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 suspension1192 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