Intergranular glassy films in ceramics

Abstract

Impurities at grain boundaries have been known to play very important roles in ceramic materials for wide range of applications. In the last decade, high resolution electron microscopy including analytical and high spatial resolution microscopy has revealed the presence and chemistry of the intergranular glassy films (IGF) in Si3N4 and other ceramic materials. Equilibrium thickness theory has been developed on the basis of these observations. Theoretical calculations on IGF have then been attempted. On the other hand, "dry" boundaries showing no IGF but impurity segregation have recently been observed in many other ceramic materials. In order to discuss both kinds of boundaries in common, a unified theory should be useful. Adsorption theory has been proposed to describe not only these boundaries but also for impurities in external surfaces. Progress of these scientific researches on impurities at grain boundaries in ceramics is reviewed with some outlooks for the near future.