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