Corrosion of silicon nitride materials in gas turbine environment

In the present study the corrosion behavior of various silicon nitride materials will be presented. Hot gas tests were conducted in an atmosphere similar to that in a gas turbine (high temperature, pressure and flow rate and the water vapor pressure as the corrosive component). While some materials displayed a high degree of microstructural stability, all materials suffered surface degradation during the rig test. The oxidation surface layer of mainly SiO2, which is essential for the oxidation protection of nonoxide materials because it induces a passive, diffusion-controlled oxidation mechanism, was found to be degraded by evaporation processes involving volatile silicon hydroxides. As the mechanism of these processes a combination of linear oxidation and corrosion was proposed. The stabilization of the protective oxidation layer of silica should be considered as the main factor in stabilizing these materials in hot gas environments. This can be achieved by environmental barrier coatings (EBC), however their long-term stability was found to be still insufficient.