KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX Y-AlOOH -C rAl2O3→6Al2O3 Al M-ALO where w is the weight of charged particles deposited Therefore, the final sintered microstructure of alu- per unit area of electrode, C, the solids-loading of the nina derived from pure boehmite is very porous after suspension, Eo the permittivity of vacuum, e, the rela- even sintering at 1600oC for a long time(6 h), if the tive permittivity of the liquid, S the zeta potential of sol is not seeded [2]. On the contrary, when boehmite medium, E the applied potential, L the distance sol is seeded with crystallographically suitable modi fiers, high-density alumina components with con between the two electrodes and t the deposition time trolled microstructure are achievable at relatively low From this equation, it is clear that for a given suspen- sion, the zeta potential, dielectric constant, viscosit sintering temperatures(1100-1300C)[2, 4, 5]. Seed- and solids-loading are the critical factors determining been tried [2, 4, 5]. Because the addition of seeds to the EPD behaviour. In order to obtain high hetero- boehmite gels enhances the e-o transformation, com. coagulated-particle EPD rates, high E(water is ideal) plete densification of a-Al_O, occurs at temperatures and s values, together with low viscosity, are neces- as low as 1180%C with a grain size of only 0.43 um. sary as particle mobility within the suspension will Electrophoretic deposition (EPD) is as a novel, be enhanced. However, to produce a homogeneous relatively simple, high forming-rate technique for pro- green infiltrated microstructure, it is also crucial to lucing ceramic components [6-18 This infiltration optimise the solids-loading of the suspension without process relies on the presence of small charged par- The main objective of this study is to show the ticles in a liquid. l.e. a sol, which, on the aplication critical steps in producing Ni-coated carbon fibre- of an electric field, will move and deposit on an reinforced alumina matrix composites using the EPD oppositely charged electrode. This technique requires only low-cost equipment and offers new possibilities technique. The experiments were carried out in vac- for the design of ceramics monoliths or fibre- uum. EPD parameters in terms of applied voltage and reinforced composites with more uniform microstruc- deposition time were examined and optimised. More- tures [19]. It has been established that the EPD pro- sol with nanosize 8-alumina powder is described in cess can be utilised to infiltrate woven or non-woven fibre ceramic preforms [10, 13, 17, 19, 20). This tech- order to lower the sintering temperature of alumina. nique allows the ceramic medium to effectively fill Crack deflection behaviour at the ductile ni interface the inter- and intra-tow regions of fibres of small was examined crack path propagation test on diameters, which may be in close proximity (to the sintered composite sample point of touching in some cases). The requirement for full infiltration of the fibre preforms is that the 2. EXPERIMENTAL WORK infiltrating ceramic be in the form of a sol, e. g. nanos zed particles in suspension, to enable them to pen- 2.1.Materials etrate in between the closely spaced fibres. Commer- A commercially available boehmite(y-AIOOH)sol cial silica sols have been utilised with success to (Remet corp, USA, Remal A20) having 40 nm aver- produce SiC fibre reinforced silica matrix system [21] age particle size was used as the alumina source.The and in woven stainless steel fibre mat reinforced glass sol contains 20 wt% solids-loading and the boehmite [22, 23]. Moreover, commercial boehmite sol has particles are in the lath shape. The as-received been used in a previous work to fabricate metal fibre boehmite sol was seeded with 0.5 wt% nanosize reinforced alumina matrix composites [17] (13 nm)8-alumina(Aluminium Oxide C, Degussa gener be ine eoced ss noificane y he te ceanic Ag Germany and d - alumina (BDi i h eo scal s uk) sitional homogeneity and stability of the starting col- mina and 0.5% a-alumina. The seeding powder was lodal suspension and by the EPD fabrication para- first dispersed in distilled water, then the dispersion meters. To obtain a uniformly infiltrated green was added to the boehmite sol whilst this was stirred microstructure, EPD requires a kinetically stable, magnetically. Finally, the seeded boehmite sol was well-dispersed suspension having the highest possible ball-mixed for 12 h using high purity Tzp balls in a solids-loading but a relatively low viscosity, which plastic container. affects the particle electrophoretic mobility, and Nickel coated carbon fibres(Inco spp, IncofiberM, hence, deposition efficiency [9]. Furthermore, the 12K50, UK) were used as reinforcement. These fibres of the electrodefibre is infuenced strongly by the were in the form of continuous tows of nickel coated single carbon fibres. Ni was deposited using a gas process time, electrode separation and applied poten- plating technology. Fibre diameter and nickel coating tial, according to the following equation [15] us and had values of 10-15 an1190 KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX gAlOOH → 400° CgAl2O3 → 800° CdAl2O3 → 1000° Cq (1) Al2O3 → 11001200° CaAl2O3 Therefore, the final sintered microstructure of alu￾mina derived from pure boehmite is very porous after even sintering at 1600°C for a long time (>6 h), if the sol is not seeded [2]. On the contrary, when boehmite sol is seeded with crystallographically suitable modi- fiers, high-density alumina components with con￾trolled microstructure are achievable at relatively low sintering temperatures (1100–1300°C) [2, 4, 5]. Seed￾ing with γ-Al2O3, α-Al2O3 and α-Fe2O3 particles has been tried [2, 4, 5]. Because the addition of seeds to boehmite gels enhances the θ–α transformation, com￾plete densification of α-Al2O3 occurs at temperatures as low as 1180°C with a grain size of only 0.43 µm. Electrophoretic deposition (EPD) is as a novel, relatively simple, high forming-rate technique for pro￾ducing ceramic components [6–18]. This infiltration process relies on the presence of small charged par￾ticles in a liquid, i.e. a sol, which, on the application of an electric field, will move and deposit on an oppositely charged electrode. This technique requires only low-cost equipment and offers new possibilities for the design of ceramics monoliths or fibre￾reinforced composites with more uniform microstruc￾tures [19]. It has been established that the EPD pro￾cess can be utilised to infiltrate woven or non-woven fibre ceramic preforms [10, 13, 17, 19, 20]. This tech￾nique allows the ceramic medium to effectively fill the inter- and intra-tow regions of fibres of small diameters, which may be in close proximity (to the point of touching in some cases). The requirement for full infiltration of the fibre preforms is that the infiltrating ceramic be in the form of a sol, e.g. nanos￾ized particles in suspension, to enable them to pen￾etrate in between the closely spaced fibres. Commer￾cial silica sols have been utilised with success to produce SiC fibre reinforced silica matrix system [21] and in woven stainless steel fibre mat reinforced glass [22, 23]. Moreover, commercial boehmite sol has been used in a previous work to fabricate metal fibre reinforced alumina matrix composites [17]. In general, the properties of the sintered ceramic matrix will be influenced significantly by the compo￾sitional homogeneity and stability of the starting col￾loidal suspension and by the EPD fabrication para￾meters. To obtain a uniformly infiltrated green microstructure, EPD requires a kinetically stable, well-dispersed suspension having the highest possible solids-loading but a relatively low viscosity, which affects the particle electrophoretic mobility, and hence, deposition efficiency [9]. Furthermore, the number of charged particles deposited per unit area of the electrode/fibre is influenced strongly by the process time, electrode separation and applied poten￾tial, according to the following equation [15]: W 2 3 Ci 0rz 1 h E L t (2) where w is the weight of charged particles deposited per unit area of electrode, Ci the solids-loading of the suspension, 0 the permittivity of vacuum, r the rela￾tive permittivity of the liquid, ζ the zeta potential of the particles, η the viscosity of the suspension medium, E the applied potential, L the distance between the two electrodes and t the deposition time. From this equation, it is clear that for a given suspen￾sion, the zeta potential, dielectric constant, viscosity and solids-loading are the critical factors determining the EPD behaviour. In order to obtain high hetero￾coagulated-particle EPD rates, high  (water is ideal) and ζ values, together with low viscosity, are neces￾sary as particle mobility within the suspension will be enhanced. However, to produce a homogeneous green infiltrated microstructure, it is also crucial to optimise the solids-loading of the suspension without causing flocculation [9]. The main objective of this study is to show the critical steps in producing Ni-coated carbon fibre￾reinforced alumina matrix composites using the EPD technique. The experiments were carried out in vac￾uum. EPD parameters in terms of applied voltage and deposition time were examined and optimised. More￾over, the seeding process of a commercial boehmite sol with nanosize δ-alumina powder is described in order to lower the sintering temperature of alumina. Crack deflection behaviour at the ductile Ni interface was examined using crack path propagation test on sintered composite samples. 2. EXPERIMENTAL WORK 2.1. Materials A commercially available boehmite (γ-AlOOH) sol (Remet corp, USA, Remal A20) having 40 nm aver￾age particle size was used as the 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 0.5 wt% nanosize (13 nm) δ-alumina (Aluminium Oxide C, Degussa AG, Germany) and α-alumina (BDH Chemicals, UK) powders. The seeding material contains 99.5% δ-alu￾mina and 0.5% α-alumina. The seeding powder was first dispersed in distilled water, then the dispersion was added to the boehmite sol whilst this was stirred magnetically. Finally, the seeded boehmite sol was ball-mixed for 12 h using high purity TZP balls in a plastic container. Nickel coated carbon fibres (Inco spp, Incofiber, 12K50, UK) were used as reinforcement. These fibres were in the form of continuous tows of nickel coated single carbon fibres. Ni was deposited using a gas plating technology. Fibre diameter and nickel coating were very homogeneous and had values of 10–15 and