Superior properties of silica thin films prepared from perhydropolysilazane solutions at room temperature in comparison with conventional alkoxide-derived silica gel films.

Preparation of silica thin films from perhydropolysilazane (PHPS) at room temperature has attracted much attention because it provides a new way to realize silica thin films in a variety of technologies where any high temperature processes should be avoided. Although silica gel films can also be prepared from alkoxides at room temperature by conventional sol-gel method, they are believed to have low mechanical and chemical durability. However, even such alkoxide-derived silica gel films have possibilities to become more durable via condensation reaction and densification when aged at room temperature. In order to clarify whether or not PHPS-derived silica thin films have critical superiority on properties, the hardness and chemical durability were compared between PHPS- and alkoxide-derived silica thin films, where PHPS films were exposed to the vapor from aqueous ammonia at room temperature for PHPS-to-silica conversion. Alkoxide-derived silica gel films were found to be densified and hardened when stored in air at room temperature, which resulted in pencil hardness even higher than 9H on Si(100) substrates. However, the ultra-microindentation tests demonstrated that the PHPS-derived films are definitely harder than the alkoxide-derived ones. The PHPS-derived films were also found to have higher chemical durability in water and in aqueous ammonia. Such higher mechanical and chemical durability of the PHPS-derived films was ascribed to their higher density, i.e., more highly condensed states, which was evidenced in infrared absorption spectra. Hard coating performance on plastic substrates was also studied, and the PHPS-derived films were demonstrated to have much higher adhesive strength on polymethylmethacrylate substrates. The in-plane stress measurement demonstrated that the PHPS-derived films have much lower or even negligible tensile stress, which may be one of the causes for such higher adhesive strength.