A protective diffusion barrier for metalized mirror structures is provided by a layer or coating of silicon nitride which is a very dense, transparent, dielectric material that is impervious to water, alkali, and other impurities and corrosive substances that typically attack the metal layers of mirrors...http://www.google.de/patents/US4963012?utm_source=gb-gplus-sharePatent US4963012 - Passivation coating for flexible substrate mirrors
