Grain Boundary Sliding is a deformation mechanism of materials which includes displacement of grains against each other at high homologous temperature and low strain rate. This mechanism is the main reason of ceramics failure at high temperatures due to formation of glassy phase in their grain boundary.
Mechanisms
Homogeneous deformation in a polycrystalline solid cannot be achieved by grain boundary sliding alone. The grain movements must be accommodated with either diffusion or with dislocation motion. In the Earth's upper mantle, dislocation-accommodated grain boundary sliding is expected to be the dominant deformation mechanism for olivine.
References
Grain Boundary Sliding Wikipedia(Text) CC BY-SA