The primary reason coatings are applied to CC composites is to provide oxidation or erosion protection at high temperatures. Until recently, CC composites were considered to be nonstructural thermal protection materials based on their excellent performance in a number of rocket propulsion and atmospheric reentry applications. These applications involve extremely high temperatures but require very short lifetimes and take advantage of the unmatched ablative properties of the all-carbon composite. Although ceramic and refractory metal external coatings were considered and evaluated for these applications, they are generally found to be of little or no benefit.
The first real need for a coated CC composite was identified during the development of the Space Shuttle orbiter vehicles. Again, CC composite was used as a thermal protection material, but in this case, thee temperatures were much lower than those for earlier applications, and the Space Shuttle material had to be reusable. The success that was achieved with coated CC composite in the Space Shuttle program provided the impetus for the current development of oxidation-protected CC composites for structural applications in aircraft and limited life of terbine engines, and as airframe components for multiuse hypersonic vehicles. A renewed interest also exists in the use of coated CC composites for rocket propulsion and missile aerodynamic heating applications because of the requirements of advanced higher performance systems.