a b s t r a c tFreeze casting has proven to be a versatile processing route to fabricate bio-inspired (''nacre-like'') hybrid composites that exhibit unique combinations of strength and toughness (damage-tolerance). To date, however, the effects of small changes in the architecture of such composites on their mechanical properties have been poorly investigated. Here we examine the influence of microstructural features such as ceramic/polymer ratio, layer thickness, and presence of bridges between ceramic lamellae, on the mechanical performance of the resulting composites. To this end, we compare the flexural strength and resistance to failure of a suite of silicon carbide/polymethyl methacrylate (SiC/PMMA) layered composites made by polymer infiltration of freeze-cast SiC scaffolds with various architectures. Our composite structures all show an increasing fracture resistance with crack extension (rising R-curve behavior) due to extrinsic toughening mechanisms such as uncracked-ligament bridging, inelastic deformation of ductile layers, lamellae pull out and ceramic bridge fracture. We show that a fine tuning of the composite architecture can lead to SiC/PMMA samples with a dendritic morphology, which exhibit the best strength and toughness. Specifically, the presence of ceramic bridges connecting the lamellae is seen to provide a strengthening effect similar to the mineral bridges between aragonite platelets in nacre, where they prevent debonding and limit platelet sliding; additionally, the fracture of these bridges between lamellae during crack extension is a potent toughening mechanism, thereby conferring optimal damage tolerance to the material.

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Alumina-zirconia (AZ) composites are attractive structural materials, which combine the high hardness and Young’s modulus of the alumina matrix with additional toughening effects, due to the zirconia dispersion. In this study, AZ composites containing different amounts of zirconia (in the range 5–20 vol %) were prepared by a wet chemical method, consisting on the surface coating of alumina powders by mixing them with zirconium salt aqueous solutions. After spray-drying, powders were calcined at 600 °C for 1 h. Green bodies were then prepared by two methods: uniaxial pressing of spray-dried granules and slip casting of slurries, obtained by re-dispersing the spray dried granulates. After pressureless sintering at 1500 °C for 1 h, the slip cast samples gave rise to fully dense materials, characterized by a quite homogeneous distribution of ZrO2 grains in the alumina matrix. The microstructure, phase composition, tetragonal to monoclinic transformation behavior and mechanical properties were investigated and are here discussed as a function of the ZrO2 content. The material containing 10 vol % ZrO2 presented a relevant hardness and exhibited the maximum value of KI0, mainly imputable to the t → m transformation at the crack tip.