1570 E. Stoll et al. Joumal of the European Ceramic Sociery 26(2006)1567-1576 Paste fabrication Single fibre mats infiltrated by Al O, paste coating on single electrophoretically infiltrated fibre mats(Nextel M 720) doctor blade Al, o, paste coating filtrated wetted with non ionized H2O 20 fibre ma Lamination and warm pressing Sintering in air Continuous fibre-reinford CMC plate 45x45X13mm) up to 1300C Fig 3. Schematic diagram showing the different processing steps for fabrication of alumina matrix composites with Nextel M 720 fibre mats reinforcement Fibre mats that had been previously electrophoretically infiltrated with Al2 O3 particles were laminated by the doctor blade method and subsequently war interlocking between the single Al2O3 layers. The pressure composite green bodies were dried for 24 h in air at room was kept at <5 MPa in order to avoid damage of the fibres temperature and densified by sintering under the same con- and the temperature was -80C to avoid excessive evap- ditions given above oration of volatiles from the green body. Composite fab- rication was completed with a sintering stage at 1300C 2.4. Microstructural characterisation in air using an electric furnace. The holding time was I h, heating rate was 10C/min and the cooling rate was All composites were characterised by scanning electron 6°Cmin microscopy(SEM)(XL 30, FA Philips/FEI Eindhoven and LEO 1525, Gemini). For SEM observation, composite sam- 2.3.3. Process 11: simultaneous EPD of several fibre ples in" green"state and after sintering were infiltrated with epoxy resin under vacuum, then sectioned using a dia- The main advantage of this direct procedure is to dra- mond saw and polished using a diamond suspension to 1 um matically reduce the number of processing steps required for the fabrication of multilayer CMC components which are unavoidable in the conventional fabrication methods 14 The technique involves the simultaneous electrophoretic deposi 3. Results tion of the matrix material onto three or more fibre mats to manufacture multilayer CMCs in just one step(see Fig. 2). 3.1. Characterisation of the suspension for EPD This method should also reduce manipulation of the infil trated fibre mats before the material has been densified by A suitable suspension for electrophoretic deposition sintering, thus reducing the possibility of microstructural should contain ceramic particles with high surface charge damage. A new EPD cell was designed and built for this pur- well dispersed in a liquid of high dielectric constant. ESA pose, as shown in Fig. 4. The cell provides a relatively large signal measurements allowed us to assess the effect of 4- effective deposition area allowing the fabrication of compo- hydroxybenzoic acid as additive on the stability of suspen- nents of 100 mm in diameter. For the present experiments, sions of Al2O3 particles in ethanol (25 wt %)and on particle the space between the stainless steel electrodes was fixed at mobility. It was found that the surface charge of the Al2O3 2 cm. The deposition time was 8 min The EPD process was particles in ethanol was always positive with a maximum always carried out in vertical position, i.e. the movement of value of 450 uPa m/V at concentrations of 4-HBS >3 wt% the particles was in direction opposite to the gravity force, in of the solids content. From these results, the concentration order to reduce agglomeration effects of the suspension and of 4-hydroxybenzoic acid for the present investigation was o to promote the infiltration of the smallest and lightest chosen as 4 wt g particles into the fibre mats avoiding preferential deposition Fig. 5 shows the results of the ESa signal measurements of the heaviest(and largest)agglomerates. After EPD, the on the milled Nextel M 720 fibre in ethanol suspension1570 E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 Fig. 3. Schematic diagram showing the different processing steps for fabrication of alumina matrix composites with NextelTM 720 fibre mats reinforcement. Fibre mats that had been previously electrophoretically infiltrated with Al2O3 particles were laminated by the doctor blade method and subsequently warm pressed and pressureless sintered. interlocking between the single Al2O3 layers. The pressure was kept at <5 MPa in order to avoid damage of the fibres and the temperature was ∼80 ◦C to avoid excessive evap￾oration of volatiles from the green body. Composite fab￾rication was completed with a sintering stage at 1300 ◦C in air using an electric furnace. The holding time was 1 h, heating rate was 10 ◦C/min and the cooling rate was 6 ◦C/min. 2.3.3. Process II: simultaneous EPD of several fibre mats The main advantage of this direct procedure is to dra￾matically reduce the number of processing steps required for the fabrication of multilayer CMC components which are unavoidable in the conventional fabrication methods.14 The technique involves the simultaneous electrophoretic deposi￾tion of the matrix material onto three or more fibre mats to manufacture multilayer CMCs in just one step (see Fig. 2). This method should also reduce manipulation of the infil￾trated fibre mats before the material has been densified by sintering, thus reducing the possibility of microstructural damage. A new EPD cell was designed and built for this pur￾pose, as shown in Fig. 4. The cell provides a relatively large effective deposition area allowing the fabrication of compo￾nents of 100 mm in diameter. For the present experiments, the space between the stainless steel electrodes was fixed at 2 cm. The deposition time was 8 min. The EPD process was always carried out in vertical position, i.e. the movement of the particles was in direction opposite to the gravity force, in order to reduce agglomeration effects of the suspension and also to promote the infiltration of the smallest and lightest particles into the fibre mats avoiding preferential deposition of the heaviest (and largest) agglomerates. After EPD, the composite green bodies were dried for 24 h in air at room temperature and densified by sintering under the same con￾ditions given above. 2.4. Microstructural characterisation All composites were characterised by scanning electron microscopy (SEM) (XL 30, FA Philips/FEI Eindhoven and LEO 1525, Gemini). For SEM observation, composite sam￾ples in “green” state and after sintering were infiltrated with epoxy resin under vacuum, then sectioned using a dia￾mond saw and polished using a diamond suspension to 1
m finish. 3. Results 3.1. Characterisation of the suspension for EPD A suitable suspension for electrophoretic deposition should contain ceramic particles with high surface charge well dispersed in a liquid of high dielectric constant.17 ESA signal measurements allowed us to assess the effect of 4- hydroxybenzoic acid as additive on the stability of suspen￾sions of Al2O3 particles in ethanol (25 wt.%) and on particle mobility. It was found that the surface charge of the Al2O3 particles in ethanol was always positive with a maximum value of 450
Pa m/V at concentrations of 4-HBS >3 wt.% of the solids content.29 From these results, the concentration of 4-hydroxybenzoic acid for the present investigation was chosen as 4 wt.%. Fig. 5 shows the results of the ESA signal measurements on the milled NextelTM 720 fibre in ethanol suspension