A.R. Boccaccini, 1. Zhitomirsky/ Current Opinion in Solid State and Materials Science 6(2002)251-260 combination with electroplating or galvanic deposition of 2. 4. Coatings on fibres, porous substrates and composite coatings (20, "217. production of metal/ ceramic membranes In recent developments, yttria stabilized zirconia/alumina composite coatings were pro- EPD is being used increasingly to coat fibres and porous duced on Fecralloys by Wang et al. [22 by using EPI ubstrates with ceramic materials for applications ranging and a reaction bonding processes. Certainly, the main from filters and porous carriers to bioactive coatings and difficulty of producing ceramic coatings on metal hollow fibre fabrication The successful use of epd for strates using Epd is that the metal substrates cannot coating carbon and metallic fibrous substrates with alumina withstand the high temperature required for sintering the and titania nanopowders was shown by Boccaccini et al deposited ceramic coating. Therefore optimized densifica- [37]. The coated porous fibrous structures are intended for tion techniques requiring lower temperatures must be high-temperature filtration of fluids, where the titania or developed. Wang et al. showed that reaction bonding is an alumina coatings act as oxidation and corrosion protective excellent alternative [22]. Further R&D efforts are how- layers. A recent paper by Zhitomirsky [38] demonstrates ever required in this important technological area the coating of carbon fibres with hydroxyapatite(HA) EPD has been recently used in ceramic joining applica- After burning out the fibrous carbon substrates hollow HA tions Mixtures of SiC or Si3, and reactive carbon were fibres of various diameters were obtained. EPD has been deposited onto SiC or Sis, parts in preparation for used by Su et al. [39 to deposit lead zirconate titanate reaction bonding with molten silicon [23]. The results of (PZT)films(<5 um)on Pt wires. In this case, EPD allows Lessing et al. are significant as they show for the first time for the use of PzT nanoparticles directly from hydrother how the combination of EPD and reaction bonding allow mal suspensions, thus avoiding agglomeration of particles for the fabrication of large complex structures manufac and resulting in lower sintering temperatures tured from smaller components made of Sic or Si,N4 Membranes and porous materials have also been pre *23] pared recently by EPD techniques, including zeolite [40] The availability of ceramic nanopowders in numerous and alumina [41] membranes. The electrophoretic assem- compositions enables the use of EPD to prepare dielectric, bly of nanozeolites has been investigated by Ke et al. [42] magnetic, semiconducting and superconducting ceramic They were able to prepare hollow zeolite fibres by coating thick films for a variety of applications in electronics. nanozeolites onto carbon fibres and subsequent burn-out of Moreover EPD allows for the fabrication of engineered the carbon core. Significant related research focusing on non-planar structures made of functional ceramics which EPD of zeolites was carried out by Ahlers et al. [43], who find application developed a method for fabricating optimized zeolite- Current work in these areas is leading to encouraging modified electrodes for applications in electroanalysis and results and therefore merit further R&D efforts. Some electrocatalysis recent significant developments include the fabrication of BaTiO, thick films for sensor and actuator applications 2.5. Fibre reinforced ceramic matrix composites 4, Zno thick films for gas sensors [25], zirconia films on porous Lao. Sro. MnO, substrates [26], MgO-modified EPD is a simple and cost-effective method for Bao Sro. 4TiO, thick films for tunable microwave devices ing high-quality fibre reinforced ceramic matrix [27, LiCoo, electrodes for rechargeable lithium batteries ites. In this application, EPD is used to infiltrate preforms [28], phosphor screens for plasma display panel applica- with tight two- or three-dimensional fibre architectures tions [29], photocatalytic titania coatings [30] and Mgo using nanosized ceramic particles. A recent comprehensive hick films for electronics [31, 32 review article reveals the great variety of conducting and Furthermore, the fabrication of high-temperature non-conducting fibre and matrix combinations that have conducting films of controlled thickness on substrates of been explored, including SiC, carbon, and oxide ceramic various shapes and dimensions by EPD is gaining increas- fibre architectures and silica, borosilicate glass, alumina, ing interest [33, *34]. The significant advantages of EPD zirconia, mullite, hydroxyapatite, SiC and Si3 N, matrices over other coating techniques for continuous fabrication of [**44]. Most recent work has been devoted to Ni-coated superconducting coatings include the suitability of EPD to carbon fibre reinforced alumina [45], borosilicate glass be scaled up and adapted to the coating of large areas as matrix composites [46], C-fibre reinforced Sic matrix well as the high deposition rate that can be achieved composites [47] and silica/silica composites [48]. More- over EPD has been recently shown to be an excellent A final area of successful application of EPD in coating pre-infiltration step for decreasing processing time of chnology is in the biomedical materials field. In par- chemical vapour infiltration of SiC-fibre reinforced Sic ticular, the improvement of the EPD technique for deposi- matrix composites [491 tion of bioactive hydroxyapatite and related calci Both aqueous and non-aqueous suspensions have been phosphate films on biocompatible metallic substrates(e.g. used and the different factors affecting the EPD behavior TiAl4V alloys) has been recently reported [35, 36 of ceramic sols and their optimization to obtain highA.R. Boccaccini, I. Zhitomirsky / Current Opinion in Solid State and Materials Science 6 (2002) 251–260 253 combination with electroplating or galvanic deposition of 2 .4. Coatings on fibres, porous substrates and metals, is being used for the production of metal/ceramic membranes composite coatings [20,*21]. In recent developments, yttria stabilized zirconia/alumina composite coatings were pro- EPD is being used increasingly to coat fibres and porous duced on Fecralloys by Wang et al. [*22] by using EPD substrates with ceramic materials for applications ranging and a reaction bonding processes. Certainly, the main from filters and porous carriers to bioactive coatings and difficulty of producing ceramic coatings on metal sub- hollow fibre fabrication. The successful use of EPD for strates using EPD is that the metal substrates cannot coating carbon and metallic fibrous substrates with alumina withstand the high temperature required for sintering the and titania nanopowders was shown by Boccaccini et al. deposited ceramic coating. Therefore optimized densifica- [37]. The coated porous fibrous structures are intended for tion techniques requiring lower temperatures must be high-temperature filtration of fluids, where the titania or developed. Wang et al. showed that reaction bonding is an alumina coatings act as oxidation and corrosion protective excellent alternative [*22]. Further R&D efforts are how- layers. A recent paper by Zhitomirsky [*38] demonstrates ever required in this important technological area. the coating of carbon fibres with hydroxyapatite (HA). EPD has been recently used in ceramic joining applica- After burning out the fibrous carbon substrates hollow HA tions. Mixtures of SiC or Si N and reactive carbon were fibres of various diameters were obtained. EPD has been 3 4 deposited onto SiC or Si N parts in preparation for used by Su et al. [39] to deposit lead zirconate titanate 3 4 reaction bonding with molten silicon [*23]. The results of (PZT) films (,5 mm) on Pt wires. In this case, EPD allows Lessing et al. are significant as they show for the first time for the use of PZT nanoparticles directly from hydrother￾how the combination of EPD and reaction bonding allow mal suspensions, thus avoiding agglomeration of particles for the fabrication of large complex structures manufac- and resulting in lower sintering temperatures. tured from smaller components made of SiC or Si N Membranes and porous materials have also been pre- 3 4 [*23]. pared recently by EPD techniques, including zeolite [40] The availability of ceramic nanopowders in numerous and alumina [41] membranes. The electrophoretic assem￾compositions enables the use of EPD to prepare dielectric, bly of nanozeolites has been investigated by Ke et al. [42]. magnetic, semiconducting and superconducting ceramic They were able to prepare hollow zeolite fibres by coating thick films for a variety of applications in electronics. nanozeolites onto carbon fibres and subsequent burn-out of Moreover EPD allows for the fabrication of engineered the carbon core. Significant related research focusing on non-planar structures made of functional ceramics which EPD of zeolites was carried out by Ahlers et al. [*43], who find applications in microsystems technologies. developed a method for fabricating optimized zeolite￾Current work in these areas is leading to encouraging modified electrodes for applications in electroanalysis and results and therefore merit further R&D efforts. Some electrocatalysis. recent significant developments include the fabrication of BaTiO thick films for sensor and actuator applications 2 .5. Fibre reinforced ceramic matrix composites 3 [24], ZnO thick films for gas sensors [25], zirconia films on porous La Sr MnO substrates [26], MgO-modified EPD is a simple and cost-effective method for fabricat- 0.9 0.1 3 Ba Sr TiO thick films for tunable microwave devices ing high-quality fibre reinforced ceramic matrix compos- 0.6 0.4 3 [27], LiCoO electrodes for rechargeable lithium batteries ites. In this application, EPD is used to infiltrate preforms 2 [28], phosphor screens for plasma display panel applica- with tight two- or three-dimensional fibre architectures tions [29], photocatalytic titania coatings [30] and MgO using nanosized ceramic particles. A recent comprehensive thick films for electronics [31,32]. review article reveals the great variety of conducting and Furthermore, the fabrication of high-temperature super- non-conducting fibre and matrix combinations that have conducting films of controlled thickness on substrates of been explored, including SiC, carbon, and oxide ceramic various shapes and dimensions by EPD is gaining increas- fibre architectures and silica, borosilicate glass, alumina, ing interest [33,**34]. The significant advantages of EPD zirconia, mullite, hydroxyapatite, SiC and Si N matrices 3 4 over other coating techniques for continuous fabrication of [**44]. Most recent work has been devoted to Ni-coated superconducting coatings include the suitability of EPD to carbon fibre reinforced alumina [45], borosilicate glass be scaled up and adapted to the coating of large areas as matrix composites [46], C-fibre reinforced SiC matrix well as the high deposition rate that can be achieved composites [47] and silica/silica composites [48]. More- [**34]. over EPD has been recently shown to be an excellent A final area of successful application of EPD in coating pre-infiltration step for decreasing processing time of technology is in the biomedical materials field. In par- chemical vapour infiltration of SiC-fibre reinforced SiC ticular, the improvement of the EPD technique for deposi- matrix composites [49]. tion of bioactive hydroxyapatite and related calcium Both aqueous and non-aqueous suspensions have been phosphate films on biocompatible metallic substrates (e.g. used and the different factors affecting the EPD behavior TiAl4V alloys) has been recently reported [35,36]. of ceramic sols and their optimization to obtain high