Thin Compressive Layers Research Articles

Design of damage tolerant and crack-free layered ceramics with textured microstructure

This work demonstrates damage tolerant behavior of ceramic laminates designed with residual stresses and free of surface edge cracks. Non-periodic architectures were designed by embedding 2 textured alumina (TA) layers between 3 equiaxed alumina-zirconia (AZ) layers. Compressive residual stresses of ∼ 250 MPa were induced in the textured layers. Indentation strength tests showed that textured compressive layers arrested the propagation of cracks. Results were compared to periodic architectures with the same volume ratio of TA and AZ materials. Crack propagation was arrested in both periodic and non-periodic designs; the minimum threshold-strength being higher in the latter. Non-periodic architectures with compressive layers as thin as ∼ 200 μm showed no evidence of surface edge cracks, yet still reached minimum threshold strength values of ∼ 300 MPa. In addition, the textured microstructure promoted crack bifurcation in the thin compressive layers and thus enhanced the damage tolerance of the material.

Validity of the Finite Fracture Mechanics Based Asymptotic Analysis for Predictions of Crack Deflection in Thin Layers of Ceramic Laminates

The work studies and compares different approaches suitable for predictions of the crack deflection (bifurcation) in ceramic laminates containing thin layers under high residual stresses and discuss a suitability and limits of using of the asymptotic analysis for such problems. The thickness of the thin compressive layers where the crack deflection occurs is only one order higher than the crack extension lengths considered within the solution. A purely FEM based calculation of the energy and stress conditions, necessary for the crack propagation, serves as the reference solution to the problem. The asymptotic analysis is used after for calculations of the same quantities (especially of energy release rate – ERR). This concept enables semi-analytical calculations of ERR or changes in potential energy induced by the crack extensions of different lengths and directions. Such approach can save a large amount of simulations and time compared with the pure FEM based calculations. It was found that the asymptotic analysis provides a good agreement for investigations of the crack increments enough far from the adjacent interfaces but for longer extensions (of length above 1/5-1/10 of the distance from the interface) starts more significantly to deviate from the correct solution. Involvement of the higher order terms in the asymptotic solution or other improvement of the model is thus advisable.

Fibrous composite with threshold strengths in multiple directions: Design and fabrication

This work reports the design of a composite consisting of square fibers separated by thin compressive layers. Fracture mechanics analysis indicated that this composite showed radial and axial threshold strengths corresponding to applied stresses in the direction perpendicular to the fiber side face and parallel to the fiber central axis, respectively. In accordance with the above designing concept, Si 3N 4/TiN fibrous composites with distinctive threshold strengths were prepared through a simple double-laminating procedure. The measured radial threshold strength of the Si 3N 4/TiN composite agreed quite well with the theoretical prediction, while a substantial discrepancy existed between the measured and predicted axial threshold strength due to the difference in the configuration of the cracks used in testing and theoretical modeling of the axial threshold strength.

High-temperature mechanical behaviour of flaw tolerant alumina–zirconia multilayered ceramics

The mechanical behaviour of alumina–zirconia multilayered ceramics designed with thin internal compressive layers has been investigated under flexural loading at room and high temperature. Young’s modulus and the sintering evolution of each layer have been experimentally determined up to 1200 °C, to account for the residual stress distribution in the layered composite. The fracture behaviour has been assessed by indentation – strength experiments at different temperatures and by a fracture mechanics analysis. Experimental findings showed that improvement in mechanical properties of the laminate at high temperatures in comparison to the alumina-based monolithic material was essentially related to the distinct modes of failure observed as a function of the temperature, in the presence of energy release mechanisms such as crack bifurcation and/or delamination that may be used as a tool for designing tolerant materials at high temperatures.

Multi-Layer Silicon Nitride Laminates Exhibiting High Fracture Toughness and Crack Deflection

Si3N4-TiN based multi-layer ceramics laminates have been produced. With external compressive layers, laminates with a three-fold increase in KIc over the monolithic ceramics have been realised. When external tensile layers are used in conjunction with thin internal compressive layers, energy absorbing crack deflection and bifurcation processes are observed.

Processing optimisation and fracture behaviour of layered ceramic composites with highly compressive layers

Multilayer ceramics with five thick layers of alumina-YTZP and four thin layers of alumina-pure zirconia have been fabricated by slip casting and mechanically characterized in order to evaluate the influence of residual stresses on the fracture behaviour. In doing so, processing parameters are optimised in terms of suspension stabilization and well-dispersion of the zirconia particles within the alumina layers. Two different compressive residual stress values are attained through sequential slip casting of specimens with thick/thin layer thickness ratios of 5.4 and 9.5, by controlling the stability and casting time of aqueous slurries. Flexural tests conducted on indented monoliths and multilayered materials show a stepwise failure in the laminates associated with the bifurcation of the impinging crack when interacting with the internal layers, this phenomenon being evidenced even for the case of laminates with very thin compressive layers where edge cracking is not observed.

Fracture behaviour of an Al2O3–ZrO2 multi‐layered ceramic with residual stresses due to phase transformations*

ABSTRACTThe fracture behaviour of a ceramic multi‐layer designed with thin internal compressive layers and obtained by slip casting is studied. It consists of nine alternated Al2O3–5vol%tZrO2 and Al2O3–30vol%mZrO2 layers of 530 μm and 100 μm thickness, respectively. Mechanical characterization includes evaluation of Vickers Hardness, Young's modulus and fracture strength under four‐point bending. In addition, the residual stress magnitude and distribution in the laminate is determined both analytically, from calculations using the differential strain between layers and the elastic properties, and experimentally, using indentation techniques. The experimental findings in terms of mechanical strength and fractography show a subcritical growth of the natural flaws in the laminate before catastrophic failure occurs, owing to the relevant role of the thin Al2O3–30vol%mZrO2 layers with compressive stresses inherent to the zirconia phase transformation. These layers are also responsible for the increase in toughness to levels of at least three times that of the reference Al2O3–5vol%tZrO2 monolith.

Effects of Porosity on the Threshold Strength of Laminar Ceramics

Ceramic laminates exhibiting a threshold strength have been fabricated by dip‐coating thick tape‐cast Al2O3 layers into slurries containing mixtures of Al2O3 and either unstabilized zirconia (MZ‐ZrO2) or mullite to produce thin compressive layers via both a molar volume change and a differential thermal contraction. Porosity was introduced into the thin compressive layers by adding rice starch to the dip‐coating slurries, which decomposed during densification of the laminate. As the volume fraction of porosity is increased, the residual compressive stress (σC), as measured by piezospectroscopy, is reduced and approaches zero at approximately 0.65 volume fraction of porosity. The elastic modulus mismatch (E1/E2) between the thin and thick laminate layers accounted for approximately one‐half of the threshold strength for volume fractions of porosity ≤0.30 (E1/E2<0.4). Above 0.40 volume fraction of porosity, the strength significantly increased as did the scatter in strength values, and it was observed that the highly porous layers completely arrested crack extension; these materials no longer exhibited a threshold strength. For these laminates, failure occurred by the independent, sequential failure of one layer after another, followed by catastrophic failure due to delamination.

Crack Interactions in Laminar Ceramics That Exhibit a Threshold Strength

Laminar ceramic composites have been fabricated with thin compressive layers, containing a mixture of alumina and mullite, sandwiched between thicker alumina layers. It has previously been shown that a single crack that extends within a thicker alumina layer can be arrested by the compressive layers to produce a threshold strength, i.e., a strength below which the probability of failure is zero. The behavior of multiple cracks within the laminate has been investigated, to observe the mechanisms of crack interaction and measure their influence on the threshold strength. It was found that when the cracks in adjacent thick layers were offset by a distance less than the thickness of two thick layers, the cracks would interact and decrease the threshold strength. The number of interacting cracks, their orientation, and location can also have an effect on the threshold strength.

Factors Affecting Threshold Strength in Laminar Ceramics Containing Thin Compressive Layers

Compressive layers placed within a laminate can arrest cracks. With increasing applied stress, the arrested crack can propagate through the compressive layer. These phenomena produce a material with threshold strength, i.e., failure cannot occur below a critical applied stress. A previously reported stress intensity function describes different variables, e.g., magnitude of compressive stress, thickness of compressive layer, and distance between compressive layers, which govern threshold strength. Laminar composites composed of thicker Al2O3 layers separated by thinner Al2O3/mullite layers were fabricated to test the different variables that are predicted to govern threshold strength. The data agreed well with the predicted values only when the magnitude of compressive stress and/or the thickness of the compressive layer were low. For these conditions, the crack extended straight through the compressive layers, as assumed by the model used to predict threshold strength. On the other hand, when the compressive stress and/or layer thickness were large, threshold strength was larger than the predicted value. In addition, for these conditions, the crack bifurcated through the compressive layer. The angle between the bifurcated cracks increased with increasing compressive stress.

Laminar Ceramics That Exhibit a Threshold Strength.

Thin compressive layers within a laminar ceramic arrest large cracks (surface and internal) and produce a threshold strength. This phenomenon increases the damage tolerance of ceramics and will allow engineers to design reliable ceramic components for structural applications. The stress intensity factor derived for a crack sandwiched between two compressive layers suggests that the threshold strength is proportional to the residual compressive stress and the thickness of the compressive layer and is inversely proportional to the distance between the compressive layers. Laminates composed of thick alumina layers (605 +/- 11 micrometers) and thin mullite/alumina compressive layers (37 +/- 1.4 micrometers) fabricated for this study had a threshold strength of 482 +/- 20 megapascals, in fair agreement with the theory.