Coatings for Protection against Hot Hydrogen in Solar-Thermal Propulsion

Advanced carbon-based heat exchangers (carbon/carbon composite or graphite) used in solar-thermal propulsion engines must be protected for extended periods from the ge;3000 K hydrogen propellant. In previous work for NASA and DoD, Ultramet successfully fabricated and demonstrated the hot hydrogen survivability of rhenium-based solar-thermal engines, and of refractory metal carbide coatings on open-cell foam components for nuclear-thermal propulsion. The potential exists to combine these technologies by using ductile rhenium as an interlayer between a carbon heat exchanger and a high melting point outer metal carbide layer with well-established survivability in high temperature hydrogen. Relative to rhenium, the carbides are lower in density and cost. This combination has the potential to alleviate issues related to thermal expansion mismatch between the substrate and protective coatings, maximize adhesion, minimize the potential for cracking, and maximize component use temperature and lifetime. In this project, Ultramet will fabricate a matrix of coated coupon test specimens, including various coating materials and layered coatings, and perform initial hot hydrogen performance testing in the Compact Fuel Element Environmental Test (CFEET) facility at NASA MSFC.