The reactions between molten calcium aluminum magnesium silicates (CMAS) at 1300°C and atmospheric plasma spray (APS) deposited environmental barrier coatings on SiC substrates have been investigated. The tri-layer coatings comprised a silicon bond coat protected by a layer of mullite and either Yb 2 SiO 5 (ytterbium monosilicate, YbMS) or Yb 2 Si 2 O 7 (ytterbium disilicate, YbDS) as the topcoat. The APS deposition process resulted in two-phase top coats; the YbMS coating contained Yb 2 O 3 regions in a matrix of Yb 2 SiO 5 while the YbDS coating contained Yb 2 SiO 5 in a matrix of Yb 2 Si 2 O 7 . Exposure of both coatings to a model CMAS resulted in dissolution of the topcoat accompanied by a rapid rise in the concentration of Yb in the melt, and formation of the same Ca 2 Yb 8 (SiO 4 ) 6 O 2 apatite reaction product phase. The thickness of the apatite layer initially varied with (time) 1/4 , but transitioned to approximately parabolic kinetics after 5-10 hours of CMAS exposure. The reaction mechanism on the YbMS layer was consistent with recent observations on Y 2 SiO 5 , wherein molten CMAS transport to the undissolved silicate was controlled by diffusion through thin amorphous films at the apatite grain boundaries. The reaction mechanism for the YbDS layer was more complex, and involved preferential reaction with the YbSiO 5 rich regions, resulting in a reaction zone that contained CMAS, the apatite reaction compound and undissolved Yb 2 Si 2 O 7 . The coating composition and microstructure significantly influenced the mechanism and rate at which the YbDS top coat was consumed by the reaction.

Environmental barrier coatings (EBCs) are needed to protect SiC structures exposed to high temperatures in water vapor-rich environments. Recent studies of a tri-layer EBC system consisting of a silicon layer attached to the SiC, a mullite diffusion barrier and a lowsteam volatility ytterbium silicate topcoat have shown some promise for use at temperatures up to 1316°C. However, the performance of the coating system appeared to be dependent upon the manner of its deposition. Here, an air plasma spray method has been used to deposit this trilayer EBC on a-SiC substrates, and the effects of the plasma arc current and hydrogen content upon the structure, composition, and defects in ytterbium monosilicate (Yb 2 SiO 5 ) and disilicate (Yb 2 Si 2 O 7 ) topcoats are investigated. Modification of spray parameters enabled the loss of SiO from the injected powder to be reduced, leading to partial control of coating stoichiometry and phase content. It also enabled significant control of the morphology of solidified droplets, the porosity, and the microcracking behavior within the coatings. Differences between the Yb 2 SiO 5 and Yb 2 Si 2 O 7 are discussed in the context of their EBC application.

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