Nickel–Alumina Functionally Graded Materials by Electrophoretic Deposition

Electrophoretic deposition has been used to synthesize nickel–alumina, functionally graded materials from NiO and alumina suspensions in ethanol. Suspension stability and the kinetics of deposition were studied to determine optimum conditions. Deposition starts with an alumina suspension into which a stream of NiO suspension is injected at various flow rates to obtain the desired profiles. The latter were controlled by varying the deposition current density and component flow rate. The green bodies obtained were sintered in Ar/H₂ atmosphere to reduce the NiO to nickel. Various gradation profiles were obtained illustrating the facility of EPD to synthesize continuously graded materials. NiO was used as the precursor for nickel to alleviate settling and rough columnar deposit problems.     

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.     

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.     

Fabrication of Net-Shape Functionally Graded Composites by Electrophoretic Deposition and Sintering: Modeling and Experimentation

It is shown that electrophoretic deposition (EPD) sintering is a technological sequence that is capable of producing net-shape bulk functionally graded materials (FGM). By controlling the shape of the deposition electrode, components of complex shapes can be obtained. To enable sintering net-shape capabilities, a novel optimization algorithm and procedure for the fabrication of net-shape functionally graded composites by EPD and sintering has been developed. The initial shape of the green specimen produced by EPD is designed in such a way that the required final shape is achieved after sintering-imposed distortions. The optimization is based on a special innovative iteration procedure that is derived from the solution of the inverse sintering problem: the sintering process is modeled in the "backward movie" regime using the continuum theory of sintering incorporated into a finite-element code. The experiments verifying the modeling approach include the synthesis by EPD of Al₂O₃/ZrO₂ 3-D (FGM) structures. In order to consolidate green parts shaped by EPD, post-EPD sintering is used. The fabricated deposits are characterized by optical and scanning electron microscopy. The experimentally observed shape change of the FGM specimen obtained by EPD and sintering is compared with theoretical predictions.     

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.     

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。     

电泳沉积母胶法碳纳米管/天然橡胶 复合材料的性能研究

用氢氧化钠和过氧化氢改性碳纳米管（CNTs）得到氧化CNTs(OCNTs),再用硅烷偶联剂KH-550改性OCNTs得到氮掺杂CNTs(NCNTs)。采用电泳沉积法制备CNTs/天然橡胶（NR）、OCNTs/NR和NCNTs/NR母胶后,再用机械共混法制得CNTs/NR复合材料,将其与直接采用机械共混法制得的CNTs/NR复合材料进行性能对比。结果表明：电泳沉积法能够使CNTs在NR中均匀分散,提高CNTs/NR复合材料的热稳定性;OCNTs/NR复合材料的300%定伸应力和拉伸强度最大,热稳定性最好,但生热和滚动阻力较高;NCNTs/NR复合材料的300%定伸应力和拉伸强度较大,抗湿滑性能最好,生热和滚动阻力最低,综合性能最好。     

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.     

Electrophoretic Deposition of Zirconia on Porous Anodic Substrates

The aim of the present work is the preparation of thin (<20 μm) zirconia layers on porous substrates with the electrophoretic deposition process. The preparation was completed with a cosintering step of substrate and layer. Through adjustment of shrinkage and the shrinkage rate of the deposited zirconia layer on the presintered porous substrate, thin, dense layers without cracks were prepared. A method for direct control of the layer thickness during the electrophoretic deposition process was developed. The solid oxide fuel cell application with porous anode substrates and thin zirconia electrolytes was chosen to demonstrate the potential of the electrophoretic deposition process.     

Influence of Washing on Zirconia Powder for Electrophoretic Deposition

The effects of powder washing on the colloidal stability and electrophoretic deposition rate of 3-mol%-Y₂O₃-doped tetragonal-ZrO₂ powder from ethanol suspensions are shown experimentally. Simultaneous acoustic and electroacoustic measurements make it possible to determine independently particle size and zeta-potential in concentrated ethanol suspension. It is suggested that ionic (Na⁺) surface contamination, which is reduced by laundering cycles, is the cause of the low zeta-potential and deposition rate of the as-received powder.     

Electrophoretic Deposition Forming of Nickel-Coated-Carbon-Fiber-Reinforced Borosilicate-Glass-Matrix Composites

The present paper introduces a novel processing technique that involves in situ electrophoretic deposition (EPD), followed by pressureless sintering, to produce dense, defect-minimized, carbon-fiber-reinforced borosilicate-glass-matrix composites with a nickel interface. The process relies on the deposition of submicrometer-sized, colloidal charged particles onto unidirectionally aligned nickel-coated carbon fibers. The preparation and characterization of a kinetically stable nanosized borosilicate sol suitable for EPD are described. The most-important EPD processing parameters in the formation of dense, fully infiltrated, green-body compacts are described, and issues that concern the infiltration of very tight carbon fiber preforms are discussed and effectively solved. Using the crack-path-propagation test, the metallic nickel interface is determined to be very effective to improve the composite mechanical performance, in terms of the nonbrittle fracture behavior. Catastrophic crack growth is prevented by such mechanisms as constrained plastic deformation of the interface and fiber debonding and pullout. The proposed processing technique has great potential to fabricate defect-minimized and damage-tolerant fiber-reinforced brittle-matrix composites with a ductile interface. Overall, this new approach offers a cost-effective and short-time processing route for the fabrication of continuous-fiber-reinforced ceramic-matrix composites.     

Fabrication of Yttria-Stabilized Zirconia Coatings Using Electrophoretic Deposition: Packing Mechanism During Deposition

Electrophoretic deposition (EPD) of yttria-stabilized zirconia (YSZ) particles on metal substrates has been examined as a function of EPD conditions and suspension properties. The packing density of particles, i.e., volume occupation of particles in a wet deposit, depends on the deposition time, applied voltage, and the electrical conductivity and solid concentration of the suspension. In addition, the packing density of particles in a wet deposit further increased when the deposit was immersed in a pure acetylacetone solvent with application of a constant electric field. The packing mechanism of YSZ particles in the wet deposit was discussed based on the electro-osmotic flow mechanism of YSZ particles during EPD.     

Room-Temperature Aging of Laminate Composites of Alumina/3-mol%-Yttria-Stablilized Tetragonal Zirconia Polycrystals

The stresses of laminate structures obtained by joining single layers of pure alumina (A), pure yttria-stabilized tetragonal zirconia, 3Y-TZP (Z), and an intimate mixture of alumina and zirconia (AZ) have been determined by fluorescence (in alumina) and Raman (in zirconia) piezospectroscopy. Three symmetrical stacking sequences were examined, namely, A/Z/A, A/AZ/A, and AZ/Z/AZ, with the aim of designing structures where the higher coefficient of thermal expansion (CTE) of zirconia could be used to induce compressive stress in the external layers (and ensuing tensile stress in the central layer). Two experimental sessions, 6 years apart, were conducted on the same samples, also taking care to record the spectra from the same locations; during the time elapsed between the two sessions, the samples were kept at room temperature and humidity. The stress values in alumina obtained during the more recent session were markedly different from those observed in the first session. Monoclinic zirconia ( m -zirconia) was absent in all samples in the first session, whereas up to 25 vol% zirconia could be observed during the second session. m -Zirconia could only be observed in AZ layers and not in Z layers, irrespective of the position in the stacking sequence. It was concluded that 3Y-TZP underwent spontaneous tetragonal-to-monoclinic ( t – m ) transformation, that is, "aging," when mixed with alumina at the grain-size level. Aging occurred only where pristine t -zirconia was subject to tensile stresses larger than ∼400 MPa.     

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.     

片状模板对电泳沉积制备织构化氧化铝的影响

本文研究不同片状氧化铝模板含量和形态对电泳沉积方法制备织构化氧化铝陶瓷的影响.结果表明,不同形态片状模板的加入都会使氧化铝的密度下降,并且模板含量增加越多,密度下降也越大.在加入量超过6％后能制备出具有一定取向的氧化铝陶瓷,但加入量进一步提高时,由于堆积密度的下降,晶粒尺寸也有所下降.不同形态的片状模板对织构化形成的影响较大,径厚比越低的模板越有利于致密化的提高,但是径厚比较大的模板更有利于织构化结构的形成.     

Influence of Zirconia Interfacial Coating on Alumina Fiber‐reinforced Alumina Matrix Composites

The present study demonstrates an approach for fabricating fiber‐reinforced ceramic matrix composites (CMCs) involving the coating of 2‐dimensional woven alumina fibers with zirconia layer by sol gel, followed by impregnation of these coated fibers with alumina matrix and pressureless sintering. To emphasize the benefits of the zirconia coating on these CMCs, a reference sample without interfacial coating layer was prepared. The zirconia‐coated CMCs showed superior flexural strength and thermal shock resistance compared with their uncoated counterparts. Foreign object damage tests carried out on the ZrO₂ coated CMCs at high impact speed showed localized damage without any shattering.     

Sintering and Microstructure Modification of Mullite/Zirconia Composites Derived from Silica-Coated Alumina Powders

This paper addresses the densification and microstructure development during firing of mullite/zirconia composites made from silica-coated-alumina (SCA) microcomposite powders. Densification occurs in two stages: in the presence of a silica–alumina mixture and after conversion to mullite. The first stage of densification occurs through transient viscous phase sintering (TVS). This is best promoted by rapid heating, which delays the crystallization of silica to higher temperatures. A further sintering stage is observed following mullitization. The introduction of seeds promotes solid-state sintering, most probably due to refinement of the mullite matrix. For seed concentrations up to about 1% the sintering kinetics depend on seed concentration. This suggests that nucleation still remains the rate-controlling mullitization step. Above this concentration the reaction becomes growth controlled. Introduction of seeds also promotes direct mullitization without transient zircon formation that was observed in a previous study of the same process without seeding. Seeding also promotes the development of elongated grains by way of a solid-state recrystallization process.     

Constant Current Electrophoretic Infiltration Deposition of Fiber-Reinforced Ceramic Composites

Modified electrophoretic infiltration deposition (EPID) under constant current conditions is used to fabricate non-conductive-fiber-reinforced composites from an ethanol suspension. Particles are infiltrated through a fiber preform by an electric field and are deposited on the front of the electrode and "backfill" through the fiber preform. A uniform morphology is achieved at the optimum deposition rate.
The constant current EPID process is modeled as capillary infiltration electrophoresis. Particles "stream" the fiber preform due to the repulsive interaction between the fiber filaments and the particles as both have the same-sign surface charge. Electro-osmotic flow makes no contribution to deposit yield as the net flow across a closed capillary cross section is zero. Hamaker's law is extended to electrophoretic infiltration; however, the total deposit yield is controlled by particle electrophoresis outside the capillaries due to the much lower electric field in the suspension. The deposit thickness increases linearly with time under optimum current conditions. Too high a deposition rate promotes air entrapment in the depositing green body.