Prediction of crack deflection in porous/dense ceramic laminates

The arrangement of ceramic layers in laminated structures is an interesting way to enhance the flaw tolerance of brittle ceramic materials. The interfaces are expected to deflect cracks, increasing the fracture energy of the laminate compared to a monolithic material and thus raising the toughness. Laminates have been fabricated with alternating dense and porous layers of the same material, i.e. SiC or B4C, in order to obtain a good chemical compatibility between the laminas and almost no thermal residual stresses. Porosity, in the porous layers, is achieved by incorporating organic particles which are removed during the debinding step. In this context, the target of this study is to predict the volume fraction of pores, in the porous layer, required to cause crack deflection. The proposed criterion derives from an energy balance. It relies on a two-scale analysis taking into account the laminated Structure of the material. It can be written in terms of two relevant material parameters: the ratio of Young's moduli of the dense and porous materials and toughness ratios. A unique function depending on the Volume fraction of pores can be used to express the two above-mentioned ratios. Assuming a cubic lattice of spherical voids, the parameters of the porous ceramic depend linearly on the porosity and vanish at the point of percolation of pores. As a consequence, the criterion can be rewritten in term of a single parameter: the porosity. Crack deflection is permitted only for very high values of the porosity. Predicted values agree satisfactorily with experiments on SiC and B4C. The comparison with the He and Hutchinson criterion shows that this latter underestimates the correct value. (c) 2004 Elsevier Ltd. All rights reserved.