Grain-Boundary Viscosity of Calcium-Doped Silicon Nitride

Internal-friction data of calcium-doped Si₃N₄ polycrystalline materials that otherwise contain only SiO₂ at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO₂ film through the analysis of the anelastic internal-friction-peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO₂ glasses with the same chemical composition. Broadening of the internal-friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion-doped Si₃N₄, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.