. Corni et al. Journal of the European Ceramic Society 28(2008)1353-136 predict the yield of the electrophoretic deposition process taking atures above 425C. It was found that EPD coatings presented into account the changes of the electric field over the suspension superior smoothness and uniformity compared to those obtained due to the potential drop over the growing deposit. This model by conventional dipping or spraying processes. These coating was validated for Al2O3 suspensions in ethanol with different have found several applications in the industrial production of More recently Van Tassel and Randall28 electrophoretically have summarised that earlier wotk Os and previous reviews concentrations and with addition of HNO3 deposited alumina powder from an acidic suspension obtainin a very uniform, dense alumina layer and observed an anoma- 3. 2. Advanced materials lous voltage rise across the deposited particulate layer. They showed that these two effects can be explained by the forma- In this section the EPD applications of conventional powders tion of an ion depleted conduction layer in the solvent at the e.g. um-sized or submicrometric (d> 100 nm), are reviewed, deposition electrode, which presents an extremely high voltage whereas the applications of nanopowders in EPD are considered gradient. Therefore the electrophoretic force on the particles in in Section 3.3 this layer is considerably higher than the force on particles in the rest of the system and this high voltage gradient layer also 3. 2.1. Coatings and films produces a large self-levelling effect for deposition thickness. First reports on the use of EPD to prepare advanced ceramic inally, Ristenpart et al. ,S have recently studied, both theo- coatings were published in the late 1980s. For example hydrated retically and experimentally, the flow around a charged spherical alumina prepared by the sol-gel method was deposited by EPD colloid next to an electrode in order to understand the nature of on aluminium alloy substrates and it was demonstrated that long-range particle-particle attraction near the electrodes. From these coatings were thicker, denser and more adherent than their studies it was clear that the direction of flow of a particle those produced by conventional dip-coating techniques. In the depends on the sign of the dipole coefficient and that the flow last two decades EPd has been increasingly employed to pro- consists of two components: the electro-osmotic flow (EOF)and duce advanced ceramic coatings on solid substrates in order to the electrohydrodynamic(EHD)flow. The electro-osmotic flow enhance the substrate properties. For example EPD has been is proportional to the current density and the particle s-potent utilized to deposit materials with improved wear and oxidation while the electrodynamic flow derives from the product of the resistance, to deposit bioactive coatings for biomedical implants current density and the applied potential. Comparing these two and to produce functional coatings for electronic, magnetic and components, Ristenpart et al. 29,30 found that the attractive EHD related applications, and key early references are given in pre flow predominated far from the particle, whereas the attractive vious review articles. -, In order to improve the wear and OF predominated over the repulsive EHD flow close to the par- abrasion resistance of materials, research has been also focused ticle. Moreover they also observed that under certain conditions, on the development of metal/ceramic and ceramic/ceramic com- the two flows are both directed toward the particle producing posite coatings For the production of metal/ceramic composite aggregation coatings EPD is usually employed in combination with electro- The novel theoretical and modelling approaches summarised plating or galvanic deposition of metals. 32-37Moreover yttria in the literature to investigate basic phenomena occurring dur- produced on Fecralloys by EPo- Composite coatings have been in this section represent examples of the few efforts available stabilized zirconia (YSZ/alumina ing EPD. We highlight here the necessity for further theoretical quent reaction bonding processes. and densified by a subse nd modelling work in the field of EPD and the need for estab- It is clear that the most difficult task in the production of lishing reliable correlations between model variables and the ceramic coatings on a metal substrates is related to the limited experimental processing EPD conditions temperature capability of the metals and the high temnes'g tures required for sintering the ceramic layer. Wang et al 3. Applications of EPD partially resolved this problem by demonstrating that reaction- bonding is an excellent alternative to conventional sintering 3.. Traditional ceramics Electrophoretic deposition has also been used in ceramic joining applications. Mixtures of SiC or Si3 N4 and reactive carbon were Electrophoretic deposition initially found commercial inter- deposited onto SiC or Si3 N4 parts to provide intermediate layers est and industrial applications for the deposition of uniform for reaction bonding with molten silicon. *U The results obtained coatings made of clay based material, vitreous enamel or by Lessing et al. are significant because they showed for the alumina, on electrically conductive surfaces from aqueous first time how the combination of EPD and reaction bonding suspensions. The use of EPD for the production of clay based allows for the fabrication of large complex ceramic structures bodies, e.g. sanitary ware, tiles, table ware, etc, on an industrial manufactured from smaller components made of SiC or Si3N4 scale has been extensively investigated because of the order of De Riccardis et al. electrophoretically deposited alumina and magnitude improvement in formation rates achieved compared alumina-zirconia coatings with uniform thickness and homo- to slip casting 4,6 EPD has also been employed to produce vitre- geneous composition on stainless steel substrates starting from ous(or porcelain) enamel coatings on metals. After deposition ethanolic suspensions. They extensively studied the suspension of a layer of glass particles, inorganic coatings were obtained properties(conductivity, stability, particle size, transmittance by fusing the powder deposited on the metal surface at temper- and s-potential) to optimize the composition and the amount of1356 I. Corni et al. / Journal of the European Ceramic Society 28 (2008) 1353–1367 predict the yield of the electrophoretic deposition process taking into account the changes of the electric field over the suspension due to the potential drop over the growing deposit. This model was validated for Al2O3 suspensions in ethanol with different concentrations and with addition of HNO3. More recently Van Tassel and Randall28 electrophoretically deposited alumina powder from an acidic suspension obtaining a very uniform, dense alumina layer and observed an anoma￾lous voltage rise across the deposited particulate layer. They showed that these two effects can be explained by the forma￾tion of an ion depleted conduction layer in the solvent at the deposition electrode, which presents an extremely high voltage gradient. Therefore the electrophoretic force on the particles in this layer is considerably higher than the force on particles in the rest of the system and this high voltage gradient layer also produces a large self-levelling effect for deposition thickness. Finally, Ristenpart et al.29,30 have recently studied, both theo￾retically and experimentally, the flow around a charged spherical colloid next to an electrode in order to understand the nature of long-range particle–particle attraction near the electrodes. From their studies it was clear that the direction of flow of a particle depends on the sign of the dipole coefficient and that the flow consists of two components: the electro-osmotic flow (EOF) and the electrohydrodynamic (EHD) flow. The electro-osmotic flow is proportional to the current density and the particle -potential, while the electrodynamic flow derives from the product of the current density and the applied potential. Comparing these two components, Ristenpart et al.29,30 found that the attractive EHD flow predominated far from the particle, whereas the attractive EOF predominated over the repulsive EHD flow close to the par￾ticle. Moreover they also observed that under certain conditions, the two flows are both directed toward the particle producing aggregation. The novel theoretical and modelling approaches summarised in this section represent examples of the few efforts available in the literature to investigate basic phenomena occurring dur￾ing EPD. We highlight here the necessity for further theoretical and modelling work in the field of EPD and the need for estab￾lishing reliable correlations between model variables and the experimental processing EPD conditions. 3. Applications of EPD 3.1. Traditional ceramics Electrophoretic deposition initially found commercial inter￾est and industrial applications for the deposition of uniform coatings made of clay based material, vitreous enamel or alumina, on electrically conductive surfaces from aqueous suspensions.6 The use of EPD for the production of clay based bodies, e.g. sanitary ware, tiles, table ware, etc., on an industrial scale has been extensively investigated because of the order of magnitude improvement in formation rates achieved compared to slip casting.4,6 EPD has also been employed to produce vitre￾ous (or porcelain) enamel coatings on metals. After deposition of a layer of glass particles, inorganic coatings were obtained by fusing the powder deposited on the metal surface at temper￾atures above 425 ◦C. It was found that EPD coatings presented superior smoothness and uniformity compared to those obtained by conventional dipping or spraying processes.6 These coatings have found several applications in the industrial production of domestic whiteware in the early 1970s and previous reviews have summarised that earlier work.1,6 3.2. Advanced materials In this section the EPD applications of conventional powders, e.g. m-sized or submicrometric (d > 100 nm), are reviewed, whereas the applications of nanopowders in EPD are considered in Section 3.3. 3.2.1. Coatings and films First reports on the use of EPD to prepare advanced ceramic coatings were published in the late 1980s. For example hydrated alumina prepared by the sol–gel method was deposited by EPD on aluminium alloy substrates and it was demonstrated that these coatings were thicker, denser and more adherent than those produced by conventional dip-coating techniques.31 In the last two decades EPD has been increasingly employed to pro￾duce advanced ceramic coatings on solid substrates in order to enhance the substrate properties. For example EPD has been utilized to deposit materials with improved wear and oxidation resistance, to deposit bioactive coatings for biomedical implants and to produce functional coatings for electronic, magnetic and related applications, and key early references are given in pre￾vious review articles.1–3,5 In order to improve the wear and abrasion resistance of materials, research has been also focused on the development of metal/ceramic and ceramic/ceramic com￾posite coatings. For the production of metal/ceramic composite coatings EPD is usually employed in combination with electro￾plating or galvanic deposition of metals.32–37 Moreover yttria stabilized zirconia (YSZ)/alumina composite coatings have been produced on Fecralloys by EPD38,39 and densified by a subse￾quent reaction bonding processes.38 It is clear that the most difficult task in the production of ceramic coatings on a metal substrates is related to the limited temperature capability of the metals and the high tempera￾tures required for sintering the ceramic layer. Wang et al.38 partially resolved this problem by demonstrating that reaction￾bonding is an excellent alternative to conventional sintering. Electrophoretic deposition has also been used in ceramic joining applications. Mixtures of SiC or Si3N4 and reactive carbon were deposited onto SiC or Si3N4 parts to provide intermediate layers for reaction bonding with molten silicon.40 The results obtained by Lessing et al.40 are significant because they showed for the first time how the combination of EPD and reaction bonding allows for the fabrication of large complex ceramic structures manufactured from smaller components made of SiC or Si3N4. De Riccardis et al.41 electrophoretically deposited alumina and alumina–zirconia coatings with uniform thickness and homo￾geneous composition on stainless steel substrates starting from ethanolic suspensions. They extensively studied the suspension properties (conductivity, stability, particle size, transmittance and -potential) to optimize the composition and the amount of