Mechanical Properties of Three-Layered Monolithic Silicon Nitride-Fibrous Silicon Nitride/Boron Nitride Monolith

摘要

A three-layered composite, composed of a strong outer layer (monolithic S_3N_4) and a tough inner layer (fibrous Si_3N_4/BN monolith), was fabricated by hot-pressing. For the inner layer, a Si_3N_4-polymer fiber made by extrusion was coated by dipping it into a 20 wt% BN-containing slurry. The three-layered composite exhibited excellent mechanical properties, including high strength, work of fracture, and crack resistance, because of the combination of a strong outer layer and a tough inner layer. In other words, the strong outer layer withheld the applied stress, while the tough inner layer promoted crack interactions through the weak BN cell boundaries. Also, the residual thermal stress on the surface due to the anisotropy in the coefficient of thermal expansion of BN affected a median/radial crack generation after indentation.