GRAIN-BOUNDARY VISCOSITY OF CALCIUM-DOPED SILICON-NITRIDE

Internal-friction data of calcium-doped Si3N4 polycrystalline material s that otherwise contain only SiO2 at grain boundaries were examined a nd compared with those collected on the same polycrystal in the undope d state or doped with anions (i.e., fluorine and chlorine). Precise mi crostructural characterizations previously performed on these material s 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 s caling with increasing calcium addition followed the same trend as bul k SiO2 glasses with the same chemical composition. Broadening of the i nternal-friction peak with increased calcium content in the material h as also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, wh ich is in agreement with our previous studies on undoped and anion-dop ed Si3N4, has demonstrated that the overall viscoelastic response of t he polycrystal is mainly dictated by the chemistry of the intergranula r glass.