Conversion Coated Surfaces for Hypersonic Environments

As designs mature and systems evolve to field hypersonic vehicles, the near-term efforts will focus on using readily available materials. In some cases, this might be carbon-carbon (c-c), which will require protective coatings. In other cases, tungsten (W), niobium (Nb), hafnium (Hf), and titanium (Ti) alloys may be used on portions of the vehicle. Over the last decade, there have been efforts applied to defining coatings that would survive the severe hypersonic environments experienced through the hypersonic missions. Refractory materials survive high temperatures and can be enhanced to higher performance with conversion coatings of the exterior surfaces; conversion meaning subjecting the material to a high carbon, oxide, or nitride environments and converting a thin portion of the outer surface to a high temperature carbide, oxide, or nitride layer. Using conversions versus coatings eliminates the weak link in coating system that being surface preparation and adhesion.  In conversion, the subjection of the metal to a carburizing, nitriding, or oxidizing environment, creates a transformation of the base metal, converting the exposed base metal surface to a higher temperature operating material.