Crack Bifurcation in Laminar Ceramic Composites

Crack bifurcation was observed in laminar ceramic composites when cracks entered thin Al₂O₃ layers sandwiched between thicker layers of Zr(12Ce)O₂. The Al₂O₃ layers contained a biaxial, residual, compressive stress of ∼2 GPa developed due to differential contraction upon cooling from the processing temperature. The Zr(12Ce)O₂ layers were nearly free of residual, tensile stresses because they were much thicker than the Al₂O₃ layers. The ceramic composites were fabricated by a green tape and codensification method. Different specimens were fabricated to examine the effect of the thickness of the Al₂O₃ layer on the bifurcation phenomena. Bar specimens were fractured in four-point bending. When the propagating crack encountered the Al₂O₃ layer, it bifurcated as it approached the Zr(12Ce)O₂/ Al₂O₃ interface. After the crack bifurcated, it continued to propagate close to the center line of the Al₂O₃ layer. Fracture of the laminate continued after the primary crack reinitiated to propagate through the next Zr(12Ce)O₂ layer, where it bifurcated again as it entered the next Al₂O₃ layer. If the loading was stopped during bifurcation, the specimen could be unloaded prior to complete fracture. Although the residual stresses were nearly identical in all Al₂O₃ layers, crack bifurcation was observed only when the layer thickness was greater than ∼70 μm.     

Crack Branching in Ceramic Disks Subjected to Biaxial Flexure

Fracture stresses in soda-lime glass, a glass-ceramic, and an alumina ceramic tested in biaxial flexure correlated with the crack lengths prior to branching. The resultant branching constant for the glass agreed closely with the mirror-mist boundary constant, and evidence indicated agreement between the branching constants and the inner-mirror boundary constants for the polycrystalline materials. The ratio of the branching constant to the critical stress intensity factor was much smaller for the polycrystalline materials than for the glass. The crack-branching measurements also indicated the presence of a compressive residual stress as Sociated with the ground sufaces of the specimens.     

In-Plane Mechanical Properties of an All-Oxide Ceramic Composite

The present article examines the in-plane tensile properties of a two-dimensional (2D) all-oxide ceramic composite. The distinguishing characteristics of the material include fine-scale porosity within the matrix and the absence of a fiber coating. The anisotropy in the elastic-plastic properties has been studied through tension tests in the axial (fiber) direction and at 45° to the fiber axes, both in the presence and the absence of holes or notches. The notch sensitivity in the axial direction is comparable to that of conventional dense-matrix, weak-interface composites, demonstrating the effectiveness of the porous matrix in enabling crack deflection and damage tolerance. Furthermore, the notch sensitivity is rationalized using models that account for the effects of inelastic straining on the local stress distributions around notches and holes, coupled with a scale-dependent failure criterion. In the off-axis orientation, the tensile strength is dictated by a plastic instability, analogous to necking in metals. Following instability, deformation continues within a diffuse localized band, with a length comparable to the specimen width. Similar deformation and fracture characteristics are obtained both with and without holes. The off-axis properties are discussed in terms of the comminution and rearrangement of matrix particles during straining.     

Crack Deflection Process for Hot-Pressed Whisker-Reinforced Ceramic Composites

A crack deflection model for two-dimensional randomly arranged rods has been derived by making an appropriate modification to the model by Faber and Evans. This model predicts more effective toughening in certain directions than that for three-dimensional randomly arranged rods. The theoretical predictions are compared with experimentally measured fracture toughness data of hot-pressed whiskerreinforced ceramic-matrix composites.     

Crack Bridging by Inclined Fibers/Whiskers in Ceramic Composites

Crack bridging by inclined fibers has been studied in a brittle fiber–brittle matrix model ceramic composite. Results of the fiber bridging force vs the crack opening displacement have been obtained for different fiber inclination angles using a fracture mechanics approach. Localized matrix cracking has been observed for inclined fibers and related to fiber inclination angle. The experimental results showing the influence of fiber inclination angle are discussed and compared with theoretical analyses to provide insight into crack bridging by inclined fibers/whiskers. Implications for toughening by whisker bridging are also discussed.     

Processing and Performance of an All-Oxide Ceramic Composite

Continuous fiber ceramic composites (CFCCs) based on oxides are of interest for high-temperature applications owing to their inherent oxidative stability. An enabling element is a matrix with an optimum combination of toughness and strength, which may be achieved by incorporating a controlled amount of fine, well-distributed porosity. Implementation of this concept by vacuum infiltration of aqueous mullite-alumina slurries into two-dimensional woven preforms of alumina fibers has been investigated. Evaluation of these materials shows stress-strain characteristics similar to other CFCCs, especially carbon-matrix composites. Moreover, promising notch and creep properties have been found. Microstructural and processing issues relevant to the attainment of these behaviors are discussed.     

Crack Branching in Transformation-Toughened Zirconia

Crack propagation and branching were investigated in calcia partially stabilized zirconia aged at 1300°C for various times. The crack-branching radii were measured and used to calculate the apparent stress intensity factors at crack branching ( K₈ ) which increase with increasing aging time. The K_B/K_IC ratios increased with aging time with a maximum of 5.3. This maximum ratio is much greater than the ratios previously observed for ceramics.     

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.     

Crack Deflection in Ceramic/Ceramic Laminates with Strong Interfaces

Crack deflection in electrophoretically deposited Al₂O₃/ TZ-3Y (3 mol% Y₂O₃-stabilized tetragonal ZrO₂) lamellar composites with strong interfaces is described. The fracture behavior of, and crack paths in, these materials were evaluated using indentation and four-point bend tests. The effects of residual and induced stresses on crack deflection are considered.     

Apatite-Diopside Bioglass Ceramic Composites

The results of research into glass-ceramic materials for medical purposes are described. The effect of the liquid phase composition in the sintering of composites on the conversion of hydroxyapatite into tricalcium phosphate is determined. The possibility of controlling the structural-mechanical properties, bioactivity, and time of implant resorption by varying the ratio between hydroxyapatite and diopside glass ceramic in the composition of biological materials is demonstrated.     

Effect of Crack Velocity on Crack Resistance in Brittle-Matrix Composites

Fast- and slow-fracture studies were conducted on a glass matrix–metal particle composite system, using the appliedmoment double-cantilever-beam technique. At 10 vol % metal, the fracture toughness ( K_Ic ) was seen to increase 71% over that of the matrix alone, as compared to an increase of only 40% in the stress intensity ( K_I ) for the same system in slow fracture. This difference was corroborated by fracture surface stereology work, which showed 9.6% metal on the fast-fracture surface as compared to only 5.3% for the slowfracture specimens. This indicates that in slow fracture, the crack "chooses" its path of propagation, thus resulting in a lowering of the expected increase in crack resistance.     

Effects of Matrix Porosity on the Mechanical Properties of a Porous-Matrix, All-Oxide Ceramic Composite

The effects of matrix porosity on the mechanical properties of an all-oxide ceramic composite are investigated. The porosity is varied through impregnation and pyrolysis of a ceramic precursor solution. Mechanical tests are performed to assess the role of the matrix in both matrix-dominated and fiber-dominated loading configurations. The results demonstrate a loss in damage tolerance and tensile strength along the fiber direction as the porosity is reduced. Concomitantly, some improvements in interlaminar strength are obtained. The latter improvements are found to be difficult to quantify over the entire porosity range using the standard short beam shear method, a consequence of the increased propensity for tensile fracture as the porosity is reduced. Measurements of interlaminar shear strength based on the double-notched shear specimen are broadly consistent with the limited values obtained by the short beam shear method, although the former exhibit large variability. In addition, effects of precursor segregation during drying on through-thickness gradients in matrix properties and their role in composite performance are identified and discussed. An analysis based on the mechanics of crack deflection and penetration at an interphase boundary is presented and used to draw insights regarding the role of matrix properties in enabling damage tolerance in porous-matrix composites. Deficiencies in the understanding of the mechanisms that enable damage tolerance in this class of composites are discussed.     

Crack Initiation in Unidirectional Brittle-Matrix Composites

This paper describes the initiation of matrix cracking for glass and glass-ceramic matrix composites reinforced with small-diameter silicon carbide and carbon fibers under uniaxial tensile loading. Acoustic emission, replication, and optical microscopy in conjunction with stress-strain curves are employed to detect the initiation of matrix cracking. The proportional limit of the stress-strain curve is found to overestimate the initiation of matrix cracking in the material systems studied. The matrix cracking iniates at axial strains from 0.07% to 0.15%. The ACK model overestimates the initiation of the matrix cracking for the material systems studied in this paper.     

Chemical Interactions in High-Temperature Ceramic Composites

The useful lives of many ceramic composites will be influenced, if not controlled, by chemical interactions. Thermodynamic and kinetic calculations are invaluable in evaluating them. Composites may contain as many as four constituents: a fiber, its coating, a matrix, and an external coating. The chemical concerns fall into three categories: (i) chemical compatibility between the various constituents, (ii) internal stability of the constituents, and (iii) environmental stability of the composite. Examples are given for each of these concerns. Also included are the approaches useful for evaluating them.     

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.     

Lanxide陶瓷基复合材料的研究进展

Lanxide熔融金属直接氧化技术是一种新型的复合材料制备技术,通过用预制体(颗粒、晶须、纤维等)增强所制备的复合材料具有高的体积稳定性、断裂韧性和强度,是目前材料科学领域的热点之一.本文就Lanxide技术及陶瓷基复合材料近年来的最新发展进行了概述.     

Two-Dimensional Whisker Percolation in Ceramic Matrix–Ceramic Whisker Composites

A computer model that treats ceramic-powder matrix–ceramic whisker composites as a percolative network of whiskers has been developed. The model calculates the critical fraction of whiskers at the percolation threshold for a two-dimensional random network of whiskers. The computed critical fraction was found to display an inverse dependence on whisker aspect ratio. In addition, the computed critical fraction (27 vol% for a whisker aspect ratio of 7) agreed well with the zero-shrinkage whisker fraction of 30 vol% in the densification of a colloid-pressed alumina–silicon carbide composite that exhibited a two-dimensional orientation of such whiskers.     

Fatigue Crack Propagation in Transformation-Toughened Zirconia Ceramic

Fatigue crack propagation under tension-tension loading is observed in a transformation-toughened partially stabilized zirconia (PSZ) ceramic containing 9 mol% MgO. Such subcritical crack growth behavior is demonstrated to be cyclically induced, based on a comparison with behavior under sustained loading (at the maximum load in the fatigue cycle) and at varying cyclic frequencies. Crack extension rates, which are measured as a function of the cyclic stress intensity range ΔK over the range 10^-10 to 10^-6 m/cycle, are found to be load ratio dependent and to show evidence of fatigue crack closure, similar to behavior in metals. Cyclic crack growth rates are observed at ΔK levels as low as 3 MPa m^1/2 and are typically many orders of magnitude faster than reported data on environmentally assisted, subcritical crack growth in PSZ under sustained-load conditions.     

Crack Healing in a Silicon Nitride Ceramic

A detailed study has been undertaken on crack healing at high temperatures in a silicon nitride containing 10 wt% additives in order to identify the dominant mechanism responsible for the phenomenon. Fracture toughness increased with annealing time and the crack growth rate decreased until arrest with increasing testing time. Differentiation between possible operating mechanisms was obtained using critical experiments involving detailed compliance measurements, crack wake removal, and crack reinitiation tests and a comprehensive TEM study of healed cracks. It was found that crack healing was not uniform in the crack wake. When the original crack path was either transgranular or intergranular, healing was associated with the appearance of a thin layer of silica glass due to the oxidation of Si₃N₄ grains. But when the crack went through multigrain junctions, the former crack path was completely obliterated and replaced by a new, crystalline phase formed by diffusion of the preexisting glass phase. It is concluded that the increased crack growth resistance and fracture toughness at high temperature is attributable to the partial recovery of the original strength from the crack segments at multigrain junctions due to vitreous phase flow and subsequent crystallization.