Development of yttrium and ytterbium silicates from their oxides and an oligosilazane precursor for coating applications to protect SI3N4 ceramics in hot gas environments
June 24-29, 2018
Environmental barrier coatings are required to protect Si3N4 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 Si3N4 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 Y2O3 or Yb2O3 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 Yb2SiO5 and Yb2Si2O7 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 Y2O3/Durazane 1800 as well-adherent, crack-free and thick (40 µm) coatings on Si3N4 was achieved after pyrolysis at 1400 °C in air. The resulting coating consisted of an Y2O3/Y2SiO5 top-layer and an Y2Si2O7 interlayer due to diffusion of silicon from the substrate and its interaction with the coating system.
Mateus Lenz Leite, Gilvan Barroso, Walter Krenkel, and Günter Motz, "Development of yttrium and ytterbium silicates from their oxides and an oligosilazane precursor for coating applications to protect SI3N4 ceramics in hot gas environments" in "Thermal Barrier Coatings V", Prof. Dr. Robert Vaßen, Forschungszentrum Jülich GmbH, Germany Brian Hazel, Pratt & Whitney, USA Prof. Dr. Uwe Schulz, German Aerospace Center, Germany Dr. Michael J. Maloney, Pratt & Whitney, USA Dr. Ram Darolia, GE Aviation (Retired), USA Eds, ECI Symposium Series, (2018). http://dc.engconfintl.org/tbcv/3