M.K. Naskar et al. /Ceramics International 35(2009)3073-3079 Manometer Vacuam R MM减 W wh Flar Specimen holder Fig 3. XRD pattern and SEM image (inset)of high alumina fibre pref 3. Results and discussion The XRD of the precursor preforms used in the present investigation indicated the presence of 8-Al2O3 as the only Sol-二-二 crystallized phase(Fig. 3). The microstructure of the fibre preforms shows the presence of inter-fibre pores and voids (inset of Fig. 3). It indicates that the fibres are circular in ag. 2. Schemati of the sol infitration set-up for the fabrication of near-net- diameter with range of 3-7 um. The presence of considerable amount of inter-fibre pores and voids is evident from the microstructure Infiltration of the sols in the preform is expected 2.3. Characterization of the materials to fill these inter-fibre pores and voids, leading to the formation of the continuous phase, i.e., matrix. The infiltrates(sols) were characterized by measuring their In the present investigation, the fibre preforms were pH with a Jencons pH meter(Model 3030, Jencons Scientific infiltrated with the sols of viscosity 10+ 1 mPa s followed Ltd, Bedfordshire, U. K. while the viscosity values were by intermediate heating at 400C. Finally, infiltrations were recorded using a Brookfield viscometer(Model LVTDV Il, carried out with the sols of viscosity 5t I mPa s which helped Brookfield Laboratories, Inc, MA, USA). to minimize matrix cracking and improve mechanical strength Crystallization behaviour of the matrix materials, fibre of CMCs [17]. It is to be noted that most of the interconnected preform and the composites after firing at different tempera- pores and voids of the fibre preform were filled with the sol of tures i. e, 800-1400C was studied by X-ray diffraction(XRD) higher viscosity followed by filling of residual pores and voids (Philips PW-1730 Philips Corporation, Almelo, the Nether- with the sol of lower viscosity lands) with Ni-filtered CuKo radiation. The flexural strength of Crystallization features of the matrix (in the absence of fibres the composites was determined by Instron Universal Testing and fibre preform calcined at different temperatures i.e., 800- Machine, U. K.(Model: 5500 R). The samples of dimensions 1400C are summarized in Tables la and lb respectively. In case 40 mm x 7 mm x 6 mm were cut from the infiltrated preform of MZY (M stands for matrix)matrix in zirconia-yttria system, a for testing. For the measurement of flexural strength, three- mixture of monoclinic(m-)and tetragonal (t-) zirconia(zrO2 point bend test under a crosshead speed of 0.5 mm/min was was found at 800-1000C while they transformed to cubic(c-) performed. The three-point bending stress, o is determined by and t-zrO2 at 1200-1400C. For MAZ (M stands for matrix) the following equation [2] matrix in alumina-zirconia system, delta(8-), gamma(Y-)Al2O3 and t-ZrO, were obtained at 800-1000C while a-AlO3, t-and c-zrO2 were found at 1200-1400C. The appearance of y-Al2O with amorphous silica was revealed for the sample MAS (M stands for matrix) matrix in alumina silica system at 800- where P is the load at break, L is the span length and b and h 1000'C followed by their transformation to orthorhombic are the width and thickness of the test specimen respectively. mullite phase at 1200-1400C. The sample MA (M stands Each strength datum is an average over six samples. Micro- for matrix)matrix in alumina system showed y-Al2O3 at 800oC, structural studies of the fibre preform and the composite y-and 8-Al2O3 at 1000C and a-Al,O3 at 1200-1400C. materials were performed by scanning electron microscopy The fibre preforms(FP) after sintering up to 1200C (SEM)(S 4301, LEO Electronic Microscopy Ltd, Cam- crystallized with 8-and B-Al2O3 along with a trace amount of bridge, U. K) a-Al2O3 and orthorhombic mullite phase (Table 1b). A2.3. Characterization of the materials The infiltrates (sols) were characterized by measuring their pH with a Jencons pH meter (Model 3030, Jencons Scientific Ltd., Bedfordshire, U.K.) while the viscosity values were recorded using a Brookfield viscometer (Model LVTDV II, Brookfield Laboratories, Inc., MA, USA). Crystallization behaviour of the matrix materials, fibre preform and the composites after firing at different tempera￾tures i.e., 800–1400 8C was studied by X-ray diffraction (XRD) (Philips PW-1730 Philips Corporation, Almelo, the Nether￾lands) with Ni-filtered CuKa radiation. The flexural strength of the composites was determined by Instron Universal Testing Machine, U.K. (Model: 5500 R). The samples of dimensions 40 mm  7 mm  6 mm were cut from the infiltrated preform for testing. For the measurement of flexural strength, three￾point bend test under a crosshead speed of 0.5 mm/min was performed. The three-point bending stress, s is determined by the following equation [2]: s ¼ 3PL 2bh2 where P is the load at break, L is the span length and b and h are the width and thickness of the test specimen respectively. Each strength datum is an average over six samples. Micro￾structural studies of the fibre preform and the composite materials were performed by scanning electron microscopy (SEM) (S 430i, LEO Electronic Microscopy Ltd., Cam￾bridge, U.K.). 3. Results and discussion The XRD of the precursor preforms used in the present investigation indicated the presence of d-Al2O3 as the only crystallized phase (Fig. 3). The microstructure of the fibre preforms shows the presence of inter-fibre pores and voids (inset of Fig. 3). It indicates that the fibres are circular in diameter with range of 3–7 mm. The presence of considerable amount of inter-fibre pores and voids is evident from the microstructure. Infiltration of the sols in the preform is expected to fill these inter-fibre pores and voids, leading to the formation of the continuous phase, i.e., matrix. In the present investigation, the fibre preforms were infiltrated with the sols of viscosity 10  1 mPa s followed by intermediate heating at 400 8C. Finally, infiltrations were carried out with the sols of viscosity 5  1 mPa s which helped to minimize matrix cracking and improve mechanical strength of CMCs [17]. It is to be noted that most of the interconnected pores and voids of the fibre preform were filled with the sol of higher viscosity followed by filling of residual pores and voids with the sol of lower viscosity. Crystallization features of the matrix (in the absence of fibres) and fibre preform calcined at different temperatures i.e., 800– 1400 8C are summarized inTables 1a and 1b respectively. In case of MZY (M stands for matrix) matrix in zirconia–yttria system, a mixture of monoclinic (m-) and tetragonal (t-) zirconia (ZrO2) was found at 800–1000 8C while they transformed to cubic (c-) and t-ZrO2 at 1200–1400 8C. For MAZ (M stands for matrix) matrix in alumina–zirconia system, delta (d-), gamma (g-) Al2O3 and t-ZrO2 were obtained at 800–1000 8C while a-Al2O3, t- and c-ZrO2 were found at 1200–1400 8C. The appearance of g-Al2O3 with amorphous silica was revealed for the sample MAS (M stands for matrix) matrix in alumina silica system at 800– 1000 8C followed by their transformation to orthorhombic mullite phase at 1200–1400 8C. The sample MA (M stands for matrix) matrix in alumina system showed g-Al2O3 at 800 8C, g- and d-Al2O3 at 1000 8C and a-Al2O3 at 1200–1400 8C. The fibre preforms (FP) after sintering up to 1200 8C crystallized with d- and u-Al2O3 along with a trace amount of a-Al2O3 and orthorhombic mullite phase (Table 1b). A Fig. 2. Schematic of the sol infiltration set-up for the fabrication of near-net￾shape ceramic fibre reinforced ceramic matrix composites. Fig. 3. XRD pattern and SEM image (inset) of high alumina fibre preform. M.K. Naskar et al. / Ceramics International 35 (2009) 3073–3079 3075