《复合材料 Composites》课程教学资源（学习资料）第五章 陶瓷基复合材料_electrophoretic deposition-3

C MATERIALIA Pergamon cta mater.4902001)1189-1197 www.elsevier.com/locate/actamat FABRICATION AND CHARACTERISATION OF NI-COATED CARBON FIBRE-REINFORCED ALUMINA CERAMIC MATRIX COMPOSITES USING ELECTROPHORETIC DEPOSITION C KAYA,21, F. KAYA A R BOCCACCINI and K. K CHAWLA and tratesrials. The Universit of birmingham. Edgbaston. Baminghpapm. B15 2 n. sK. M et lurgacar ay Material Engineering Department, Yildiz Technical University, Besiktas, Istanbul, 80750 Turkey, Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP UK and"Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham. AL 35294 USA Received 26 May 2000: received in revised form 21 December 2000: accepted 27 December 2000) Abstract-The present study explores the feasibility of fabricating Ni-coated carbon fibre-reinforced alu mina ceramic matrix composites via a single-infiltration electrophoretic deposition(EPD) process perfo ned in vacuum. The nano-size boehmite sol was seeded using nano-size 8-alumina powder in order to ntrol the final sintered microstructure and then characterised using transmission electron microscopy differential thermal and thermogravimetric analysis (DTA/TG) and X-ray disc centrifuge system(Bl- DC) in order to determine the sol microstructure, phase transformation temperatures and particle siz (also degree of agglomeration), respectively. An EPD manufacturing cell for fabrication of Ni-coated oon fibre reinforced alumina matrix cor tes was designed and experiments were conducted under acuum (first time to date), resulting in full deposition of the sol material throughout the voids ithin/between the fibre tows. Composites with high green density (67%0 theoretical density) were pro duced using an applied voltage of 15v d. c. and deposition time of 400 s. The sintered dens pressureless sintering at 1250C for 2 h was 91% theoretical density Crack path propagation tes that the metallic Ni coating was able to provide a weak interface, as an indenter induced cr he alumina matrix was deflected and arrested at the Ni interface. c 200/ Acta Materialia Ine by Elsevier Science Ltd. All rights reserved. Keywords: Electrophoretic deposition: Composites: Nickel: Interface: Microstructure 1 INTRODUCTION ered by several hundred degrees. A commercial sol can be seeded with isostructural seeds in order to A sol is generally defined as a colloidal dispersi ower crystallisation temperature and enhance of sol (or colloidal) processing route has many modifiers/seeds in terms of grain size, pore size and advantages, such as greater purity, higher homogen- pore size distribution. Boehmite(y-AlOOH)sol ity and ultrafine (5-100 nm)particle size distri- one of the ideal candidate materials to manufacture bution, in comparison to conventional ceramic pow- high quality alumina base ceramic components with der manufacturing processes. The main goal of this controlled tinal sintered microstructure, as it con technique is to achieve an ultra homogeneous or tains highly sinter-active ceramic particles on a atomic scale mixing of different chemical compo- ometer scale. Without seeding, however, a commer nents. The high surface area to volume ratio of a cial or hydrothermally produced boehmite sol ceramic sol makes the material usually highly sin- requires very high sintering temperatures(1600C) ter-active, thus sintering temperatures can be low- for complete densification. This is due to the large and extensive pore network that develops during the reconstructive transformation to the final stable t To whom all correspondence should be addressed. Fax: phase a-Al2O3, according to the dehydration and +44-121-4143441. high temperature phase transformation of boehm- E-mail address: c kaya@bhamac uk(C. Kaya) te[3]: 1359-6454/01/$20.00@ 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved PI:S1359-6454(01)00018-0

Acta mater. 49 (2001) 1189–1197 www.elsevier.com/locate/actamat FABRICATION AND CHARACTERISATION OF Ni-COATED CARBON FIBRE-REINFORCED ALUMINA CERAMIC MATRIX COMPOSITES USING ELECTROPHORETIC DEPOSITION C. KAYA1, 2†, F. KAYA1 , A. R. BOCCACCINI3 and K. K. CHAWLA4 1 Interdisciplinary Research Centre (IRC) For High Performance Applications and School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK, 2 Metallurgical and Material Engineering Department, Yildiz Technical University, Besiktas, Istanbul, 80750 Turkey, 3 Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, SW7 2BP UK and 4 Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294, USA ( Received 26 May 2000; received in revised form 21 December 2000; accepted 27 December 2000 ) Abstract—The present study explores the feasibility of fabricating Ni-coated carbon fibre-reinforced alu￾mina ceramic matrix composites via a single-infiltration electrophoretic deposition (EPD) process perfor￾med in vacuum. The nano-size boehmite sol was seeded using nano-size δ-alumina powder in order to control the final sintered microstructure and then characterised using transmission electron microscopy, differential thermal and thermogravimetric analysis (DTA/TG) and X-ray disc centrifuge system (BI￾XDC) in order to determine the sol microstructure, phase transformation temperatures and particle size (also degree of agglomeration), respectively. An EPD manufacturing cell for fabrication of Ni-coated carbon fibre reinforced alumina matrix composites was designed and experiments were conducted under vacuum (first time to date), resulting in full deposition of the sol material throughout the voids within/between the fibre tows. Composites with high green density (67% theoretical density) were pro￾duced using an applied voltage of 15 V d.c. and deposition time of 400 s. The sintered density after pressureless sintering at 1250°C for 2 h was 91% theoretical density. Crack path propagation test showed that the metallic Ni coating was able to provide a weak interface, as an indenter induced crack within the alumina matrix was deflected and arrested at the Ni interface.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Electrophoretic deposition; Composites; Nickel; Interface; Microstructure 1. INTRODUCTION A sol is generally defined as a colloidal dispersion of very fine solid particles in the range of 10 nm to 2 µm in a liquid medium, where the suspension is sustained indefinitely by Brownian motion. The use of sol (or colloidal) processing route has many advantages, such as greater purity, higher homogen￾eity and ultrafine (5–100 nm) particle size distri￾bution, in comparison to conventional ceramic pow￾der manufacturing processes. The main goal of this technique is to achieve an ultra homogeneous or atomic scale mixing of different chemical compo￾nents. The high surface area to volume ratio of a ceramic sol makes the material usually highly sin￾ter-active, thus sintering temperatures can be low- † To whom all correspondence should be addressed. Fax:
44-121-4143441. E-mail address: c.kaya@bham.ac.uk (C. Kaya) 1359-6454/01/$20.00  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S13 59-6454(01)00018-0 ered by several hundred degrees. A commercial sol can be seeded with isostructural seeds in order to lower crystallisation temperature and enhance densification with refined microstructure through solid-state epitaxy [1, 2]. The final sintered micro￾structure can be controlled using these modifiers/seeds in terms of grain size, pore size and pore size distribution. Boehmite (γ-AlOOH) sol is one of the ideal candidate materials to manufacture high quality alumina base ceramic components with controlled final sintered microstructure, as it con￾tains highly sinter-active ceramic particles on a nan￾ometer scale. Without seeding, however, a commer￾cial or hydrothermally produced boehmite sol requires very high sintering temperatures (>1600°C) for complete densification. This is due to the large and extensive pore network that develops during the reconstructive transformation to the final stable phase α-Al2O3, according to the dehydration and high temperature phase transformation of boehm￾ite [3]:

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 an

1190 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

KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX 0.5 um, respectively. The nickel coating provided weight gain per millisecond during the deposition excellent conductivity which is essential for EPD, as process, i.e. in real time. The dimensions of the anode well as ease of fibre hand and adequate wett-(25 mmX25 mm) were half of the cathode dimen ability. These fibres have been used recently as sions (50 mmX50 mm) in order to eliminate th reinforcement in borosilicate glass matrix com- 'edge effect which may give an inhomogeneous posites [201 thickness from the centre to the edges of the anode 22. EPD The EPD-prepared green body specimens containing about 25-30 vol% fibre loading were dried under An in situ EPD cell was designed in order to infil- humidity controlled atmosphere for one day and left trate the Ni-coated carbon fibre tows with the boehm- in normal air for another day before being press ite sol. The tows were unidirectionally aligned in a ureless sintered at 1250C for 2 h under nitrogen grooved perspex frame. EPD experiments were car- atmosphere ried out under vacuum. The Epd cell used is sche- 23. Microstructural characterisation matically shown in The distance between each tow was chosen to be in the range 0. 20.4 mm. Nickel To prepare green and sintered fibre reinforced coated carbon fibres held in the frame were used as CMC samples for cross-sectional scanning electron the deposition electrode( cathode). Two stainless steel microscopy (SEM), the specimens were placed in a plates on either side of the anode served as the posi- vacuum chamber and vacuum-impregnated with Epo- ive(anode) electrodes. After the fibre preform was fix resin. Impregnated green and sintered CMC placed in the sol, the system was vacuum degassed samples were left to harden overnight and then cut to remove any entrapped air, and then the cell elec- into slices using a diamond saw. a high resolution trodes were connected to a 0-60V d c power supply. scanning electron microscope(Field Emission Gun, EPD was performed subsequently under constant FEG SEM, Hitachi S-4000, Japan) was employed to voltage conditions (5, 10, 15 and 20 V) using varying characterise the various microstructural features of leposition times(from 50 to 500 s). An electrode sep- the infiltrated and sintered composite bodies, includ aration distance of 15 mm was used in all experi- ing: grain shape and size; porosity distribution and ments. Under the applied electric field, the very fine location; ductile interface, deposit thickness and infil- boehmite particles possessing a net positive surface tration of the matrix into the fibre architecture on both charge, as determined from the electrophoretic green and sintered samples. mobility data(see below ), migrated towards the nega- A Phillips CM 20 transmission electron microscop tive electrode, i.e. the Ni coated carbon fibre tows. (TEM) was used to observe and characterise the sol The particles infiltrated the fibre tows and deposited particle shape, size and degree of agglomeration, until a sufficient matrix thickness, which enveloped well as the nano-scale particle-particle interactions the fibre tows, was achieved. The fibre preform acting TEM chemical analysis of the sintered specimen was as the electrode was connected to a balance linked to then conducted, using a JEOL 4000 FX TEM a computer. The EPD apparatus is able to record the equipped with energy dispersive X-ray analysis. Pow der samples of the material deposited in between the layers of carbon fibre in each of the EPD-infiltrated CMPUTER green compacts were extracted. These samples were then subjected to differential thermal analysis (DtA) Vacuum Chamber Digital balance in order to determine the phase transformation tem- peratures. Other samples of this powder were calcined ((+) at given temperatures for 2 h and then analysed using X-ray( CuKo radiation) powder diffraction to identify the phases present. Finally, in order to characterise the interfacial behaviour of the composite produced under optimised EPD conditions, the crack path observation technique [24] was used on sintered and polished samples. 3. RESULTS AND DISCUSSION igure 2 shows a bright-field TEM micrograph of the spatial arrangement of the boehmite particles in Elcctrode suspension. With reference to the boehmite particles Ni coated fibers Electrode shown in the picture, the lath morphology of the boehmite particles is evident from the arrowed planar Fig. 1. Schematic diagram of the custom-built vacuum in situ and side views. The modal particle size of 40 nm is lectrophoretic deposition(EPD) cell incorporating Ni-coated so seen with an indicated size range of 2060nm carbon fibres as the deposition electrode Particle size analysis (cumulative mass distribution

KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX 1191 0.5 µm, respectively. The nickel coating provided excellent conductivity which is essential for EPD, as well as ease of fibre handling and adequate wett￾ability. These fibres have been used recently as reinforcement in borosilicate glass matrix com￾posites [20]. 2.2. EPD An in situ EPD cell was designed in order to infil￾trate the Ni-coated carbon fibre tows with the boehm￾ite sol. The tows were unidirectionally aligned in a grooved perspex frame. EPD experiments were car￾ried out under vacuum. The EPD cell used is sche￾matically shown in Fig. 1. The distance between each tow was chosen to be in the range 0.2–0.4 mm. Nickel coated carbon fibres held in the frame were used as the deposition electrode (cathode). Two stainless steel plates on either side of the anode served as the posi￾tive (anode) electrodes. After the fibre preform was placed in the sol, the system was vacuum degassed to remove any entrapped air, and then the cell elec￾trodes were connected to a 0–60 V d.c. power supply. EPD was performed subsequently under constant voltage conditions (5, 10, 15 and 20 V) using varying deposition times (from 50 to 500 s). An electrode sep￾aration distance of 15 mm was used in all experi￾ments. Under the applied electric field, the very fine boehmite particles possessing a net positive surface charge, as determined from the electrophoretic mobility data (see below), migrated towards the nega￾tive electrode, i.e. the Ni coated carbon fibre tows. The particles infiltrated the fibre tows and deposited until a sufficient matrix thickness, which enveloped the fibre tows, was achieved. The fibre preform acting as the electrode was connected to a balance linked to a computer. The EPD apparatus is able to record the Fig. 1. Schematic diagram of the custom-built vacuum in situ electrophoretic deposition (EPD) cell incorporating Ni-coated carbon fibres as the deposition electrode. weight gain per millisecond during the deposition process, i.e. in real time. The dimensions of the anode (25 mm25 mm) were half of the cathode dimen￾sions (50 mm50 mm) in order to eliminate the ‘edge effect’ which may give an inhomogeneous thickness from the centre to the edges of the anode. The EPD-prepared green body specimens containing about 25–30 vol% fibre loading were dried under humidity controlled atmosphere for one day and left in normal air for another day before being press￾ureless sintered at 1250°C for 2 h under nitrogen atmosphere. 2.3. Microstructural characterisation To prepare green and sintered fibre reinforced CMC samples for cross-sectional scanning electron microscopy (SEM), the specimens were placed in a vacuum chamber and vacuum-impregnated with Epo- fix resin. Impregnated green and sintered CMC samples were left to harden overnight and then cut into slices using a diamond saw. A high resolution scanning electron microscope (Field Emission Gun, FEG SEM, Hitachi S-4000, Japan) was employed to characterise the various microstructural features of the infiltrated and sintered composite bodies, includ￾ing: grain shape and size; porosity distribution and location; ductile interface, deposit thickness and infil￾tration of the matrix into the fibre architecture on both green and sintered samples. A Phillips CM 20 transmission electron microscope (TEM) was used to observe and characterise the sol particle shape, size and degree of agglomeration, as well as the nano-scale particle–particle interactions. TEM chemical analysis of the sintered specimen was then conducted, using a JEOL 4000 FX TEM equipped with energy dispersive X-ray analysis. Pow￾der samples of the material deposited in between the layers of carbon fibre in each of the EPD-infiltrated green compacts were extracted. These samples were then subjected to differential thermal analysis (DTA) in order to determine the phase transformation tem￾peratures. Other samples of this powder were calcined at given temperatures for 2 h and then analysed using X-ray (CuKα radiation) powder diffraction to identify the phases present. Finally, in order to characterise the interfacial behaviour of the composite produced under optimised EPD conditions, the crack path observation technique [24] was used on sintered and polished samples. 3. RESULTS AND DISCUSSION Figure 2 shows a bright-field TEM micrograph of the spatial arrangement of the boehmite particles in suspension. With reference to the boehmite particles shown in the picture, the lath morphology of the boehmite particles is evident from the arrowed planar and side views. The modal particle size of 40 nm is also seen with an indicated size range of 20–60 nm. Particle size analysis (cumulative mass distribution

l19 KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX Table 1. The effect of the vacuum atmosphere on the green and sintered densities(in theoretical density, TD) of alumina matrix composite fabricated by EPD, The samples were sintered at 1250 C for 2h Processing ro Green density(% TD) Sintered density(S PD in air EPD under vacuum green body. Thus, all EPD experiments were carried out under a high vacuum in order to obtain full depo sition of the sol material throughout the voids within the fibre mat. Two different EPD experiments were Fig. 2. Bright-field TEM micrograph showing the parti initially carried out in air and under vacuum usin show the surface and the side view of the lath-shape boehmite constant applied voltage of 10 V for 4 min with a con stant electrode separation of 15 mm in order to explore the effect of vacuum. Results are presented in Table I. It was found that vacuum EPD provided 9o)of the boehmite sol indicated that 100% of the a higher degree of deposition by eliminating the effect formed as a result of the electrolysis of Fig.3. It was also found that the boehmite sol used water. Under vacuum, very fine boehmite particles 油 hin the suspension. The graph of particle size dis- g逃的 deep into the inter/intra: fibre tows,fl as there were no big heteroflocculated agglomerates ing all the voids, resulting in the formation of high- uality, dense green(and sintered) composites. The tribution shows no extreme large particle sizes (Fig. maximum green and sintered densities were 54 and 75% of theoretical density (TD) for EPD experiments In situ EPD system has been developed recently carried out in air, respectively, whilst vacuum EPD [19] and successfully applied to produce different process provided green and sintered density values of fibre-reinforced composites, such as alumina fibre- 67 and 84%TD, respectively. These results confirmed reinforced mullite [9], mullite fibre-reinforced mullite the effectiveness of the vacuum environment 25], woven stainless steel fibre-reinforced silica [131 and nickel coated carbon fibre-reinforced borosilicate d, Figure 4 shows the particle electrophoretic mobility ata for the nano-size aqueous boehmite sol as a func glass composites[20]. For the first time in this work, tion of sol pH. From these data, it is clear that the however, in situ EPD experiments were carried out boehmite particles are positively charged below ph under vacuum(see Fig. 1), in order to eliminate the 9.5 and negatively charged above this point. At the undesirable formation and entrapment of bubbles working ph value of 4, therefore, positively charged within the deposit due to the electrolysis (evolution boehmite particles will move and deposit on to the of gases)of the aqueous sol dispersion medium. The negative electrode(fibres )under an applied d.c.volt (100 nm)particle material, in form of a sol, would The microstructure of uncoated and Ni-coated car penetrate deep into the inter/intra-fibre tows regions, filling the voids and thus providing a dense composite Diameter, nm Fig 3. X-ray disc centrifuge(BI-XDC)particle size distribution Fig. 4. Particle electrophoretic mobility data for Remal A20 (in %o cumulative mass, smaller than) of boehmite sol for a boehmite suspension. The suspension solids-loading is solids-loading of 2 wt%. Note that 100% of the total boehmite 0.01 wt% of the dispersion medium. Note that the boehmite particles are smaller than 60 nm, showing the absence of big particles have positive surface charge at the working pH value eteroflocculated chains within the suspension

1192 KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX Fig. 2. Bright-field TEM micrograph showing the particle shape and size of the used Remal A20 boehmite sol. Arrows show the surface and the side view of the lath-shape boehmite particles. %) of the boehmite sol indicated that 100% of the total particles were smaller than 60 nm, as shown in Fig. 3. It was also found that the boehmite sol used in this work was kinetically stable and well dispersed, as there were no big heteroflocculated agglomerates within the suspension. The graph of particle size dis￾tribution shows no extreme large particle sizes (Fig. 3). In situ EPD system has been developed recently [19] and successfully applied to produce different fibre-reinforced composites, such as alumina fibre￾reinforced mullite [9], mullite fibre-reinforced mullite [25], woven stainless steel fibre-reinforced silica [13] and nickel coated carbon fibre-reinforced borosilicate glass composites [20]. For the first time in this work, however, in situ EPD experiments were carried out under vacuum (see Fig. 1), in order to eliminate the undesirable formation and entrapment of bubbles within the deposit due to the electrolysis (evolution of gases) of the aqueous sol dispersion medium. The main purpose of the EPD process is that ultra fine (100 nm) particle material, in form of a sol, would penetrate deep into the inter/intra-fibre tows regions, filling the voids and thus providing a dense composite Fig. 3. X-ray disc centrifuge (BI-XDC) particle size distribution (in % cumulative mass, smaller than) of boehmite sol for a solids-loading of 2 wt%. Note that 100% of the total boehmite particles are smaller than 60 nm, showing the absence of big heteroflocculated chains within the suspension. Table 1. The effect of the vacuum atmosphere on the green and sintered densities (in % theoretical density, TD) of alumina matrix composites fabricated by EPD. The samples were sintered at 1250°C for 2 h Sintered density (% Processing route Green density (% TD) TD) EPD in air 54 75 EPD under vacuum 67 84 green body. Thus, all EPD experiments were carried out under a high vacuum in order to obtain full depo￾sition of the sol material throughout the voids within the fibre mat. Two different EPD experiments were initially carried out in air and under vacuum using constant applied voltage of 10 V for 4 min with a con￾stant electrode separation of 15 mm in order to explore the effect of vacuum. Results are presented in Table 1. It was found that vacuum EPD provided a higher degree of deposition by eliminating the effect of gases formed as a result of the electrolysis of water. Under vacuum, very fine boehmite particles can penetrate deep into the inter/intra-fibre tows, fill￾ing all the voids, resulting in the formation of high￾quality, dense green (and sintered) composites. The maximum green and sintered densities were 54 and 75% of theoretical density (TD) for EPD experiments carried out in air, respectively, whilst vacuum EPD process provided green and sintered density values of 67 and 84%TD, respectively. These results confirmed the effectiveness of the vacuum environment. Figure 4 shows the particle electrophoretic mobility data for the nano-size aqueous boehmite sol as a func￾tion of sol pH. From these data, it is clear that the boehmite particles are positively charged below pH 9.5 and negatively charged above this point. At the working pH value of 4, therefore, positively charged boehmite particles will move and deposit on to the negative electrode (fibres) under an applied d.c. volt￾age. The microstructure of uncoated and Ni-coated car￾Fig. 4. Particle electrophoretic mobility data for Remal A20 boehmite suspension. The suspension solids-loading is 0.01 wt% of the dispersion medium. Note that the boehmite particles have positive surface charge at the working pH value of 4

KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX 1193 bon fibres, see Fig. 5(a)and(b), respectively, showed that the metallic Ni coating around the fibres was very △5V homogeneous and with a uniform thickness of about 0.5 um. The carbon fibres were 10-15 um in diam- When EPD is used as a forming technique fo MCs, it is possible to use either constant current or 3 constant voltage conditions. Constant current con- ditions would result in a continually increasing 。=9=3=a882 age due to the increasing resistance of the deposit as it grows in thickness and mass, thus constant voltage conditions were used in this work. Voltages higher than 20V were not used in these experiments in order Electrophoretic Deposition Time(s) to prevent gas bubbles being incorporated within the deposited ceramic matrix. The results from experi- -5V mental trials using the in situ EPD cell design under 000 vacuum are shown in Fig. 6. The graphs in Fig. 6(a) g weight and thickness as a function of deposition time 3300 experiments were performed for duration of up to 200 500 s, as this gave a deposit thickness of about 3 100 o um, which was enough to produce a composite 5 with an acceptable green density. The deposit thick- ness increased with increasing deposition time, as the amount of particles deposited on to the fibres preform Electrophoretic Deposition Time(s) increased. When aqueous based sols are used in EPD 3 experiments, one problem associated with this is the 3 (c) 0 3.2 um electrophoretic thickness as a function of deposition time for different applied voltages. In(c), the deposit formation rate as a function of EPD time under optimised applied voltage of electrolysis of the water. Higher voltages resulted in rapid deposit formation, but also in the undesirable formation and entrapment deposit due to the electrolysis of the aqueous sol dis- persion medium, while low voltages reduced the ele trolysis, but they also needed higher deposition times Thus, a compromise had to be found and voltage and 231pm deposition time were optimised Figure 6a shows that the eight al Most linear with increasing deposition time(up to Fig. 5. FEG SEM micrographs, showing the microstructure of 500 s)and voltage.An oltage of 20 V seems that the metallic Ni coating around the fibres is very homo. ideal according to the I app of deposited material geneous and it has a uniform thickness of 0.5 um. The carbon as shown in Fig. 6(a). However, the deposited matrix fibres are 10-15 um in diameter a result of the gas

KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX 1193 bon fibres, see Fig. 5(a) and (b), respectively, showed that the metallic Ni coating around the fibres was very homogeneous and with a uniform thickness of about 0.5 µm. The carbon fibres were 10–15 µm in diam￾eter. When EPD is used as a forming technique for CMCs, it is possible to use either constant current or constant voltage conditions. Constant current con￾ditions would result in a continually increasing volt￾age due to the increasing resistance of the deposit as it grows in thickness and mass, thus constant voltage conditions were used in this work. Voltages higher than 20 V were not used in these experiments in order to prevent gas bubbles being incorporated within the deposited ceramic matrix. The results from experi￾mental trials using the in situ EPD cell design under vacuum are shown in Fig. 6. The graphs in Fig. 6(a) and (b) show the results of electrophoretic deposit weight and thickness as a function of deposition time for different applied voltages, respectively. EPD experiments were performed for duration of up to 500 s, as this gave a deposit thickness of about 660 µm, which was enough to produce a composite with an acceptable green density. The deposit thick￾ness increased with increasing deposition time, as the amount of particles deposited on to the fibres preform increased. When aqueous based sols are used in EPD experiments, one problem associated with this is the Fig. 5. FEG SEM micrographs, showing the microstructure of (a) uncoated and (b) Ni-coated carbon fibres. It can be seen that the metallic Ni coating around the fibres is very homo￾geneous and it has a uniform thickness of 0.5 µm. The carbon fibres are 10–15 µm in diameter. Fig. 6. Graphs of the (a) electrophoretic deposit weight and (b) electrophoretic thickness as a function of deposition time for different applied voltages. In (c), the deposit formation rate as a function of EPD time under optimised applied voltage of 15 V is shown. electrolysis of the water. Higher voltages resulted in rapid deposit formation, but also in the undesirable formation and entrapment of bubbles within the deposit due to the electrolysis of the aqueous sol dis￾persion medium, while low voltages reduced the elec￾trolysis, but they also needed higher deposition times. Thus, a compromise had to be found and voltage and deposition time were optimised. Figure 6a shows that the increase in weight is almost linear with increasing deposition time (up to 500 s) and voltage. An applied voltage of 20 V seems ideal according to the amount of deposited material, as shown in Fig. 6(a). However, the deposited matrix microstructure is porous as a result of the gases