Fracture Characterization of Silicon Nitride Based Layered Composites

Silicon nitride based structural ceramics are a family of advanced materials that exhibit a combination of high hardness, high strength, good corrosion and erosion behaviour, high elastic modulus and dimensional stability. Major application of these ceramics includes wear components, cutting tools and parts of engines (turbochargers, bearings, etc.). Their wide application is, however, still limited mainly due to their brittleness, low flaw tolerance and low reliability1,2. In recent years nitride based ceramics have been very intensively investigated all over the world with the aim to improve their mechanical properties and make them suitable for structural applications. The main ways of improving the room temperature mechanical properties of silicon nitride based ceramics can be summarized as follows: improving the strength level and reducing the strength values scatter, i.e., enhancing the reliability by reduction of the critical defect size (improved properties of powders, clean room manufacturing, etc.) — the flaw diminution approach3,4; promoting the localized bridging behind the crack tip (in the form of frictional and mechanical interlocking, or pull out) by which the flaw tolerance of the material can be improved — the flaw tolerance approach5−7; improving the strength values by incorporating into the matrix the nano-sized, second-phase particles with different expansion coefficients — the nano-particle dispersion strengthening8; improving the structural reliability by designing novel laminar composites with a promoted crack deflection at the interlayer boundaries and utilizing the compressive residual stresses arisen during cooling down from the sintering temperature because of the differences in the thermal expansions between the layers which have different compositions — the laminar structure approach9−12.