Anelastic Grain-Boundary Sliding in Ceramics. II. Polycrystals with Glassy Boundaries

Systematic experimentation on grain-boundary sliding of ceramics with a glassy film at grain boundaries has been carried out by low-frequency internal friction technique up to ≈2200K. A model silicon nitride polycrystal was selected for this basic investigation in which the grains were continuously encompassed by a nanometre-sized film of pure SiO2 glass. The SiO2 film was also doped with increasing amounts of two different glass-network modifier anions (i.e., F and Cl) in order to systematically lower its bulk viscosity. For comparison, a very dilute sialon material was also tested in which the grain-boundary film was eliminated and the grains were directly bonded. An internal-friction peak, whose top displayed just below the melting point of the grain-boundary glass, was systematically found, the shift of the peak upon frequency variation or dopant addition being closely related to the viscosity change of bulk SiO2 glass. For comparison, a dilute sialon material, in which the grain-boundary film was eliminated, was also tested. The anelastic peak of internal friction disappeared in this latter material and only a viscoelastic background rising could be detected.