Silicide Coatings on Refractory Metal Alloys for High Temperature Oxidation Protection of Hypersonic Vehicle Leading Edges

Hypersonic flight requires materials capable of withstanding high temperatures, high heat fluxes, and high mechanical loading. Many hypersonic vehicle components, including control surfaces, engine components, and actively cooled leading edge structures, utilize high temperature niobium and molybdenum alloys such as C103 and TZM respectively. These refractory alloys have found use in hypersonic vehicle applications because of their shared properties, including high melting point and excellent high temperature strength. To date, conventional oxidation-protective coatings such as R512E have provided sufficient oxidation protection to the metal alloys. However, as hypersonic vehicle performance advances, these metallic components will be exposed to operating environments that far exceed the capabilities of conventional oxidation protection coatings. To enable the next generation of metallic components for hypersonic vehicle applications, new oxidation-protective coating systems will be required to protect the alloys from oxidation at very high temperatures. In this project, Ultramet proposes to develop a pack cementation process that will form a protective silicide coating on C103 niobium-based alloy. In conjunction with the pack cementation coating process development, UltrametÆs Small Business Technology Transfer (STTR) research institution partner will develop a high temperature oxidation model for niobium silicide (NbSi2) coatings during hypersonic flight that predicts SiO2 formation, SiO2 shear thinning during flight, and ultimately coating recession with regard to the temperature and atmospheric conditions of hypersonic flight. The goal is to develop a high-fidelity oxidation model that defines the operating envelope of the coated C103 alloy. Ultramet intends to leverage the proposed coating technology and in follow-on work combine it with its proven chemical vapor deposited (CVD) silicon carbide (SiC)-based ultrahigh temperature ceramic topcoat solutions. The combination of the proposed silicide coating process with existing oxidation-resistant topcoat offerings will reduce manufacturing complexity, lead time, and cost. The silicide coating will provide comparable high temperature oxidation protection for the C103 at elevated temperatures and will also serve as a bondcoat layer that enables application of ultrahigh temperature SiC-based coatings to the refractory alloys that can significantly extend their operating temperature range. The process development and material modeling performed in Phase I will provide a fundamental understanding of these coatings on the metallic alloy substrates and a firm foundation for material system and modeling optimization to be validated by component testing in relevant environments in Phase II. Successful completion of the combined efforts will support faster progression of the coating system TRL toward program insertion. Approved for Public Release | 24-MDA-11673 (8 Jan 24)