A family of diboride/SiC CMC ultrahigh-temperature thermal protection systems are particularly well suited for nonablating sharp leading edges at hypersonic velocity, where aerodynamic heating is severe.

Thermal insulation materials for sharp leading edges on hypersonic vehicles must be stable at very high temperatures (e.g., 2000 C, or 3630 F); must resist evaporation, erosion and oxidation; and they should have low thermal diffusivity to limit heat transfer to support structures. The most promising materials for the application are a family of ultrahigh temperature ceramics (UHTCs) comprising ZrB2 and HfB2, and composites of those ceramics with SiC. ZrB2 and HfB2 ceramics have high hardness and very high melting temperatures (3245 C for ZrB2 and 3380°C for HfB2), and they form protective, oxidation resistant coatings and have low vapor pressures at potential use temperatures. However, despite their attractive properties, these material have seen limited use due to difficulties in processing that precluded achieving optimal properties. Development work at Sandia National Laboratories is overcoming these problems.