TABLE OF CONTENTS An interface is a meeting surface between two dissimilarly oriented perfect crystals or between two chemically different crystals, i.e., a crystallographic or chemical discontinuity. In terms of crystal structure and chemistry of adjacent crystals, interfaces can be classified into homophase and heterophase boundaries (Cahn and Kalonji, 1982; Finnis and R hle, 1993). Homophase boundaries form between grains of identical crystal structure and composition, e.g., grain boundaries, twin boundaries, domain boundaries and stacking faults. Heterophase boundaries form between regions of different crystal structure and/or chemical composition, e.g., interfaces between co-existing polytypes, between metal-ceramic joints and most interfaces between the matrix and the reinforcing agents in composite materials. Hence, the term interface is a general term, whereas the terms grain boundary and interphase boundary are more specialised terms used to distinguish between the two types of interface.
In contrast to grain boundaries and interphase boundaries in metals, in which grains make intimate contact at the atomic level regardless of the orientation relationship across the boundary (Sutton and Balluffi, 1995), grain boundaries in non-oxide engineering ceramics and interphase boundaries in composites invariably contain thin films of extraneous material which arise through the presence of impurities and additives present in the starting powders used to make the ceramics and composites (Kleebe et al., 1992; Turan and Knowles, 1995). Here, we will use the term intergranular film to describe a phase at an interface.
