Lucie Sestakova a , Raul Bermejo b , Zdenek Chlup c , Robert Danzer a,b a MaterialsCenter Leoben Forschung GmbH, Leoben, Austria b Montanuniversität Leoben, Institut für Struktur- und Funktionskeramik,Leoben, Austria c Academy of Sciences of the Czech Republic, Institute of Physics of Materials,Brno, Czech Republic Strategies for fracture toughness, strength and reliability optimisation of ceramic–ceramic laminates Dedicated to Prof. F. D. Fischer on the occasion of his 70 th birthday Layered ceramics are,compared to conventional mono- lithicceramics,a good choice for highly loaded structural applications having improved fracture toughness, strength and mechanical reliability. The use of tailored residual compressive stresses in the layers isthe key parameterto adjust theseproperties. In thiswork two types of ceramics are analysed whichhaveexternal(ECS-laminates)or inter- nal(ICS-laminates) compressive stresses. The most impor- tant factors having influence on the strength and toughness of theselaminates are discussed. Clear recommendations on the properselection of a suitable mismatch strain, vol- ume ratio of the layer materials and thickness and distribu- tion of individuallayers are given, eitherto achieve a high toughness and/or a high lower limit (threshold) for strength. Keywords: Ceramic laminates; Layered ceramics; Residu- al stress; Fracture toughness; Threshold strength 1. Introduction For many decades naturehas inspired research to develop high performancematerials by combining ceramicswith other ceramics, metals or polymers [1– 3]. Examples of ad- vanced biological structures include the extraordinarily high toughness and strength of mollusc shells, which are related to the fine-scale of their microstructures; a laminate of thin calcium carbonate crystallitelayers consisting of 99% cal- cium carbonate(CaCO 3 ) and tough biopolymers,arranged in an energy-absorbing hierarchical microstructure [4]. The strength and toughness of suchlayered structures are signifi- cantly higherthan those of theirconstituents, for instance yielding an increase in toughness values of one order of mag- nitude [5, 6]. Nevertheless, the replication of architectural features found in nature(at the micro and nano scales) into macro scale structural engineering materials at a reasonable cost is still not possible. But itwould be very welcome to build these toughness raising architectural features intodur- able materialswhich possess also additionalfunctional and/ or structuralproperties such as, for example, high tempera- ture stability and specific electric functionality. The concepts of layered ceramics, functionally graded materials and coat- ings could allowtailoring of the surface and bulk properties of advanced engineering components with the purpose of en- hancing their structural integrity aswell as adding multi- functionality, which would translate into higher efficiency and better performance of these components. The use of colloidalprocessing hasto some extent en- abled the reduction of the critical size of the flaw causing the failure of the material, thus yielding relative high strength ceramics [7]. However in the last decades a “flaw- tolerant” design approach in layered ceramic–ceramiccom- posites has also been attempted to improve toughness, strength and mechanical reliability of the ceramic materials [3, 8–14]. Ceramic laminate systems are in generalproduced via the powderroute,which also involvesa high temperature sintering step. At the sintering temperature significant dif- fusion occurs, and any stress at the micro as well as at macro scale will be relaxed within a relativelyshort time span. But atroom temperature significant diffusion in ce- ramic systems does not exist and stresses cannotrelax. Therefore the differential thermal shrinking of the constitu- ents during the cooling from sintering to room temperature causessignificant mismatch strainswhich translate into re- sidual stresses in the layers [15, 16]. Mismatch strains can also result from otherreasons, e. g. phase transformations of constituents of the layer materials [17 – 19]. The residual stress field developed in the layers can be used as key elementto improve the fracture behaviour of the system. In thisregard, various toughening mechanisms can take place in layered ceramic structures. The most im- portant ones,among others,are crack deflection,crack bi- furcation and interface delamination aswell as crack shield- ing, which are all triggered by compressive residual stresses and/or elastic mismatch between layers [20 – 30]. Efforts to combine both approaches have recently been attempted by some authors, where, undersome particular conditions, in- terface delamination caneven beproduced in laminates with strong interfaces, thus taking advantage of various toughening mechanisms in a unique design [31, 32]. Crack deflection and interface delamination exist fora specialgroup of laminateswith“weak interfaces” [2, 33 – 37], i. e. in this kind of multilayerthe interface between the adjacent layers has a very low fracture resistance. An ap- IJMR_ijmr-110523 – 29.4.11/stm media köthen L. Sestakovaet al.:Strategies for fracture toughness, strength and reliability optimisation of ceramic–ceramic laminates Int. J. Mat. Res. (formerly Z. Metallkd.) 102 (2011) 6 1