Availableonlineatwww.sciencedirect.com °scⅰ ence Direct COMPOSITES SCIENCE AND TECHNOLOGY ELSEVIER Composites Science and Technology 67(2007)1930-1938 w.elsevier. com/locate/compscitech Processing optimisation and fracture behaviour of layered ceramic composites with highly compressive layers R. Bermejo,, C. Baudin ,R. Moreno, L Llanes, A.J. Sanchez-Herencia epartamento de clew b Instituto de Ceramica y Vidrio(CSIC). CIKelsen 5, 28049 Madrid, Spain Departamento de Ciencia de Materiales e Ingenieria Metalurgica (UPC), Aoda. Diagonal 647(ETSEIB)08028 Barcelona, spain Received 23 June 2006: received in revised form 3 October 2006: accepted 8 October 2006 Available online 20 November 2006 Abstract Multilayer ceramics with five thick layers of alumina-YTZP and four thin layers of alumina-pure zirconia have been fabricated by slip casting and mechanically characterized in order to evaluate the influence of residual stresses on the fracture behaviour. In doing so, pro- essing parameters are optimised in terms of suspension stabilization and well-dispersion of the zirconia particles within the alumina ayers. Two different compressive residual stress values are attained through sequential slip casting of specimens with thick/thin layer thickness ratios of 5.4 and 9.5, by controlling the stability and casting time of aqueous slurries Flexural tests conducted on indented monoliths and multilayered materials show a stepwise failure in the laminates associated with the bifurcation of the impinging crack when interacting with the internal layers, this phenomenon being evidenced even for the case of laminates with very thin compressive layers where edge cracking is not observed. c 2006 Elsevier Ltd. All rights reserved. Keywords: E Powder processing: A Layered structures; Phase transformation; C Residual stress; B Fracture 1. Introduction design approach. If mechanical resistance is sought, com- pressive stresses are to be located at the surface [7, 8] hence In the last years layered configurations have been stud- fracture strength increases as a result of the corresponding ied as a structural option for improving the mechanical stress state superposition acting on the intrinsic critical sur behaviour and reliability of ceramics [1]. Some of the lam- face flaw On the other hand, if the compressive residual inates proposed include reinforcement mechanisms based stresses are induced in the internal layers [9-11], damage on either the deflection of opagating crack by weak tolerance and, consequently, reliability is the property pri- interlayers [24] or through the variation of the elastic marily favoured. From this perspective, attainment of a modulus within the material [5, 6]. Another case refers to threshold strength, i.e. a failure stress that is independent layered structures with strongly joined interfaces where of the original processing flaw size, is a sound evidence of an alternate tensile-compressive residual stress state may the potential effectiveness of this approach [ll] arise during cooling from sintering. Such stress field can Residual stresses in ceramic laminates can be due to dif- be controlled by designing the proper thickness, composi- ferent factors, either intrinsic like epitaxial growth, varia- tion and/or distribution of the layers in order to tailor tions of density or volume, densification or oxidation at the mechanical properties depending on the attempted the surface etc or extrinsic such as thermal or thermo- plastic strains developed during cooling or external forces and momentums. The most common approach is that asso- Corresponding author. Address: Institut fur Struktur- und Fun ionskeramik. Montanuniversitat Leoben. Peter-Tunner Strasse 5. A-8700 ciated with the differences in the coefficient of thermal expansion(Cte) between adjacent layers. Here, although E-mail address: raul bermejo@mu-leoben at(R. Bermejo) during sintering it is considered that stresses are negligible 0266-3538S. see front matter a 2006 Elsevier Ltd. All rights reserved doi: 10.1016/j. compscitech. 2006.10.010Processing optimisation and fracture behaviour of layered ceramic composites with highly compressive layers R. Bermejo a,*, C. Baudı´n b , R. Moreno b , L. Llanes a , A.J. Sa´nchez-Herencia b a Departamento de Ciencia de Materiales e Ingenierı´a Metalu´rgica (UPC), Avda. Diagonal 647 (ETSEIB) 08028 Barcelona, Spain b Instituto de Cera´mica y Vidrio (CSIC), C/Kelsen 5, 28049 Madrid, Spain Received 23 June 2006; received in revised form 3 October 2006; accepted 8 October 2006 Available online 20 November 2006 Abstract Multilayer ceramics with five thick layers of alumina-YTZP and four thin layers of alumina-pure zirconia have been fabricated by slip casting and mechanically characterized in order to evaluate the influence of residual stresses on the fracture behaviour. In doing so, pro￾cessing parameters are optimised in terms of suspension stabilization and well-dispersion of the zirconia particles within the alumina layers. Two different compressive residual stress values are attained through sequential slip casting of specimens with thick/thin layer thickness ratios of 5.4 and 9.5, by controlling the stability and casting time of aqueous slurries. Flexural tests conducted on indented monoliths and multilayered materials show a stepwise failure in the laminates associated with the bifurcation of the impinging crack when interacting with the internal layers, this phenomenon being evidenced even for the case of laminates with very thin compressive layers where edge cracking is not observed. 2006 Elsevier Ltd. All rights reserved. Keywords: E. Powder processing; A. Layered structures; Phase transformation; C. Residual stress; B. Fracture 1. Introduction In the last years layered configurations have been stud￾ied as a structural option for improving the mechanical behaviour and reliability of ceramics [1]. Some of the lam￾inates proposed include reinforcement mechanisms based on either the deflection of a propagating crack by weak interlayers [2–4] or through the variation of the elastic modulus within the material [5,6]. Another case refers to layered structures with strongly joined interfaces where an alternate tensile–compressive residual stress state may arise during cooling from sintering. Such stress field can be controlled by designing the proper thickness, composi￾tion and/or distribution of the layers in order to tailor the mechanical properties depending on the attempted design approach. If mechanical resistance is sought, com￾pressive stresses are to be located at the surface [7,8], hence fracture strength increases as a result of the corresponding stress state superposition acting on the intrinsic critical sur￾face flaw. On the other hand, if the compressive residual stresses are induced in the internal layers [9–11], damage tolerance and, consequently, reliability is the property pri￾marily favoured. From this perspective, attainment of a threshold strength, i.e. a failure stress that is independent of the original processing flaw size, is a sound evidence of the potential effectiveness of this approach [11]. Residual stresses in ceramic laminates can be due to dif￾ferent factors, either intrinsic like epitaxial growth, varia￾tions of density or volume, densification or oxidation at the surface, etc., or extrinsic such as thermal or thermo￾plastic strains developed during cooling or external forces and momentums. The most common approach is that asso￾ciated with the differences in the coefficient of thermal expansion (CTE) between adjacent layers. Here, although during sintering it is considered that stresses are negligible 0266-3538/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2006.10.010 * Corresponding author. Address: Institut fu¨r Struktur- und Funk￾tionskeramik, Montanuniversita¨t Leoben, Peter-Tunner Strasse 5, A-8700 Leoben, Austria. E-mail address: raul.bermejo@mu-leoben.at (R. Bermejo). www.elsevier.com/locate/compscitech Composites Science and Technology 67 (2007) 1930–1938 COMPOSITES SCIENCE AND TECHNOLOGY