Synthesis and characterization of yttrium and ytterbium silicates from their oxides and an oligosilazane by the PDC route for coating applications to protect Si3N4 in hot gas environments

Abstract

Environmental barrier coatings are required to protect Si3N4 against hot gas corrosion and enable its application in gas turbines, among which yttrium and ytterbium silicate-coatings stand out. Thus, the polymer-derived ceramic route was used to synthesize these silicates 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 silicates are predominant phases. The corrosion behaviour of the resulting composites was tested at 1400°C for 80h in moist environments. The material containing x2-Yb2SiO5 and Yb2Si2O7 undergoes the lowest corrosion rate (−1.8μgcm−2h−1). Finally, the processing of Y2O3/Durazane 1800 as well-adherent, crack-free and thick (40μm) coatings for Si3N4 was achieved after pyrolysis at 1400°C in air. The coating consisted of an Y2O3/Y2SiO5 top-layer and an Y2O3/Y2Si2O7 interlayer due to the interaction of the coating system with the substrate.