DEVELOPMENT OF YTTRIUM AND YTTERBIUM SILICATES FROM THEIR OXIDES AND AN OLIGOSILAZANE PRECURSOR FOR COATING APPLICATIONS TO PROTECT SI3N4 CERAMICS IN HOT GAS ENVIRONMENTS

摘要

Environmental barrier coatings are required to protect Si_3N_4 against hot gas corrosion and enable its application in gas turbines. In comparison to other environmental barrier coatings, rare-earth silicate-based coatings stand out due to the very low corrosion rates in moist environments at high temperatures and the compatibility of thermal expansion coefficient to Si_3N_4 ceramics. Thus, the polymer-derived ceramic route was used to synthesize yttrium and ytterbium silicates in the temperature range of 1000-1500 °C for basic investigations regarding their intrinsic properties from a mixture of Y_2O_3 or Yb_2O_3 powders and the oligosilazane Durazane 1800. After pyrolysis above 1200 °C in air, the corresponding silicates are already the predominant phases. The corrosion behaviour of the resulting composites was assessed after exposure to flowing moist air at 1400 °C for 80 h. The material containing Yb_2SiO_5 and Yb_2Si_2O_7 as main crystalline phases undergoes the lowest corrosion rate (-1.8 μg cm~(-2) h~(-1)). In contrast, the corrosion rate of yttrium-based composites remained at least ten times higher. Lastly, the processing of Y_2O_3/Durazane 1800 as well-adherent, crack-free and thick (40 μm) coatings on Si_3N_4 was achieved after pyrolysis at 1400 °C in air. The resulting coating consisted of an Y_2O_3/Y_2SiO_5 top-layer and an Y_2Si_2O_7 interlayer due to diffusion of silicon from the substrate and its interaction with the coating system.