M.K. Naskar et al. /Ceramics International 30(2004)257-265 the decomposable and carbonaceous materials at 800C [17] with the formation of white coloured fibres. Based on this result, the intermediate sintering temperature of 6 800C was selected for multiple infiltration of ZY sol for fabrication of CMCs in the present investigation For the other sols. ie. A. aZ and as. based on their thermogravimetry (TG) results, an intermediate sinter ing temperature of 400C corresponding to the removal 251090501002005009000 of maximum amount of volatiles and decomposable Particle size(nm) materials was selected. Sintering temperatures above 800C proved to be ineffective probably due to the Fig. 6. Particle size distribution of the sol-gel derived ZY particles fibre/matrix interaction. The infiltrated sol, in the inter-fibre region of the sample preforms, after gel formation followed by calci nation may form agglomerates of particles at the inter mediate sintering temperature of 400, 500 or 800 oC which undergoes densification after further sintering at higher temperatures, e. g 1400C. Hence the strength of the CMCs has been found to depend to a great extent on its final sintering temperature which affects the degree of interaction between fibres and matrix. At a because of interfacial reaction. while at a too low tem- perature, the matrix does not sinter adequately This is supported by the strength values of the CMCs sintered at different temperatures (Table 2). Thus, an optimum sintering temperature is needed. It is to be 10 um noted that, in the present study, since the CMCs fabri cated at the final sintering temperature of 1000 oC Fig.7.SEM of the sol-gel derived ZY particles(fillers)showing the exhibited very low flexural strength, i.e. below I MPa, ormation of submicrometre sized particle those values were not presented in Table 2. ze distribution and SEM, respectively of the ZY par- 3.3.3. In-situ deposition of carbon in CMCs ticles used as fillers in the present study. Both the figures Intermediate heat treatment of the Sample nos. ZY5 indicate the formation of submicrometre sized particles. and ZY6 of Table 2 at 500C for I h in air resulted in It is to be noted that although attempts were made to the formation of black coloured materials due to the in increase the flexural strength of CMCs by optimising situ deposition of carbon from the decomposable ace- different process parameters, however, insignificant tate groups present in the infiltrated preforms [17]. The improvements in the results were obtained in these pre- black colour of the above materials is retained after liminary experiments. Further work in this area to final calcination at 1000 and 1400C in N2 atmosphere improve the strength values is under study It is to be noted that the presence of carbon in the developed products corresponding to Sample nos. ZY5 3.3.2. Intermediate and final sintering temperature and zY6 of Table 2 caused an increase in their flexural It has already been mentioned in Section 3.3. 1 that strengths in comparison with that obtained for Sample the multiple infiltration followed by intermediate sin no. ZYl(free from carbon). Further, comparing the tering minimise matrix cracking, thereby enhancing the results of the Sample no. ZYl(modulus value 3 GPa) mechanical properties. Unless the decomposable mate- and ZY5(modulus value 51 GPa), it is observed that the rials present in the green body, after each infiltration, presence of carbon in the fibre/matrix composite mate- are properly removed during intermediate sintering rial significantly increased the modulus values and steps, high strength of the CMCs is difficult to attain. pseudo ductility in the materials. This may be explained The choice of this intermediate sintering temperature, to be due to the fact that the in situ deposition of carbon however, depends on the system under consideration. in the composite material presumably protected the Fourier transform infrared(FTIR) spectra of the sol- fibre/matrix interface from strong interaction and acted gel CaO-doped ZrO, fibres prepared from the zirconium as the crack arrester [18-20]. This is discernible from the acetate sols and calcined at different temperatures from load-displacement curve of the three-point bend test in 30 to 1000C had confirmed the removal of almost all Fig 8 and the fibre pull-out from the SEM of Fig 9 ofsize distribution and SEM, respectively of the ZY par￾ticles used as fillers in the present study. Both the figures indicate the formation of submicrometre sized particles. It is to be noted that although attempts were made to increase the flexural strength of CMCs by optimising different process parameters, however, insignificant improvements in the results were obtained in these pre￾liminary experiments. Further work in this area to improve the strength values is under study. 3.3.2. Intermediate and final sintering temperature It has already been mentioned in Section 3.3.1 that the multiple infiltration followed by intermediate sin￾tering minimise matrix cracking, thereby enhancing the mechanical properties. Unless the decomposable mate￾rials present in the green body, after each infiltration, are properly removed during intermediate sintering steps, high strength of the CMCs is difficult to attain. The choice of this intermediate sintering temperature, however, depends on the system under consideration. Fourier transform infrared (FTIR) spectra of the sol– gel CaO-doped ZrO2 fibres prepared from the zirconium acetate sols and calcined at different temperatures from 30 to 1000 Chad confirmed the removal of almost all the decomposable and carbonaceous materials at 800 C [17] with the formation of white coloured fibres. Based on this result, the intermediate sintering temperature of 800 Cwas selected for multiple infiltration of ZY sol for fabrication of CMCs in the present investigation. For the other sols, i.e. A, AZ and AS, based on their thermogravimetry (TG) results, an intermediate sinter￾ing temperature of 400 Ccorresponding to the removal of maximum amount of volatiles and decomposable materials was selected. Sintering temperatures above 800 Cproved to be ineffective probably due to the fibre/matrix interaction. The infiltrated sol, in the inter-fibre region of the sample preforms, after gel formation followed by calci￾nation may form agglomerates of particles at the inter￾mediate sintering temperature of 400, 500 or 800 C which undergoes densification after further sintering at higher temperatures, e.g. 1400 C. Hence the strength of the CMCs has been found to depend to a great extent on its final sintering temperature which affects the degree of interaction between fibres and matrix. At a too high temperature, the material becomes brittle because of interfacial reaction, while at a too low tem￾perature, the matrix does not sinter adequately [1,3]. This is supported by the strength values of the CMCs sintered at different temperatures (Table 2). Thus, an optimum sintering temperature is needed. It is to be noted that, in the present study, since the CMCs fabri￾cated at the final sintering temperature of 1000 C exhibited very low flexural strength, i.e. below 1 MPa, those values were not presented in Table 2. 3.3.3. In-situ deposition of carbon in CMCs Intermediate heat treatment of the Sample nos. ZY5 and ZY6 of Table 2 at 500 Cfor 1 h in air resulted in the formation of black coloured materials due to the in￾situ deposition of carbon from the decomposable ace￾tate groups present in the infiltrated preforms [17]. The black colour of the above materials is retained after final calcination at 1000 and 1400 Cin N2 atmosphere. It is to be noted that the presence of carbon in the developed products corresponding to Sample nos. ZY5 and ZY6 of Table 2 caused an increase in their flexural strengths in comparison with that obtained for Sample no. ZY1 (free from carbon). Further, comparing the results of the Sample no. ZY1 (modulus value 3 GPa) and ZY5 (modulus value 51 GPa), it is observed that the presence of carbon in the fibre/matrix composite mate￾rial significantly increased the modulus values and pseudo ductility in the materials. This may be explained to be due to the fact that the in situ deposition of carbon in the composite material presumably protected the fibre/matrix interface from strong interaction and acted as the crack arrester [18–20]. This is discernible from the load–displacement curve of the three-point bend test in Fig. 8 and the fibre pull-out from the SEM of Fig. 9 of Fig. 7. SEM of the sol-gel derived ZY particles (fillers) showing the formation of submicrometre sized particles. Fig. 6. Particle size distribution of the sol-gel derived ZY particles (fillers). 262 M.K. Naskar et al. / Ceramics International 30 (2004) 257–265