Edge Stresses in Alumina/Zirconia Laminates

Using the technique of fluorescence piezospectroscopy, we determine the distribution of thermal residual stresses across the edges of three laminated alumina/zirconia composites. We develop a methodology for separating the measured stress state into microstresses that result from grain-to-grain thermal mismatch and macrostresses that result from lamination-induced thermal mismatch between individual plies. Comparison between the measured edge-stress distributions and those calculated based on a simple force-superposition model shows good agreement, indicating that the laminate system is well approximated as linear elastic. Given the experimental confirmation of significant edge stresses in multi-ply laminates, the possibility of failure initiating at composite edges must be considered in the design of surface-compressed laminate structures with the aim of mediating the detrimental effect of surface flaws.     

Processing of Tape–Cast Laminates Prepared from Fine Alumina/Zirconia Powders

This paper addresses the effect of thermocompression pressure, shear deformation of green laminates, and postsinter HIPing on the microstructural homogeneity of cast tapes and laminates prepared from fine Al₂O₃, and Al₂O₃/ ZrO₂, powders. Green density increases with increasing thermo–compression pressure. Sintered densities, however, depend more on the macroscopic uniformity in the green tapes. When density gradients develop within the individual green tapes (because of improper drying), sintering is constrained in two dimensions and densities remain low. Postsinter HIPing does not significantly increase the sintered densities because of the retention of open porosity within the individual tape–cast layers. The use of a revised thermocompression process involving shear deformation results in higher sintered densities and complete densification after HIPing. Sintered densities increase with the degree of shear strain during green–state deformation processing. Thus, green-state deformation can improve homogeneity in laminates. A further variation of the shear deformation process has also been developed that allows the formation of complex shapes from tape–cast laminates in the green state, while retaining layer integrity.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, II: Viscoelastic Stress Computation

To analyze the inhibited densification during sintering and differential shrinkage during cooling of Al₂O₃/ZrO₂symmetric and asymmetric laminates, viscoelastic formulations, in which the viscosity and elastic modulus vary with time, have been developed. The viscoelastic mismatch stresses have been numerically computed over the entire processing cycle, including the heating period, the isothermal period, and the cooling period. The viscosity and free sintering rates that are needed for stress computation have been obtained by modifying the parameters that are measured for a normal isotropic densifying compact using cyclic loading dilatometry. The modification is based on the available sintering models to account for the effect of strain history on compact viscosity and sintering rates. The stress calculation shows that, with the exception of the initial heating period, the viscoelastic stress is identical to the viscous stress that is calculated solely from the strain rate mismatch. Sintering damage in the laminates is shown to occur during densification under conditions where the differential sintering stress is smaller than the intrinsic sintering pressure. The magnitude of residual stress in hybrid laminates on cooling is dependent on the cooling rate, and slower cooling rates are capable of almost completely relaxing the expansion mismatch stress at temperatures of >1200°C.     

Constrained Densification of Alumina/Zirconia Hybrid Laminates, I: Experimental Observations of Processing Defects

Various forms of damage were observed in pressure-less-sintered Al₂O₃/ZrO₂symmetric laminates and asymmetric laminates (bilayers) fabricated by tape casting and lamination. These defects included channel cracks in the ZrO₂ layers, Al₂O₃ edge-effect cracks parallel to the layers, delamination in the Al₂O₃layers, and debonding between the Al₂O₃and ZrO₂layers. Based on detailed microscopic observations, the defects were attributed to sintering rate and thermal expansion mismatch between the layers. Cracks or cracklike defects were formed in the early stages of densification, and these cracks either opened during sintering or acted as preexisting flaws for thermal expansion mismatch cracks. Consequently, the extent of cracking could be reduced or even eliminated by decreasing mismatch stresses during the sintering and cooling stages. This can be accomplished by reducing the heating and/or cooling rates or by adding Al₂O₃in the ZrO₂layers. The sintering mismatch stresses were estimated from the degree of curling in asymmetric laminates and from layer viscosities that were obtained by cyclic loading dilatometry. The measured curvature was an indication of the mismatch in sintering strain between Al₂O₃and ZrO₂and were consistent with the dilatometric data that were obtained for the component layers.     

Fracture Behavior of Alumina/Monazite Multilayer Laminates

Monazite (LaPO₄) has been proposed as an interphase to promote debonding between the reinforcement and the matrix during the fracture of oxide-based composites. The correlation between fracture behavior and micromechanical properties in model alumina/monazite (Al₂O₃/LaPO₄) multilayer laminates has been investigated in this study. The delamination fracture energy (Γ_i) was dependent on crack length, which is consistent with previous results; the initial value of Γ_i was ∼10 J/m². The interfacial frictional sliding resistance increased as the normal stress on the interface increased. Using a Coulombic friction model, the coefficient of static friction between the Al₂O₃ and LaPO₄ layers was determined to be 0.63. The influence of Γ_i and flaw size in the Al₂O₃ layers on fracture path has been predicted, using an existing model, and confirmed experimentally. The results indicate that, in addition to satisfying energy-based fracture criteria, several other factors affect whether LaPO₄ is a suitable interphase for oxide composites.     

Diffusion Bonding of Zirconia/Alumina Composites

ZrO₂/Al₂O₃ composites with from 0% to 100% Al₂O₃ content were diffusion bonded at 12.5 MPa for 30 min in the temperature range 1450° to 1500°C. Under appropriate bonding conditions, a bonding strength greater than 1000 MPa was achievable between dissimilar materials with different thermal expansion coefficients.     

Alumina and Alumina/Zirconia Multilayer Composites Obtained by Slip Casting

The slip casting technique has been revealed as a powerful method to obtain multilayer composites close to theoretical density. From zeta potential and viscosity measurements of Al₂O₃ and Al₂O₃/ZrO₂ (4 vol% ZrO₂) suspensions, the corditions for the preparation of multilayer composites by slip casting have been determined. A microstructural analysis of the different layers by scanning electron microscopy is also reported.     

反应烧结法制备ZTM/Al2O3陶瓷材料的研究

通过测定试样的烧结密度、烧成收缩率、原料细度、两种研磨方法和两种烧成制度下抗弯强度和韧性,对工业上利用ZrSiO_4与AL_2O_3原位反应,制备还原海绵铁高温炉膛用ZTN/AL_2O_3陶瓷材料进行了研究。结果表明,球磨与过长的研磨时间都不能提高超细原料的细度。AL_2O_3含量较高的试样,原位反应烧结所产生的体积膨胀较小,试样的性能越高。两步烧成制度有利于反应烧结试样性能的提高。     

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.     

Alumina/Zirconia Micro/Nanocomposites: A New Material for Biomedical Applications With Superior Sliding Wear Resistance

In the present investigation, the sliding wear behavior is described for Al₂O₃/ZrO₂ micro/nanocomposites and monolithic alumina of similar grain size under defined conditions of a constant sliding speed and different loads (20–150 N). Nano ZrO₂ particles (1.7 vol%) were observed uniformly distributing throughout the composites, and most of them were located within the matrix alumina grains. The wear rate of the alumina and the micro/nanocomposites increased as the contact load increased and a clear transition in friction and wear behavior was observed in both materials. However, the nanocomposite wear resistance at low contact loads was one order of magnitude higher than that of the alumina. In the severe regime, no difference was observed among the materials. The low wear rate (10⁻⁷ mm³·(N·m)⁻¹) along with low pullout indicates higher wear resistance of micro/nanocomposites in the mild regime compared with monolithic alumina. Based on the morphological observation of worn surfaces by scanning electron microscope and on residual stress analysis performed by neutron diffraction, some wear mechanisms of Al₂O₃–ZrO₂ micro/nanocomposites are proposed. The high wear resistance of the nanocomposites is discussed in terms of fracture resistance properties and residual stress. Improvements in mechanical and tribological properties of these composites make them promising candidates for biomedical applications.     

Mechanically Induced Zone Darkening of Alumina/Ceria-Stabrlized Zirconia Composites

Alumina/ceria-stabilized zirconia bars tested in four-point bending exhibited deformation bands on the tensile side due to localized tetragonal-to-monoclinic transformation. These bands became increasingly darker and were fully visible some weeks after the bending experiment. XPS spectra revealed that the darker regions are characterized by a higher concentration of cerium in the + 3 oxidation state, as opposed to other undeformed regions, in which + 4 is the predominant oxidation state. A numerical calculation established that residual stresses exist in the deformation bands. It is suggested that these stresses facilitate the reduction of cerium with the gradient in stress motivating the diffusion of oxygen vacancies.     

铝锆质骨料的显微结构对耐火材料的耐热震性的影响

为提高耐火材料的耐热震性，进行了添加铝锆颗粒的试验。由于铝锆颗粒内部的显微结构的不同，因此对耐热震性的影响也不同。在耐火材料中添加了氧化铝初晶细小、氧化铝和二氧化锆的共晶部分呈镶嵌状组织的颗粒一，由于这种颗粒自身不容易破裂，因此能提高耐火材料的耐热震性。     

Zirconia/Alumina Functionally Gradiented Composites by Electrophoretic Deposition Techniques

An electrophoretic deposition and sintering route was used to prepare YSZ/Al₂O₃ composites with a compositional gradient. The YSZ content was continuously decreased from the YSZ-rich surface to the Al₂O₃-rich surface, Microstructural and Vickers hardness (16–24 GPa) evidence tracked the compositional development, and the indentation fracture toughness was found to vary across the section (10–3 MPa·m^1/2).     

Tape Casting of Fine Alumina/Zirconia Powders for Composite Fabrication

Ceramic films, containing AI₂O₃, with up to 40 vol% ZrO₂, have been fabricated using the tape casting process. Finer powders (average mean diameter of 250–300 nm) than have generally been reported for tape casting were used in this study. The optimum formulation for tape casting is affected substantially by decreasing particle size. For example, the amount of dispersant needed is increased. Moreover, the amount of plasticizer/binder must be increased so as to maintain the solids content in the dried tapes below a critical level (about 55 vol% in this case), which decreases with particle size. Rheological studies on the effectiveness of menhaden fish oil and phosphate ester as dispersants show that phosphate ester can be used in lower concentrations, for the preparation of higher solids loading slurries, and was therefore selected for further study. The amount of dispersant required to obtain minimum slurry viscosity was found to be primarily dependent upon the effective particle surface area, defined as that available to the dispersant molecules. In the case of particles composed of agglomerated crystallites (such as the ZrO₂, powder used here), this may be considerably less than that measured by nitrogen absorption. Moreover, the porous internal structure of such powders is filled with solvent, which increases the effective solids loading of the slurry, and thus its viscosity. Particle morphology also influences the packing efficiency; i.e., the green density decreases as ZrO₂, is added.     

Activation Energy for the Sintering of Two-Phase Alumina/Zirconia Ceramics

In an earlier paper we reported measurements of the activation energy for sintering from constant-heating-rate experiments with alumina/5% zirconia. Here, results from a full range of compositions in this two-phase system are described. They show that the activation energy remains in the range 700 ± 100 kJ/mol when the composition changes from 5 to 95 vol% zirconia. In comparison, pure zirconia sinters with an activation energy of 615 ± 80 kJ/mol and pure alumina with the energy of 440 ± 45 kJ/mol. The addition of 2.8 mol% yttria to zirconia does not have a measurable effect on the activation energy. The grain size dependence of the sintering rate suggests boundary-diffusion-controlled sintering. These activation energies are phenomenologically correlated with the interfacial energies in alumina, zirconia, and two-phase alumina/zirconia, suggesting that the bonding at the interface influences diffusional transport.     

ZTA复相陶瓷凝胶注模成形工艺的研究

本文研究了ＺｒＯ２（３Ｙ）－Ａｌ２Ｏ３复相陶瓷的凝胶注模成形工艺,着重研究了低粘度高固相体积分数浓悬浮体的制备。     

Effect of Dynamic Compaction on the Retention of Tetragonal Zirconia and Mechanical Properties of Alumina/Zirconia Composites

Dynamic consolidation techniques were employed to investigate the retention of tetragonal zirconia and degree of consolidation in alumina/zirconia powder compacts. Heating the specimens prior to explosive shock compaction increased the tetragonal-phase retention significantly. Low shock pressures yielded no macrocracking, although final densities were low (60% to 70% of the theoretical density). Heat treatment following dynamic consolidation enhanced the retention of the tetragonal zirconia polymorph regardless of the shock pressure employed. Compact densities were increased to over 90% of theoretical at relatively low sintering temperatures (1300°C). Hardness, toughness, and Young's modulus of the compacts were comparable to those achieved in composites that were synthesized using more conventional techniques. Dynamic compaction offers an alternative method for the fabrication of zirconia-toughened alumina ceramics.     

CF/PPS复合材料层板电阻焊接研究

针对航空结构用碳纤维增强聚苯硫醚(CF/PPS)复合材料层板采用电阻焊接进行了实验研究,电阻热元件选择了CF/PPS混编织物,重点研究了输入功率、输入能量、焊接时间与压力对焊接接头性能的影响。结果表明,焊接过程中,CF/PPS混编织物加热元件在焊接结合面上产生的温度场两端略高于中间,整体温度分布均匀;在1~1.5 MPa压力范围下和80~170 kW/m^2的功率范围内,可获得质量良好的焊接接头;通过电阻焊接实现最大剪切强度(均值17.92 MPa)与模压制备基准试样剪切强度(均值17.88 MPa)相当;剪切断口观测分析表明,焊接质量良好的焊接接头主要失效形式为层内失效破坏;同时,提出了采用CF/PPS混编织物作为电阻元件焊接CF/PPS复合材料层板的最佳工艺窗口。     

Anisotropic Ionic Conductivity Observed in Superplastically Deformed Yttria-Stabilized Zirconia/Alumina Composite

Ionic conductivity measurements on a yttria-stabilized tetragonal zirconia polycry stall alumina composite subjected to superplastic deformation demonstrate anisotropic character. Parallel to the pressing direction, the grain-boundary resistance to oxygen ion mobility is 25% to 30% higher than that measured perpendicular to the pressing direction. The same directional dependency on the volume conductivity is observed but is less pronounced, showing approximately a 9% difference. Microstructural evidence reveals an agglomeration and elongation of alumina particles perpendicular to the pressing direction, and it is suggested that this phenomenon restricts the passage of ions parallel to the compression direction, giving rise to the anisotropic nature of the conductivity measurements.     

Roles of Alumina in Zirconia for Functional Applications

The Al₂O₃ addition to stabilized ZrO₂ has been studied for more than 20 years. In this article, literature and new results on the positive and negative effects of Al₂O₃ additions on the electrical properties of ZrO₂ are summarized and analyzed. In particular, a comprehensive grain-boundary conduction model is proposed. The Al₂O₃ addition always increases the bulk resistivity, mainly because of the formations of defect associates and insulating Al₂O₃ second-phase particles. The Al₂O₃ addition within the solubility limit increases the grain-boundary resistivity, as a result of increased grain-boundary space-charge potential; the Al₂O₃ addition above the solubility limit, however, scavenges the silicon-rich second phase from the grain boundaries, thereby decreasing the grain-boundary resistivity.