Towards Highly Dense Yb-Silicate Microstructures Deposited by Air Plasma Spray for Environmental Barrier Coating Applications II: Plasma Gas Composition, Feedstock, and Anode Orifice Comparisons

In all air plasma sprayed (APS) environmental barrier coating (EBC) applications, the predominant goal is to achieve maximum coating density, gas tightness, and/or hermeticity prior to subjecting it to harsh environments (i.e., high-temperature impingement of high-velocity water vapor). The microstructures of APS coatings are historically understood to be influenced by the input processing parameters. Beyond the local deposition rate (surface speed, feeding rate) explored in Part I, there are further extrinsic processing parameters such as plasma gas composition, feedstock choice, and anode orifice dimensions which can be tuned, but have not been fully explored in the context of EBCs. Screening these ancillary extrinsic inputs in a rigorous and systematic way presents challenges in determining which control variable(s) to select to gain meaningful insights. A constant particle temperature distribution (not average particle temperature) in the spray stream was held as a constraint, and the aforementioned extrinsic parameters were varied. As in Part I, a qualitative microstructural approach toward examining the presence (or absence) of advantageous vertical thin microcracks in the as-deposited coating was taken. For certain conditions, a Dense Vertically Macrocracked structure was achieved. Concurrent synthesis of these results offers further insights into process selection and parameter design can be gained.