| This paper gives an overview of recent work on microscale mechanical properties of
silicon nitride (Si3N4) ceramics determined by microcantilever bending tests and their
relationships with the microstructures. First, we focus on deformation behaviours
and fracture strength of Si3N4 single crystals. -Si3N4 single crystals are plastically
deformed at room temperature under high bending strength, and the obtained yield
stress depends on the crystal orientation. The critically resolved shear stress of the
primary slip system determined from the yield stress is lower than 1.5 GPa. Then we
describe microscale mechanical properties of Si3N4 polycrystalline ceramics. Particular
emphasis is placed on their grain-boundary strength or toughness in conjunction
with intergranular glassy film (IGF), which is governed by processing parameters
such as sintering additives. We consider two cases of fractures; one is that at the
IGF-grain interface and the other is within the IGF. In each case, effects of rare-earth
oxide additives are assessed and discussed, based on differential binding energies
and cationic field strengths of rare-earth ions. In Si3N4 ceramics doped with Al2O3,
-SiAlON layer forming on -Si3N4 grains enhances the microscale grain-boundary
strength. However, when Al2O3 is not doped, the strength is affected by the content of
O anion, which depends on the type of the rare-earth ion. |
