L Mariappan er al. / Materials Chemistry and Physics 75(2002) 284-290 y controlling the particle size to less than I um, complete case of only zirconia as the matrix, though the absolute stabilisation in tetragonal form could be obtained up to content of tetragonal form increases, the ratio of tetragonal its content of 15 vol. in the sample without the addition to monoclinic is reduced. XRD figures presented reveal the of stabilisers. Further increase in the volume percent re- fine changes occurring in various formation reactions of sults in a part of the zirconia remaining in the monoclinic tetragonal zirconia with respect to changes in composition form. This was attributed to the higher elastic modulus of Typical XRD patterns obtained at 1650oC with AC for alumina acting as a constraining matrix on the tetragonal various compositions and at various temperatures for the zirconia. Above 15 vol. the effect of constraint is reduced composition 1535 are presented in Figs. 3 and 4. Typical and therefore a part reverts to monoclinic. Since the possi- SEM pictures of product powders obtained at 1650C with bility of generating tetragonal zirconia from a carbothermal AC and CB are presented in Figs. 5 and 6 reaction was reported to be difficult by Chaklader et al. It is seen from the SEM micrographs from Fig. 5 that [10], the initial compositions were formulated to generate the whiskers are thicker in the case of AC even when all a ZTA matrix wherein the toughening is mainly due to the the other parameters are constant. This could be due to the effect of microcracking. Hence 20 and 25 vol. of zirconia high reactivity of AC at these temperatures, and the gener was also taken in the compositions for this study. In the ation of Sio vapours would be far faster resulting in fewer 1650C/hr 60 m→)ax-Al2O3;t→β-SiC;A→m-ZrO2G→+t-zrO2 Fig. 4. Product phases obtained at different temperatures for the composition 1535 with AC.(m)a-Al2O3,(t)B-SiC,(A)m-ZrO2,(G)t-ZrO2,(B) mullite, ()zircon and (u) aluminium oxy carbide.288 L. Mariappan et al. / Materials Chemistry and Physics 75 (2002) 284–290 by controlling the particle size to less than 1 m, complete stabilisation in tetragonal form could be obtained up to its content of 15 vol.% in the sample without the addition of stabilisers. Further increase in the volume percent re￾sults in a part of the zirconia remaining in the monoclinic form. This was attributed to the higher elastic modulus of alumina acting as a constraining matrix on the tetragonal zirconia. Above 15 vol.% the effect of constraint is reduced and therefore a part reverts to monoclinic. Since the possi￾bility of generating tetragonal zirconia from a carbothermal reaction was reported to be difficult by Chaklader et al. [10], the initial compositions were formulated to generate a ZTA matrix wherein the toughening is mainly due to the effect of microcracking. Hence 20 and 25 vol.% of zirconia was also taken in the compositions for this study. In the Fig. 4. Product phases obtained at different temperatures for the composition 1535 with AC. (m)
-Al2O3, (t) -SiC, (A) m-ZrO2, (G) t-ZrO2, (B) mullite, (I) zircon and (u) aluminium oxy carbide. case of only zirconia as the matrix, though the absolute content of tetragonal form increases, the ratio of tetragonal to monoclinic is reduced. XRD figures presented reveal the fine changes occurring in various formation reactions of tetragonal zirconia with respect to changes in composition. Typical XRD patterns obtained at 1650 ◦C with AC for various compositions and at various temperatures for the composition 1535 are presented in Figs. 3 and 4. Typical SEM pictures of product powders obtained at 1650 ◦C with AC and CB are presented in Figs. 5 and 6. It is seen from the SEM micrographs from Fig. 5 that the whiskers are thicker in the case of AC even when all the other parameters are constant. This could be due to the high reactivity of AC at these temperatures, and the gener￾ation of SiO vapours would be far faster resulting in fewer