10 - Grain Boundary Energy and Sintering

Abstract

The grain boundary in polycrystalline materials is not an ideal source and sink of atoms, and a critical driving force is needed if boundaries are to operate as a source. The critical driving force for movement of atoms at the boundary varies considerably with the nature, structure, and orientation of the grain boundary. For rough boundaries with random orientations, the critical driving force must be very low. For special boundaries with energy cusps, the critical driving force should be high. When the boundary is faceted, the critical driving force is expected to be much higher than that for rough boundaries. Grain boundary faceting causes a considerable increase in the critical driving force for atom movement from the grain boundary to the neck. The grain boundary is not an ideal atom source with no energy barrier for operation, but a source needing a critical driving force that varies with the nature of the boundary and particularly its structure. In solid state sintering, because the surface is replaced by the grain boundary, the grain boundary energy impedes sintering. In real systems, the grain boundary energy is not zero but has finite values.