H Miyazaki et al Journal of the European Ceramic Society 26(2006)3539-3546 3545 Table 3 4-point bending strength and Weibull modulus of the fibrous composites, the powder- mixture composites and constituent monoliths Fibrous composites Zirconia Powder-mixture composites Average strength(MPa) 496 Weibull modulus 19 9 S D. standard deviation: No. number of specimen. a volume fraction of ZrO2(%). since the strength becomes less sensitive to the distribution of Appendix A flaw size when its fracture toughness increases. The fact that there was no significant increase in strength of the composites AI and 2 are defined as follows. over that of the monolithic alumina, despite the improved frac- suppose that the mismatch in sintering behavior between the A1=--(E/EDv+( -)CrEr/e ture toughness, suggests that the flaw size in the composites was 1-(1-E/E)(1-w)/2+cm(m-u) larger than that in the monolithic alumina. it is reasonable to two phases not only inhibited full densification of the compos ites but also introduced the fracture origin, which lowered the strength. Although the fracture toughness of the powder-mixture [1-(-E/En/2](+m)+(1+cm-)/2 composites was lower than those of the fibrous composites, the A2 strength of the powder-mixture composites was higher than that of the fibrous composites, which indicates that the flaw size he powder-mixture composites was smaller than that of the where fibrous composites. The decrease in Aaw size in the powder- mixture composites is contributed to the fact that the grain size 4=1+vf Vm -)cref (A.3) in the powder-mixture composites was much finer than that in the fibrous composites E=Cr et+cmE (A4) 4. Conclusion References Fibrous zirconia/alumina composites with different compo- sitions were fabricated by piston co-extrusion. A fine-scale and 1. Clegg, W.J., Kendall, K, McN. Alford, N,Button, T. W and Birchall, J ligned microstructure with no thermal cracking was obtained D, A simple way to make tough ceramics. Nature, 1990, 347, 455-457 for all the compositions following three extrusion steps. The 2.Kovar, D,King.BH,Trice,RWand Halloran, J.W,Fibrous monolithic Youngs modulus of the composites followed the well-known 3. Prakash, O. Sarkar. P. and Nicholson, P. S. Crack deflection in Voigt rule-of-mixture. All these composites attained higher ceramic/ceramic laminates with strong interfaces. J. Am. Cera. Soc.. 1995 toughness than that of the constituent monolithic ceramics with no degradation in the bending strength. The fracture 4. Menon, M and Chen, 1 w, Bimaterial composites vie colloidal ro toughness was optimized at the composition of 47/53 vol% niques: Ill, mechanical properties. J.A. Ceram Soc., 1999 zirconialalumina and the maximum fracture toughness of 5. Rao, M. P, Sanchez-Herencia, A.J. Beltz, G. E, McMee Lange, F. F, Laminar ceramics that exhibit a threshold 6.2 MPam" was attained. The effect of the aligned fibrous 1999,286,102-105 microstructure on the toughness improvement was through a 6. Dakskobler, A, Kosmac, T and Chen, I. W, Paraffin-based process for crack deflection mechanism produced by thermal residual stress at the interface, as well as by"stress-induced"transformation Soc.,2002,85,1013-1015 of zirconia. The variation in the fracture toughness among these 7. Adachi, T, Sekino, T, Kusunose, T, Nakayama, T, Hikasa, A, Choa,Y.H and Niihara, K, Crack propagation behavior of nano-sized SiC dispersed composites was explained by the variation in the contributions multilayered Al2O3/3Y-TZP hybrid composites. J. Ceram. Soc. Jpn, 2003 from each mechanism. The Weibull modulus of the fibrous composites increased owing to the increment in fracture tough- 8. Poulon-Quintin, A. Berger, M. H, Bunsell, AR Kaya, C, Butler, E. G ness. while the strength remained almost the same as that of Wootton, A. et al, Processing and structures of bi-phase oxide ceram monolithic alumina, indicating that favorable influence of the filaments. J. Eur Ceram Soc., 2004. 24, 101-110 increased fracture toughness was offsetted by the increment in 9. Lee, B. T, Kim, K. H. and Han, J.K., Microstructures and material proper- ties of fibrous Al2O3-(m-ZrO2 )i-ZrO2 composites fabricated by a fibrousH. Miyazaki et al. / Journal of the European Ceramic Society 26 (2006) 3539–3546 3545 Table 3 4-point bending strength and Weibull modulus of the fibrous composites, the powder-mixture composites and constituent monoliths Sample Alumina Fibrous composites Zirconia Powder-mixture composites 0a 10a 31a 47a 66a 88a 100a 10a 50a 90a Average strength (MPa) 496 434 478 545 567 586 859 637 891 878 S.D. (MPa) 40 13 31 28 33 58 134 123 97 105 Weibull modulus 11 30 15 19 18 10 6 5 9 8 No. 9 9 9 9 9 10 11 12 11 10 S.D.: standard deviation; No.: number of specimen. a Volume fraction of ZrO2 (%). since the strength becomes less sensitive to the distribution of flaw size when its fracture toughness increases. The fact that there was no significant increase in strength of the composites over that of the monolithic alumina, despite the improved frac￾ture toughness, suggests that the flaw size in the composites was larger than that in the monolithic alumina. It is reasonable to suppose that the mismatch in sintering behavior between the two phases not only inhibited full densification of the compos￾ites but also introduced the fracture origin, which lowered the strength. Although the fracture toughness of the powder-mixture composites was lower than those of the fibrous composites, the strength of the powder-mixture composites was higher than that of the fibrous composites, which indicates that the flaw size in the powder-mixture composites was smaller than that of the fibrous composites. The decrease in flaw size in the powder￾mixture composites is contributed to the fact that the grain size in the powder-mixture composites was much finer than that in the fibrous composites. 4. Conclusion Fibrous zirconia/alumina composites with different compo￾sitions were fabricated by piston co-extrusion. A fine-scale and aligned microstructure with no thermal cracking was obtained for all the compositions following three extrusion steps. The Young’s modulus of the composites followed the well-known Voigt rule-of-mixture. All these composites attained higher toughness than that of the constituent monolithic ceramics with no degradation in the bending strength. The fracture toughness was optimized at the composition of 47/53 vol% zirconia/alumina and the maximum fracture toughness of 6.2 MPa m1/2 was attained. The effect of the aligned fibrous microstructure on the toughness improvement was through a crack deflection mechanism produced by thermal residual stress at the interface, as well as by “stress-induced” transformation of zirconia. The variation in the fracture toughness among these composites was explained by the variation in the contributions from each mechanism. The Weibull modulus of the fibrous composites increased owing to the increment in fracture tough￾ness, while the strength remained almost the same as that of monolithic alumina, indicating that favorable influence of the increased fracture toughness was offsetted by the increment in the flaw size. Appendix A λ1 and λ2 are defined as follows. λ1 = 1 − (1 − E/Ef)(1 − νf)/2 + cm(νm − νf)/2 −(E/Ef) νf + (νm − νf)cfEf/E2 (1 − νm)∆ , (A.1) λ2 = 1 − (1 − E/Ef)/2  (1 + νf) + (1 + cf)(νm − νf)/2 ∆ , (A.2) where ∆ = 1 + νf + (νm − νf)cfEf E (A.3) and E = cfEf + cmEm (A.4) References 1. Clegg, W. J., Kendall, K., McN. Alford, N., Button, T. W. and Birchall, J. D., A simple way to make tough ceramics. Nature, 1990, 347, 455–457. 2. Kovar, D., King, B. H., Trice, R. W. and Halloran, J. W., Fibrous monolithic ceramics. J. Am. Ceram. Soc., 1997, 80, 2471–2487. 3. Prakash, O., Sarkar, P. and Nicholson, P. S., Crack deflection in ceramic/ceramic laminates with strong interfaces. J. Am. Ceram. Soc., 1995, 78, 1125–1127. 4. Menon, M. and Chen, I. W., Bimaterial composites vie colloidal rolling tech￾niques: III, mechanical properties. J. Am. Ceram. Soc., 1999, 82, 3430–3440. 5. Rao, M. P., Sanchez-Herencia, A. J., Beltz, G. E., McMeeking, R. M. and ´ Lange, F. F., Laminar ceramics that exhibit a threshold strength. Science, 1999, 286, 102–105. 6. Dakskobler, A., Kosmac, T. and Chen, I. W., Paraffin-based process for ˇ producing layered composites with cellular microstructures. J. Am. Ceram. Soc., 2002, 85, 1013–1015. 7. Adachi, T., Sekino, T., Kusunose, T., Nakayama, T., Hikasa, A., Choa, Y. H. and Niihara, K., Crack propagation behavior of nano-sized SiC dispersed multilayered Al2O3/3Y-TZP hybrid composites. J. Ceram. Soc. Jpn, 2003, 111, 4–7. 8. Poulon-Quintin, A., Berger, M. H., Bunsell, A. R., Kaya, C., Butler, E. G., Wootton, A. et al., Processing and structures of bi-phase oxide ceramic filaments. J. Eur. Ceram. Soc., 2004, 24, 101–110. 9. Lee, B. T., Kim, K. H. and Han, J. K., Microstructures and material proper￾ties of fibrous Al2O3-(m-ZrO2)/t-ZrO2 composites fabricated by a fibrous monolithic process. J. Mater. Res., 2004, 19, 3234–3241.