1002 A.R. Boccaccini et al. Composites: Part A 32(2001)997-1006 3. A case in point: Ni-coated carb alumina matrix composite 3.1. Description of the EPD experimental procedure The feasibility of fabricating Ni-coated carbon fibre rein- forced alumina matrix composites via a single-infiltration EPD process is considered here, as a typical example of the application of EPD technique in the fabrication of ceramic A commercially available boehmite (Y-AlOOH) sol (Remet corp, USA, Remal A20) having 40 nm average ek U particle size was used as alumina source. The sol contains 20 wt% solids-loading and the boehmite particles are in the lath shape. The as-received boehmite sol was seeded with EM micrograph of a metallic fibre mat which has been fully nanosize(13 nm)8-alumina powder(Aluminium Oxide C ltrated by silica sol using EPD. The working voltage was 4 V and Degussa AG, Germany) containing 0.5 wt% a-alumina deposition time 2 min [52]- (BDH Chemicals, UK). The seeding powder was first dispersed in distilled water, then the dispersion was added to the boehmite sol while this was stirred magnetically preforms was conducted by using silica nanoparticles in Finally, the seeded boehmite sol was ball-mixed for 12 h colloidal suspensions [53, 54]. In these composites, the using high purity TzP balls in a plastic container. EPD porous silica deposit was used to provide adequate Ni-coated carbon fibres(Inco spp, Incofiber, 12K50 matrix/fibre interfacial bonding and to avoid possible UK) were used as reinforcement. These fibres were in the reactions between the silicate glass matrices and the form of continuous tows of Ni-coated single carbon fibres, metallic fibres with 12,000 tows coated to 50% by weight with nickel. The Using EPD, the infiltrated metallic fabrics were suffi- nickel coating provided excellent conductivity which is iently infiltrated with matrix material so as to be used as essential for EPD, as well as ease of fibre handling and preforms for the fabrication of metal fibre reinforced cera- adequate wettability. The tows were unidirectional aligned mic or glass matrix composites. An example of a metallic in a grooved perspex frame. EPD experiments were carried fibre mat, fully infiltrated by silica sol using EPD, is shown out under vacuum. Ni-coated carbon fibres held in the frame in Fig 4[52]. The working voltage was 4 V and deposition were used as the deposition electrode(cathode). Two stain- time 2 min less steel plates on either side of the cathode served as the It must be pointed out that the reinforcement of ceramic positive(anode) electrodes. After the fibre preform was matrices by continuous ductile elements has not been as placed in the sol, the system was vacuum degassed to much investigated as their ceramic-ceramic counterparts, remove any entrapped air, and then the cell electrodes despite the advantages they may offer. These include were connected to a 0-60v dc power supply. EPD was performed subsequently under constant voltage conditions due to the intrinsic ductility of metallic fibres and the possi- (5, 10, 15 and 20 V)using varying deposition times( from 50 bility of exploiting their plastic deformation for composite to 500 s). An electrode separation distance of 15 mm was toughness enhancement [55] used in all the experiments. Under the applied electric field, Overall, it appears that EPD is a very versatile and cost- very fine bo effective technique to infiltrate complex fibre architectures, surface charge, as determined from the EM data, migrated paving the way for the development of ceramic and glass towards the negative electrode, i. e. the Ni-coated carbon matrix composites with 2D and 3D reinforcement. More- fibre tows The particles infiltrated the fibre tows and depos over, due to the high matrix homogeneity and high relative ited until a sufficient matrix thickness, which enveloped the density achieved in the green bodies by EPD, several e tows, was achieved. The fibre preform acting authors showed that the required subsequent densification electrode was connected to a balance linked to a computer [32,35, 40,42,44,51,52, 53], avoiding, therefore, e tering The apparatus is able to record the weight gain per milli- second during the deposition process, i.e. in real time. The intensive traditional hot-pressing route. Specific experimen- dimensions of the cathode (25 X 25 mm") were half the al information about the EPd fabrication of Ni-coated anode's dimensions (50 X 50 mm]) in order to eliminate carbon fibre reinforced alumina matrix composites, a system the ' edge effect which may give an inhomogeneous being developed currently by the authors, is given below. thickness from the centre to the edges of the cathode. The The objective is to describe the processing steps and to show EPD-prepared green body specimens containing about 25 optimisation of the EPD arameters 30 vol% fibre loading were dried under humidity controlled    (  (        (  <A>AE= +     !#$  (    (    '(  I                    
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