Development of hot gas corrosion stable coating systems on non-oxide Si based ceramics for turbine applications

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A significant increase in the efficiency of gas turbines based on superalloys is not expected even by using sophisticated cooling concepts. Hence, their substitution by non-oxide Si-based ceramics as Si3N4, SiC and SiC/SiC CMCs is a promising approach, thus allowing higher service temperatures. Besides the lower density, these ceramics possess a high oxidation stability due to the formation of a SiO2 protective layer. However, water vapour, one of the main combustion products, reacts with this protective film and leads to its volatilization as Si(OH)4, thus compromising the demanded long service life of the ceramic components. Among the suitable candidates for the protection of Si3N4, SiC and SiC/SiC, Ytterbium Disilicate (Yb2Si2O7) coatings stand out due to the increased hot gas corrosion stability and the matching coefficient of thermal expansion to these ceramics. Recently, we have successfully developed a dense Yb2Si2O7-based coating by spraying suspensions containing the Silazane Durazane 1800 (ceramic binder and source of SiO2) as well as Yb2O3 and elemental Si as active fillers onto Si3N4 substrates, followed by reactive pyrolysis at 1415 °C in air. Besides Yb2Si2O7, a SiO2 phase was however detected at its grain boundaries, which led to the enhanced degradation of the coating during realistic hot gas corrosion tests (v = 100 m/s, 1200 °C for 200 h). Thus, the hindrance of a SiO2 boundary phase is of major importance to achieve a suitable protection of non-oxide Si-based ceramics as turbine components during their expected service life (t > 10,000 h). Hereafter, two promising strategies will be explored within the scope of this project proposal. In Strategy 1, the use of Yb, Sc or Hf metal or their oxide nanopowders in combination with the already approved coating components as additives should promote the conversion of the SiO2 phase at the grain boundaries of Yb2Si2O7 into suitable silicates during pyrolysis. In Strategy 2, the chemical modification of the Silazane precursor with the already mentioned elements should avoid the formation of free SiO2 within the Yb2Si2O7 boundary phase, by leading to its conversion into the respective silicates during pyrolysis. In order to specifically control the silicate formation for both strategies and to investigate the interactions of the coating components among each other during pyrolysis, basic experiments are first carried out on monoliths. Subsequently, the spraying and pyrolysis of selected suspensions onto Si3N4, SiC and SiC/SiC substrates will be performed, in order to assure the processing of a dense and thick (> 100 µm) silicate coating. A comprehensive characterization of the mechanical and physical properties of all coating systems but especially their stability against hot gas corrosion should determine their application potential.