Grain‐Boundary Sliding in Fluorine‐Doped Silicon Nitride

A systematic study of the effect of small additions of fluorine impurity on the relaxation and creep behavior of Si 3 N 4 ceramics was conducted. A model polycrystal, consisting of equiaxed grains and containing only pure (glassy) SiO 2 at the grain boundary, was selected for this investigation. The boundary SiO 2 ‐glass film, completely surrounding the grains, was doped with increased amounts of the glass‐network modifier fluorine to systematically lower its bulk viscosity. The addition of the fluorine made both the grain‐boundary relaxation peak and the background of internal friction notably shift toward lower temperatures; the scaling was in accordance with the lowered viscosity of the bulk fluorine‐doped SiO 2 glass. Invoking basic viscoelasticity principles, the grain‐boundary relaxation phenomenon could be rationalized and the internal friction data consistently related to the macroscopic (torsional) creep behavior of the polycrystal.