Thin Anisotropic Coatings Based on Sol-Gel Technology
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THIN ANISOTROPIC COATINGS BASED ON SOL-GEL TECHNOLOGY.
S. Randall Holmes-Farley and Lynn C. Yanyo Lord Corporation, P. 0. Box 8225, Cary, NC 27512-8225.
ABSTRACT Using sol-gel technology, thin organic/ceramic (ceramer) coatings have been applied to metal surfaces in order to enhance such surface properties as adhesion promotion and corrosion prevention. Isotropic coatings have been found to be effective in certain applications such as corrosion prevention, but the formation of anisotropic coatings permits greater flexibility over the resulting properties. Isotropic coatings derived from tetraethoxysilane, for example, effectively inhibit corrosion while being only 100 to 1000 A thick. These coatings do not, however, promote adhesion. Thin cpatings made from traditional silane adhesion promoters alone are unable to prevent corrosion of metallic substrates. Using monomers with appropriate reactivities permits the single-step synthesis of anisotropic coatings that can both promote adhesion and prevent corrosion. These types of anisotropic coatings allow the physical and chemical properties of a coating to be varied as a function of distance from the substrate and confer properties to the substrate that would not be possible from a single isotropic coating. The principle behind the construction of these anisotropic coatings is general enough that it can be used in many applications where microengineering of surface structures is important. INTRODUCTION Sol-gel coatings can be applied to substrates using two different methods. The first, and most widely used, involves making a coating material that is spread onto the substrate and subsequently cured. Typically these materials are made by the acid-catalyzed hydrolysis and partial condensation of metal alkoxides. 1 -7 An alternative approach reported in this paper is the basecatalyzed growth of coatings directly on the substrate. In this method metal alkoxides are mixed with solvent, water, and a basic catalyst in the presence of a substrate (such as aluminum). As the monomers condense, some of them react with the surface of the substrate. As condensation continues a coating is deposited onto, or more correctly reacted with, the substrate. If the initial coating solution is formulated such that it contains several metal alkoxides of differing reactivity then the nature of the coating being deposited will be a function of time. The lowest layers of the coating will be derived from the most reactive monomers while the upper layers will be derived from the less reactive monomers. In this manner one can develop anisotropic coatings. The properties of these coatings can then be optimized so that each part of the coating provides optimum properties. One might choose, for example, for the first layer to interact strongly with the substrate, an intermediate layer to provide some sort of barrier, and a final layer to provide compatibility with a subsequently applied top coat (e.g. paint or Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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