Intergranular glassy films:: An overview

In certain ceramics like Si3N4, SiC, SrTiO3, Al2O3, etc., the grain boundary (GB) region can have an amorphous film of about 1-2 nm thickness. These inter-ranular glassy films (IGFs) are characterized by a nearly constant thickness which is basically independent of the orientation of the bounding grains, but is dependent on the composition of the ceramic. The IGF is resistant to crystallization and is thought to represent an equilibrium configuration. The presence of the IGF, along with its structure, plays an important role in determining the properties of the ceramic as a whole. Important amongst these properties, keeping in mind the system based specificities, are fracture, creep, oxidation and electrical behaviour. Depending on the system, various synthetic routes like liquid phase sintering, solid-state activated sintering, crystallization of glass surrounding the crystal. etc., have led to the formation of IGFs. Equilibrium thickness amorphous films on surfaces have also been synthesized which are considered to be the surface analogue of IGFs. Important advances in the microscopy techniques have provided invaluable insights into the structure of IGFs, along with its interface with the bounding crystals. These techniques include: high-resolution microscopy, Fresnel contrast imaging, diffuse dark field imaging. diffraction analysis, electron holography, high-angle annular dark field imaging, energy-dispersive X-ray analysis and electron energy loss spectroscopy. It is now being progressively realized that the composition and structure within the IGF is graded, i.e., it has a diffuse interface with the bounding crystals and that the amorphous material in the IGF is different from the bulk glass forming in that system. The order induced by the bounding crystals on the IGF is seen as a contributing factor to the gradation. In spite of the achievements, a lot of open questions remain regarding the formation of IGFs, its behaviour with temperature, its dependence on crystallography of the bounding grains, etc. This overview aims at introducing IGFs to a non-specialist audience, summarizing the important advances in the field and outlining the outstanding issues. (c) 2006 Elsevier B.V. All rights reserved.