Viscous Slip along Grain Boundaries in Chlorine‐Doped Silicon Nitride

The effect of chlorine doping on the anelastic‐relaxation and torsional‐creep behavior of a silicon nitride (Si 3 N 4 ) polycrystalline body was studied. Two model polycrystals—one undoped and the other doped with a small fraction of chlorine—were investigated. Their microstructures consisted of equiaxed and well‐faceted Si 3 N 4 grains whose boundaries were separated by a continuous, nanometer‐sized film of silica (SiO 2 ) glass. The actual presence of chlorine in the doped polycrystal was ascertained by ion chromatography and is thought to be enriched at the grain boundaries. The effect of chlorine on the intergranular film structure was characterized by high‐resolution electron microscopy. The micromechanical response of the SiO 2 grain boundary under shear stress was monitored up to very high temperatures (i.e., ∼2000°C) by internal‐friction and torsional‐creep experiments. The presence of the chlorine dopant, which is a network modifier of SiO 2 glass that also causes a widening of the grain‐boundary film, significantly lowered the bulk viscosity of the residual glass. As a consequence of the change in grain‐boundary chemistry, the internal‐friction curve of the chlorine‐doped material shifted toward lower temperatures and the torsional‐creep rate markedly increased, as compared to the undoped material. According to a viscoelastic model of the Si3N4 polycrystal, the internal‐friction data resulted as a superposition of two individual components: (i) a relaxation peak that is related to the anelastic slip mechanism along grain boundaries and (ii) a background component that results from an irreversible diffusional‐creep mechanism.