To attach a porcelain enamel coating to any substrate by enameling, both substrate and glass must be heated to the fusion temperature of the glass. However, fusion methods have not been successful for the more refractory materials. Because most refractory porcelain enamel coatings are amorphous or crystalline in nature, they have to be applied by relatively novel techniques.
Although most ceramic materials are refractory, some of them can be vaporized in an electric arc or hot vacuum. Thin porcelain enamel coatings of amorphous silica can be applied readily to relatively cool substrates by vaporizing metallic silicon or silicon halides in the presence of small quantities of oxygen. Apparently the transfer is accomplished largely as silicon monoxide, which recombines with oxygen on cooling. The process is used to obtain thin, protective, optically transparent films on lenses, certain electrical components, and metal reflectors.
Other porcelain enamel coatings may be produced by vaporizing one or more components of the coating. In this way porcelain enamel coatings of the respective carbides of silicon, boron, aluminum, and chromium can be deposited on graphite, silicon nitride can be formed on metallic silicon, and silicide coatings can be deposited on metals such as tungsten and molybdenum. These processes are necessarily expensive and are poorly adaptable to large specimens or complex shapes.
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