Transformation-toughened Ceramics Research Articles

Design and processing of a ceramic laminate with high toughness and strong interfaces

An alumina–aluminium titanate laminate designed to combine high crack deflection capability with strong interfaces is proposed. It is constituted by relatively stiff and brittle external layers with microcracked internal layers to produce multiple crack deflection at the microstructural scale. The most important difference of the laminated structure proposed here and that of other laminates with high capability for crack deflection is that the crack deflection process occurs at local level, thus, delamination lengths are limited and delamination does not lead to the lost of structural integrity. A symmetric structure formed by five layers has been design to minimise residual stresses taking into account the strain on cooling and the Young’s modulus of monolithic materials of the same compositions as those of the layers and fabricated using the same processing procedure as that of the laminate. Special care was given to adjust the processing variables that permitted the fabrication of the designed laminated by sequential slip casting and sintering. Mechanical characterisation has been done in terms of strength (4-points bending), dynamic Young’s modulus, work of fracture and apparent toughness. The two latter parameters have been determined by 3-points bending of Single-Edge-V-Notch-Beams (SEVNB) and fractographic analysis has been performed on the tested samples. The apparent toughness value at the point of failure (12 MPa m 1/2) was comparable to values reported for the stationary state of transformation-toughened ceramics. Work of fracture (62 ± 3 Jm −2) was significantly higher (≈26%) than that obtained by calculation from the values corresponding to monolithic materials of the same composition as that of the layers, revealing the synergic effect of the laminated structure on the mechanical behaviour of the material.

Fracture Enhancement of Mixed Mode I-II Transformation Toughened Ceramics

The pressure sensitive transformation criterion is used to determine the constitutive relation of transformation toughened ceramics. By the use of the strain energy release rate criterion and the method of weight function, the fracture enhancement of mixed-mode I-II crack in transformation toughened ceramics is predicated. The theoretical expressions of the toughening effect for both the stationary and steady-state growing crack are given respectively. The result show there is no toughening effect for the stationary crack and the toughening effect for the steady-state growing crack is associated with the modulus of elasticity, the width of transformation and its volume fraction.

Finite element analysis of the fatigue crack growth rate in transformation toughening ceramics

The fatigue crack growth rate in the zirconia tetragonal polycrystal is analyzed through the finite element method. In order to achieve this purpose, a continuum based constitutive law for materials subjected to phase transformations has been suitably implemented into a commercial finite element code. The fatigue crack growth in a notched beam, subjected to a cyclic four points bending load, has been investigated through a sensitivity analyses with respect to the two most relevant constitutive parameters: one accounting for the amount of the transformation strain and one accounting for the activation energy threshold. The fatigue crack growth rate typical of transforming materials is characterized by two distinct stages: at the beginning of the crack propagation process, the crack growth rate exhibits a negative dependency on the applied stress intensity factor; thereafter, a linear positive dependency is observed. This two stage process is well caught by the finite element model presented in this paper. Moreover, the response of the computational analyses has shown that the strength of the transformation process is determinant for the crack growth process to be arrested.

Arrest of Fatigue Cracks in Transformation-Toughened Ceramics

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate fatigue crack growth in a transformation-toughened ceramic. The effective stress-intensity factor is calculated during crack growth, because it is believed to determine the crack-growth rate similar to the Paris-type growth law. For certain combinations of transformation strength and load, the effective stress-intensity factor decreases to zero, indicating crack arrest. A detailed parametric study of this phenomenon reveals that the applied load and minimum transformation strength parameter necessary to cause crack arrest are linearly related, independent of initial crack length. This suggests that a threshold stress similar to the endurance limit in the conventional stress/life (S/N) approach should be used instead of the threshold stress-intensity factor in the design of transformation-toughened ceramics against fatigue.

Texture formation on the friction surface in transformation-toughened ceramics

The surface of Y-TZP ceramics after dry friction on steel in a wide range of relative sliding velocities (0.2–11.1 m/s) was studied by X-ray diffraction. An analysis of the surface structure showed that the texture state of the friction surface is similar to that observed for analogous materials upon rough grinding. The main trends in the I(002)/I(200) intensity ratio characterizing the tetragonal ZrO2 phase as a function of the sliding velocity and the ceramic grain size are established. The mechanisms of lattice reorientation operative at various sliding velocities are discussed.

Comparison between fatigue behavior of some ceramics: a new concept of intrinsic stress-corrosion exponent n0

The fatigue behavior was systematically investigated with three typical ceramic materials, including Y 2O 3–ZrO 2, Si 3N 4 and a machinable glass–ceramic. Cyclic, static and dynamic fatigue tests were conducted in three environments, i.e., moist air, distilled water and kerosene. The effects of environment and loading condition on fatigue behavior were analyzed. The difference between materials was discussed. Experimental results showed that, for all the three materials, fatigue life under cyclic load was the shortest and a low value of n (fatigue exponent) was obtained. Compared with the other materials, Si 3N 4 ceramic had a very large n value under static load. Therefore the static fatigue for Si 3N 4 ceramics may be ignored. Cyclic load decreases the fatigue life of transformation-toughening ceramics (Y–TZP) more seriously, while no remarkable difference was observed between cyclic and static fatigue for original glass of the glass–ceramic. A new idea about the physical meaning of n value and the concept of intrinsic stress-corrosion exponent n 0 were introduced. The exponent n can be separated into two terms, i.e., n= n 0+ n μ , where n 0 is a material constant and just related to the most basic properties, such as atom-bonds or crystalline structure, n μ reflects the contribution of microstructure toughening and is very sensitive to environments or loading conditions. This hypothesis can be used to describe and explain the experiment results successfully.

Strength and fracture toughness of isostatically hot-pressed (Y,Nb)-TZP/Al2O3 composites

Among transformation-toughened ceramics, tetragonal zirconia polycrystals (TZPs) have been considered as the biomaterials for dental and orthopedic prostheses due to their excellent biocompatibility and mechanical properties [1, 2]. Although yttria-stabilized TZPs (Y-TZPs) possess high strength and toughness at room temperature, they suffer low-temperature strength degradation (LTD) because of the spontaneous tetragonal (t) to monoclinic (m) phase transformation when annealed at temperatures from 100 to 500◦C in air [3–5]. The LTD is accelerated under hydrothermal environments due to the reaction between Zr-O-Zr bonds and H2O [4, 5]. The phase stability of TZPs under hydrothermal conditions is a critical requirement for application of TZPs for use in medical devices because TZPs should pass the autoclave sterilization for 20– 30 min at temperatures from 120 to 135◦C prior to use [1]. Recently, Lee et al. [6] examined the t-phase stability composition region in the ZrO2-Y2O3-Nb2O5 system and showed that Nb2O5 doping to Y-TZP with a composition of 90.24 mol% ZrO2-5.31 mol% Y2O3-4.45 mol% Nb2O5 ((Y,Nb)-TZP) influenced remarkably the fracture toughness and LTD. The phase stability of TZPs in the ternary system is likely due to the Y-Nb ordering in t-ZrO2 lattice into a scheelite-like arrangement, which resulted in a relief of the internal strain in the t-ZrO2 lattice since the internal stress was known to assist the degradation under aging environments [5–7]. Even though (Y,Nb)-TZPs, having a large grain size of above 4μm, showed excellent phase stability, a relatively large grain size caused lower fracture strength of 450 MPa, as compared with that (800 MPa) of the commercial 3Y-TZP [6, 8]. The strength of (Y,Nb)-TZP was further enhanced by the addition of Al2O3 due to the combined effects of phase transformation, grain bridging toughening, and grain growth inhibition [8, 9]. The strength of (Y,Nb)-TZP/Al2O3 composites containing 20 vol% of 2.8μm Al2O3, sintered for 2 h at 1550◦C in air, was 700 MPa [9], which was still lower than that of 3Y-TZP. Masaki reported that greater strength of Y-TZP was achieved by hot-pressing (HPing) or hot-isostatic pressing (HIPing) [10, 11]. To improve fracture strength, (Y,Nb)-TZP was hot-pressed for 1 h at 1400◦C under 30 MPa. (Y,Nb)-TZP exhibits the strength of about 1 GPa, the fracture toughness of 7.5 MPa ·m1/2, and no LTD behavior at temperatures lower than 400◦C [12]. However, TZPs, aged at temperatures higher than 400◦C, suffered from high-temperature strength degradation due to the extensive cavitation caused by graphite oxidation. X-ray photoelectron spectroscopy results of the as-HPed (Y,Nb)-TZP demonstrated that the carbon incorporated in (Y,Nb)-TZP was either an ether-type C-O or a carbonyl-type C=O and the oxidation of CO to CO2 during aging at temperatures from 400◦C to 1000◦C was attributed to strength degradation [12]. (Y,Nb)-TZP was greatly strengthened by the addition of Al2O3 and the application of sinter-HIPing. The green compacts were sintered for 2 h at 1500◦C with heating rates of 6◦C min−1 to 900◦C and 3◦C min−1 up to the sintering temperature, and then furnace cooled to room temperature. The presintered (Y,Nb)-TZPs and (Y,Nb)-TZP/Al2O3 composites were placed into the hot zone of a high pressure vessel and isostatically hot-pressed for 0.5 h at 1500◦C under 100 MPa using Ar gas as the pressure-transmitting medium. The pressure was applied to HIPing temperature with a rate of 27 MPa h−1 and the temperature was with a rate of 400◦C h−1. The purpose of the present study is to investigate the mechanical properties of (Y,Nb)-TZPs and (Y,Nb)-TZP/Al2O3 composites prepared by HIPing. The powder preparation procedures of (Y,Nb)-TZP and (Y,Nb)-TZP/Al2O3 composite were reported elsewhere [6, 8]. The bulk density of both sintered and HIPed specimens was determined by the Archimedes method. For mechanical property measurements, the disk specimens were polished to a 1μm diamond finish and subsequently annealed for 2 h at 1200◦C to remove the possible residual stress. A hydrothermal stability of both sintered and HIPed specimens was evaluated after aging for 20 h at 180◦C under 3.5 MPa water vapor pressure in an autoclave. The extent of m-ZrO2 was estimated from the X-ray diffraction diffractometry (XRD) after Garvie and Nicholson [13]. The dimension of the specimens after polishing was 18 mm in diameter and 1.8 mm in thickness. A flaton-three ball biaxial-fixture was used for the determinations of strength and toughness with a stress rate of 23 MPa · s−1 that was determined from a correlation between the rate and the specimen thickness in the ASTM standard [14]. The fracture toughness was determined

Measurement of single-crystal elastic constants by neutron diffraction from polycrystals

The relationships of diffraction averaged elastic compliances for an ideally random polycrystal to the single-crystal elastic compliances are given, within the Reuss approximation, for crystal systems with orthorhombic and higher symmetry. For anisotropic materials, these diffraction elastic compliances are dependent on the reflection indexhkl. Expressions for the conventional elastic constants (Young's modulus, Poisson's ratio) are also given. A connection is made to the `X-ray elastic constants' used for diffraction-based measurements of residual stress. The relationships are used to calculate diffraction averaged constants for comparison with neutron diffraction data recorded from samples under applied uniaxial stress. The Reuss approximation works well for materials with the capacity for plastic deformation, such as metals and transformation toughening ceramics, whereas for other materials the Voigt–Reuss–Hill approximation gives better results. Based on the given relationships and experimental determinations of the diffraction elastic compliances for polycrystalline materials, a method is developed for determining the single-crystal elastic constants. The method for estimating single-crystal compliances is demonstrated here by application to extant data on Ni–Cr–Fe and Ti–6 wt% Al–4 wt% V alloys, and new measurements on cubic zirconia. It has been applied very recently [Kisi & Howard (1998).J. Am. Ceram. Soc.81, 1682–1684] to determine the previously unknown elastic constants for a tetragonal zirconia.

Asymptotic analysis of plane-strain mode I steady-state crack growth in transformation toughening ceramics(II)

Based on a constitutive law which includes the shear components of transformation plasticity, the asymptotic solutions to near-tip fields of plane-strain mode I steadity propagating cracks in transformed ceramics are obtained for the case of linear isotropic hardening. The stress singularity, the distributions of stresses and velocities at the crack tip are determined for various material parameters. The factors influencing the near-tip fields are discussed in detail.

Unagglomerated powders for transformation-toughened structural ceramics

Results of a study of the characteristics of ZrO2 powders stabilized by 3 mol% Y2O3 and fabricated by the method of ultrasound spray pyrolysis from aerosols of aqueous solutions of mineral salts are presented. Unlike powders of the same composition obtained by the conventional coprecipitation technique, aerosol powders have coarser particles with a narrower size distribution and do not contain agglomerates. The temperature of sintering coarse aerosol powders to a maximum density is not high (1450°C). Despite the larger grain size in ceramics sintered from aerosol powders, their strength is much higher than that of ceramics obtained from ultradisperse coprecipitated powders of a similar composition.

Transformation criterion of phase transformation toughening ceramics with misoriented microcracks

Phase transformation criterion is the key to investigating the toughness of phase transformation ceramics. In this paper, the modified equivalent inclusion theory by the authors is employed to study the interaction between microcracking and transformation in ceramics. The transformation criterion is derived. The influence of microcracks and transformation particles on the critical transformation load is discussed.

Enhanced Wear Performance of Transformation Toughened Ceramics by Microstructural Optimization

The poor rolling/sliding wear performance of transformation toughened ceramics (TTC) such as partially stabilized zirconia (PSZ) is believed to be a result of the volume expansion accompanying phase transformation of precipitates located immediately beneath the contacting surface. The expanding precipitates induce a surface uplift leading to surface waviness and altered rolling/sliding conditions. The very same precipitates, on the other hand enhance the toughness of the ceramic by preventing the growth of surface cracks. In order to find a compromise solution, an optimization problem is formulated for an idealized model consisting of an edge crack normal to the contacting surface and a periodic distribution of transformable grains in the layer immediately beneath this surface. The objective is to maximize the crack tip shielding by varying the volume fraction and size of the layer without exceeding a prescribed allowable surface uplift. It is shown that the maximum volume fraction of t-ZrO2 is attained at a certain depth below the surface but that its magnitude is considerably smaller than that pertaining in peak-aged PSZ.

Improved Endurance Limit of Zirconia Ceramics by Overloading

The influence of initial overloads upon the subsequent fatigue crack growth in transformation‐toughened ceramics (TTCs) is investigated. It is revealed that initial overloading can be beneficial to these ceramics. In particular, it is shown that an overload will facilitate crack arrest in a low transformation strength material which would otherwise be prone to cyclic fatigue degradation and eventual failure. Based on the results of this investigation, a proof test may be designed to screen out TTC components which do not attain a prescribed endurance limit.

Subsurface and surface cracks under contact loading in transformation-toughened ceramics

The wearing surface in rolling and sliding wear is approximated by a half-plane subjected to frictional contact loading. Under such loading inherent flaws, such as microcracks, voids and inhomogeneities, eventually develop into macrocracks with a resultant deterioration in wear performance. Two macrocrack configurations are studied: a straight subsurface crack parallel to the surface and a straight edge crack perpendicular to the surface. The cracks are modelled using the dislocation formalism, and the contact load is assumed to be Hertzian. In the vicinity of the crack tips, where a high tensile stress field is anticipated, the material is allowed to undergo irreversible phase transformation in accordance with an appropriate transformation criterion. The transformation zone around the crack tip (or tips) is modelled as a collection of minute circular regions. Such a model is a good representation of the transformable precipitates, for example in partially stabilized zirconia. The elastic field from the transformed circular regions is obtained by applying the Eshelby formalism. The phase transformation exerts a strong influence on the behaviour of the cracked material compared to the purely elastic material through a significant closing effect on the crack faces. This results in the mode I stress intensity factor being almost zero. However, as the transformation zone is highly asymmetric relative to the crack faces, because of the non-uniform stress field due to the applied contact load, the mode II loading of the crack tip(s) can be severely enhanced. This results in high residual near tip deformations after unloading, i.e. a high residual mode II stress intensity factor.

Application of the weight function and boundary collocation method to the calculation of initial phase transformation zones

Transformation-toughened ceramics exhibit stress-induced phase transformation with the tetragonal phase changing to the monoclinic phase. These transformations will affect the stress field in the surroundings of crack tips. A special procedure—based on the fracture mechanics weight function—is used to compute the stress field at the onset of stable crack propagation. Calculations are performed for the special case of small-scale transformation zones as well as for zones around finite cracks in a finite body. It can be shown that the real stress field deviates from that computed on the base of the applied stress intensity factor. An interesting result which can be shown is that the crack opens due to the phase transformation zone, although the stress intensity factor is zero.

Crack front trapping in transformation-toughened ceramics

A theoretical study is made of the trapping of a crack front in brittle matrix ceramic reinforced by dispersed transformable zirconia particles. The trapped discontinuous crack front is modelled by a collinear array of microcracks interspersed by transformable particles. The effect of trapping of the macrocrack front is approximated by a closing pressure on a bridged zone. The maximum closing pressure and the critical opening of the macrocrack are related to the volume (area) fraction and size of the transformable particles. It is found that the maximum toughening due to trapping occurs at moderate volume (area) fractions of large particles. This is in agreement with available theoretical and experimental results.

Fatigue Crack Growth from Small Surface Cracks in Transformation‐Toughened Ceramics

A theoretical model based on the theory of complex potentials and dislocation formalism is used to simulate the fatigue crack growth of small cracks in a transformation‐toughened ceramic. Assuming power‐law crack growth in which the growth rate depends on the effective stress intensity at the crack tip instead of the applied stress intensity, it is shown that the crack growth rate decreases with the applied stress intensity in the initial stage of fatigue crack growth. This is in agreement with existing experimental evidence for the growth of small cracks in Mg‐PSZ. New experimental results obtained by in situ observation in a scanning electron microscope of a similar material confirm this behavior. The numerical results also confirm the plausibility of using the steady‐state toughness value obtained from quasi‐static crack growth as a normalizing parameter in the power‐law for fatigue crack growth.

Optimum Microstructure of Transformation‐Toughened Ceramics for Enhanced Wear Performance

The poor rolling/sliding wear performance of transformation‐toughened ceramics such as partially stabilized zirconia (PSZ) is believed to be a result of the phase transformation of precipitates located immediately beneath the contacting surface. These very precipitates, on the other hand, enhance the toughness of the ceramic by preventing the growth of surface cracks. In order to find a compromise solution, an optimization problem is formulated for an idealized model consisting of an edge crack normal to the contacting surface and a periodic distribution of transformable grains in the layer immediately beneath this surface. The objective is to maximize the crack tip shielding by varying the volume fraction and size of the layer without exceeding a prescribed allowable surface uplift. It is shown that maximum volume fraction is attained at a certain depth below the surface but that its magnitude is considerably smaller than that pertaining in peak‐aged PSZ.

The stiffness and strength of transformation toughening ceramics with misoriented microcracks

Theoretical studies on misoriented transformation particles and microcracks in transformation toughening ceramics are presented using the Eshelby equivalent inclusion method. The stress field, stiffness and strength were calculated. Experiments were done by the three-point bend method using Al2O3/ZrO2 ceramics and the stiffness and strength were also measured. Comparison between theoretical and test results confirmed the important role of microcracks.

Surface cracks in transformation toughening ceramics

This paper presents a complete R-curve analysis for a single surface crack and an array of equally spaced parallel surface cracks in transformation toughened ceramics (TTC). The problem is reduced to the solution of a pair of singular integral equations in which the dislocation density function and the transformation zone boundary are the unknown functions. The form of the equations does not depend on the number of surface cracks being studied; only the weight functions are different. The singularities of the weight functions and of the dislocation density functions at the crack tip and at the intersection of the crack by the transformation zone wake are analytically isolated for accurate numerical integration of the integral equations.