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CERAMIC COATINGS ARE AN ALTERNATIVE TO organic or polymer based coatings for selected surface coating applications. When protecting a surface, it is usually less expensive to use organic coatings rather than other materials such as ceramic coatings. However, organic paints have limitations and will not meet all service requirements. Though more expensive, for some applications ceramic coatings may be a more suitable form of protection. Vitreous (glassy) ceramic coatings are chosen for application over a substrate for one or more of several reasons . These reasons for selecting a ceramic coating in preference to an organic paint include rendering the surface more chemically inert, impervious to liquids and gases, more readily cleanable, and more resistant to service temperature, abrasion, and scratching. The chemical durability of ceramic coatings in service substantially exceeds that of organic paints . Vitreous coatings are formulated to be resistant to a variety of reagents, from acids to hot water to alkalies, as well as to essentially all organic media. The only important exception is hydrofluoric acid, which readily attacks all silicate glasses. This outstanding durability, combined with a very smooth surface, renders many ceramic coatings suitable for applications requiring the highest standards of cleanability, such as ware that comes in contact with food and drink. These coatings are also suitable for applications requiring true hermeticity, usually to protect sensitive electronic equipment. No organic resins are truly hermetic. Even the most thermally stable organic resins depolymerize at temperatures on the order of 300°C. Hence, organic paints are not suitable for applications requiring thermal stability above 300°C. For example, stove side panels are painted, but stove tops are porcelain enameled. A similar argument can be made for abrasion resistance. Organic resins are soft (Moh 2–3). By contrast, vitreous coatings are harder (Moh 5–6), and some plasma coatings are much harder. For example, alumina coatings, plasma sprayed, have Moh = 9. Vitreous coatings are thin layers of glass fused onto the surface of the substrate. When the substrate is a ceramic, the coating is called a glaze. When the substrate is a metal, the coating is called a porcelain enamel. When the substrate is a glass, the coating is called a glass enamel.
Eppler, Richard A.
Consultant, Eppler Associates, Cheshire, CT