KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX powder samples obtained from the dried boehmite precursor sols exhibited major endothermic peaks at 100 and 250C, corresponding to the removal of water from boehmite. As seen from the dta curve in Fig. ll, first boehmite transformation from x AlOOH to y-Al2O3 has been completed at 500C and the other phase transformations of transitional alum inas(H-8-0)to a-alumina, as shown in Eq(1), take place at 800, 1050 and 1250C, respectively. The final a-alumina formation is evident with a major 29μm peak at 1250C. The weight lost of boehmite as a 185 um function of temperature is also seen from the TGA ured(starting and finishing weights of the sample tested were 16.2 and 12.8 mg, respectively) 4. CONCLUSIONS EPD was used under vacuum as a novel, quick and cost-effective processing technique to explore the possibility of fabricating nickel coated carbon fibre reinforced alumina matrix composites, which could be densified by a pressureless sintering route. Boehm- ite sol seeded with nano-size d-alumina powders was found to be co matrix after firing for 2 h at 1250C. Unidirectionally Fig 9. FEG SEM micrograph of EPD-infiltrated Ni-coated car- aligned Ni coated carbon fibre-reinforced alumina propagation test(sample sintered at 1250C for 2 h under nitro- matrix composites were produced using an optimised gen atmosphere)showing: (a)the propagation of an indenter- single-stage EPD technique. The process parameters induced crack along the interface between the Ni coating and in terms of applied d. c. voltage and deposition time hatrix, proving that the crack is deflected and arrested at the were found to be 15 V and 400 s, respectively, for interface, (b)the presence of a weak interface between the duc. tile Ni coating and the alumina matrix(Ni: nickel coating, M ull infiltration of the fibre mats used. vacuum alumina matrix and C: carbon fibre) environment during EPD was proven to be effective in reducing the formation of gas bubbles within the deposited green body, resulting in a high degree of excess alumina. Thus, Fig. 10 confirms that the a- deposition and high final density of the component alumina matrix can be produced from seeded boehm- produced. Under vacuum, ultra fine boehmite par ite sol using low sintering temperatures, as low as ticles within the sol could easily penetrate deep into 1250C, in agreement with literature reports [2, 4, 5]. the inter/intra-fibre tows regions, filling the voids and The DTA traces(heating rate: 10C/min) for the thus providing a dense composite Crack path propa "2 Theta Fig. 10. X-ray(CuKo) diffraction patterns for EPD deposited boehmite material, showing the final a-alumina matrix structure after sintering at 1250C for 2 h.1196 KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX Fig. 9. FEG SEM micrograph of EPD-infiltrated Ni-coated car￾bon fibre reinforced alumina matrix composite after crack path propagation test (sample sintered at 1250°C for 2 h under nitro￾gen atmosphere) showing: (a) the propagation of an indenter￾induced crack along the interface between the Ni coating and matrix, proving that the crack is deflected and arrested at the interface, (b) the presence of a weak interface between the duc￾tile Ni coating and the alumina matrix (Ni: nickel coating, M: alumina matrix and C: carbon fibre). excess alumina. Thus, Fig. 10 confirms that the α- alumina matrix can be produced from seeded boehm￾ite sol using low sintering temperatures, as low as 1250°C, in agreement with literature reports [2, 4, 5]. The DTA traces (heating rate: 10°C/min) for the Fig. 10. X-ray (CuKα) diffraction patterns for EPD deposited boehmite material, showing the final α-alumina matrix structure after sintering at 1250°C for 2 h. powder samples obtained from the dried boehmite precursor sols exhibited major endothermic peaks at 100 and 250°C, corresponding to the removal of water from boehmite. As seen from the DTA curve in Fig. 11, first boehmite transformation from γ- AlOOH to γ-Al2O3 has been completed at 500°C and the other phase transformations of transitional alum￾inas (γ–δ–θ) to α-alumina, as shown in Eq. (1), take place at 800, 1050 and 1250°C, respectively. The final α-alumina formation is evident with a major peak at 1250°C. The weight lost of boehmite as a function of temperature is also seen from the TGA curve. A total weight loss of about 20.98% was meas￾ured (starting and finishing weights of the sample tested were 16.2 and 12.8 mg, respectively). 4. CONCLUSIONS EPD was used under vacuum as a novel, quick and cost-effective processing technique to explore the possibility of fabricating nickel coated carbon fibre￾reinforced alumina matrix composites, which could be densified by a pressureless sintering route. Boehm￾ite sol seeded with nano-size δ-alumina powders was found to be converted completely to α-alumina matrix after firing for 2 h at 1250°C. Unidirectionally aligned Ni coated carbon fibre-reinforced alumina matrix composites were produced using an optimised single-stage EPD technique. The process parameters in terms of applied d.c. voltage and deposition time were found to be 15 V and 400 s, respectively, for full infiltration of the fibre mats used. Vacuum environment during EPD was proven to be effective in reducing the formation of gas bubbles within the deposited green body, resulting in a high degree of deposition and high final density of the component produced. Under vacuum, ultra fine boehmite par￾ticles within the sol could easily penetrate deep into the inter/intra-fibre tows regions, filling the voids and thus providing a dense composite. Crack path propa-