     

Fracture of Si3N4–SiC Whisker Composites under Cyclic Loads

This communication demonstrates the role of cyclic compressive loads in inducing mode I fatigue crack growth at room temperature in Si₃N₄ matrix–SiC whisker composite materials containing stress concentrations. The characteristics of stable, cyclic fracture are examined for several volume fractions of the SiC whisker and are compared with those of the matrix material. It is found that the composites with higher volume fractions of SiC whiskers exhibit an inferior resistance to fracture under cyclic compressive loads despite improvements in fracture toughness values.