Wetting of grain boundaries in metals and ceramics

Observations of the wetting of grain boundaries in metals and ceramics are reviewed. Systematics of grain boundary wetting are introduced and the conditions for penetration of a liquid down a grain boundary presented. These concepts are sufficient to describe the macroscopic wetting behavior, such as the macroscopic contact angle, but are insufficient to describe the occurrence of thin (similar to nm) intergranular films seen in some ceramics. These films require the existence of attractive and repulsive forces on the nanometer length scale. Observations by high resolution electron microscopy are necessary to establish whether intergranular films form in the metallic systems that exhibit wetting and hence whether such forces are also operative in metal systems.