Physical properties of sol-gel coatings

One of the most important applications of sol-gel technology is the fabrica tion of coatings. This is because of the possibility of applying oxide coat ings with practically all types of chemical compositions at low ambient tem peratures on many substrates of various shapes through the use of liquid so lutions. Both oxides and different types of organic-inorganic hybrid coatin gs have been reported. Both oxides and hybrid coatings are usually amorphou s at ambient temperatures but some oxides can be converted to the crystalli ne phase with heating. Regardless of the intended applications of the coati ngs their physical properties are always of importance. For instance, an an ti-reflective coating for an automobile mirror is of little practical value unless it is fairly scratch-resistant. In this review which covers publish ed information in the past fifteen years, some of the more important result s of physical properties of sol-gel derived coatings are discussed firstly for oxides and then for organic-inorganic hybrids. It appears that properti es such as the hardness of oxide coatings are inadequate unless the heat-tr eatment temperatures are in excess of about 400 degreesC. The hybrid coatin gs, especially when they contain a dispersed phase of a hard solid like col loidal silica, can be processed at temperatures below about 150 degreesC an d can improve the performance of organic plastics such as the polycarbonate s. There is insufficient scientific understanding of the relationship betwe en physical properties and other interdependent variables such as processin g conditions, chemistry and coating thickness. More research in this area w ill undoubtedly contribute to the availability of better and new coatings v ia the sol-gel approach.