Coatings are applied on top of materials to form a separate layer whereas surface treatments are applied into a material to form an integrated or composite material. Surface treatments and coatings for reusable thermal protection systems (TPS) generally have the same goals:
- high temperature capability to withstand nominal and abort environments
- high emissivity (> 0.9) except for areas where sunlight is the primary heat source
- low catalycity to avoid heating via chemical recombination of hot atmospheric/plasma constituents
- mechanically stable as member of a material system throughout the mission given high temperatures, material thermal expansion, and thermal shocks
- water proofing is often desired for TPS that are exposed to water/high humidity.
Reaction Cured Glass (RCG, RCG-M)
RCG is a black coating consisting of tetraboronsilicide and low porosity borosilicate glass. Typically applied to all but one side of the tile to protect the porous silica, RCG is very effective on silica based tiles up to 3000° F. RCG-M is a modified version of RCG with a higher temperature capability (operates up to 3150° F). Most Shuttle tiles and many X-37b tiles were/are coated with RCG.
Toughened Unipiece Fibrous Insulation (TUFI)
TUFI is a surface treatment that consists of borosilicate glass, silica-boride, and molybdenum disilicide, yielding a stronger, tougher silica tile. Standard TUFI tiles were used on the Shuttle Orbiter’s underside. White variants with higher impact resistance and conductivity were used on the upper body.
High Efficiency Tantalum-based Composite (HETC)
HETC is a surface treatment similar to TUFI except that HETC includes tantalum disilicide. HETC is designed to operate at higher temperatures than TUFI and to mitigate higher thermal expansion differences between substrate and coatings. HETC is current used in TUFROC.