E Stoll et al. /Joumal of the European Ceramic Society 26(2006 )1567-1576 (1)Slope for fixing fibre mats (5)Deposition electrode(stainless steel) (7) Strain tensor 4)Electrode(stainless steel) (8)Quick clamp 4. Design of the new electrophoretic deposition cell for the simultaneous infiltration of several Nextel 720 fibre mats:(a) schematic diagram showing position of the cell for vertical deposition, with direction of particles movement opposite to the gravitation force; and (b) schematic diagram showing the I for horizontal deposition direction with additions of the same additive (4-HBS). This curve 3. 2. Fabrication of Al2O3 matrix/NextelM 720 fibre demonstrates that milled Nextel 720 fibres have a pos composites by EPD itive polarity in ethanol suspension. It is also observed that the original value of ESA signal increases from 107 .2.I. Process 1: EPD of single fibre mat and subsequent to 170 m/V for concentrations of 4-HBS of up 0.8 wt % For higher concentrations of 4-HBS, the signal A fibre mat showing high-quality infiltration of Al2O3 par- value stabilized at 170 uPam/V. Thus, at a 4-HBS con- ticles by EPD is shown in Fig 6a. The mean particle size of centration of 4 wt %, chosen for the present study, the the Al2O3 starting powder, in the range 100-300 nm. Is con- fibres and the Al2O3 particles posses the same positive firmed in Fig. 6b, which demonstrates that this particle size polarity, which has advantages regarding the mechanist is appropriate to effectively infiltrate the Nextel 720 fibre of electrophoretic deposition, as discussed below (Section mats. This image qualitatively indicates also the high parti 1) cle packing density in the deposited matrix. The high-quality of the infiltration and the homogeneous matrix microstruc- ture confirm thus the suitability of the suspension compo- sition and the selected concentration of 4-hydroxidbezoic acid. This behaviour is in broad agreement with extensive evidence in the literature on the high versatility of the EPD technique to infiltrate 2D and 3D fibre performs with cerami (nano)particles. 5Ed The SEM micrographs in Fig. 7 compare the microstruc tures of the EPD-infiltrated Nextel720 fibre mat in"green and sintered state. It is possible to observe isolated pores in the sintered matrix. This is an indication that the organic additive 20 EPD suspension has been burned out during the sintering process, leaving some residual porosity. Inspection of Fig. 7b indicates also that the sintering temperature used been too high because some chemical Fig. 5. ESA signal in ethanol suspension of milled Nextel TM 720 fibres as reaction at the fibre/matrix interface is apparent. This effect function of 4-hydroxybenzoic acid concentration. is detrimental for the mechanical properties of the composite,E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 1571 Fig. 4. Design of the new electrophoretic deposition cell for the simultaneous infiltration of several NextelTM 720 fibre mats: (a) schematic diagram showing the position of the cell for vertical deposition, with direction of particles movement opposite to the gravitation force; and (b) schematic diagram showing the cell for horizontal deposition direction. with additions of the same additive (4-HBS). This curve demonstrates that milled NextelTM 720 fibres have a pos￾itive polarity in ethanol suspension. It is also observed that the original value of ESA signal increases from ∼107 to 170
Pa m/V for concentrations of 4-HBS of up to 0.8 wt.%. For higher concentrations of 4-HBS, the signal value stabilized at 170
Pa m/V. Thus, at a 4-HBS con￾centration of 4 wt.%, chosen for the present study, the fibres and the Al2O3 particles posses the same positive polarity, which has advantages regarding the mechanism of electrophoretic deposition, as discussed below (Section 4.1). Fig. 5. ESA signal in ethanol suspension of milled NextelTM 720 fibres as function of 4-hydroxybenzoic acid concentration. 3.2. Fabrication of Al2O3 matrix/NextelTM 720 fibre composites by EPD 3.2.1. Process I: EPD of single fibre mat and subsequent lamination A fibre mat showing high-quality infiltration of Al2O3 par￾ticles by EPD is shown in Fig. 6a. The mean particle size of the Al2O3 starting powder, in the range 100–300 nm, is con- firmed in Fig. 6b, which demonstrates that this particle size is appropriate to effectively infiltrate the NextelTM 720 fibre mats. This image qualitatively indicates also the high parti￾cle packing density in the deposited matrix. The high-quality of the infiltration and the homogeneous matrix microstruc￾ture confirm thus the suitability of the suspension compo￾sition and the selected concentration of 4-hydroxidbezoic acid. This behaviour is in broad agreement with extensive evidence in the literature on the high versatility of the EPD technique to infiltrate 2D and 3D fibre performs with ceramic (nano)particles.15,16,18–25 The SEM micrographs in Fig. 7 compare the microstruc￾tures of the EPD-infiltrated NextelTM 720 fibre mat in “green” and sintered state. It is possible to observe isolated pores in the sintered matrix. This is an indication that the organic additive used in the EPD suspension has been burned out during the sintering process, leaving some residual porosity. Inspection of Fig. 7b indicates also that the sintering temperature used (1300 ◦C) might have been too high because some chemical reaction at the fibre/matrix interface is apparent. This effect is detrimental for the mechanical properties of the composite