Mechanical spectroscopy methods for the quantitative analysis of intergranular glass viscosity in polycrystalline ceramics

Mechanical spectroscopy methods for the analysis of viscous behavior of grain boundaries are described and applied to glass-bonded polycrystalline ceramics. Emphasis is placed on the analysis of a high-temperature peak, which originates from grain-boundary sliding relaxation, with respect to: (i) peak shift with damping frequency change; (ii) peak intensity for a given testing geometry and amount of glassy phase; and, (iii) peak morphology characteristics. According to analyses (i) and (ii), an activation energy for intergranular viscous flow and the magnitude of glass-phase viscosity are quantitatively determined. The morphology of the peak depends on the activation energy for intergranular viscous flow, but it is also affected by the volume fraction of intergranular glass and by the presence of chemical gradients at grain boundaries. In addition microstructural inhomogeneities, such as grain size distribution and grain texture, can affect the peak morphology. Chemical and microstructural inhomogeneities generate spectra of activation energies and relaxation times, respectively. With inherently different characteristics, they both contribute to peak broadening. Internal friction experiments as a function of glass volume fraction and chemical composition experimentally confirmed that the above features are actually represented in the internal friction curve of a ceramic polycrystal as a function of temperature.