KAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX especially at the fibre-fibre contact points. The final the matrix, which resulted from the contact with the green and sintered densities and, hence, the mechan- diamond saw during the very slow-speed cutting ical properties of the final composites made from operation. Dense composite samples were obtained these fabrics, will be poor if these regions are not by pressureless sintering at 1250C for 2 h under N2 fully infiltrated with matrix material during EPD. The atmosphere. A fully infiltrated and dense sintered FEG-SEM pictures shown in Fig. 7 represent the microstructure is shown in Fig 8. Sintered densities green microstructures of composites produced under of about 91% of TD were measured by using the optimised EPD parameters. Figure 7(a) shows a SEM Archimedes technique. This result confirms that micrograph of an EPD-formed green body, containing present processing approach is very effective in 30 vol% fibre loading. High green densities (about obtaining dense CMCs samples at relatively low sin- 61% of TD) were achieved at 15 V for 400 s Green tering temperatures and avoiding the cost-intensive density was measured by dividing the mass of the hot-pressing fabrication procedure, which has been ample by its geometrically determined volume. It the common practise in previous studies on fibre- can also be seen in Fig. 7(b) that the Ni-coated carbon reinforced CMCs [27 fibre preform was fully infiltrated with the boehmite A crack path propagation test was performed on sol. Even regions where the Ni-coated fibres were sintered samples to characterise the Ni interface nearly touching each other were fully impregnated by behaviour in terms of crack deflection and debonding. the nano-size boehmite powders in a very short time, Fig. 9(a) shows the interactions between an indenter- i.e. 400 s, leading to high-quality green bodies. The induced crack created within the alumina matrix and effectiveness of the EPD providing full deposition the ductile Ni interface. The crack first interacts wit between two fibres with a separation of 400-500 nm the Ni coating and then it is deflected at the interface is clearly visible in Fig. 7(b). It must be noted that One can conclude from the crack path in Fig. 9(a) these green samples were not polished in order to that a weak bonding between the Ni coating and the avoid damaging the ductile Ni interface. Thus, some alumina matrix exists. Thus, no significant chemical cutting effects are visible on the carbon fibres and reaction between the Ni interface and the alumin matrix at the sintering temperature, i.e. 1250.C, has occurred, as shown in Fig. 9(b). From this picture, it is also evident that the Ni coating provides a useful weak interface between carbon fibres and alumina matrix, resulting in crack deflection and debonding These mechanisms should result in an overall pseudo-ductile fracture mode of the composite since catastrophic failure is expected to be eliminated by the operation of crack deflection and pull-out mec anisms [28 X-ray diffraction patterns for the powders extracted from the regions between each layer of the nickel 7.5 um coated carbon fibre preforms after calcination at 1250C for 2 h are shown in Fig. 10. All EPd formed mples produced pure a-alumina peaks, with no additional peaks corresponding to non-transformed 273pr Fig. 7. FEG SEM micrographs of EPD- infiltrated unsintered Ni-coated carbon fibre reinforced alumina matrix composite containing 30 vol% fibre loading. The fibre preform was infil- trated using an applied voltage of 15 V for 400 s Both(a) high Fig. 8. FEG SE uum EPD-infiltrated N and(b) low magnification micrographs show that the Ni-coated coated Carbon d alumina matrix composite con- carbon fibre preform was fully infiltrated with the boehmite taining 30 vol% after sintering at 1250%C for 2 h sol. Even regions where the Ni-coated fibres were nearly touch- in nitrogen atmosphere. The micrograph shows the full depo- ing each other have been fully impregnated by the nanosize sition of the boehmite matrix into the fibre preform and a dense boehmite particles, leading to high-quality green bodies. microstructure afterKAYA et al.: FABRICATION AND CHARACTERISATION OF ALUMINA CERAMIC MATRIX 1195 especially at the fibre–fibre contact points. The final green and sintered densities and, hence, the mechan￾ical properties of the final composites made from these fabrics, will be poor if these regions are not fully infiltrated with matrix material during EPD. The FEG-SEM pictures shown in Fig. 7 represent the green microstructures of composites produced under optimised EPD parameters. Figure 7(a) shows a SEM micrograph of an EPD-formed green body, containing 30 vol% fibre loading. High green densities (about 61% of TD) were achieved at 15 V for 400 s. Green density was measured by dividing the mass of the sample by its geometrically determined volume. It can also be seen in Fig. 7(b) that the Ni-coated carbon fibre preform was fully infiltrated with the boehmite sol. Even regions where the Ni-coated fibres were nearly touching each other were fully impregnated by the nano-size boehmite powders in a very short time, i.e. 400 s, leading to high-quality green bodies. The effectiveness of the EPD providing full deposition between two fibres with a separation of 400–500 nm is clearly visible in Fig. 7(b). It must be noted that these green samples were not polished in order to avoid damaging the ductile Ni interface. Thus, some cutting effects are visible on the carbon fibres and Fig. 7. FEG SEM micrographs of EPD-infiltrated unsintered Ni-coated carbon fibre reinforced alumina matrix composite containing 30 vol% fibre loading. The fibre preform was infil￾trated using an applied voltage of 15 V for 400 s. Both (a) high and (b) low magnification micrographs show that the Ni-coated carbon fibre preform was fully infiltrated with the boehmite sol. Even regions where the Ni-coated fibres were nearly touch￾ing each other have been fully impregnated by the nanosize boehmite particles, leading to high-quality green bodies. the matrix, which resulted from the contact with the diamond saw during the very slow-speed cutting operation. Dense composite samples were obtained by pressureless sintering at 1250°C for 2 h under N2 atmosphere. A fully infiltrated and dense sintered microstructure is shown in Fig. 8. Sintered densities of about 91% of TD were measured by using the Archimedes technique. This result confirms that the present processing approach is very effective in obtaining dense CMCs samples at relatively low sin￾tering temperatures and avoiding the cost-intensive hot-pressing fabrication procedure, which has been the common practise in previous studies on fibre￾reinforced CMCs [27]. A crack path propagation test was performed on sintered samples to characterise the Ni interface behaviour in terms of crack deflection and debonding. Fig. 9(a) shows the interactions between an indenter￾induced crack created within the alumina matrix and the ductile Ni interface. The crack first interacts with the Ni coating and then it is deflected at the interface. One can conclude from the crack path in Fig. 9(a) that a weak bonding between the Ni coating and the alumina matrix exists. Thus, no significant chemical reaction between the Ni interface and the alumina matrix at the sintering temperature, i.e. 1250°C, has occurred, as shown in Fig. 9(b). From this picture, it is also evident that the Ni coating provides a useful weak interface between carbon fibres and alumina matrix, resulting in crack deflection and debonding. These mechanisms should result in an overall ‘pseudo-ductile’ fracture mode of the composite since catastrophic failure is expected to be eliminated by the operation of crack deflection and pull-out mech￾anisms [28]. X-ray diffraction patterns for the powders extracted from the regions between each layer of the nickel coated carbon fibre preforms after calcination at 1250°C for 2 h, are shown in Fig. 10. All EPD formed samples produced pure α-alumina peaks, with no additional peaks corresponding to non-transformed Fig. 8. FEG SEM micrograph of vacuum EPD-infiltrated Ni￾coated Carbon fibre reinforced alumina matrix composite con￾taining 30 vol% fibre loading after sintering at 1250°C for 2 h in nitrogen atmosphere. The micrograph shows the full depo￾sition of the boehmite matrix into the fibre preform and a dense composite microstructure after sintering