E Stoll et al. /Joumal of the European Ceramic Society 26(2006)1567-1576 Approaching trajectory Fibre Nextel 720 F.: Stroke s friction force 0O Relaxation force m Oscillatory motion of the particle due to particle and fibres Direction of growth of the same charge Repulsion force between fibre and partide with the same charge. Fig 10. Schematic diagram describing the motion of positively charged a-Al2O3 particles as they infiltrate a positively charged Nextel M 720 fibre mat during distance away from the electrodes, where the particles move cles reach the electrode or the surface of previously deposited externally applied electric field. The second region, the"e particles, they have no further possibility to move and so the towards the deposition electrode only under influence of th electrophoretic ceramic deposit grows with a high particle filtration trajectory, occurs close to the deposition electrode packing density. For the case where fibre and particle have here the charge of the fibres influences the motion of the opposite surface charge, on the contrary, it is expected that harged particles. In the ideal case of a stable suspension, coagulation will occur on the first layer of fibres encountered the particles move as separate entities(no agglomeration), by the travelling particle. Consequently, the formation of a and they all move simultaneously in random manner due to deposit on the outer fibre layer will block or at least make e attraction and repulsions forces between them(dVlo- more difficult the movement of the particles towards the in- theory ). A simple linear motion of the particle under the terior of the fibre mat, which could result in poor infiltration external voltage can be assumed in the"approaching trajec- and low quality microstructure of the green bod In the"infiltration trajectory teraction forces appear between particles and the charged 42. Comparison of the two processes fibres. In the present case, where fibres and particles pos sess the same charge, there are repulsion forces(F1) acting Both techniques developed here are convenient routes to between particles and fibres as shown in Fig. 10. For each the manufacture of multilayer NextelTM720fibre-reinforced particle, these forces depend on the relative distance between alumina matrix composites. The method based on the simul- particles and fibres. Due to the applied external electrical taneous Epd of several fibre mats resulted in higher fibre vol field, each positively charged particle is attracted to the fi- ume fraction, higher matrix density and more homogeneous bre mat because this is fixed to the cathode. However, the matrix microstructure than the standard method of epd of particles are repelled before they can reach the fibre surfaces single fibre mats As assessed by SEM, less structural damage (coagulation point) due to the positive charges on the fibres. developed in the matrix during sintering. In addition, damage It can be hypothesised that under the effect of the repulsive was minimised due to less extent of manipulation of the green forces due to the surrounding fibres, the particles will follow body, as the intermediate steps of forming the composite by the path with the fewest possible obstacles until reaching the lamination were eliminated. This benefit becomes apparent next interstice between adjacent fibres. Thus, when the parti- when comparing Figs. &b and 9. Nevertheless, the advantage1574 E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 Fig. 10. Schematic diagram describing the motion of positively charged -Al2O3 particles as they infiltrate a positively charged NextelTM 720 fibre mat during EPD. distance away from the electrodes, where the particles move towards the deposition electrode only under influence of the externally applied electric field. The second region, the “in- filtration trajectory”, occurs close to the deposition electrode where the charge of the fibres influences the motion of the charged particles. In the ideal case of a stable suspension, the particles move as separate entities (no agglomeration), and they all move simultaneously in random manner due to the attraction and repulsions forces between them (DVLO￾theory).17 A simple linear motion of the particle under the external voltage can be assumed in the “approaching trajec￾tory”. In the “infiltration trajectory”, however, additional in￾teraction forces appear between particles and the charged fibres. In the present case, where fibres and particles pos￾sess the same charge, there are repulsion forces (F1) acting between particles and fibres as shown in Fig. 10. For each particle, these forces depend on the relative distance between particles and fibres. Due to the applied external electrical field, each positively charged particle is attracted to the fi- bre mat because this is fixed to the cathode. However, the particles are repelled before they can reach the fibre surfaces (coagulation point) due to the positive charges on the fibres. It can be hypothesised that under the effect of the repulsive forces due to the surrounding fibres, the particles will follow the path with the fewest possible obstacles until reaching the next interstice between adjacent fibres. Thus, when the parti￾cles reach the electrode or the surface of previously deposited particles, they have no further possibility to move and so the electrophoretic ceramic deposit grows with a high particle packing density. For the case where fibre and particle have opposite surface charge, on the contrary, it is expected that coagulation will occur on the first layer of fibres encountered by the travelling particle. Consequently, the formation of a deposit on the outer fibre layer will block or at least make more difficult the movement of the particles towards the in￾terior of the fibre mat, which could result in poor infiltration and low quality microstructure of the green body. 4.2. Comparison of the two processes Both techniques developed here are convenient routes to the manufacture of multilayer NextelTM 720 fibre-reinforced alumina matrix composites. The method based on the simul￾taneous EPD of several fibre mats resulted in higher fibre vol￾ume fraction, higher matrix density and more homogeneous matrix microstructure than the standard method of EPD of single fibre mats. As assessed by SEM, less structural damage developed in the matrix during sintering. In addition, damage was minimised due to less extent of manipulation of the green body, as the intermediate steps of forming the composite by lamination were eliminated. This benefit becomes apparent when comparing Figs. 8b and 9. Nevertheless, the advantage