Structure, composition, and defect control during plasma spray deposition of ytterbium silicate coatings

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 low-steam volatility ytterbium silicate topcoat have shown some promise for use at temperatures up to 1316 A degrees 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 tri-layer EBC on alpha-SiC substrates, and the effects of the plasma arc current and hydrogen content upon the structure, composition, and defects in ytterbium monosilicate (Yb2SiO5) and disilicate (Yb2Si2O7) 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 Yb2SiO5 and Yb2Si2O7 are discussed in the context of their EBC application.