It is well known that the improvement in the failure behavior of ceramic materials could be obtained by microstructural design. In the present study, as a method for suppressing the catastrophic failure and improving the work of fracture in brittle ceramics, $Si_3N_4/BN$ ceramic laminates and fibrous monolithic ceramics were investigated. These materials are composed of the load bearing phase with high strength and weak boundary phase with low shear strength. The improvements in failure behavior are generally achieved by deflection of the propagating crack away from its original opening path, leading to a crack blunting and a higher work of fracture.
To prepare the laminated structure materials, green tapes formed by tape casting process, coated with BN were stacked and hot-pressed at 1750℃ for 1 hour. The failure behavior of ceramic laminates were similar to that of fiber reinforced composites showing a so-called 'saw-tooth type fracture'. In this study, the relationship between structure and mechanical properties, such as the fracture strength and the work of fracture, were evaluated.
Fibrous monolithic ceramics could be prepared with polycrystalline $Si_3N_4$ cell surrounded by BN layer as cell boundary. The shape of cell and the thickness of cell boundary could be controlled by the processing parameters, the plasticity of fibrous cell and solid content of coating slurry. The microstructure of cell was not the same with that of $Si_3N_4$ monolith due to the change of the amount and composition of sintering additives. The fracture behavior of fibrous monolithic ceramics affected by crack deflection in weak boundary phase was similar to that of ceramic laminates. In high temperature and oxidizing environment, the fracture strength decreased by surface degradation, but the work of fracture are increased. Thermal shock resistance of fibrous monolithic ceramics was evaluated by measuring the retained strength of water quenched specimen. The room temperature strength was retained up to the quenching temperature of 1070℃ and the load-deflection curves of quenched specimen showed apparently no difference with that of unquenched specimen.
