High resolution transmission electron microscopy studies of metal/ceramics interfaces

When single crystals of two different materials are in contact at a sharp interface, the orientation relationship between them is said to be epitaxial and the configuration of the atoms at the two sides of the interface is such that the lattice mismatch between them is accommodated in the least energetic way. Among other factors, this depends on the bonding between the atoms on the two sides of the interface. In this paper, the relaxation of strain in thin films grown epitaxially on dissimilar substrates is first discussed theoretically for cases of small and large lattice mismatch. In a following section, two metal-ceramics heteroepitaxial systems are investigated in detail by various techniques of transmission electron microscopy. One case, vanadium on MgO, corresponds to a small-mismatched system and the interface changes from coherent to semicoherent above a critical thickness; this turns out to be much larger than the expected value. In the other case-vanadium on the basal and rhombohedral (R) planes of sapphire-the lattice mismatch is large and misfit dislocations exist from the very initial stages of deposition. It is argued that although misfit dislocations in small and large lattice-mismatched systems are geometrically similar, their physical nature is different.