Ytterbium disilicate environmental barrier coatings: processing and combined steam-CMAS corrosion

As efficiency gains are sought within gas turbines through ceramic matrix composites (CMCs), ytterbium disilicate coatings, as both abradable and environmental barriers, are seriously considered to achieve the desired performance. This paper will present how SPS and APS abradable and EBC coatings perform when exposed to steam, molten calcium magnesium alumino-silicates (CMAS) and combined steam and CMAS. Three ytterbium disilicate (Yb2Si2O7 or YbDS) abradable EBCs containing 8, 15 and 22 % porosity by area were deposited using atmospheric plasma spraying. These coatings were then exposed to steam, CMAS (35 CaO – 10 MgO – 7 Al2O3 – 48 SiO2 mol. %) and combined steam and CMAS at 1350 °C for 100 hrs. Exposure to steam caused the formation of a thin ytterbium monosilicate (Yb2SiO5 or YbMS) reaction layer. Reaction with the CMAS occurred by a dissolution-precipitation mechanism, with a reprecipitated ytterbium disilicate phase and Yb-apatite (Ca2Yb8(SiO4)6O2) crystals noted as the only reaction products. After the combined exposure, the CMAS infiltration depth was higher than that observed in the standalone CMAS exposure. Also, an increased amount of Yb-apatite formation was observed within with YbMS reaction layer, and an ytterbium aluminium garnet (Yb3Al5O12 or YbAG) phase was also observed.