Plasma Gas Chemistry Effects on the In-Flight Stoichiometric Changes of Ytterbium Silicate EBCs

Air plasma-sprayed (APS) environmental barrier coatings (EBCs) is presently a well-established technology. Even so, there are still unexplored process–property relationships. For example, it is notionally understood that rare-earth silicates before impacting the substrate will undergo an in-flight, in situ chemical shift. It is hypothesized among the community that the free silica in the molten droplets vaporizes, leading to chemically shifted individual splats. The available literature is divided on whether the presence of hydrogen or the plasma power drives this chemical shift. This work established two spraying conditions (at nearly equivalent plasma power) with Ar-H₂ and Ar-He plasma gases. Process diagnostics revealed the two plasmas yield particles of nearly equivalent melt state to directly address the question—what drives the chemical shift of APS EBCs, and how does the chemical shift influence the properties of the coating? The results in this paper will show there is a measurable optical emission signature to indicate the in-flight decomposition when hydrogen is used. Moreover, for equivalently-molten spray streams at similar plasma powers, the phases, crystallization, and thermal expansion properties vary largely depending on the presence or absence of hydrogen in the plasma.