L. Mariappan et al. / Materials Chemistry and Physics 75(2002)284-29 Fig. 5. Typical SEM micrographs of the composite precursor powders with AC:(a)1535 and (b)1540 the volume fractions present, which could in part be due to nucleation sites for the whiskers to grow. On the other particles of SiC other than whiskers. Reaction conditions hand, the whiskers grown with CB from Fig. 6 show leaner and longer whiskers, which indicates that the reaction had seem to play a role in this distribution Decomposition of Sic does not occur because SiC is gone on for sufficient time and was kinetically slower. This stable with respect to both Al2 O3 and ZrO2 at these temper- resulted in the whiskers nucleating at a more number of atures. Zircon gives the monoclinic form immediately after sites and followed by an orderly growth decomposition, but at these temperatures the thermodynam ically stable form is tetragonal zirconia. As temperature increases there is grain growth. When the reacted powders 4. Discussion are cooled to room temperature, any particles below a crit ical size remain in the tetragonal form and those above this The variations observed with different carbon sources critical size transform to monoclinic. The critical size for could be attributed due to the fine particle si of this transformation is determined by the Al2O3 and Sic ursors and the high reactivity associated with AC at high matrix. This explains the presence of only monoclinic zir temperatures. AC with its fine microporous structure and conia in some cases after the treatment at 1550C, largely higher surface area gives a larger area with which the Sio tetragonal at 1650C for AC and at 1700C for CB. The vapours can react. Also at pressures higher than ambient reduction in the volume fractions of t-ZrOz at 1700C for and carbon ratios greater than unity, aluminium carbide AC could be due to grain growth formation could take place since this occurs at 1867C in the system Al2O3-SiC-C at ambient pressure and a carbon content of unity [11]. Indeed, this is what is observed for 5. Conclusions compositions with AC at 1700C. Whisker morphology is en for SiC at all reaction temperatures. It is to be noted Al2O3 ZrO2 SiCw composite powders were syn that the Sic is not all in whisker form. The XRD data gives thesised by the carbothermal reduction of cheap naturalL. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 289 Fig. 5. Typical SEM micrographs of the composite precursor powders with AC: (a) 1535 and (b) 1540. nucleation sites for the whiskers to grow. On the other hand, the whiskers grown with CB from Fig. 6 show leaner and longer whiskers, which indicates that the reaction had gone on for sufficient time and was kinetically slower. This resulted in the whiskers nucleating at a more number of sites and followed by an orderly growth. 4. Discussion The variations observed with different carbon sources could be attributed due to the fine particle sizes of the pre￾cursors and the high reactivity associated with AC at high temperatures. AC with its fine microporous structure and higher surface area gives a larger area with which the SiO vapours can react. Also at pressures higher than ambient and carbon ratios greater than unity, aluminium carbide formation could take place since this occurs at 1867 ◦C in the system Al2O3–SiC–C at ambient pressure and a carbon content of unity [11]. Indeed, this is what is observed for compositions with AC at 1700 ◦C. Whisker morphology is seen for SiC at all reaction temperatures. It is to be noted that the SiC is not all in whisker form. The XRD data gives Fig. 6. Typical SEM micrographs of the composite precursor powders with CB: (a) 1535 and (b) 1540. the volume fractions present, which could in part be due to particles of SiC other than whiskers. Reaction conditions seem to play a role in this distribution. Decomposition of SiC does not occur because SiC is stable with respect to both Al2O3 and ZrO2 at these temper￾atures. Zircon gives the monoclinic form immediately after decomposition, but at these temperatures the thermodynam￾ically stable form is tetragonal zirconia. As temperature increases there is grain growth. When the reacted powders are cooled to room temperature, any particles below a crit￾ical size remain in the tetragonal form and those above this critical size transform to monoclinic. The critical size for this transformation is determined by the Al2O3 and SiC matrix. This explains the presence of only monoclinic zir￾conia in some cases after the treatment at 1550 ◦C, largely tetragonal at 1650 ◦C for AC and at 1700 ◦C for CB. The reduction in the volume fractions of t-ZrO2 at 1700 ◦C for AC could be due to grain growth. 5. Conclusions Al2O3 + ZrO2 + SiCw composite powders were syn￾thesised by the carbothermal reduction of cheap natural