Microstructurally induced internal stresses in β-Si₃N₄ whisker-reinforced Si₃N₄ ceramics

The presence of aluminium and oxygen in the outer rim of a whisker, which is grown on the surface of the original β-Si₃N₄ whisker introduced to the Si₃N₄ ceramic prior to hot pressing wax observed; it indicates the presence of the β-SiAlON phase. An estimation of the internal stress generated by whisker impingement in β-Si₃N₄ whisker-reinforced Si₃N₄ composites is reported. The procedure involves the measurement of the angular deviation from the exact Bragg reflection condition, using a transmission electron microscope, associated with lattice bending within Si₃N₄ grains when one grain impinges on another. The simple calculations using these results shows that the tensile stress produced on the surface of the impinged whisker as a result of whisker bending is about 340 MPa and the shear stress acting in the area of impingement on the whiskers is substantially higher, 1-4 GPa. This value is comparable to the tensile strength of the silicon nitride whisker. The implications resulting from these microstructural observations on fracture toughness and steric hindrance effects during grain growth are discussed.