Transformation-toughened Ceramics Research Articles

Mechanical properties of ZrO2 + 12%1CeO2 ceramics with additives of CaO, Y2O3, and Nb2O5

The mechanical properties of transformation-toughened ceramics of the composition ZrO2 + 12% CeO2 with additives of CaO, Y2O3, and Nb2O5 in an amount of 1% (molar fractions) have been studied. Doping increases the strength and cracking resistance of the material. The increase of strength is due to grain refinement in the material. According to their effect in increasing the strength (and in decreasing the average grain size), the doping elements considered can be arranged in the sequence Ca, Y, and Nb. As regards their effect in increasing the critical coefficient of stress intensityK1c (measure of cracking resistance), the doping elements can be written in the following order: Ca, Y, no additive, and Nb. The level of cracking resistance of the material correlates with the volume of the elementary cell of the tetragonal solid solution. It is shown that cracking resistance increases due to the more intensive destabilizing effect of a doping an additive on the tetragonal solid solution. The highest effect in increasing the strength (from 380 N/mm2 for a material without an additive to 680 N/mm2 for one with an additive) and the cracking resistance (from 15 to 23 N/mm3/2) is produced by CaO. Recommendations on the selection of doping elements are given.

Effects of shear stresses on crack-growth microstructures in transformation-toughened ceramics

Abstract In accordance with a prior study, transformation toughening of zirconia-reinforced ceramics is simulated by treating regions of particles surrounding quasi-statically advancing cracks as Mode-I symmetric distributions of circular spots that undergo a ‘supercritical’ dilatant transformation when a critical stress criterion is satisfied. Three stress criteria are considered: mean stress, maximum in-plane shear stress, and an equal mixture of these two. Simulations with the mean-stress criterion show good agreement with results of previous continuum-based analyses, as the profile of the transformed region ahead of the tip approximates a partial cardioid. In contrast, simulations for the other criteria display claw-like transformed regions extending far ahead of the crack tip. This difference in behaviour is conjectured to arise from the presence of direct spot-to-spot interaction terms in the shear stress and may account for large (i.e. mm size) transformed regions seen in some materials.

Effect of grinding and subsequent annealing on the properties of transformation-toughened alumina ceramics

The extent of phase transformation due to grinding as well as the recovery of transformable phase upon annealing is discussed in the light of X-ray diffraction analysis and variation in mechanical properties.

An Investigation of Ternary Zirconia Ceramics

Transformation toughening ceramics (TTCs) are engineering materials which combine ceramic properties such as hardness, corrosion resistance and low thermal conductivity with good toughness and mechanical strength. At elevated temperatures their use is limited due to destabilisation of the transformation toughening microstructure (partially stabilised zirconia or PSZ) or creep and hydrothermal degradation (tetragonal zirconia polycrystals or TZPs). Despite these limitations, the use of TTCs, particularly zirconia based, has become widespread. To date, most commercial TTCs are based on combinations of zirconia and one stabilising oxide. This work investigates a zirconia ceramic containing two stabilisers, namely yttria and titania in roughly equal proportions.

Transient subcritical crack-growth behavior in transformation-toughened ceramics

Transient subcritical crack-growth behavior following abrupt changes in the applied load are studied in transformation-toughened ceramics. A mechanics analysis is developed to model the transient nature of transformation shielding of the crack tip K s, with subcritical crack extension following the applied load change. Conditions for continued crack, growth crack growth followed by arrest, and no crack growth after the load change, are considered and related to the magnitude and sign of the applied load change and to materials properties such as the critical transformation stress. The analysis is found to provide similar trends in K s compared to values calculated from experimentally measured transformation zones in a transformation-toughened Mg-PSZ. In addition, accurate prediction of the post load-change transient crack-growth behavior is obtained using experimentally derived steady-state subcritical crack-growth relationships for cyclic fatigue in the same material.

An analytical approach for the calculation of stress-intensity factors in transformation-toughened ceramics

Stress-induced transformation toughening in Zirconia-containing ceramics is described analytically by means of a quantitative model: A Griffith crack which interacts with a transformed, circular Zirconia inclusion. Due to its volume expansion, a ZrO2-particle compresses its flanks, whereas a particle in front of the crack opens the flanks such that the crack will be attracted and finally absorbed. Erdogan's integral equation technique is applied to calculate the dislocation functions and the stress-intensity-factors which correspond to these situations. In order to derive analytical expressions, the elastic constants of the inclusion and the matrix are assumed to be equal.

Finite cracks in transformation-toughened ceramics

The dependence of the tensile fracture stress on crack length in dilatationally phase-transforming ceramics is studied by modeling the evolution of the transformed regions around the tips of finite cracks during crack growth. The presence of the transformation is found to reduce the stress required for crack-growth initiation. However, the peak, or “ultimate”, tensile stress is found to occur during subsequent crack growth, and the transformation-strengthening, that is, the increase of tensile strength due to phase transformation, is found as a function of initial crack size.

Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, ΔK, for fracture initiation in cyclic tension is as low as 3.4 M Pam1/2, and catastrophic failure occurs at ΔK=6.6 M Pam1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of “static failure modes”. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y2O3. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.

Crack-growth resistance in transformation-toughened ceramics

Crack-growth resistance in transformation-toughened ceramics is studied by modeling the region surrounding an advancing crack tip as a zone which has undergone a uniform dilatational phase transformation. This zone is allowed to evolve around the advancing crack tip under conditions of increasing far-field load while the tip is maintained at a critical stress intensity necessary for fracture. This procedure leads to the surprising conclusion that maximum toughening occurs for finite amounts of crack advance.

Theory and experiments of fracture in cyclic compression: Single phase ceramics, transforming ceramics and ceramic composites

The application of cyclic compressive stresses to notched plates of brittle solids can lead to stable crack growth along the plane of the notch in a direction normal to the far-field compression axis. In this paper, we present experimental results and constitutive models for single phase ceramics, transforming ceramics and ceramic composites with the aim of developing the framework for a general theory of cyclic compression fracture in notched brittle solids. Firstly, the results of a recent constitutive model for microcracking brittle solids in cyclic compression are briefly summarized. Next, a generalized constitutive formulation for martensitic transformations which considers both volumetric and shear strains is developed. A finite element analysis gives the near-tip residual stress field, where numerical results for microcracking single phase ceramics and transformation-toughened ceramics illustrate the inducement of notch-tip residual tensile stresses upon unloading from a far-field compressive stress. Large residual tensile stresses are created over a distance which is substantially greater than the characteristic microstructural dimension, but comparable to the experimentally-observed distance of crack growth after one compression cycle. The results obtained for ceramics are compared and contrasted with our previous studies of notch-tip residual tensile fields for metallic materials.

Transformation Toughening in ZrO2‐Containing Ceramics

Transformation toughening in ZrO2‐containing ceramics is discussed. Specifically, microstuctures of the three distinct types of ZrO2‐toughened ceramics are presented, after which microstructural evolution in MgO‐partially‐stabilized ZrO2 Mg‐PSZ) is reviewed. The mechanical properties of such transformation‐toughened ceramics are dominated by “resistance‐curve” (R‐curve) behavior, wherein the crack resistance increases during the course of crack propagation. Ceramics subject to R‐curve behavior require a more detailed failure criterion than those subject to the usual linear elastic fracture mechanics criterion involving a critical stress intensity factor, KIC.R‐curve‐controlled fracture in ceramics provides a degree of very desirable flaw insensitivity, but can lead to counterintuitive relationships concerning strength, toughness, and initial flaw size. Examples of R curves of Mg‐PSZ with different thermal histories are given.

Relation of transformation temperature to the fracture toughness of transformation-toughened ceramics

The stress induced tetragonal to monoclinic ZrO2 martensitic transformation contribution to fracture toughness is described in terms of the required external strain energy and the thermo-dynamic stability of the constrained tetragonal phase. The strain energy, derived from an externally applied stress acting on the main crack, required to achieve transformation toughening is shown to be a function of the term (T - Ms) whereT is the test temperature andMs is the martensite start temperature for the case ofT > Ms. Thus for a givenT (T > Ms), the transformation toughening component increases asMs approachesT and for a fixedMs, the toughness decreases asT increases. Experimental data for partially stabilized zirconia ceramics confirm these results and show that increasing tetragonal precipitate size is the primary feature which affects an increase inMs. In the case ofT ⩽Ms, autotransformation occurs, resulting in decreasing toughness with decrease inT due to a continuous loss in the tetragonal phase content. A temperature region is thus obtained over which transformation toughening exhibits a maximum in its contribution. The temperatures over which this occurs then is shown to be dependent on theMs temperature of the material.

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Strengthening strategies for ZrO 2-toughened ceramics at high temperatures

Transformation-toughened (i.e. ZrO 2-toughened) ceramics represent a new class of high performance ceramics with spectacular strength properties at low and intermediate temperatures. However, at temperatures above about 700°C, most of these tough oxide-base ceramics can no longer be used as load-bearing engineering parts because of characteristic deficiencies. The aim of the present paper is to provide and discuss microstructural design strategies which may enable ZrO 2-toughened ceramics to be applied to higher temperatures. From the various strategies suggested, three appear to show good prospects, namely (i) the prevention of glassy intergranular films, (ii) the addition of hard high modulus particles and (iii) whisker or fibre reinforcement. Experimental approaches are presented for some ZrO 2-toughened ceramics, e.g. tetragonal ZrO 2 polycrystals and ZrO 2-toughened cordierite, spinel and mullite.

R-Curve Behavior and The Mechanical Properties of Transformation-Toughened ZrO2-Containing Ceramics

AbstractCurrent understanding of R-curve behaviour and the strength-toughness relationship of transformation-toughened ZrO2-containing ceramics is reviewed. If the toughness increases with crack, extension along an R-curve, a maximum in strength versus toughness is observed, which is related to the crack instability criterion for R-curves. An analytical expression from a well-known “wake” theory of crack tip shielding due to McMeeking and Evans is used to compute R-curves and strength/toughness relations, which are compared with experiment.

Crack-size dependence of fracture toughness in transformation-toughened ceramics

Fracture toughness (K IC) has been determined for Y2O3-partially stabilized zirconia, Y2O3-partially stabilized hafnia, CaO-partially stabilized zirconia and Al2O3+ZrO2 composites. It is shown thatK IC determined using the identation technique may not yield a unique number but may depend upon the crack size (C) (on the indent load). The slope ofK IC againstC 1/2 yields the magnitude of the surface stress created by the tetragonal → monoclinic transition on the surface induced by grinding.K IC determined using the double cantilever beam (DCB) technique, on the other hand, is shown to be independent of crack length.

Determination of Residual Stresses in Transformation-Toughened Ceramics

The strength of ceramics or glasses can be increased by placing their surfaces into compression. Techniques include ion exchange, temperature glazing, surface chemical reactions and stress-induced phase transformations. Although most of these techniques are well recognized, little effort has been expended In experimentally determining the magnitude of the compressive stress, and in particular, to use experimental evidence to identify important material and process parameters that need to be controlled. The goal of this investigation was to determine some of the factors that effect the magnitude, profile and depth of the compressive layer introduced by a structural phase transformation. X-ray residual stress measurements were used to directly determine the state of the surface residual stress.