S. Bueno et al. /Journal of the European Ceramic Sociery 25(2005)847-856 5. Conclusions 100 Hm The fracture behaviour of monolithic Al2O3-Al2TiO5 ma terials with 0, 10, 30, and 40 vol. second phase was studied showing that increasing proportions of aluminium titanate decrease strength and increase the non-linear behaviour. The studied system is adequate to obtain layered compos- ites with very different mechanical behaviours, selected as a function of the behaviour of the constituent Al203-Al2T1O5 layers and their thermal expansion mismatch In particular, two laminate designs were fabricated to combine the desired properties of external layers with lin- ear stress-strain behaviour up to fracture and relatively high strength, with larger strains to fracture provided by the internal layers. One of them presented fracture strength values lower than those of alumina and significant step-like fracture. The second presented strength values close to those of alumina and step-like fracture in a lesser Fig. 8. Scanning electron micrographs of characteristic edge crack ob. Acknowledgements served in A40(A+T) layer of polished and chemically etched (HF 10% I min)surface in the A10A40 material Work supported in part by the European Community Human Potential Programme under contract HPRN-CT the fracture behaviour of the different layers, as focused by and by the grant CSIC 13P-BPD2001-1(Spall? 003-00836 the material design, crack bifurcation due to compression in the internal A40(A+T) layers may also occur. The presence of these compressive stresses is demonstrated by the occur rence of edge cracks as those shown in Fig. 8. Both, crack References deflection and crack bifurcation imply the occurrence of ad- ditional energy consuming processes during fracture in the I Runyan, J. L. and Bennison, S. J, Fabrication of flaw-tolerant um-titanate-reinforced alumina J. Eur Ceram. Soc. 1991.7 The fact that strength values for the system A10A40(147 2. Padture, N P, Bennison, S.J. and Chan, H.M., Flaw-tolerance and t 20 MPa)were lower(25%)than those of the monolith crack-resistance properties of alumina-aluminium titanate composites with the same composition as that of the external layer, ith tailored microstructures. Am. Ceram. Soc. 1993. 76.23 AlO(A+T), can be attributed to the residual stresses devel- oped in the external layer during cooling from the sinter 3. Bartolome, J, Requena, J, Moya, J.S., Li, M. and Guiu, F, Cyclic atigue crack growth resistance of Al2O3-Al2T1Os composites.Acta ing temperature. Calculations show that, in the laminate Mater1996,44,1361-1370 A10A40, compressive stresses of a500 MPa and tensile 4. Bartolome, J, Requena, J, Moya, J.S., Li, M. and Guiu, F, stresses of A90 MPa would develop in the A40(A+T)and atigue of Al,O3-Al TI0 AlO(A+T)layers, respectively, and, therefore, a strength Fract. Eng. Mater: Struct. 1997, 20, 789-798 decrease of up to 40% could be expected for the AlO(A+T 5. Uribe, R. and Baudin, C, Infuence of a dispersion of aluminum titanate particles of controlled size on the thermal shock resistance ayer in the laminate. As discussed above, residual stresses alumina. Am. Ceram. Soc. 2003. 86. 846-850 are not significant in AAlO and, consequently, strength 6. Padture, N. P, Runyan, J. L, Bennison, S. J, Braun, L.M. and values in this system are of the same order as those for Lawn, B. R,, Model for toughness curves in ty alumina monoliths Microstructural variables. Am. Ceram. Soc. 1993.76. 2241-2247 From the above discussion it is clear that different be- 7. Chan, H M, Layered ceramics Ann Rey Mater: Sci. 1997. 27 haviours can be achieved by combination of composite lay 8. Russo, C J, Harmer, M. P, Chan, H. M. and Miller, G. A, Design of ers in the system Al2O3-Al2TiO5. Further studies will be a laminated ceramic e for improved strength and toughness. dedicated to establish the effect of different stacking orders Am. Ceran.soe.1992,75,3396-4000 and layer thickness on the mechanical behaviour of the lam- 9. Lakshminarayanan, R, Shetty, D. K. and Cutler, R. A, Toughening inates. moreover detailed fracture studies in terms of crack of layered ceramics composites with residual surface compression. J. Am. Ceram. Soc. 1996.79. 79-87 propagation will help to understand the fracture behaviour 10. Wang, H and Hu,x, Surface properties of ceramic laminates fabri- of the laminates cated by die pressing. J. Am. Ceram. Soc. 1996, 79, 553-556S. Bueno et al. / Journal of the European Ceramic Society 25 (2005) 847–856 855 Fig. 8. Scanning electron micrographs of characteristic edge crack ob￾served in A40(A+T) layer of polished and chemically etched (HF 10%, 1 min) surface in the A10A40 material. the fracture behaviour of the different layers, as focused by the material design, crack bifurcation due to compression in the internal A40(A+T) layers may also occur. The presence of these compressive stresses is demonstrated by the occur￾rence of edge cracks17 as those shown in Fig. 8. Both, crack deflection and crack bifurcation imply the occurrence of ad￾ditional energy consuming processes during fracture in the laminates. The fact that strength values for the system A10A40 (147 ± 20 MPa) were lower (∼25%) than those of the monolith with the same composition as that of the external layer, A10(A+T), can be attributed to the residual stresses devel￾oped in the external layer during cooling from the sinter￾ing temperature. Calculations18 show that, in the laminate A10A40, compressive stresses of ≈500 MPa and tensile stresses of ≈90 MPa would develop in the A40(A+T) and A10(A+T) layers, respectively, and, therefore, a strength decrease of up to 40% could be expected for the A10(A+T) layer in the laminate. As discussed above, residual stresses are not significant in AA10 and, consequently, strength values in this system are of the same order as those for alumina monoliths. From the above discussion it is clear that different be￾haviours can be achieved by combination of composite lay￾ers in the system Al2O3–Al2TiO5. Further studies will be dedicated to establish the effect of different stacking orders and layer thickness on the mechanical behaviour of the lam￾inates. Moreover, detailed fracture studies in terms of crack propagation will help to understand the fracture behaviour of the laminates. 5. Conclusions The fracture behaviour of monolithic Al2O3–Al2TiO5 ma￾terials with 0, 10, 30, and 40 vol.% second phase was studied, showing that increasing proportions of aluminium titanate decrease strength and increase the non-linear behaviour. The studied system is adequate to obtain layered compos￾ites with very different mechanical behaviours, selected as a function of the behaviour of the constituent Al2O3–Al2TiO5 layers and their thermal expansion mismatch. In particular, two laminate designs were fabricated to combine the desired properties of external layers with lin￾ear stress–strain behaviour up to fracture and relatively high strength, with larger strains to fracture provided by the internal layers. One of them presented fracture strength values lower than those of alumina and significant step-like fracture. The second presented strength values close to those of alumina and step-like fracture in a lesser extent. Acknowledgements Work supported in part by the European Community’s Human Potential Programme under contract HPRN-CT- 2002-00203, [SICMAC], by the project MAT2003-00836 and by the grant CSIC I3P-BPD2001-1 (Spain). References 1. Runyan, J. L. and Bennison, S. J., Fabrication of flaw-tolerant aluminum-titanate-reinforced alumina. J. Eur. Ceram. Soc. 1991, 7, 93–99. 2. Padture, N. P., Bennison, S. J. and Chan, H. M., Flaw-tolerance and crack-resistance properties of alumina–aluminium titanate composites with tailored microstructures. J. Am. Ceram. Soc. 1993, 76, 2312– 2320. 3. Bartolomé, J., Requena, J., Moya, J. S., Li, M. and Guiu, F., Cyclic fatigue crack growth resistance of Al2O3–Al2TiO5 composites. Acta Mater. 1996, 44, 1361–1370. 4. Bartolomé, J., Requena, J., Moya, J. S., Li, M. and Guiu, F., Cyclic fatigue of Al2O3–Al2TiO5 composites in direct push-pull. Fatigue Fract. Eng. Mater. Struct. 1997, 20, 789–798. 5. Uribe, R. and Baud´ın, C., Influence of a dispersion of aluminum titanate particles of controlled size on the thermal shock resistance of alumina. J. Am. Ceram. Soc. 2003, 86, 846–850. 6. Padture, N. P., Runyan, J. L., Bennison, S. J., Braun, L. M. and Lawn, B. R., Model for toughness curves in two-phase ceramics: II. Microstructural variables. J. Am. Ceram. Soc. 1993, 76, 2241–2247. 7. Chan, H. M., Layered ceramics: processing and mechanical behaviour. Annu. Rev. Mater. Sci. 1997, 27, 249–282. 8. Russo, C. J., Harmer, M. P., Chan, H. M. and Miller, G. A., Design of a laminated ceramic composite for improved strength and toughness. J. Am. Ceram. Soc. 1992, 75, 3396–4000. 9. Lakshminarayanan, R., Shetty, D. K. and Cutler, R. A., Toughening of layered ceramics composites with residual surface compression. J. Am. Ceram. Soc. 1996, 79, 79–87. 10. Wang, H. and Hu, X., Surface properties of ceramic laminates fabri￾cated by die pressing. J. Am. Ceram. Soc. 1996, 79, 553–556