Creep behavior of ceramics studied by mechanical loss measurements

Mechanical loss due to grain boundary (GB) sliding has been measured at high temperature in fine-grained yttria-stabilized tetragonal zirconia and silicon nitride. In zirconia, GB sliding gives rise to a mechanical loss peak and exponential background. The transition from the peak to the exponential background accounts for creep appearance in the material. When the amount of inter-granular lubricating glassy phases is higher the mechanical loss level is globally higher and so is the creep rate. When part of the amorphous phase forms amorphous pockets, like in silicon nitride, a mechanical loss peak related to dissipation in these pockets is observed. Silicon nitride sintered under gas pressure contains a very small amount of glassy phase and consequently exhibits a strong resistance to deformation until 1450 °C. As a consequence the mechanical loss background and relaxation peak are very low in the as-sintered material. However, the peak increases after annealing and “quenching” from high temperature. This argues for high temperature melting of the inter-granular crystalline phases, followed by freezing of these liquid phases into the amorphous state upon rapid cooling.