Postsintering Hot Isostatic Pressing of Ceria-Doped Tetragonal Zirconia/Alumina Composites in an Argon–Oxygen Gas Atmosphere

Ceria-doped tetragonal zirconia (Ce-TZP)/alumina (Al₂O₃) composites were fabricated by sintering at 1450° to 1600°C in air, followed by hot isostatic pressing (postsintering hot isostatic pressing) at 1450°C and 100 MPa in an 80 vol% Ar–20 vol% O₂ gas atmosphere. Dispersion of Al₂O₃ particles into Ce-TZP was useful in increasing the relative density and suppressing the grain growth of Ce-TZP before hot isostatic pressing, but improvement of the fracture strength and fracture toughness was limited. Postsintering hot isostatic pressing was useful to densify Ce-TZP/Al₂O₃ composites without grain growth and to improve the fracture strength and thermal shock resistance.     

Microstructure and Bending Strength of 3Y-TZP/12Ce-TZP Ceramics Fabricated by Liquid-Phase Sintering at Low Temperature

With the addition of 1 wt% of MgO–Al₂O₃–SiO₂ glass as a sintering aid, 3Y-TZP/12Ce-TZP ceramics (composed from a mixture of 3Y-TZP and 12Ce-TZP powder) have been fabricated via liquid-phase sintering at 1250°–1400°C. In the sintered bodies, the grain growth of Y-TZP is almost unaffected, whereas that of Ce-TZP is inhibited. MgO·Al₂O₃ spinel and an amorphous phase that contains Al₂O₃ and SiO₂ (from the sintering aid) fully fill the grain junctions. The bending strength of 3Y-TZP/12Ce-TZP, when sintered at 1250°–1300°C, is ∼800–900 MPa, which is greater than that of 3Y-TZP ceramics without Ce-TZP particles. Ce-TZP grains and MgO·Al₂O₃ spinel in 3Y-TZP/12Ce-TZP ceramics may impede crack growth, and the bending strength is enhanced.     

High-Toughness Ce-TZP/Al2O3 Ceramics with Improved Hardness and Strength

Simulataneous additions of SrO and Al₂O₃ to ZrO₂ (12 mol% CeO₂) lead to the in situ formation of strontium aluminate (SrO · 6Al₂O₃) platelets (∼0.5 μm in width and 5 to 10 μm in length) within the Ce-TZP matrix. These platelet-containing Ce-TZP ceramics have the strength (500 to 700 MPa) and hardness (13 to 14 GPa) of Ce-TZP/Al₂O₃ while maintaining the high toughness (14 to 15 MPa ± m^1/2) of Ce-TZP. Optimum room-temperature properties are obtained at SrO/Al₂O₃ molar ratios between 0.025 and 0.1 for ZrO₂ (12 mol% CeO₂) with starting Al₂O₃ contents ranging between 15 and 60 vol%. The role of various toughening mechanisms is discussed for these composite ceramics.     

Transformation Plasticity and Toughening in CeO2-Partially-Stabilized Zirconia–Alumina (Ce-TZP/Al2O3) Composites Doped with MnO

Microstructure, phase stability, and mechanical properties of CeO₂-partially-stabilized zirconia (12 mol% Ce-TZP) containing 10 wt% Al₂O₃ and 1.5 wt% MnO were studied in relation to the base Ce-TZP and the Ce-TZP/Al₂O₃ composite without MnO. The MnO reacted with both CeO₂ and Al₂O₃ to form a new phase of approximate composition CeMnAl₁₁O₁₉. The reacted phase had a magnetoplumbite structure and formed elongated, needlelike crystals. The MnO-doped Ce-TZP/Al₂O₃ composites sintered at an optimum temperature of 1550°C exhibited high strength (650 MPa in four-point bending) and rising crack-growth-resistance behavior, with fracture toughness increasing from 7.6 to 10.3 MPa.In¹² in compact tension tests. These improved mechanical properties were associated with relatively high tetragonal-to-monoclinic transformation temperature ( M _s=−42°C) at small grain size (2.5 μm), significant transformation plasticity in mechanical tests (bending, uniaxial tension, and uniaxial compression) and transformation zones at crack tips in compact tension specimens. The transformation yield stress, zone size, and fracture toughness were sensitive to the sintering temperature varied in the range 1500° to 1600°C. Analysis of the transformation zones using Raman microprobe spectroscopy and calculation of zone shielding for the observed zones indicated that a large fraction of the fracture toughness (∼70%) was derived from transformation toughening.     

Co2+-Substitution Effect in Ce-TZP/La M-type Hexaferrite Composites

A Ce-TZP/platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was synthesized in situ while sintering from a mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO powders. Platelike La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ crystals were grown in a dense Ce-TZP matrix after sintering at temperatures of 1200°–1350°C. The temperature range for sintering Ce-TZP/La(Fe,Al)₁₂O₁₉ composites was expanded widely by substituting Co²⁺ ions for Fe²⁺ ions in its structure. The highest value of the bending strength of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composites was 880 MPa, which was higher than that of the Ce-TZP/La(Fe,Al)₁₂O₁₉ composite (780 MPa) and Ce-TZP (513 MPa). The saturation magnetization of the Ce-TZP/La(Co(Fe_0.9Al_0.1)₁₁)O₁₉ composite was a constant value of 7.7 emu/g after the composite was sintered at 1200°–1350°C.     

Synthesis and Magneto-mechanical Properties of Ce-TZP/La M-Type Hexaferrite Composite

An in situ composite composed of ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP) and La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was synthesized from a powder mixture of Ce-TZP, La(Fe_0.9Al_0.1)O₃, Fe₂O₃, Al₂O₃, and CoO. The dense Ce-TZP dispersed with platelike La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ crystals as a second phase were formed after sintering from 1250° to 1350°C. The saturation magnetization of the in situ composite Ce-TZP/La{Co_0.5Fe_0.5(Fe_0.9Al_0.1)₁₁}O₁₉ was proportional to the mass fraction of the hexaferrite second phase in Ce-TZP. The coercivity of the composite with a 20 mass% of second phase decreased from 9.14 to 2.52 kOe (from 728 to 201 kA/m) after the pulverization of the composite. The susceptibility (χ) increased by 15%–25% under uniaxial stress on the composite. The change of the susceptibility (Δχ/χ) value increased with decreasing the mass fraction of the second phase in the composite. The Δχ was found to increase linearly with applied stress and abruptly change on cracking, which is expected for the application in fracture sensing of the composite.     

Role of Autocatalytic Transformation in Zone Shape and Toughening of Ceria–Tetragonal-Zirconia–Alumina (Ce-TZP/Al2O3) Composites

Stress–strain behaviors in three-point bending, transformation zones in single-edge-notch-bend specimens, and transformation toughening were studied in two types of Ce-TZP/Al₂O₃ composites. A commercial grade exhibited yield-point behavior triggered by autocatalytic transformation and elongated zones. A new grade of Ce-TZP/Al₂O₃ composite showed monotonic stress–strain behavior and a zone shape close to theoretical prediction based on a shear–dilatation yield criterion. The effects of zone sizes and shapes on fracture toughness of the two ceramic composites are shown to be qualitatively consistent with the predictions of transformation-zone shielding theories.     

Bimaterial Composites via Colloidal Rolling Techniques: II, Sintering Behavior and Thermal Stresses

Shrinkage behavior and crack formation during firing have been investigated for Al₂O₃/Ce-TZP composites that have been fabricated by colloidal rolling and folding. These composites show improved sinterability and sinter isotropically after repeated rolling. Interface instability in rolling creates corrugated interfaces with large layer waviness; therefore, rolling can substantially alleviate the in-plane sintering constraints, which leads to improved sinterability. A loss of sintering anisotropy also is observed and is directly correlated to the microstructure instability, which is coincident with the laminate-cellular transition. Sintering cracks during heating and thermal cracks during cooling both are limited to the thick Ce-TZP layers in the composites. The critical layer thickness and the normalized crack spacing of the thermal cracks follow the predicted behavior of elasticity theory. Thus, crack-free, high-density Al₂O₃/Ce-TZP composites with either a laminate or cellular microstructure can be obtained, with a layer thickness of 4-60 µm, via pressureless sintering.     

Stability of ZrO2 Phases in Ultrafine ZrO2-Al2O3 Mixtures

Mixtures of ultrafine monoclinic zirconia and aluminum hydroxide were prepared by adding NH₄OH to hydrolyzed zirconia sols containing varied amounts of aluminum sulfate. The mixtures were heat-treated at 500° to 1300°C. The relative stability of monoclinic and tetragonal ZrO₂ in these ultrafine particles was studied by X-ray diffractometry. Growth of ZrO₂ crystallites at elevated temperatures was strongly inhibited by Al₂O₃ derived from aluminum hydroxide. The monoclinic-to-tetragonal phase transformation temperature was lowered to ∼500°C in the mixture containing 10 vol% Al₂O₃, and the tetragonal phase was retained on cooling to room temperature. This behavior may be explained on the basis of Garvie's hypothesis that the surface free energy of tetragonal ZrO₂ is lower than that of the monoclinic form. With increasing A1₂O₃ content, however, the transformation temperature gradually increased, although the growth of ZrO₂ particles was inhibited; this was found to be affected by water vapor formed from aluminum hydroxide on heating. The presence of atmospheric water vapor elevates the transformation temperature for ultrafine ZrO₂. The reverse tetragonal-to-monoclinic transformation is promoted by water vapor at lower temperatures. Accordingly, it was concluded that the monoclinic phase in fine ZrO₂ particles was stabilized by the presence of water vapor, which probably decreases the surface energy.     

Strength and Phase Stability of Yttria-Ceria-Doped Tetragonal Zirconia/Alumina Composites Sintered and Hot Isostatically Pressed in Argon–Oxygen Gas Atmosphere

Yttria-ceria-doped tetragonal zirconia (Y,Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing at 1400° to 1450°C and 196 MPa in an Ar–O₂ atmosphere using the fine powders prepared by hydrolysis of ZrOCl₂ solution. The composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5–4 mol% CeO₂–ZrO₂ and 2.5 mol% YO_1.5–5.5 mol% CeO₂–ZrO₂ exhibited mean fracture strength as high as 2000 MPa and were resistant to phase transformation under saturated water vapor pressure at 180°C (1 MPa). Postsintering hot isostatic pressing of (4Y, 4Ce)-TZP/Al₂O₃ and (2.5Y, 5.5Ce)-TZP/Al₂O₃ composites was useful to enhance the phase stability under hydrothermal conditions and strength.     

Cyclic Fatigue of Ce-TZP/AI2O3 Composites: Role of the Degradation of Transformation Zone Shielding

The origin of cyclic fatigue in two Ce-TZP/Al₂O₃ composites was investigated by (a) measurements of residual stresses in the transformation zones and crack-tip stress intensities in in situ loaded compact specimens using microprobe Raman spectroscopy, (b) examination of the crack-tip transformation zones by transmission electron microscopy, and (c) measurements of crack-growth rates in cyclic fatigue and in sustained loading at 400°C, a temperature at which stress-induced transformation of the tetragonal zirconia to the monoclinic polymorph was suppressed. Transformation zones formed during cyclic fatigue consistently showed lower compressive residual stresses and higher crack-tip stress intensities than the zones formed in sustained loading. Transmission electron microscopy revealed monoclinic laths of smaller average twin spacing and of multiple types of lattice correspondence in the transformation zones of the fatigue specimens as compared to the sustained-load specimens. Crack-growth measurements at 400°C indicated a significant suppression of the cyclic fatigue effect in the absence of transformation plasticity. These results in combination pointed to degradation of transformation-zone shielding as an important contributing cause of cyclic fatigue in Ce-TZP/Al₂O, composites. A more efficient accommodation of the transformation strains within the zones appears to be the underlying mechanism of the degradation of zone shielding in cyclic fatigue.     

Formation and Transformation of Cubic ZrO2 Solid Solutions in the System ZrO2—Al2O3

In the system ZrO₂-Al₂O₃, cubic ZrO₂ solid solutions containing up to 40 mol% Al₂O₃ crystallize at low temperatures from amorphous materials prepared by the simultaneous hydrolysis of zirconium and aluminum alkoxides. The values of the lattice parameter, a, increase linearly from 0.5095 to 0.5129 nm with increasing Al₂O₃ content. At higher temperatures, the solid solutions transform into tetragonal ZrO₂ and α-Al₂O₃. Pure ZrO₂ crystallizes in the tetragonal form at 415° to 440°C.     

Coupled Grain Growth Effects in Al2O3/10 vol% ZrO2

Coupled crystallization has been observed in the Al₃O₃/10%-ZrO₂ system by heating an amorphous precursor Al /Zr copolymerized alkoxide network structure. A finely divided two-phase material results which stabilizes tetragonal ZrO₂ to 1700°C and exhibits an unprecedented microstructure. During crystallization, the grain growth of ZrO₂ is coupled to the γ→α phase transformation of Al₂O₃.     

Crack Shielding in Ce-TZP/Al2O3 Composites: Comparison of Fatigue and Sustained Load Crack Growth Specimens

Crack shielding stress intensities in in situ loaded compact tension specimens of two types of ceria-partially-stabilized zirconia/alumina (Ce-TZP/Al₂O₃) composites with prior histories of subcritical crack growth in sustained and tension-tension fatigue loading were directly assessed using laser Raman spectroscopy. Crack-tip stress fields within the transformation zones were measured by measuring a stress-induced frequency shift of a peak corresponding to the tetragonal phase. The peak shift as a function of the applied stress was separately calibrated using a ball-on-ring flexure test. Total crack shielding stress intensity was estimated from the far-field applied stress intensity and the local crack-tip stress intensity assessed from the measured near-crack-tip stresses. The shielding stress intensities were consistently lower in the fatigue specimens than in the sustained load crack growth specimens. The reduced crack shielding developed in the fatigue specimens was independently confirmed by measurements of larger crack-opening displacement under far-field applied load as compared to the sustained load crack growth specimens. Thus, diminished crack shielding was a major factor contributing to the higher subcritical crack growth rates exhibited by the Ce-TZP/Al₂O₃ composites in tension–tension cyclic fatigue. Calculations of zone shielding considering only the dilatational strains in the transformation zones accounted for 81% and 86% of the measured values in the sustained load crack growth specimens, but significantly overestimated the shielding in the fatigue specimens. Possible reasons for the diminished crack shielding in the fatigue specimens are discussed.     

Stability of Ultrafine Tetragonal ZrO2 Coprecipitated with Al2O3 by the Spray-ICP Technique

The transformation of ultrafine powders (particle size, 0.01 to 0.04 μm) of the system ZrO_2–Al₂O₃, prepared by spraying their corresponding nitrate solutions into an inductively coupled plasma (ICP) of ultrahigh temperature, was investigated. The powders were composed of metastable tetragonal ZrO₂ ( mt- ZrO₂) and γ-Al₂O₃. On heating, the mt- ZrO₂ (or tetragonal ZrO₂, t -ZrO₂) was retained up to 1200°C. At 1380°C the transformation to monoclinic ZrO₂ ( m -ZrO₂) occurred and the amount of the m -ZrO₂ decreased with the increase in Al₂O₃ content, thus indicating the stabilization of the t -ZrO₂ by the Al₂O₃, which seems to be explained in terms of the retardation of grain growth.     

Properties of (Y)ZrO2–Al2O3 and (Y)ZrO2–Al2O3–(Ti or Si)C Composites

The effect of Al₂O₃ and (Ti or Si)C additions on various properties of a (Y)TZP (yttria-stabilized tetragonal zirconia polycrystal)–Al₂O₃–(Ti or Si)C ternary composite ceramic were investigated for developing a zirconia-based ceramic stronger than SiC at high temperatures. Adding Al₂O₃ to (Y)TZP improved transverse rupture strength and hardness but decreased fracture toughness. This binary composite ceramic revealed a rapid loss of strength with increasing temperature. Adding TiC to the binary ceramic suppressed the decrease in strength at temperatures above 1573 K. The residual tensile stress induced by the differential thermal expansion between ZrO₂ and TiC therefore must have inhibited the t - → m -ZrO₂ martensitic transformation. It was concluded that a continuous skeleton of TiC prevented grain-boundary sliding between ZrO₂ and Al₂O₃. In contrast, for the ternary material containing β-SiC in place of TiC, the strength decreased substantially with increasing temperature because of incomplete formation of the SiC skeleton.     

In-Situ Formation of Ce-TZP-M-Type Hexaferrite Composites

Composites consisting of a Ce-TZP matrix and in-situ grown particles of La(Fe,Al)₁₂O₁₉ were prepared by the solid-state reaction of Ce-TZP, La(Fe,Al)O₃, and Al₂O₃ powders. The in-situ formation process of La(Fe,Al)₁₂O₁₉ in a Ce-TZP matrix was investigated in relation to the microstructure evolution. The Ce-TZP-La(Fe,Al)₁₂O₁₉ composites had significant magnetic properties and improved mechanical properties. The maximum value of magnetization of the Ce-TZP-La(Fe,Al)₁₂O₁₉ composites was obtained at 90 mol% LaFe₁₂O₁₉ in La(Fe,Al)₁₂O₁₉. The value of the three-point bending strength of Ce-TZP-20 wt% La(Fe,Al)₁₂O₁₉ (90 mol% LaFe₁₂O₁₉) sintered at 1350°C was 780 MPa, compared with 500 MPa of Ce-TZP ceramics. The composite demonstrated crack sensing based upon the change in magnetic gradient.     

Sol–Gel Processed Y-PSZ Ceramics with 5 wt% Al2O3

Y-PSZ ceramics with 5 wt% Al₂O₃ were synthesized by a sol–gel route. Experimental results show that powders of metastable tetragonal zirconia with 2.7 mol% Y₂O₃ and 5 wt% Al₂O₃ can be fabricated by decomposing the dry gel powder at 500°C. Materials sintered in an air atmosphere at 1500°C for 3 have high density (5.685 g/cm³), high content of metastable tetragonal zirconia (>96%), and high fracture toughness (8.67 MPa.m^1/2). Compared with the Y-PSZ ceramics, significant toughening was achieved by adding 5 wt% Al₂O₃.     

Deformation Bands in Ceria-Stabilized Tetragonal Zirconia/Alumina: I, Measurement of Internal Stresses

The residual stresses within a deformation band in a ceriastabilized tetragonal zirconia/alumina (Ce-TZP/Al₂O₃) ceramic composite are measured by piezo-spectroscopy, in the zirconia phases from their Raman spectra and in the alumina phase from its Cr³⁺ fluorescence. The concentrations of monoclinic zirconia across the deformation band are obtained from Raman spectra recorded from locations across the band. These measurements show that the deformation bands are associated with a localization of the tetragonal-to-monoclinic transformation and that although the overall residual stress in the bands is compressive, the tensile stress in the tetragonal zirconia phase is enhanced. The experimental data are compared with the predictions of a stochastic theory for three-phase materials and with models for the stress distribution within a deformation band. The region ahead of the deformation band is subject to an additional, superimposed tensile stress whose magnitude compares favorably with the predictions of a superdislocation model presented.     

Bimaterial Composites via Colloidal Rolling Techniques: III, Mechanical Properties

The mechanical properties of layered Ce-TZP/Al₂O₃ composites with laminate and cellular morphologies have been investigated. The strength and toughness increase as the layer thickness decreases, and the amount of transformation in the Ce-TZP layer increases discontinuously at the laminate/cellular transition. Very high strengths (1.1 GPa) and toughnesses (16 MPa·m^1/2) have been obtained in some cases. These results indicate that the progressive refinement of layer microstructure and the disruption of planar connectivity of phases are beneficial to the mechanical performance, because they provide more stress concentrators to trigger stress-assisted transformation for toughening functions. The composites of finer microstructure, with a layer thickness of lessthan equal to20 µm, have a homogeneous hardness of 11.5 GPa, which is a considerable improvement over that of Ce-TZP alone.