HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDIES OF METAL CERAMICS INTERFACES/

When single crystals of two different materials are in contact at a sh arp 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 accommodat ed in the least energetic way. Among other factors, this depends on th e bonding between the atoms on the two sides of the interface. In this paper, the relaxation of strain in thin films grown epitaxially on di ssimilar substrates is first discussed theoretically for cases of smal l and large lattice mismatch. In a following section, two metal-cerami cs heteroepitaxial systems are investigated in detail by various techn iques of transmission electron microscopy. One case, vanadium on MgO, corresponds to a small-mismatched system and the interface changes fro m 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 misma tch is large and misfit dislocations exist from the very initial stage s of deposition. It is argued that although misfit dislocations in sma ll and large lattice-mismatched systems are geometrically similar, the ir physical nature is different. (C) 1998 Wiley-Liss, Inc.