Fibrous Monolithic Ceramics: II, Flexural Strength and Fracture Behavior of the Silicon Carbide/Graphite System

The fibrous monolith microstructure consists of high aspect ratio polycrystalline cells of SiC separated by thin cell boundaries containing graphite. The SiC/100% graphite fibrous monolith has noncatastrophic fracture behavior, is damage tolerant, and is notch insensitive. The failure process is characterized by fracture along weak graphite cell boundaries. The room-temperature flexural strength is 300–350 MPa. The estimated shear strength along the graphite cell boundaries is ∼ 15 MPa. Increasing the strength of the cell boundary by additions of SiC (40–60 vol%) results in a monolithic SiC material showing brittle fracture behavior but retaining damage tolerance. Strength and fracture behavior are also influenced by cell texture and orientation.     

Fibrous Monolithic Ceramics: III, Mechanical Properties and Oxidation Behavior of the Silicon Carbide/Boron Nitride System

Fibrous monolithic ceramics were fabricated in the SiC/BN system. The microstructure consists of high-aspect-ratio polycrystalline cells of SiC separated by cell boundaries of BN. The fibrous monolith with aligned cells fails noncatastrophically in flexure, with delamination cracking along the BN cell boundaries. Indentations cause controlled damage on the surface, but no strength-degrading flaws. The flexural strength is in the range 300–375 MPa, and the estimated shear strength is ∼14 MPa. The SiC/BN fibrous monoliths also show excellent resistance to high-temperature oxidation in air. Noncatastrophic fracture behavior is observed at room temperature after heat treatments between 1200° and 1500°C. The SiC cells on the surface are oxidized to form a protective silicate scale, which prevent deterioration of the BN cell boundaries.     

Fibrous Monolithic Ceramics: IV, Mechanical Properties and Oxidation Behavior of the Alumina/Nickel Systein

Fibrous monolithic ceramics were fabricated in the alumina/nickel system. The microstructure consists of high-aspect-ratio polycrystalline cells of alumina separated by thin cell boundaries of nickel. The nickel content in the material is 3 to 8 vol%. The fibrous monolith with uniaxially aligned cells fails noncatastrophically in flexure. Bridging ligaments of nickel, crack deflection along cell boundaries, and crack branching in the axial direction are observed in flexure bars and notched beams. Strength values range from 246 to 375 MPa. Indentations cause controlled damage on the surface but do not introduce strength-degrading flaws. The alumina/nickel fibrous monoliths also show potential for use at high temperatures in oxidizing environments. Noncatastrophic fracture behavior is observed at room temperature after 10 h at 1200°C in air. The Ni cell boundary network is oxidized to a depth of 50 to 100 μm by this heat treatment. The NiO oxidation product in the cell boundaries reacts partly with alumina from the cells to form NiAI₂O₄, which would provide better protection.     

Oxidation Behavior of SiC-Whisker-Reinforced Alumina-Zirconia Composites

During high-temperature oxidation in air, SiC-whisker-reinforced Al₂O₃—ZrO₂ composites degrade by the formation of a whisker-depleted mullite-zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al₂O₃—ZrO₂—SiC and the results reported in the literature for Al₂O₃—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deeper into the sample.     

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.     

Indirect Determination of the Piezospectroscopic Coefficients of Ceria-Stabilized Tetragonal Zirconia Polycrystals

A method is proposed for the indirect determination of the stress dependence (expressed as piezospectroscopic (PS) coefficients) of spectroscopic bands of ceramic materials/phases. This method is based on the intimate mixture (intimate at the microstructural, grain-size level) of two phases/materials when the stress in one is independently known; it is used to determine the PS coefficients of the most intense Raman bands in ceria-stabilized tetragonal zirconia polycrystals (Ce-TZP). Different amounts of Ce-TZP were mixed with alumina and the composite pellets sintered; subsequently, the stress in alumina was determined through the PS coefficient of its R2 luminescence band and the stress in Ce-TZP derived from the static equilibrium condition. The frequency shifts of each Raman band of Ce-TZP have been plotted against the stress and the slopes provide the PS coefficients. The method has the advantage of not requiring any type of loading device (i.e., diamond anvil-cell, bending jig). Finally, the limits are also discussed, the most important one being the requirement of immiscibility of the two materials/phases.     

Influence of Microstructure and Temperature on the Interfacial Fracture Energy of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

The microstructure and interfacial fracture energy of silicon nitride/boron nitride fibrous monoliths, Gamma_BN, were determined as a function of starting silicon nitride composition and temperature using the method described by Charalambides. The glassy phase created by the sintering aids added to the silicon nitride cells was shown to migrate into the boron nitride cell boundaries during hot-pressing. The amount of glassy phase in the boron nitride cell boundaries was shown to strongly influence Gamma_BN at room temperature, increasing the fracture energy with increasing amounts of glass. Similar trends in the interfacial fracture energy as a function of temperature were demonstrated by both compositions of fibrous monoliths, with a large peak in Gamma_BN observed over a narrow temperature range. For silicon nitride cells densified with 6 wt% yttria and 2 wt% alumina, the room-temperature interfacial fracture energy was 37 J/m², remaining constant through 950°C. A sharp increase in Gamma_BN, to 60 J/m², was observed between 1000° and 1050°C. This increase was attributed to interactions of the crack tip with the glassy phase in the boron nitride cell boundary. Measurements at 1075°C indicated a marked decrease in Gamma_BN to 39 J/m². The interfacial fracture energy decreased with increasing temperature in the 1200° to 1300°C regime, plateauing between 17 to 20 J/m². A crack propagation model based on linkup of existing microcracks and peeling/cleaving boron nitride has been proposed.     

Elevated-Temperature Mechanical Properties of Silicon Nitride/Boron Nitride Fibrous Monolithic Ceramics

A unique, all-ceramic material capable of nonbrittle fracture via crack deflection and delamination has been mechanically characterized from 25° through 1400°C. This material, fibrous monoliths, was comprised of unidirectionally aligned 250 μm diameter silicon nitride cells surrounded by 10 to 20 μm thick boron nitride cell boundaries. The average flexure strengths of fibrous monoliths were 510 and 290 MPa for specimens tested at room temperature and 1300°C, respectively. Crack deflection in the BN cell boundaries was observed at all temperatures. Characteristic flexural responses were observed at temperatures between 25° and 1400°C. Changes in the flexural response at different temperatures were attributed to changes in the physical properties of either the silicon nitride cells or boron nitride cell boundary.     

Thermal Shock Resistance and Fracture Behavior of ZrB2–Based Fibrous Monolith Ceramics

The thermal shock resistance and fracture behavior of zirconium diboride (ZrB₂)-based fibrous monoliths (FM) were studied. FMs containing cells of ZrB₂–30 vol% SiC with cell boundaries composed of graphite–15 vol% ZrB₂ were hot pressed at 1900°C. The average flexure strength of the FMs was 375 MPa, less than half of the strength of hot-pressed ZrB₂–30 vol% SiC. Flexure specimens failed noncatastrophically and retained 50%–85% of their original strength after the first fracture event. A critical thermal shock temperature (Δ T _c) of 1400°C was measured by water quench thermal shock testing, a 250% improvement over the previously reported Δ T _c values for ZrB₂ and ZrB₂–30 vol% SiC of similar dimensions (4 mm × 3 mm × 45 mm). The flexure strength was maintained with Δ T _c values of 1350°C and below. As Δ T _c increased, the stiffness of the flexure specimen decreased linearly. The lower stiffness and improvement in thermal shock resistance is attributed to crack propagation in the cell boundary and crack deflection around the load-bearing cells. The critical thermal shock was attributed to the fracture of the ZrB₂–30% SiC cell material.     

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.     

Effect of SiC Submicrometer Particle Size and Content on Fracture Toughness of Alumina–SiC "Nanocomposites"

A simple model was developed which describes the effect of SiC submicrometer particles on the fracture toughness of alumina–SiC "nanocomposites." This effect was attributed to the change in the fracture mode observed in alumina on adding SiC submicrometer particles, which was suggested to be a result of both matrix weakening and grain boundary strengthening. The model suggests that the increase in fracture toughness should be obtained only for small additions (less than 5 wt%) of SiC.     

Local Measurements of Temperature Changes Associated with the Stress-Induced Tetragonal to Monoclinic Transformation in Zirconia

An experimental arrangement capable of monitoring temperature changes from 0.01 to 0.1 K has been successfully tested for registering the temperature evolution occurring during the tetragonal to monoclinic transformation of an alumina/ceria-stabilized tetragonal zirconia polycrystal (Al₂O₃/Ce-TZP). The arrangement is based on a very small thermistor. The data obtained have been used for evaluating the thermal diffusivity of the Al₂O₃/Ce-TZP composites.     

Effects of Yttrium on the Sintering and Microstructure of Alumina–Silicon Carbide "Nanocomposites"

Alumina and alumina-based "nanocomposites" with 2 and 5 vol% silicon carbide and varying amounts of yttria (0–1.5 wt%) have been prepared by pressureless sintering in the temperature range 1450°–1650°C. The effects of composition and sintering temperature on density and microstructure are reported. Yttria inhibited sintering in alumina, but enhanced the sinterability of the nanocomposites. It also induced abnormal grain growth in both alumina and nanocomposites, but strongly bimodal grain size distributions could be prevented by careful choice of the composition and the sintering temperature. Fully dense (>99%), fine-grained alumina–5 vol% SiC–1.5 wt% yttria nanocomposites were produced from uniaxially pressed powders with a yttria content of 1.5 wt% and a sintering temperature of 1600°C. Reasons for this behavior are discussed, and it is suggested that the enhancement of sintering in the alumina–SiC materials is because of the reaction of silica on the surface of the silicon carbide particles with alumina, yttria, and possibly magnesia, modifying the grain boundary composition, resulting in enhanced grain boundary diffusion. scanning transmission electron microscopy/energy-dispersive X-ray data show that such co-segregation does occur in the yttria-containing nanocomposites.     

Capacitor-Discharge Joining of Oxide Ceramics

A capacitor-discharge joining technique has been used to fabricate alumina–metal–alumina and zirconia–metal– zirconia joints using thin foils of aluminum, titanium, or amorphous Al_x Ni_y Y_z alloys as interlayers. The technique involves passing a high-energy pulse through a conductive interlayer, so converting the interlayer into a liquid-vapor "energized foil" which wets and bonds the pieces of ceramic being joined. The bond strengths of the joints were mea-sured by shear testing which showed that the highest bond strengths for both alumina and zirconia substrates were obtained when an amorphous Al_x Ni_y Y_z interlayer was used. An investigation of the interfacial structure of the joints revealed that there is a distinct reaction layer between the ceramic substrate grains and the amorphous Al_x Ni_y Y_z interlayer.     

髋关节整体替代陶瓷材料

许多材料在医学领域应用广泛,例如,整体替换硬组织或软组织的元件(如骨盆、骨头、关节、植牙等)、修补、诊断或矫正仪器(如起搏器、心脏阀等)。这些材料不仅要有好的力学性能,还要保持长期稳定,不能与人体相排斥。由于陶瓷材料在生理环境中具有强度高、生物相容性强和稳定性好的优点,人们研究用陶瓷材料替换骨骼。从20 世纪70 年代起,欧洲人用陶瓷组件置换整个髋关节。这些组件主要由氧化铝和氧化锆单体制成。然而,在有水环境中,氧化锆会发生低温降解。目前人们的研究重点在于提高陶瓷组件的强度和耐磨性,同时缩小其尺寸并延长其使用寿命。研究中使用的材料是氧化锆增韧的氧化铝复合陶瓷和其它氧化铝复合陶瓷,不再是单体陶瓷。另外,还可以使用氧化铝和氧化锆功能梯度复合材料。该梯度材料可以利用电泳沉积法(EPD)制得,其表面为纯氧化铝,中心部分为均匀的氧化铝、氧化锆复合材料,中间过渡部分是呈连续梯度渐变的氧化铝、氧化锆复合材料,烧成后会产生剩余热应力。设计这样的梯度结构是为了使复合材料具有最大表面压应力和最小内部张应力,与纯氧化铝组件相比,提高了强度和耐磨性。     

Kinked Grain Boundaries in Alumina Doped with Y2O3

Polycrystalline alumina specimens with and without MgO doping show smoothly curved grain boundaries after heat treatment at 1400°C indicating their rough structure. When heat-treated at 1400° and 1500°C for 24 h after packing in an alumina–YAG powder mixture, many grain boundaries (without any liquid phase) develop kinks of large and small scales as observed by scanning electron microscopy and transmission electron microscopy. The addition of Y₂O₃ at concentrations close to the solubility limit is thus shown to induce the grain boundary transition to singular structures.     

Alumina toughened zirconia reinforced with equiaxed and elongated lanthanum hexa-aluminate precipitates

Three different alumina sources (boehmite, aluminium nitrate and α-alumina particles) and 12Ce-TZP powder containing 1 wt% lanthanum oxide were used to prepare 12Ce-TZP-based alumina-toughened-zirconia (ATZ) composites. The obtained ATZs had similar density and phase composition, whereas the microstructures were significantly different. Alumina-particle addition gave rise to a typical ATZ microstructure consisting of equiaxial sub-micrometer zirconia and alumina phases, while the lanthanum hexa-aluminate phase was formed in large and non-homogenously distributed precipitates (∼3.5 μm in length). The boehmite and aluminium nitrate-based composites contained not only sub-micrometer equiaxial alumina and zirconia grains but also small-sized lanthanum hexa-aluminate precipitates (∼1.2 μm in length) that were inter- and transgranularly positioned in the zirconia matrix and effectively promoted crack deflection and toughening. In combination with a higher t-ZrO₂ transformability, the boehmite-based composites had a higher indentation fracture resistance, strength and reliability compared to the aluminium-nitrate and alumina-particle based equivalents.     

Residual Stresses in Alumina/Ceria-Stabilized Zirconia Composites

Measurements of the residual stresses in Al₂O₃/Ce-TZP (12 mol% CeO₂) sintered composites, containing 10, 20, and 40 vol% zirconia, obtained by neutron diffraction and by piezospectroscopy using optical fluorescence and Raman are compared. The techniques give essentially the same values for the spatial average of the hydrostatic residual stresses in the two phases despite the difference in the parameters measured in the two techniques. The measured stresses are also in accord with those predicted from a stochastic stress analysis for materials cooling from a stressfree temperature of ∼1180°C. Over the range of volume fraction investigated the hydrostatic stress in the alumina phase varies linearly with zirconia content, corresponding most closely to the upper Hashin bound.     

Effect of Sintering Aid Composition on the Processing of Si3N4/BN Fibrous Monolithic Ceramics

Si₃N₄/BN fibrous monoliths were prepared with 4 wt% Y₂O₃ added as a sintering aid to the Si₃N₄. Residual carbon, present in the billet before hot-pressing, was shown to influence the final microstructure. The sintering aid glass, known to migrate into the BN cell boundaries during hot-pressing, was not sufficient in quantity to prevent premature shear failure when samples were tested in flexure. Increasing the hot-pressing temperature alleviated this problem. For flexure samples tested at 1400°C, fibrous monoliths fabricated with 4 wt% Y₂O₃ demonstrated linear-elastic loading behavior at a greater stress than fibrous monoliths fabricated with 6-wt%-Y₂O₃/2-wt%-Al₂O₃ sintering aids.     

Stronger and tougher nanosized dense ceria-doped tetragonal zirconia polycrystals by sinter-HIP

Ceria-doped tetragonal zirconia polycrystal (Ce-TZP) ceramics possess significant toughness and aging resistance, while their strength is relatively limited due to the high tendency of grain growth, which has long hindered their versatile applications. This work demonstrates an effective method to combine stereolithography with pre-sintering and hot isostatic pressing (HIP) to obtain dense Ce-TZP ceramics with a reduced grain size of about 420 nm. In addition to the contribution of smaller grains, the Ce-rich phase Ce_0.75Zr_0.25O₂ precipitates at the grain boundaries during HIP hinders grain growth and strengthens the grain boundaries, together leads to a transgranular-dominant fracture mode. As a result, the achieved Ce-TZP ceramic simultaneously reaches a high flexural strength of 770.90 ± 76.06 MPa and an excellent fracture toughness of 11.96 ± 1.31 MPa·m^1/2, which are 48% and 29% higher, respectively, compared to the pressureless-sintered samples. This method is expected to enable broader applications of pure Ce-TZP ceramics and to be further extended for designing and manufacturing densified and fine-grained components with complex structures.