n.K. Naskar et al. Ceramics International 30(2004)257-265 Table XRD results of CMCs calcined at different temperatures Sol Calcination Duration Crystalline designation temperature (h) hases c+t-ZrO, 0.00004 0.08 0.12 Fig 8. Load-displacement curve of sample no. ZY6 of Table mullite (orthorhombic) D-Al,O3+t-ZrO 25μm Fig 9. SEM of the fracture surface of the carbon containing CMc after three-point bend test(sample no. ZY6 of Table 2) the same sample. Therefore, instead of using carbon 15m deposited carbon in the bulk matrix may also modify Fig. 10. SEM of the fracture surface of the CMC(sample no. A2 of the fibre/matrix interface and act as the crack arrester 3.3.4. Crystallisation behaviour of the CMCs CMCs fabricated after calcining at 1000, 1200 and transformation of the transient phases, e.g. y-and 8-Al20 1400C, each with a dwell time of I h, were examined to the stable phase a-AlO3 during heat-treatment up to by XRD and the identified crystalline phases are pre- 1400C followed by grain growth with temperature sented in Table 3. It is to be noted that the phases other and thereby degrading the mechanical properties than mullite(fibre preform) crystallised fferent temperatures from the matrix part of the CMCs have Addition of 13 mol% ZrO, to the AlO3 matrix, been listed in Table 3 however, exhibited an increase in the flexural strength at Considering the flexural strength values in Table 2 the same temperature. Crystallisation of t-ZrO2 in the and XRD phases in Table 3, it is to be noted that the alumina matrix is believed to inhibit the growth of the MCs fabricated from the alumina sol exhibited the a-AlO3, resulting in the increase in strength lowest flexural strength compared with those developed [23, 24]. Similarly, incorporation of 40 mol% SiO, in the from the other three sols, irrespective of the final sin- alumina matrix caused crystallisation of orthorhombic tering temperature. The fracture surface of the Sample mullite at 1200-1400C and thereby helped to increase no. A2 of Table 2 as a typical case, under SEM showed the flexural strength of the fabricated CMCs(Table 2) the presence of considerable amounts of cracks in the [25]. Further, XRD of the CMCs infiltrated with the Z matrix part of the matenals, as indicated in Fig. 10 The sol followed by sintering at the 1400oC confirmed the above phenomenon may be due to the reconstructive presence of both c-and t-ZrO2, along with the mullitethe same sample. Therefore, instead of using carbon￾coated fibres as the reinforcement agents, the in situ deposited carbon in the bulk matrix may also modify the fibre/matrix interface and act as the crack arrester. 3.3.4. Crystallisation behaviour of the CMCs CMCs fabricated after calcining at 1000, 1200 and 1400 C, each with a dwell time of 1 h, were examined by XRD and the identified crystalline phases are pre￾sented in Table 3. It is to be noted that the phases other than mullite (fibre preform) crystallised at different temperatures from the matrix part of the CMCs have been listed in Table 3. Considering the flexural strength values in Table 2 and XRD phases in Table 3, it is to be noted that the CMCs fabricated from the alumina sol exhibited the lowest flexural strength compared with those developed from the other three sols, irrespective of the final sin￾tering temperature. The fracture surface of the Sample no. A2 of Table 2 as a typical case, under SEM showed the presence of considerable amounts of cracks in the matrix part of the matenals, as indicated in Fig. 10 The above phenomenon may be due to the reconstructive transformation of the transient phases, e.g. g- and d-Al2O3 to the stable phase a-Al2O3 during heat-treatment up to 1400 Cfollowed by grain growth with temperature, and thereby degrading the mechanical properties [21,22]. Addition of 13 mol% ZrO2 to the Al2O3 matrix, however, exhibited an increase in the flexural strength at the same temperature. Crystallisation of t-ZrO2 in the alumina matrix is believed to inhibit the growth of the a-Al2O3, resulting in the increase in strength values [23,24]. Similarly, incorporation of 40 mol% SiO2 in the alumina matrix caused crystallisation of orthorhombic mullite at 1200–1400 Cand thereby helped to increase the flexural strength of the fabricated CMCs (Table 2) [25]. Further, XRD of the CMCs infiltrated with the ZY sol followed by sintering at the 1400 Cconfirmed the presence of both c- and t- ZrO2, along with the mullite Fig. 8. Load–displacement curve of sample no. ZY6 of Table 2. Fig. 9. SEM of the fracture surface of the carbon containing CMC after three-point bend test (sample no. ZY6 of Table 2). Table 3 XRD results of CMCs calcined at different temperatures Sol designation Calcination temperature ( C) Duration (h) Crystalline phases ZY 1000 1 c-+t-ZrO2 1200 1 c-+t-ZrO2 1400 1 c-+t-ZrO2 A 1000 1 g+d-Al2O3 1200 1 a-Al2O3 1400 1 a-Al2O3 AS 1000 1 g-Al2O3+SiO2 (amorphous) 1200 1 Mullite (orthorhombic) 1400 1 Mullite (orthorhombic) AZ 1000 1 g-Al2O3+t-ZrO2 1200 1 a-Al2O3+t-ZrO2 1400 1 a-Al2O3+t-ZrO2 Fig. 10. SEM of the fracture surface of the CMC (sample no. A2 of Table 2) showing multiple fracture of the matrix. M.K. Naskar et al. / Ceramics International 30 (2004) 257–265 263