Surface perfection, thin films

Ultra-high vacuum (UHV) surface science methods allow preparation and characterization of perfectly clean, well ordered surfaces of single crystalline materials. By preparing pairs of such surfaces it is possible to fonn interfaces under highly controlled conditions. Furthennore, thin films of adsorbed species can be produced and characterized using a wide variety of methods. Surface science methods have been coupled with UHV measurements of macroscopic friction forces. Such measurements have demonstrated that adsorbate film thicknesses of a few monolayers are sufficient to lubricate metal surfaces [12, 181. 

Epitaxial growth of thin films usually involves the formation of strained material as a result of mismatch between the film and substrate and because of the large surface to volume ratio in the film. Surface stress can be a major factor, even when the lattice constants of film and substrate are perfectly matched. Although it appears to be difficult to eliminate the stress totally, it is important to be able to control it and even use it to produce desired qualities. 

Overcoats are an integral part of thin-film disk structures. Their primary role is to provide wear protection. The most common overlayers, such as Rh, plasma-polymerized coatings, SiOz, and carbon, are all chemically stable if they were fully to cover the disk surface, they would provide good corrosion resistance. The thinness of the overcoats and the roughness of the surface preclude perfect coverage and open up the path for localized corrosion at the sites where the magnetic layer is exposed to the environment. 

One should keep two things in mind when choosing a support (1) structure and (2) surface characteristics. Structure contributes to the efficiency of the support, whereas the surface characteristics govern the support s participation in the resulting separations. The perfect column material would be chemically inert towards all types of samples. It would have a large surface area so that liquid phase could be spread in a thin film and structure of the surface would be such that it would properly retain the liquid film. However, large surface area is not a guarantee of an efficient column. 

On the other hand, iron remains perfectly bright and free from all traces of corrosion when immersed in solutions of the chromates or bichromates of the alkali metals, unless, indeed, the solutions are excessively dilute. This is generally attributed to the formation of a thin film of oxide on the surface of the metal which shields the under-lying portions from attack, but this is not the only explanation, as has been seen (p. 71). 

Figure 9.1 Contact angle as a measure of wettability. For 6=0°, we have perfect wetting and the liquid will spread spontaneously on the surface in the form a thin film. For 6=180°, the liquid will ball up, and there will be no wetting.

Fig. 5.17. Cartoon sideviews illustrating the effect of an increasingly oxygen-rich atmosphere on a late TM surface. Whereas the clean surface prevails in perfect vacuum (left), finite O2 pressures in the environment first lead to oxygen adsorption phases. Apart from some bulk-dissolved oxygen, the lower deformation cost will at increasing pressures lead to a preferential accommodation of oxygen in the near-surface fringe. Thin surface oxide structures are the salient consequence before eventually thickening of the oxide film and formation of an ordered bulk compound set in. A thermodynamic or kinetic stabilization of the nanometer thin surface oxide could lead to novel functionalities, different from both bulk metal or bulk oxide surfaces...

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