氧化铝/莫来石复合陶瓷断裂行为研究

用X射线衍射方法测定了氧化铝/莫来石复合陶瓷的残余应力,并通过计算加以验证。在莫来石含量比较小的情况下,氧化铝/莫来石复合陶瓷基体拉应力与莫来石体积含量成线性关系。通过模型分析了氧化铝基体和莫来石颗粒的应力状态及其对裂纹扩展的影响。由莫来石颗粒引入的基体拉应力使裂纹倾向于向晶内扩展。观察了氧化铝/莫来石复合陶瓷断裂方式的转变,计算了穿晶断裂比率与基体应力的关系,进而建立了莫来石含量、基体应力、穿晶断裂比率三者的对应关系。这为复合陶瓷材料的制备和性能分析提供了可靠基础。     

Interfacial behavior on Al2O3/HAYNES® 214™ joints fabricated by solid state bonding technique with Ni or Cu–Ni–Cu interlayers

The residual stresses due to the difference in thermal expansion between ceramic and metal is a significant parameter to control during the fabrication of ceramics/metal joint. In this work, residual stress distribution, after solid state bonding of different joints, was measured using X-ray diffraction (XRD) and Vickers Indentation Fracture (VIF) methods. Tensile stress concentration in alumina caused by the thermal expansion mismatch in the Al₂O₃/Ni/Ni alloy (HAYNES^® 214?) joint severely deteriorated the assembly and caused cracks in alumina. To solve this problem, this paper shows that the use of a Cu/Ni/Cu multi-layer, associated with the direct copper bonding method (DCB), by pre-oxidation of copper, reduces significantly the tensile residual stresses in alumina material. Consequently, this process offers the possibility of producing an interlayer with a high melting temperature and hence joints which can withstand high-temperatures.     

Micro- and macroscopic thermal expansion of stabilized aluminum titanate

The lattice expansion of aluminum titanate (AT) obtained by firing a mixture of alumina, rutile, strontium and calcium carbonate and silica was measured using neutron and laboratory X-ray diffraction. The microscopic data are compared with macroscopic measurement completed by dilatometry.A powder and a compact rod sample were compared to assess the influence of micro residual stresses locked into the solid structure at grain level, which could possibly be relieved upon grinding.Results show good correlation between neutron and X-ray diffraction techniques. They also show that a compact material behaves differently than a powder, contrary to what happens for other porous ceramics such as cordierite. The integrity factor model was used to rationalize the results and predict grain level stresses in all crystal directions and all phases (AT, Strontium aluminum silicate, alumina and residual glass). Calculation show that the AT c-axis is always under compression while all other crystal directions and phases are under tension. Those micro-stresses do not undermine the macroscopic mechanical properties of the material and confer to it its interesting properties like low thermal expansion and enhanced strain tolerance.     

In situ quantitative three-dimensional characterisation of sub-indentation cracking in polycrystalline alumina

The three-dimensional deformation beneath a Vickers indentation in polycrystalline alumina has been measured, in situ, by digital volume correlation of high resolution synchrotron X-ray computed tomographs. The displacement fields at the peak indentation load and after unloading are used to study the shape and orientation of sub-surface cracks induced by the indentation; lateral cracking due to residual stresses, bounded by a system of radial cracks, is revealed. For the first time, it is shown that radial cracks have mixed mode opening displacements, which are affected by the relaxation of residual stresses via lateral cracking. This novel technique may find applications in the study of surface damage by abrasive wear in brittle materials.     

In Situ Measurements of Frictional Bridging Stresses in Alumina Using Fluorescence Spectroscopy

Fracture of a large-grained alumina polycrystal has been examined in situ by optical microscopy. Concurrently, local bridging stresses, as generated by friction or tension of unbroken ligaments in the wake of the crack path, were measured by piezospectroscopy. Stress measurements were performed both at fixed sites as a function of the external load and at a fixed external load along the crack profile. Frictional stresses were similar/congruent50 MPa, while unbroken ligaments between the crack faces were found to support tensile stresses up to similar/congruent100 MPa. The maximum bridging stress was dictated by the weak (intrinsic) interface bonding of the polycrystal. Average bridging stresses, either theoretically calculated from R -curve data or experimentally measured by piezospectroscopy on frictional/bridging sites, were similar. Such a circumstance enables us to explain the fracture behavior of polycrystalline alumina by considering crack-wake shielding as the main micromechanism contributing to toughening.     

Highly Dispersive Carbon Nanotube/Alumina Composites and their Electrospun Nanofibers

Noncovalent functionalization approach was used to make negatively charged carbon nanotubes (CNTs) suspension in water medium. Stable alumina/CNT sol solution was consequently formed by simple titration process, which allowed high dispersibility of CNTs in final composite powders and final CNT/alumina (CA) nanocomposites. Moreover, these CA composite powders were transformed to continuous composite fibers via electrospinning technique. Very significant shear stresses exerted on alumina were well recorded and reflected by shifts of X-ray diffraction peaks. The as-fabricated CA fibers possess a specific surface area 30 times larger than that of pure poly(acrylic acid) fibers. The axial alignment of CNTs observed in the composite fibers is promising for developing textured ceramics, and also for the study of the effect of orientation of fillers on the mechanical and functional properties.     

Comparison of Texture Analysis Techniques for Highly Oriented α-Al2O3

Texture measurements were performed on liquid-phase-sintered alumina textured by a templated grain growth process from 1250° to 1650°C. Texture distributions were measured using X-ray pole figures, rocking curves, Rietveld refinement, and stereology. The March–Dollase equation fitted the measured distributions very well and gave quantitative values of the degree of texture and the texture fraction. The fitting parameters of the X-ray diffraction measurements were comparable to those measured by stereology. Rocking curve analysis was found to be straightforward and to give accurate characterization of texture in the alumina system of this study in a relatively short time.     

Thermal residual stress gradients in an alumina–zirconia composite obtained by electrophoretic deposition

In order to understand the mechanical behavior of layered composites with compositional gradient, it is necessary to determine their state of residual stresses. Compositionally graded materials can offer the advantage of eliminating abrupt changes in composition between layers having different thermal expansion coefficient. The existence of a compositional gradient can reduce discontinuities in thermal residual stresses, something beneficial from the point of view of the mechanical properties.We present here a study of the microstructure and state of residual stressses in a layered material made of homogeneous layers of alumina and alumina–zirconia separated by thin (less than 300 μm) intermediate compositionally graded layers. The composite was obtained by controlled deposition of powders from solution using an electrophoretic deposition (EPD) method. The phase distribution and compositional gradient in the sintered composite were investigated using scanning electron microscopy (SEM). Thermal residual stresses generated during cooling after sintering were measured by using fluorescence ruby luminiscence piezo-spectroscopy and the profile of hydrostatic stress on alumina was determined at steps of about 300 μm along the direction of the compositional gradient, and at steps of about 30 μm in the compositionally graded layers. The obtained profile of hydrostatic stresses on alumina grains follows closely the profile of compositional changes along the layered composite. The presence of thin intermediate graded layers reduce significantly changes in stress in the layered composite.     

Phase Distribution and Residual Stresses in Melt-Grown Al2O3-ZrO2(Y2O3) Eutectics

Al₂O₃-ZrO₂ eutectics containing 0 to 12.2 mol% Y₂O₃ (with respect to zirconia) were produced by directional solidification using the laser floating zone (LFZ) method. Processing variables were chosen to obtain homogeneous, colony-free, interpenetrating microstructure for all of the compositional range, optimum from the viewpoint of mechanical properties. The amount of cubic, tetragonal, or monoclinic zirconia phases was determined using a combination of Raman and X-ray diffraction techniques. Monoclinic zirconia was present up to concentrations of 3 mol% Y₂O₃, while the amount of tetragonal zirconia gradually increased with yttria content up to 3 mol%. Cubic zirconia was the only phase detected when the yttria content reached 12 mol%. The residual stresses in alumina were measured using the shift of the ruby R lines. Compressive stresses were isotropic when measured in the samples containing tetragonal and cubic zirconia, while higher tensile, anisotropic stresses were found when monoclinic zirconia was present. They were partially relieved in the eutectic sample without yttria. These results were compared with a thermoelastic analysis based on the self-consistent model.     

Residual Stresses in Machined MgO Crystals

The residual stresses introduced in MgO crystals by grinding on {100} surfaces in 〈100〉 directions were measured using photoelastic techniques. Grinding was conducted with two wheels; a 100-grit diamond wheel removed material by brittle fracture, and a 46-grit alumina wheel caused plastic flow and burnishing. Both wheels introduced a discrete, highly deformed layer adjacent the machined surface. In all cases the machined surfaces were under a residual tensile stress which became compressive within the deformed region. Beneath the deformed layer the residual stress patterns were distinctly different. In crystals ground with the alumina wheel the stresses became tensile again within 0.5 mm of the ground surface, whereas the subsurface stresses in crystals ground with the diamond wheel remained compressive to distances ≥1 mm. These residual stress distributions are discussed in terms of a simple model based on the superposition of mechanically and thermally induced stresses.     

A Direct Method of Measuring Bridging Stresses

A direct method of measuring bridging stresses using a double-notched plate (DNP) specimen is developed. Bridging stresses are measured via the stable fracture of the DNP specimen using the bridging stabilizer developed in this study. The opening displacement of the fracture surfaces can be evaluated from the deformation of the bridging stabilizer and the DNP specimen. As an example of the application, the bridging stresses of polycrystalline alumina are measured under monotonic load. The bridging characteristics that have been measured using the present method are compared with those obtained via the other methods.     

Simulation of crack propagation in Alumina particle-dispersed SiC composites

Addition of alumina particles to silicon carbide results in strongly improved toughness values. In order to come to a better understanding of this phenomenon, crack propagation is simulated for a 20 vol% alumina particles-dispersed silicon carbide composite material using the Body Force Method. Special emphasis is paid to the influence of graded compositions. Numerically obtained crack paths are compared to crack paths generated experimentally by Vickers indentations. Moreover, mechanical properties of the investigated material were measured experimentally. Microstructural toughness variations as well as the direction of crack propagation are found to be strongly influenced by residual stresses due to the mismatch between thermal expansion coefficients of alumina and silicon carbide and by the actual crack location. According to tensile residual stresses in the radial direction cracks approaching a particle are deviated circumferentially in the matrix around the particle. Moreover, the failure behavior of cracks propagating into a zone of increasing or decreasing volume fraction of alumina particles is found to behave differently as residual stress fields superimpose in the case of particle clustering. ©.     

Effect of Joint Thickness and Residual Stresses on the Properties of Ceramic Adhesive Joints: II, Experimental Results

The residual stresses in joints were varied by varying the thickness and thermal expansion mismatch of joints prepared using alumina adherends and silicate glass adhesives. An apparent fracture toughness was measured by the single-edge notchedbeam method; strengths were measured in flexure, and the fracture surfaces were studied. Stress distributions, determined using the finite element method in Part I, together with the results of literature analyses for stresses in joints subjected to externally applied loads, were used to aid in interpreting the experimental observations. The measured fracture toughness and strength of ceramic adhesive joints increase with decreasing adhesive thickness and decrease with increasing thermal expansion mismatch (residual stress), both positive and negative.     

Microstructure,phase and dielectric strength of thermally sprayed alumina layers in coating-based heating systems

In coating-based resistive heating systems applied on an electrically conductive substrate, an intermediary dielectric layer is required to minimize or eliminate leakage of current from the heating surface to the substrate. Aluminum oxide has widely been used for electrical insulation purposes. In this study, alumina was deposited onto carbon steel substrates by using suspension plasma spraying and flame spraying processes. Then, Ni-20Cr, as the heating element, was deposited on the alumina layer by using air plasma spraying. The resultant microstructure and phase composition of the alumina layers were evaluated by using scanning electron microscope images, X-ray diffraction analysis, and Raman spectroscopy. The breakdown voltage of alumina layers was measured independently and within the heating system. It was found that the dielectric properties of alumina layers were significantly affected by its microstructural features. It was observed that penetration of metal topcoat into the alumina layer decreased the overall breakdown voltage of the system.     

Critical Stresses for Extension of Filament-Bridged Matrix Cracks in Ceramic-Matrix Composites: An Assessment with a Model Composite with Tailored Interfaces

Matrix cracking was studied in a model unidirectional composite of SiC filaments in an epoxy-bonded alumina matrix. The residual clamping stress on the filaments due to the shrinkage of the epoxy was moderated with the addition of the alumina filler, and the filament surface was coated with a releasing agent to produce unbonded frictional interfaces. Uniaxial tension specimens with controlled through-cracks with bridging filaments were fabricated by a two-step casting technique. Critical stresses for extension of the filament-bridged cracks of various lengths were measured in uniaxial tension using a high-sensitivity extensometer. The measured crack-length dependence of the critical stress was in good agreement with the prediction of a stress-intensity analysis that employed a new force-displacement law for the bridging filaments. The analysis required independent experimental evaluation of the matrix fracture toughness, the interfacial sliding friction stress, and the residual tension in the matrix. The matrix-cracking stress for the test specimens without the deliberately introduced cracks was significantly higher than the steady-state cracking stress measured for the long, filament-bridged cracks.     

Crack-Tip-Bridging Stresses in Ceramic Materials

A simple approach is presented for the determination of crack-tip-bridging stresses in ceramic materials. The technique utilizes the difference in compliance predicted for a crack of known size and that measured. The bridging stresses for an alumina material that displays considerable R -curve behavior are calculated from measured compliance and crack-length observations. The results are almost identical with those recently reported in the literature. Additional measurements on a more complex "duplex" ceramic that exhibits a very substantial R -curve due to a crack-branching chain reaction are also presented.     

Plastic Flow of Saturated, Consolidated Alumina Powder Compacts: Particle Size and Binary Mixtures

The flow stresses and relaxed yield stresses of saturated, alumina powder compacts that have been consolidated via pressure filtration were measured in unconstrained uniaxial compression. Two different sized powders, as well as binary mixtures of the two powders, were investigated. Bodies that were consolidated from slurries of larger particles had lower flow and relaxed yield stresses, relative to bodies that were made of small particles with the same relative density and solution conditions. This result is primarily due to the lower number of particle-particle contacts per unit volume in the body that was made of the large particles. The flow stress of the body can be controlled by adjusting the fraction of large particles to small particles.     

The role of residual stresses in layered composites of Y–ZrO2 and Al2O3

Laminar composites, containing layers of Y–TZP and either Al₂O₃ or a mixture of Al₂O₃ and Y–ZrO₂ have been fabricated using a sequential centrifuging technique of water solutions containing suspended particles. Controlled crack growth experiments with notched beams of composites were done and showed the significant effect of barrier layer thickness and composition on crack propagation path during fracture. Distinct crack deflection in alumina layers was observed. The increase of crack deflection angle with the alumina layer thickness was also found. In the case of the barrier layer made of a mixture, crack deflection did not occur independently on layer thickness. The observed changes have been correlated with the radial distribution of residual stresses in barrier layers created during cooling of sintered composites from fabrication temperature. The stresses found were the result of the difference in the thermal expansion and sintering shrinkage of alumina and zirconia and the crystallographically anisotropic thermal expansion of the alumina. The residual stress distribution has been measured by piezo-spectroscopy based on the optical fluorescence of Cr⁺³ dopants in alumina.     

Characterization and modeling of microstructural stresses in alumina

Brittle failure is often influenced by difficult to measure and variable microstructure‐scale stresses. Recent advances in photoluminescence spectroscopy (PLS), including improved confocal laser measurement and rapid spectroscopic data collection have established the potential to map stresses with microscale spatial resolution (< 2 μm). Advanced PLS was successfully used to investigate both residual and externally applied stresses in polycrystalline alumina at the microstructure scale. The measured average stresses matched those estimated from beam theory to within one standard deviation, validating the technique. Modeling the residual stresses within the microstructure produced qualitative agreement in comparison with the experimentally measured results. Microstructure scale modeling is primed to take advantage of advanced PLS to enable its refinement and validation, eventually enabling microstructure modeling to become a predictive tool for brittle materials.