Grain boundary segregation substrate

If the composition of the outermost layer is different from that of the bulk (for example, as a result of surface or grain boundary segregation), a different approach has to be made. For example, if a partial overlayer of element A of fractional coverage 9a covers a substrate of element B, the spectrum contains three contributions that from the overlayer, that from the exposed part of the substrate, and that from the covered part of the substrate. 

Compositional analysis involves the determination of three quantities. The most fundamental of these is the elemental identity of surface species, i.e., the atomic number of each species. It also is desirable to know, however, the chemical identities of these species. For example, is CO adsorbed as a molecule or is it dissociated into separate C and 0 complexes with the substrate. Finally, it is necessary to determine the approximate spatial location of the various chemical species. Are they "on top" an otherwise undisturbed substrate Do they reconstruct the substrate or diffuse into it, e.g., along grain boundaries Or perhaps they form localized islands or even macroscopic segregated phases at various positions across the surface. An important trend in modern compositional analysis is the increasing demand for spatial resolution laterally across the surface on a scale (d 0.1 u m = 10 A) comparable to the dimensions of modern integrated circuits (10-12). Compositional analysis is by far the most extensively used form of surface analysis and is the subject of most of the papers in this symposium as well as of numerous reviews in the literature (5-9., 13, 14). 

Figure5.4.1) Contact angle versus time for a eutectic (Ag-Cu) dropon polycrystalline W at 900°C in a high vacuum. Before the experiment, the W substrate was heat-treated in high vacuum at 1100°C for 2 h. Despite this treatment, the surface remained oxidised and a slow spreading, controlled by W deoxidation, was observed. 2) The same without prior heat treatment of W. In this case segregation of O at the W surface, by fast grain-boundary diffusion, prevents deoxidation of the substrate, resulting in non-wetting behaviour. From Lorrain (1996).

It is not possible to determine by SIMS the precise location of zirconium (e.g., whether segregation has occurred to the oxide/aUoy interface or to oxide grain boundaries). This is where higher-resolution electron microscopy with energy-dispersive X-ray analysis is important. STEM/X-ray data [16] on samples previously analyzed by SIMS show that for NiAl+Zr, no particles are present right at the oxide/substrate interface. Also, no interfacial voids... 

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