Availableonlineatwww.sciencedirect.com DIRECT E≈RS ELSEVIER Journal of the European Ceramic Society 26(2006)1567-1576 www.elsevier.com/locate/jeurcera Fabrication technologies for oxide-oxide ceramic matrix composites based on electrophoretic deposition E. Stoll a P MahraH-G. Kruger aH. Kern a B.J. C. Thomas.AR. BoccacciniD,* a Institute of Materials Technology. Technical University of ILmenau. D-98684 Ilmenau, Germany b Department of Materials, Imperial College London, Prince Consort Road, London SW72BP. UK Received 29 November 2004: received in revised form 17 March 2005; accepted 19 March 2005 Available online 17 May 2005 Abstract Electrophoretic deposition(EPD) was used to fabricate alumina matrix composites with high volume fraction of woven fibre mat(Nextel M 720)reinforcement in a multilayer structure. Colloidal suspensions of Al,O3 nanoparticles in ethanol medium with addition of 4-hydrobezoic acid were used for EPD. Two different techniques were developed for fabrication of Al2O3 matrix/Nextel TM 720 fibre composites. The first method is a combination of standard EPD of single fibre mats with a subsequent lamination procedure to fabricate the multilayered composite The second method involves the simultaneous infiltration of several( three or more)NextelTM 720 fibre mats by EPD in a tailor-made cell. The mposites exhibit a homogeneous matrix microstructure, characterised by a very high particle packing density and relatively low porosity after sintering at 1300C. The EPD cell allows production of relatively large bodies(10 cm diameter). By combination of the multilayer EPD infiltration and lamination processes developed here, thick ceramic matrix composite components(>10mm thickness)can be fabricated, which opens the possibility of greater industrial application of the materials o 2005 Elsevier Ltd. All rights reserved. Keywords: Suspensions; Composites; Al2O3; Electrophoretic deposition 1. Introduction and fibres 15,16 18-25 The main advantages of Epd over a con- ventional slurry route are the reduced processing times and Continuous fibre-reinforced oxide ceramic matrix com- improved control over green body microstructure. 5,6Aque- posites( CMCs) have attracted significant scientific and tech- ous or non-aqueous suspensions of ceramic(nano)particles nological interest for high temperature structural applications are usually considered for forming the matrix, and both con- in gas turbines, rocket engines, heat exchangers and hot fil- ductive(e.g SiC Nicalon, carbon) and non-conductive(e.g. ters, due to their low specific weight, damage-tolerant be NextelTM)fibres have been used as reinforcement. 8-25In haviour, oxidation resistance and high resistance to cree the case of non-conductive fibres, the fibre weave is placed in front of the deposition electrode and the ceramic deposit expended in the optimisation of CMCs, with particular em- forms on the electrode and grow around and through the fibre phasis on the establishment of reliable and cost effective fab- mat. 5.25 A schema of a typical EPD cell, commonly used for rication procedures. -14 the infiltration of single non-conductive fibre mats, is shown Electrophoretic deposition(EPD)is the process by which in Fig charged particles in a liquid medium move under an applied This paper presents EPD based methods for the fabrication potential towards an oppositely charged electrode and coagu of multilayer NextelTM 720 fibre-reinforced alumina matrix late there to form a stable deposit. -EPD has been used for composites. Two techniques are described, based or the production of CMCs with a variety of ceramic matrices Corresponding author. Tel. +44 207594 6731: fax: +44 20 7594675 (i)EPD of single NextelTM 720 fibre mats and subsequer E- Jmail address: a boccaccini@imperial ac uk(A R. Boccaccini lamination using Al2O3 paste to form the matrix and 0955-2219/S-see front matter 2005 Elsevier Ltd. All rights reserved. doi: 10. 1016/j-jeurceramsoc. 2005.03.251
Journal of the European Ceramic Society 26 (2006) 1567–1576 Fabrication technologies for oxide–oxide ceramic matrix composites based on electrophoretic deposition E. Stoll a, P. Mahr a, H.-G. Kruger ¨ a, H. Kern a, B.J.C. Thomas b, A.R. Boccaccini b,∗ a Institute of Materials Technology, Technical University of Ilmenau, D-98684 Ilmenau, Germany b Department of Materials, Imperial College London, Prince Consort Road, London SW7 2BP, UK Received 29 November 2004; received in revised form 17 March 2005; accepted 19 March 2005 Available online 17 May 2005 Abstract Electrophoretic deposition (EPD) was used to fabricate alumina matrix composites with high volume fraction of woven fibre mat (NextelTM 720) reinforcement in a multilayer structure. Colloidal suspensions of Al2O3 nanoparticles in ethanol medium with addition of 4-hydrobezoic acid were used for EPD. Two different techniques were developed for fabrication of Al2O3 matrix/NextelTM 720 fibre composites. The first method is a combination of standard EPD of single fibre mats with a subsequent lamination procedure to fabricate the multilayered composite. The second method involves the simultaneous infiltration of several (three or more) NextelTM 720 fibre mats by EPD in a tailor-made cell. The composites exhibit a homogeneous matrix microstructure, characterised by a very high particle packing density and relatively low porosity after sintering at 1300 ◦C. The EPD cell allows production of relatively large bodies (10 cm diameter). By combination of the multilayer EPD infiltration and lamination processes developed here, thick ceramic matrix composite components (>10 mm thickness) can be fabricated, which opens the possibility of greater industrial application of the materials. © 2005 Elsevier Ltd. All rights reserved. Keywords: Suspensions; Composites; Al2O3; Electrophoretic deposition 1. Introduction Continuous fibre-reinforced oxide ceramic matrix composites (CMCs) have attracted significant scientific and technological interest for high temperature structural applications in gas turbines, rocket engines, heat exchangers and hot filters, due to their low specific weight, damage-tolerant behaviour, oxidation resistance and high resistance to creep and thermal shock.1–6 Significant research effort is being expended in the optimisation of CMCs, with particular emphasis on the establishment of reliable and cost effective fabrication procedures.1–14 Electrophoretic deposition (EPD) is the process by which charged particles in a liquid medium move under an applied potential towards an oppositely charged electrode and coagulate there to form a stable deposit.15–17 EPD has been used for the production of CMCs with a variety of ceramic matrices ∗ Corresponding author. Tel.: +44 20 7594 6731; fax: +44 20 75946757. E-mail address: a.boccaccini@imperial.ac.uk (A.R. Boccaccini). and fibres.15,16,18–25 The main advantages of EPD over a conventional slurry route are the reduced processing times and improved control over green body microstructure.15,16 Aqueous or non-aqueous suspensions of ceramic (nano)particles are usually considered for forming the matrix, and both conductive (e.g. SiC Nicalon®, carbon) and non-conductive (e.g. NextelTM) fibres have been used as reinforcement.18–25 In the case of non-conductive fibres, the fibre weave is placed in front of the deposition electrode and the ceramic deposit forms on the electrode and grow around and through the fibre mat.15,25 A schema of a typical EPD cell, commonly used for the infiltration of single non-conductive fibre mats, is shown in Fig. 1. This paper presents EPD based methods for the fabrication of multilayer NextelTM 720 fibre-reinforced alumina matrix composites. Two techniques are described, based on: (i) EPD of single NextelTM 720 fibre mats and subsequent lamination using Al2O3 paste to form the matrix and 0955-2219/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jeurceramsoc.2005.03.251
E Stoll et al. /Joumal of the European Ceramic Society 26(2006)1567-1576 (adjustable posite electrode deposition electrode Aqueous or non aqueous suspension o8 Ceramic particles (usually 10-500 nm) 8°68。。08 Frame for fixing fibre mats to the elctrodode otion of the particles Fig. 1. Schematic representation of the typical EPD cell suitable for the infiltration of non-conductive fibre mats with ceramic particles(positively charged in this schema), for the fabr (ii) simultaneous infiltration of multiple Nextel M 720 fibre equipment(Sonifier 450 Branson Ultrasonic SA, Carong mats with Al2O3 particles by means of a single EPD step Geneve, Switzerland). Subsequently, the suspension was left in a newly developed EPD cell. to rest for 24 h at room temperature before being used for The electrochemical behaviour of the Nextel TM 720 fib 2. Experimental procedures mmersed in ethanol with addition of 4-HBS was charac terised using ESA signal measurements. For this experiment, 2.. Materials desized NextelTM 720 fibres were ground using a ball mill (Type P6, Fritsch Company, Germany) to a fine powder of The fibres used in this work are NextelTM 720(3M mean particle size 1.35 um, with particle sizes varying be- Corporation, St Paul,MN,USA)woven into eight-harness tween 0.5 and 2.5 um, as determined by laser scattering pa satin fabrics 26,27 The woven fabrics contain 400 individ- ticle analysis (Zetasizer, Malvern Instruments GmbH). The ual filaments with diameters between 10 and 12 um. Be- objective of the ESA signal measurements on the powered fore using the fibres for composite fabrication, they were NextelTM fibres was to characterise the surface charge ac- desized by a heat treatment at 700oC in air for 10 min. quired by the fibres when immersed in ethanol and the The 100% a-alumina powder used for the matrix was the fect of the 4-HBS dispersant on the surface charge of the commercial powder AKP-50 (Sumitomo, Chemicals, Tokyo, fibre Japan), which has a particle size distribution between 100 and 300 nm and a BET surface area of 10.6m/g(manufacturer 2.3. Composite processing 2.3.1. General description of the process 2.2. Preparation of suspensions for EPD Two different techniques were developed for the fabrication of the Al2 O3 matrix/Nextel 720 fibre com- Ethanol was chosen as the suspension medium to avoid posites. The first method is a combination of EPD of any formation of porosity in the deposits during EPD single fibre mats(individual layers) with a subsequent lam- due to gas evolution, which usually occurs in EPD from ination procedure to fabricate the multilayered composite aqueous suspensions. 8 According to previous investiga- The second method involves the simultaneous infiltration tions on the electrochemical behaviour of Al2O3 parti- of several (three or more)Nextel M 720 fibre mats by cles in ethanol, 29 suspensions containing an optimised a single EPD step in an advanced tailor-made EPD cell concentration of 25 wt %o Al2O3 powder were prepared. 4- Fig. 2 shows the flow chart describing the basic steps in- ydroxybenzoic acid (4-HBS)was used as dispersant agent volved. The following main processing parameters were aconcentration of 4 wt. of solid content. This was the op- used after a preliminary trial-and-error optimisation ap timised concentration of dispersant found by electronic sonic proach similar to the one discussed in the literature amplitude(ESA)measurements, which showed that Al2O3 constant electric voltage= 100 V and distance between elec particles in ethanol exhibit a positive charge. The suspension trodes =2 cm. In all cases, the CMCs green bodies after ras dispersed and homogenized in high frequency ultrasound EPD were dried in air at room temperature and subsequently
1568 E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 Fig. 1. Schematic representation of the typical EPD cell suitable for the infiltration of non-conductive fibre mats with ceramic particles (positively charged in this schema), for the fabrication of oxide–oxide ceramic matrix composites. (ii) simultaneous infiltration of multiple NextelTM 720 fibre mats with Al2O3 particles by means of a single EPD step in a newly developed EPD cell. 2. Experimental procedures 2.1. Materials The fibres used in this work are NextelTM 720 (3M Corporation, St. Paul, MN, USA) woven into eight-harness satin fabrics.26,27 The woven fabrics contain ∼400 individual filaments with diameters between 10 and 12 m. Before using the fibres for composite fabrication, they were desized by a heat treatment at 700 ◦C in air for 10 min. The 100% -alumina powder used for the matrix was the commercial powder AKP-50 (Sumitomo, Chemicals, Tokyo, Japan), which has a particle size distribution between 100 and 300 nm and a BET surface area of 10.6 m2/g (manufacturer data). 2.2. Preparation of suspensions for EPD Ethanol was chosen as the suspension medium to avoid any formation of porosity in the deposits during EPD due to gas evolution, which usually occurs in EPD from aqueous suspensions.28 According to previous investigations on the electrochemical behaviour of Al2O3 particles in ethanol,29 suspensions containing an optimised concentration of 25 wt.% Al2O3 powder were prepared. 4- Hydroxybenzoic acid (4-HBS) was used as dispersant agent in a concentration of 4 wt.% of solid content. This was the optimised concentration of dispersant found by electronic sonic amplitude (ESA) measurements, which showed that Al2O3 particles in ethanol exhibit a positive charge. The suspension was dispersed and homogenized in high frequency ultrasound equipment (Sonifier 450 Branson Ultrasonic SA, CarongeGeneve, Switzerland). Subsequently, the suspension was left to rest for 24 h at room temperature before being used for EPD. The electrochemical behaviour of the NextelTM 720 fibre immersed in ethanol with addition of 4-HBS was characterised using ESA signal measurements. For this experiment, desized NextelTM 720 fibres were ground using a ball mill (Type P6, Fritsch Company, Germany) to a fine powder of mean particle size 1.35 m, with particle sizes varying between 0.5 and 2.5m, as determined by laser scattering particle analysis (Zetasizer, Malvern Instruments GmbH). The objective of the ESA signal measurements on the powered NextelTM fibres was to characterise the surface charge acquired by the fibres when immersed in ethanol and the effect of the 4-HBS dispersant on the surface charge of the fibre. 2.3. Composite processing 2.3.1. General description of the process Two different techniques were developed for the fabrication of the Al2O3 matrix/NextelTM 720 fibre composites. The first method is a combination of EPD of single fibre mats (individual layers) with a subsequent lamination procedure to fabricate the multilayered composite. The second method involves the simultaneous infiltration of several (three or more) NextelTM 720 fibre mats by a single EPD step in an advanced tailor-made EPD cell. Fig. 2 shows the flow chart describing the basic steps involved. The following main processing parameters were used after a preliminary trial-and-error optimisation approach similar to the one discussed in the literature30: constant electric voltage = 100 V and distance between electrodes = 2 cm. In all cases, the CMCs green bodies after EPD were dried in air at room temperature and subsequently
E Stoll et al. /Joumal of the European Ceramic Society 26(2006 )1567-1576 Stainless stee AKP-50/Al2O3 (Ethanol) (NexrelTM720 SS)substrate Fibre mat(s) strongly ultrasonically fixed to ss substrate EPD EPD of multilayer (single fibre mat (Three or more fibre mats (air, room temperature) nats) T<80C) “滥 more layers of 20 fibre mats (CMC) aau:08 arT<1300°) ar,T<1300°c) CMC Fig. 2. Flow chart describing the two methods developed to produce ceramic matrix composites(CMCs) with alumina matrix and three or more layers of Nextel TM 720 fibre mats as reinforcement a pressureless sintering process was used to densify the ng polyvinylalcohol(PVA)with a-Al2O3 particles and composites. deionised water according to the composition described in Table 1. The paste coating on wet surfaces of fibre mats previously infiltrated by EPD was applied using a Doc 2.3.2. Process 1: EPD of single fibre mats and subsequent lamination tor Blade type method(see Fig 3). The thickness of the Fibre mats of square shape with an effective area for in- peated for three l: oating was between 0.5 and 1 mm. The procedure was re yers and subsequently the composite(green iltration of 45 mm x 45 mm were used. The stable suspen- body) was consolidated, forming a three-layer"sandwich filt sion was incorporated in a typical(standard)electrophoretic packet, by warm-pressing. The application of heat during deposition cell, as the one shown in Fig. 1. Under the application of the external electrical field, the positively pressing was required to reduce the viscosity of the paste and thus to generate better conditions for impregnation and charged Al2O3 particles in the suspension moved towards the oppositely charged electrode, before which the non- conductive Nextel M 720 fibre mat was placed. The elec- Table 1 tric field of 50 V/cm was kept constant for all experiments Composition of the paste used in the lamination procedure for CMCs fabri and the deposition time was 3 min. Electrophoretically in- cation(see Fig 3) filtrated single fibre mats were dried in air at normal pres- Concentration ure and stored at room temperature in dissicators for later Non-ionized water Owt.%c use Owt.%c icaly the lamination process, the procedure shown schemat- Polyvinilalcohol binder( Polyviol LL 6035) 10-20w%of in Fig. 3 was used. A paste was made by mix- (20% solution
E. Stoll et al. / Journal of the European Ceramic Society 26 (2006) 1567–1576 1569 Fig. 2. Flow chart describing the two methods developed to produce ceramic matrix composites (CMCs) with alumina matrix and three or more layers of NextelTM 720 fibre mats as reinforcement. a pressureless sintering process was used to densify the composites. 2.3.2. Process I: EPD of single fibre mats and subsequent lamination Fibre mats of square shape with an effective area for in- filtration of 45 mm × 45 mm were used. The stable suspension was incorporated in a typical (standard) electrophoretic deposition cell, as the one shown in Fig. 1. Under the application of the external electrical field, the positively charged Al2O3 particles in the suspension moved towards the oppositely charged electrode, before which the nonconductive NextelTM 720 fibre mat was placed. The electric field of 50 V/cm was kept constant for all experiments and the deposition time was 3 min. Electrophoretically in- filtrated single fibre mats were dried in air at normal pressure and stored at room temperature in dissicators for later use. For the lamination process, the procedure shown schematically in Fig. 3 was used. A paste was made by mixing polyvinylalcohol (PVA) with -Al2O3 particles and deionised water according to the composition described in Table 1. The paste coating on wet surfaces of fibre mats previously infiltrated by EPD was applied using a Doctor Blade type method (see Fig. 3). The thickness of the coating was between 0.5 and 1 mm. The procedure was repeated for three layers and subsequently the composite (green body) was consolidated, forming a three-layer “sandwich” packet, by warm-pressing. The application of heat during pressing was required to reduce the viscosity of the paste, and thus to generate better conditions for impregnation and Table 1 Composition of the paste used in the lamination procedure for CMCs fabrication (see Fig. 3) Composition of the paste Concentration Non-ionized water 50 wt.% -Al2O3 (AKP-50) powder 50 wt.% Polyvinilalcohol binder (Polyviol LL 6035) (20% solution) 10–20 wt.% of solid content
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 suspension
1570 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 evaporation of volatiles from the green body. Composite fabrication 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 dramatically 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 deposition 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 infiltrated 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 purpose, as shown in Fig. 4. The cell provides a relatively large effective deposition area allowing the fabrication of components 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 conditions 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 samples in “green” state and after sintering were infiltrated with epoxy resin under vacuum, then sectioned using a diamond 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 suspensions 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 450Pa 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
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 positive polarity in ethanol suspension. It is also observed that the original value of ESA signal increases from ∼107 to 170Pa 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 concentration 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 particles 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 particle packing density in the deposited matrix. The high-quality of the infiltration and the homogeneous matrix microstructure confirm thus the suitability of the suspension composition 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 microstructures 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