Oxidation Protection of Refractory Metals via Refractory Bond Coat Alloy Design and Thermally Grown Complex Oxides

The key goal of this proposal is to develop a refractory alloy bond coat capable of growing a protective layer of CrTaO4 upon exposure to a high-temperature oxidizing environment, such as the environment of an actively firing rocket. HIgh temperature oxidation resistance is integral to radiatively cooled rockets and thrusters, which are present in a myriad of space vehicle applications from small maneuvering thrusters to large, in-space transit thrusters and rockets. Currently used iridium-coated rhenium rockets have finite lifespans, as iridium does not form a protective oxide layer but rather simply reacts much slower with oxygen than other metals. The lifespan of iridium coatings with currently used hydrazine/NTO propellants is acceptable, but many advanced propellants that are nearing adoption produce more severely oxidizing ignition byproducts, creating concern around the lifespan of iridium coatings in these more severely oxidizing environments. This proposal is comprised of three projects that build upon each other to develop a CrTaO4-forming coating suitable for application on other refractory alloys. Bulk alloy samples and powder feedstocks for coating will be created using arc-melting and atomization, respectively. Oxidation experiments will be done using controlled-environment furnaces. Materials characterization methods such as scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD) will be used to determine the formation mechanisms of CrTaO4 in the alloy systems under study. This proposal's successful completion will afford more flexibility in propellant selection and oxidizing potential, as well as allowing for the use of lighter and cheaper refractory alloys in radiatively cooled rockets (iridium and rhenium are both two of the densest and rarest elements). Doing so will help enable future NASA missions into deep space, as mission distances and payload requirements continue to increase.

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