opposites Science and Technology 67(2007)1930- 37 solely to the interaction of the impinging crack with the [4]Kuo DH, Kriven WM. Fracture of multilayer oxide compressIve layers Mater Sci Eng A-Struct 1998: 241(1-2): 241-50 []He MY, Hutchinson Jw. Crack deflection at dissimilar elastic.materials. Int J Solids Struct I 4. Conclusions [6]Pender dC. Thompson SC, Padture NP, Giannakopoulos AE, Suresh S. Gradients in elastic modulus for improve Alumina-zirconia layered composites with controlled resistance. Part II: The silicon nitride-silicon carbide system. Acta layer thicknesses have been fabricated by sequential slip Mater2001;4916:3263-8 casting of optimised stable suspensions with a high solid [7 Virkar AV, Huang JL, Cutler RA. hening of oxide ceramics by transformation-induced stresses. J Am Ceram Soc 1987: 70(3): 164-70 content(70 wt%), dispersed with 0.8 wt% of a polyelectro- [8]Green DI, Tandon R, Sglavo VM. Crack arrest and multiple cracking lyte in order to avoid heteroflocculation. As a result,two in glass through the use of designed residual stress profiles. Science ts of layered materials with thickness ratios of 5.4 and 1999283(5406):12957 9.5 were obtained. Residual stresses estimated by the differ- [9] Hillman C, Suo ZG, Lange FF. Cracking of laminates subjected to nce of thermal strain between the corresponding mono- liths were of 102 MPa and 60 MPa in the thick tensile [10] Sanchez-Herencia AJ. Pascual C, He J, Lange FF. ZrO/ZrO2 ayered composites for crack bifurcation. J Am Ceram Soc layers and-688 MPa and -718 MPa in the thin compres 1999:82(6:1512-8 sive ones, respectively. The high compressive stresses in the [I] Rao MP, Sanchez-Herencia AJ, Beltz GE, McMeeking RM,Lange thin layers, mainly due to the zirconia phase transforma- FF. Laminar ceramics that exhibit a threshold strength. Science tion, promoted the formation of edge cracks in the layered 1999286(5437):102-5 system with the smaller thickness ratio, i.e. with thicker [2] Krishnamurthy R, Sheldon BW. Stresses due to gradients in non-stoichiometric oxides. Acta Mater 2004: 52(7 compressive layers l807-1822 Bending tests conducted on indented laminates showed [13]Oechsner M, Hillman C, Lange F. Crack bifurcation in laminar a stepwise fracture in comparison with the catastrophic ceramic composites. J Am Ceram Soc 1996: 79(7): 1834-8. brittle failure found for the reference monolith. The first [14] Moya JS, Sanchez-Herencia JA, Bartolome JF, Tanimoto T.Elastic load disruption for the laminates, corresponding to the 1997 37(7): 1095-100O3/ZrO, ceramic laminates. Scripta Mater pop-in of the surface indentation cracks, showed the effect [15]Pontin MG, Rao MP, Sanchez-Herencia AJ, Lange FF.Laminar of the different tensile residual stresses in the outer layer of ceramics utilizing the zirconia tetragonal-to- monoclinic phase trans- the two laminates investigated. In this regard, observation formation to obtain a threshold strength. J Am Ceram Soc 200285(12):3041-8 of the fracture surfaces revealed the interaction of cracks [16] Sanchez- Herencia AJ, James L, Lange FF. Bifurcation in alumina with the thin layers under compression, which act as an effective barrier to crack propagation. For the case of the laminate system with smaller thickness ratio edge cracks [17] Sglavo VM, Paternoster M, Bertoldi M. Tailored residual stresses in were induced and, as it would be predicted, bifurcation high reliability alumina-mullite ceramic laminates. J Am Ceram Soc as observed along the centre of the thin compressive 200588(10):2826-32 layer. However, even though edge cracking did not occur [18] Cutler RA, Bright JD, Virkar AV, Shetty DK Strength improvement in transformation-toughened alumina by selective phase-transforma- in the laminate with higher thickness ratio, crack bifurca tion. J Am Ceram Soc 1987: 70(10): 714-8. tion was also appreciated at fracture. Hence, the results [19] Tandon R, Green DJ. Crack stability and T-curves due to macro- here presented point out that these two phenomena might copic residual compressive stress profiles. J Am Ceram Soc ot be so clearly related, and thus a more profound inves igation, aiming to evaluate the real stress state taking place 20 As S, Suo z. Tunneling cracks in constrained layers rs J Appl Mech-T in the presence of edge cracking and bifurcation phenom- [21] Lugovy M, Orlovskaya N, Slyunyayev V, Gogotsi G, Kabler J ena, Is recalled Sanchez-Herencia AJ. et al. Crack bifurcation features in lamina specimens with fixed total thickness. Comput Sci Techol Acknowledgements 002;62(6)819-30 [22] Boch P, Chartier T, Huttepain M. Tape casting of AlO3/ZrO2 laminated composites. J Am Ceram Soc 1986: 69(8): C191-2. This work was supported by projects MAT 2003-00836 [23] Davis JB, Kristoffersson A, Carlstrom E, Clegg WJ.Fabrication and and MAT2006-13480 CICYT(Spain); and by the Euro- crack deflection in ceramic laminates with porous interlayers. J Am pean Communitys Human Potential Programme under Ceram Soc200083(10):2369-74 contract HPRN-CT-2002-00203, [SICMAC]. [24] Sanchez-Herencia AJ, Gurauskis J, Baudin C Processing of TZP laminates from water based cast tapes. Compos Part B-Eng 2006:37(6:499508. References 225] Marshall DB, Ratto JJ, Lange FF. Enhanced fracture-toughness in layered microcomposites of Ce-zrO2 and Al,O3.J Am Ceram Soc [1] Chan HM. Layered ceramics: processing and mechanical behavior Ann Rey mater Sci 1997: 27: 249-82 [26] Lucchini E, Sbaizero O. Alumina/zirconia multilayer composites [2] Clegg W], Kendall K, Alford NM, Button TW, Birchall JD. A simple obtained by centrifugal consolidation. J Eur Ceram Soc way to make tough ceramics. Nature 1990: 347: 455-7 1995;15(10):975-81 3] Phillipps AJ, Clegg WJ, Clyne Tw. Fracture-behavior of ic [27]Requena J, Moreno R, Moya JS. Alumina and alumi Lminates in bending. 1. Modeling of crack-propagation. Acta Metall asting. J Am Ceram Soc ater I993:4l03):805-17. 1989;72(8):151l-3solely to the interaction of the impinging crack with the compressive layers. 4. Conclusions Alumina–zirconia layered composites with controlled layer thicknesses have been fabricated by sequential slip casting of optimised stable suspensions with a high solid content (70 wt%), dispersed with 0.8 wt% of a polyelectro￾lyte in order to avoid heteroflocculation. As a result, two sets of layered materials with thickness ratios of 5.4 and 9.5 were obtained. Residual stresses estimated by the differ￾ence of thermal strain between the corresponding mono￾liths were of 102 MPa and 60 MPa in the thick tensile layers and 688 MPa and 718 MPa in the thin compres￾sive ones, respectively. The high compressive stresses in the thin layers, mainly due to the zirconia phase transforma￾tion, promoted the formation of edge cracks in the layered system with the smaller thickness ratio, i.e. with thicker compressive layers. Bending tests conducted on indented laminates showed a stepwise fracture in comparison with the catastrophic brittle failure found for the reference monolith. The first load disruption for the laminates, corresponding to the pop-in of the surface indentation cracks, showed the effect of the different tensile residual stresses in the outer layer of the two laminates investigated. In this regard, observation of the fracture surfaces revealed the interaction of cracks with the thin layers under compression, which act as an effective barrier to crack propagation. For the case of the laminate system with smaller thickness ratio edge cracks were induced and, as it would be predicted, bifurcation was observed along the centre of the thin compressive layer. However, even though edge cracking did not occur in the laminate with higher thickness ratio, crack bifurca￾tion was also appreciated at fracture. Hence, the results here presented point out that these two phenomena might not be so clearly related, and thus a more profound inves￾tigation, aiming to evaluate the real stress state taking place in the presence of edge cracking and bifurcation phenom￾ena, is recalled. Acknowledgements This work was supported by projects MAT 2003-00836 and MAT2006-13480 CICYT (Spain); and by the Euro￾pean Community’s Human Potential Programme under contract HPRN-CT-2002-00203, [SICMAC]. References [1] Chan HM. Layered ceramics: processing and mechanical behavior. Ann Rev Mater Sci 1997;27:249–82. [2] Clegg WJ, Kendall K, Alford NM, Button TW, Birchall JD. A simple way to make tough ceramics. Nature 1990;347:455–7. [3] Phillipps AJ, Clegg WJ, Clyne TW. Fracture-behavior of ceramic laminates in bending. 1. Modeling of crack-propagation. Acta Metall Mater 1993;41(3):805–17. [4] Kuo DH, Kriven WM. Fracture of multilayer oxide composites. Mater Sci Eng A-Struct 1998;241(1–2):241–50. [5] He MY, Hutchinson JW. Crack deflection at an interface between dissimilar elastic-materials. Int J Solids Struct 1989;25(9):1053–67. [6] Pender DC, Thompson SC, Padture NP, Giannakopoulos AE, Suresh S. Gradients in elastic modulus for improved contact-damage resistance. Part II: The silicon nitride-silicon carbide system. Acta Mater 2001;49(16):3263–8. [7] Virkar AV, Huang JL, Cutler RA. Strengthening of oxide ceramics by transformation-induced stresses. J Am Ceram Soc 1987;70(3):164–70. [8] Green DJ, Tandon R, Sglavo VM. Crack arrest and multiple cracking in glass through the use of designed residual stress profiles. Science 1999;283(5406):1295–7. [9] Hillman C, Suo ZG, Lange FF. Cracking of laminates subjected to biaxial tensile stresses. J Am Ceram Soc 1996;79(8):2127–33. [10] Sanchez-Herencia AJ, Pascual C, He J, Lange FF. ZrO2/ZrO2 layered composites for crack bifurcation. J Am Ceram Soc 1999;82(6):1512–8. [11] Rao MP, Sanchez-Herencia AJ, Beltz GE, McMeeking RM, Lange FF. Laminar ceramics that exhibit a threshold strength. Science 1999;286(5437):102–5. [12] Krishnamurthy R, Sheldon BW. Stresses due to oxygen potential gradients in non-stoichiometric oxides. Acta Mater 2004;52(7): 1807–1822. [13] Oechsner M, Hillman C, Lange FF. Crack bifurcation in laminar ceramic composites. J Am Ceram Soc 1996;79(7):1834–8. [14] Moya JS, Sanchez-Herencia JA, Bartolome JF, Tanimoto T. Elastic modulus in rigid Al2O3/ZrO2 ceramic laminates. Scripta Mater 1997;37(7):1095–103. [15] Pontin MG, Rao MP, Sanchez-Herencia AJ, Lange FF. Laminar ceramics utilizing the zirconia tetragonal-to-monoclinic phase trans￾formation to obtain a threshold strength. J Am Ceram Soc 2002;85(12):3041–8. [16] Sanchez-Herencia AJ, James L, Lange FF. Bifurcation in alumina plates produced by a phase transformation in central, alumina/ zirconia thin layers. J Eur Ceram Soc 2000;20(9):1297–300. [17] Sglavo VM, Paternoster M, Bertoldi M. Tailored residual stresses in high reliability alumina-mullite ceramic laminates. J Am Ceram Soc 2005;88(10):2826–32. [18] Cutler RA, Bright JD, Virkar AV, Shetty DK. Strength improvement in transformation-toughened alumina by selective phase-transforma￾tion. J Am Ceram Soc 1987;70(10):714–8. [19] Tandon R, Green DJ. Crack stability and T-curves due to macro￾scopic residual compressive stress profiles. J Am Ceram Soc 1991;74(8):1981–6. [20] Ho S, Suo Z. Tunneling cracks in constrained layers. J Appl Mech-T ASME 1993;60(4):890–4. [21] Lugovy M, Orlovskaya N, Slyunyayev V, Gogotsi G, Ku¨bler J, Sanchez-Herencia AJ, et al. Crack bifurcation features in laminar specimens with fixed total thickness. Comput Sci Techol 2002;62(6):819–30. [22] Boch P, Chartier T, Huttepain M. Tape casting of Al2O3/ZrO2 laminated composites. J Am Ceram Soc 1986;69(8):C191–2. [23] Davis JB, Kristoffersson A, Carlstrom E, Clegg WJ. Fabrication and crack deflection in ceramic laminates with porous interlayers. J Am Ceram Soc 2000;83(10):2369–74. [24] Sanchez-Herencia AJ, Gurauskis J, Baudı´n C. Processing of Al2O3/Y￾TZP laminates from water based cast tapes. Compos Part B-Eng 2006;37(6):499–508. [25] Marshall DB, Ratto JJ, Lange FF. Enhanced fracture-toughness in layered microcomposites of Ce–ZrO2 and Al2O3. J Am Ceram Soc 1991;74(12):2979–87. [26] Lucchini E, Sbaizero O. Alumina/zirconia multilayer composites obtained by centrifugal consolidation. J Eur Ceram Soc 1995;15(10):975–81. [27] Requena J, Moreno R, Moya JS. Alumina and alumina zirconia multilayer composites obtained by slip casting. J Am Ceram Soc 1989;72(8):1511–3. R. Bermejo et al. / Composites Science and Technology 67 (2007) 1930–1938 1937