Interaction of CMAS on thermal sprayed ytterbium disilicate environmental barrier coatings: A story of porosity

Molten calcium magnesium alumina-silicates (CMAS) represent a challenge for the current generation of rare earth silicates environmental barrier coatings (EBCs). Their interaction with ytterbium disilicate (Yb2Si2O7) free-standing coatings deposited using thermal spraying technique has been studied to further understand the reaction mechanisms. Three coatings, deposited with different porosity levels and thickness, representing traditional EBCs (<3% porosity and similar to 350 mu m thickness) and abradable coatings (similar to 20% porosity and 500-1000 mu m thickness) were exposed to CMAS at 1350 degrees C. The results show that higher porosity levels facilitates CMAS infiltration in the first hour of exposure, in combination with infiltration through the inter-splat boundaries. Preferential dissolution of ytterbium monosilicate (Yb2SiO5) takes place, forming a 10-15 mu m Ca2Yb8(SiO4)(6)O-2 apatite layer as the reaction product, producing a network of fine porosity (<10 mu m) as the inter-splat boundary material is consumed. After exposure for 48 h, CMAS has completely infiltrated all three coatings, with apatite crystals present across the coatings, up to a depth of similar to 550 mu m. Despite the extensive CMAS infiltration and apatite formation, no damage could be observed in any of the coatings, providing a promising first step for environmental barrier abradable coatings.