FTIR and XPS Studies on Corrosion - resistant Si 02 Coatings as a Function of the Humidity during Deposit ion

The degradation of SiO, coatings deposited on alloys by metal organic chemical vapour deposition (MOCVD) in sulphidizing high-temperature environments is determined by delamination and crack formation. With increasing water concentration during deposition, the crack density in silica decreases and the critical thickness for delamination of SiO, coatings increases. This improvement is supposed to be caused by compositional changes in the SiO, coating. In this study presence of water and silanol groups as measured by Fourier transform infrared spectroscopy (FTIR) and the Si : 0 ratio as measured by XPS are discussed in relation to the protective properties. The FTIR measurements show that the coatings deposited in more humid environments contain more silanol groups and have lower stress levels. The coatings obtained under all deposition conditions consisted of stoichiometric SiO,., as determined by XPS. The presence of silanol groups reduces the viscosity of the coating, and stress relaxation by viscous flow becomes enhanced, thereby improving the coating performance.