M.K. Naskar et al. /Ceramics International 35(2009)3073-307 al strength of the composites calcined at different temperatures. Sample designation Sol viscosity(mPa s) Total no. of infiltration Final sintering temperature (C) Flexural strength(MPa) CZY 10±1(3),5±1(1) 13.67 10±1(3),5±1(1) 10±1(3),5±1(1) 1200 10±1(3),5±1(1) 1400 CAZ 10±1(2),5±1(2) 10±1(2),5±1(2) 444444444 1000 10±1(2),5±1(2) 1200 14.34 10±1(2),5±1(2) 1400 CAS 10±1(1),5±1(3) 10±1(1),5±1(3) 1000 10±1(1),5±1(3) 1200 19.54 10±1(1),5±1(3) 1400 02 10±1(1),5±1(3) 10±1(1),5±1(3) 10±1(1),5±1(3) 1200 14.31 10±1(1),5±1(3) 1400 16.27 ntering temperatu certain loads. Conversely, a brittle ceramic character is noticed observed due to presence of Y2O3 as dopant in ZrO2 host which for the samples sintered at 1200-1400C indicating sudden fall helped to retain t-zrO2(Fig. 4)inhibiting grain growth in the of load elongation curves after certain loads Table 2 shows the fibre-matrix interface. The material sintered at 1200-1400C flexural strength values of the composites sintered at 800- showed ceramic character resulting in higher flexural strength 1400C along with the preceding infiltration steps of the sols values(Fig. 6). The strong interaction at the fibre-matrix e forms. The change in flexural strength with interface developed at 1400C( into the fibre perros for the composites CZY, CAZ, CAS and ceramic in nature. With increasing sintering temperatures, the b). the material becoming sintering temperatur ca is shown in fig. 6. materials became highly dense with the formation of strong The flexural strength of the composite, CZY increased fibre-matrix interface which resulted in higher flexural strength continuously with the increase in sintering temperature from with some sorts of ceramic brittleness 800 to 1400C(Fig. 6). At relatively low sintering It is interesting to point out that the flexural strength of the temperatures i.e.,800-1000C, the flexural strength values composites with an alumina-containing matrix(CAZ, CAS and were quite low but increased sharply after 1000C. The SEM CA)increased slightly with increase in temperature from 800 to fractograph(Fig. 7a) of the sample CZY sintered at 1000C 1000C(Fig. 6). However, it decreased significantly at 1200C after three-point bending test showed pseudo-ductile character followed by slight increase with increase in temperature from with fibre pull-out resulting from the development of weak 1200 to 1400C. The decrease in flexural strength values at fibre-matrix interface. Pseudo-ductility of the material was 1200C is due to the phase transformation of transient Y-Al2O3 to the stable a-Al2O3(Fig 4)accompanying the grain growth of the matrix [18, 19 and fibres at the fibre-matrix interface. The better densification of the materials at 1400C resulted in slight CZY increase of flexural strength values Fig &a and b shows the SEM fractographs of the composites, CAZ and CA respectively sintered at 1000C each after three-point bend test. The fibre pull-out with relatively weak interaction at fibre-matrix interface is revealed from the microstructures. It indicated CAS pseudo-ductility of the composites ga and b shows the fracture surfaces of the composites, CAS and CA respectively sintered at 1400C each. It revealed the formation of strong CAZ bonding between fibres and matrix of the composites indicating brittle ceramic character It is to be noted that for all the composites Czy, CAZ, CAs and CA, pseudo-ductile character was prominent at 1000C showing fibre pull-out of their fracture surfaces. The composites, CAZ, CAS and Ca show the maximum values 出(8= gth at 10oo C,the presence oI y1iuecertain loads. Conversely, a brittle ceramic character is noticed for the samples sintered at 1200–1400 8C indicating sudden fall of load elongation curves after certain loads. Table 2 shows the flexural strength values of the composites sintered at 800– 1400 8C along with the preceding infiltration steps of the sols into the fibre preforms. The change in flexural strength with sintering temperatures for the composites CZY, CAZ, CAS and CA is shown in Fig. 6. The flexural strength of the composite, CZY increased continuously with the increase in sintering temperature from 800 to 1400 8C (Fig. 6). At relatively low sintering temperatures i.e., 800–1000 8C, the flexural strength values were quite low but increased sharply after 1000 8C. The SEM fractograph (Fig. 7a) of the sample CZY sintered at 1000 8C after three-point bending test showed pseudo-ductile character with fibre pull-out resulting from the development of weak fibre–matrix interface. Pseudo-ductility of the material was observed due to presence of Y2O3 as dopant in ZrO2 host which helped to retain t-ZrO2 (Fig. 4) inhibiting grain growth in the fibre–matrix interface. The material sintered at 1200–1400 8C showed ceramic character resulting in higher flexural strength values (Fig. 6). The strong interaction at the fibre–matrix interface developed at 1400 8C (Fig. 7b), the material becoming ceramic in nature. With increasing sintering temperatures, the materials became highly dense with the formation of strong fibre–matrix interface which resulted in higher flexural strength with some sorts of ceramic brittleness. It is interesting to point out that the flexural strength of the composites with an alumina-containing matrix (CAZ, CAS and CA) increased slightly with increase in temperature from 800 to 1000 8C (Fig. 6). However, it decreased significantly at 1200 8C followed by slight increase with increase in temperature from 1200 to 1400 8C. The decrease in flexural strength values at 1200 8C is due to the phase transformation of transient g-Al2O3 to the stable a-Al2O3 (Fig. 4) accompanying the grain growth of the matrix [18,19] and fibres at the fibre–matrix interface. The better densification of the materials at 1400 8C resulted in slight increase of flexural strength values. Fig. 8a and b shows the SEM fractographs of the composites, CAZ and CA respectively sintered at 1000 8C each after three-point bend test. The fibre pull-out with relatively weak interaction at fibre–matrix interface is revealed from the microstructures. It indicated pseudo-ductility of the composites. Fig. 9a and b shows the fracture surfaces of the composites, CAS and CA respectively sintered at 1400 8C each. It revealed the formation of strong bonding between fibres and matrix of the composites indicating brittle ceramic character. It is to be noted that for all the composites CZY, CAZ, CAS and CA, pseudo-ductile character was prominent at 1000 8C showing fibre pull-out of their fracture surfaces. The composites, CAZ, CAS and CA show the maximum values of flexural strength at 1000 8C; the presence of g-Al2O3 in those samples enhanced the flexural strength values. Table 2 Flexural strength of the composites calcined at different temperatures. Sample designation Sol viscosity (mPa s) Total no. of infiltration Final sintering temperature (8C) Flexural strength (MPa) CZY 10  1 (3), 5  1 (1) 4 800 13.67 10  1 (3), 5  1 (1) 4 1000 15.86 10  1 (3), 5  1 (1) 4 1200 21.30 10  1 (3), 5  1 (1) 4 1400 31.08 CAZ 10  1 (2), 5  1 (2) 4 800 18.48 10  1 (2), 5  1 (2) 4 1000 19.90 10  1 (2), 5  1 (2) 4 1200 14.34 10  1 (2), 5  1 (2) 4 1400 14.76 CAS 10  1 (1), 5  1 (3) 4 800 21.71 10  1 (1), 5  1 (3) 4 1000 23.94 10  1 (1), 5  1 (3) 4 1200 19.54 10  1 (1), 5  1 (3) 4 1400 22.02 CA 10  1 (1), 5  1 (3) 4 800 19.98 10  1 (1), 5  1 (3) 4 1000 21.23 10  1 (1), 5  1 (3) 4 1200 14.31 10  1 (1), 5  1 (3) 4 1400 16.27 Note: (i) intermediate sintering temperature = 400 8C/h; (ii) figures in parenthesis indicate the number of infiltration. Fig. 6. Change in flexural strength of the composites, CZY, CAZ, CAS and CA with different temperatures: (&) CZY, (*) = CAZ, (~) = CAS and (^) = CA. M.K. 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