Micromechanics of viscous slip along ceramic grain boundaries

Internal friction characterization has been used to quantitatively assess the inherent viscosity of residual SiO2 glass segregated to grain boundaries of polycrystalline Si3N4 and SiC ceramics. The intergranular SiO2 glass was intentionally doped with foreign anions to systematically alter its inherent viscosity. The anelastic relaxation peaks of internal friction, arising from viscous slip along grain boundaries wetted by glass, was collected and analyzed with respect to its shift upon changing the oscillation frequency. As a result of this characterization, Arrhenius plots of the microscopic intergranular glass viscosity could be obtained. Creep rates were also measured in torsional geometry as a function of temperature, and the macroscopic viscosity of the polycrystals evaluated. A plot of macroscopic (polycrystal) viscosity vs microscopic (intergranular phase) viscosity revealed that the viscous flow of intergranular glass dominates the overall deformation behavior of the polycrystal only when the glass viscosity is relatively low (i.e., log eta (i)<6.5). At higher intergranular glass viscosities, the creep behavior of the polycrystal obeys a different law and it is dominated by diffusive phenomena.