The mechanisms of electrophoretic deposition (EPD) are discussed and their shortcomings identified. The kinetics of the processes involved are analyzed for constant‐current and constant‐voltage conditions. A method of determining the Hamaker constant of suspended particles is developed by modeling the relationship between the particle inter‐action energy and the suspension stability. A three‐probe dc technique is used to map the voltage profile around the depositing electrode, and the results are used to explain discrepancies between the calculated and experimentally observed voltage drops during deposition. A mechanism of deposition is proposed based on DLVO theory and particle double‐layer distortion/thinning on application of a dc field to the suspension. Kinetic equations are developed for constant‐current and constant‐voltage EPD using mass balance conditions; these are verified by experiments. After the phenomenon is introduced and discussed, a critique of the application of EPD to the synthesis of ceramic shapes and coatings is given.

Linear calibration curves were developed for determining the content of free ZrO2 in partially stabilized zirconia ceramics by X‐ray diffraction techniques. Two methods were studied. The matrix method, in which free ZrO2 was considered to be distributed in a matrix (the cubic phase), gave approximately equal mass absorption coefficients for the monoclinic and cubic phases. The polymorph technique, in which the cubic phase was considered to be a polymorph of ZrO2 and in which integrated intensities were used, gave the better results.

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The role of elastic, thermoelastic, and interfacial properties in the toughening of a brittle matrix by metallic second-phase particles was studied. Two composites were studied: glass+ partly oxidized Ni particles (thermal expansion coefficient of the glasses lower than, equal to, and higher than that of Ni) and glass+partly oxidized AJ particles (thermal expansion and elastic moduli equal). Weak interfacial bonding between the nickel and its oxide and developed stress concentrations are the major toughness limitations found in the glass/Ni composites. When the thermal expansion coefficient and elastic modulus of the second phase are sufficiently greater than that of the glass matrix, a propagating crack will bypass the particles. When the thermal and elastic stresses are minimized and satisfactory bonding is achieved (glass/Al composites), a 60x toughness increase was realized.

A model is developed for deposition during the electrophoretic deposition (EPD) process. It suggests that ions that move with the charged particles in suspension are depleted at the depositing electrode, locally changing the pH toward the isoelectric point (pHiep) to give coagulation. The variation of zeta (zeta) potential is modeled via chemical‐equilibrium and surface‐adsorption isotherms. The model successfully fits the experimental data for Al2O3 particles in ethanol when the Freundlich surface‐adsorption isotherm is assumed. Calculations predict the co‐ion concentration gradient as a function of location within the suspension, and the deposition time and its role in the coagulatoin process during EPD.

Partially stabilized zirconia (PSZ) ceramics in the system CaO-ZrOz were characterized. The microstructure, as revealed by optical microscopy, consisted of grains of pure ZrO, distributed in a matrix of fully stabilized material. Electron microscopy showed that the matrix grains have a complex substructure of 10004 domains of cubic and monoclinic ZrO,. The grains appeared to fit Ubbelohde's concept of a hybrid single crystal. Evidence obtained indicated that the substructure provides an effective stress-relieving mechanism during thermal shock. It is proposed that initiation of phase inversion in pure ZrO, domains, even a t subtransition temperatures (by thermal stresses), creates an extremely large microcrack density. On the basis of Hasselman's thermal-shock criterion, only quasi-static crack propagation occurs during thermal shock of PSZ; evidence is presented to support this concept. "L 0 0 20 40 60 MONOCLINIC PHASE, %

widespread skepticism concerning the importance of electroAlumina (Al 2 O 3 ) powders have been dispersed in ethanol statics in organic media" and warned against its negligence. (EtOH) solutions of different acidity. In aqueous media, If a certain degree of electrolyte dissociation can be obtained acidity is defined by pH. This definition can be extended to in a medium of low dielectric constant, and a high surface nonaqueous media using ion-transfer functions. The electripotential be created, significant repulsive forces should be poscal charge on the particle surfaces has been found to be sible. This is feasible in polar organic media, such as the lower acidity dependent. The electrostatic stability of Al 2 O 3 -alcohols, with moderate dielectric constant. EtOH suspensions has been evaluated via electrophoresisThe Derjaguin-Landau-Verwey-Overbeek (DLVO) theory 9 and turbidity. An electrostatic stabilization mechanism is makes it possible to quantify the electrostatic stability of lyoproposed and analyzed via Derjaguin-Landau-Verweyphobic colloids. Although the theory was developed for aqueOverbeek (DLVO) theory.ous systems, it also is applicable to nonaqueous media of low dielectric constant. As the double-layer potential decays slowly I. Introduction in nonaqueous media, the particle surface potential and zeta potential () (the potential at the shear plane) are approximately T HE fabrication of ceramic materials involves powder proequal. 3-5 cessing, shaping, and firing. The reliability of ceramic prodSolid oxide powders in aqueous suspension possess a ucts is dependent on the uniformity of their microstructure.pH-dependent surface electrical charge. Recently, Kosmulski The latter can be accomplished most effectively by processing and Matijevic 10 investigated the zeta potential of silica in a colloidal suspensions. Reproducible and optimized colloidal water-alcohol mixture and demonstrated the effect of pH on processing is dependent on control of the stability of colloidal the zeta potential. suspensions.The present paper is concerned with correlating the pH scales The search for tough structural ceramics has led to the for different solvents and investigating the effect of pH on the development of laminar ceramic composites. Electrophoretic surface charge on Al 2 O 3 particles in ethanolic suspensions. deposition is a powerful technique for the synthesis of laminar Dispersion stability is evaluated by electrophoresis, and turmicrocomposites with a minimum layer thickness of ϳ2 m bidity and particle interaction are analyzed via interactionand an interfacial smoothness of submicrometer scale. 1 energy calculations using the DLVO theory. The traditional medium for ceramic powder dispersion is water. However, electrophoretic deposition from aqueous suspensions has the disadvantage of electrolysis at low potentials.

Yttria-stabilized zirconia (Y SZ)/AI,O, laminar microcomposites with a total of 80 layers, each as thin as -2 pm, were fabricated by electrophoretic deposition. Positively charged YSZ or A1,0, powders were deposited on a cathode, layer by layer from 10 wt% solids/ethanol suspensions. The suspensions were characterized by their rate of deposition as a function of voltage. The deposited samples had a green density of -60% of theoretical. The microstructure of the sintered, theoretically dense samples was Characterized by optical and electron microscopy, and microindentation was used to explore the mechanical properties.