Powerful shock waves form in front of an aircraft, generating high temperatures in the airflow. The parts of the aircraft exposed to the highest temperatures are the rocket's nose cone, the front end of a space shuttle, and the leading edge of the main wing. The materials in these areas must withstand the harsh aerodynamic heating environment during re-entry and be reusable. Currently, carbon-carbon composites are commonly used for these heat-resistant structural materials.
Oxidation-resistant carbon-carbon composites are produced by the following method: Prepreg tapes, made by impregnating viscose-based carbon fiber braids with phenolic resin, are stacked in 19 to 38 layers in a desired shape. After curing in an autoclave, they are post-cured and carbonized at 815°C for 70 hours. Then, they are densified by three impregnation, curing, and carbonization cycles with furfural resin. High-temperature heat treatment and oxidation-resistant coating are then performed simultaneously.
The calcined prepreg is then embedded in a powder mixture of aluminum, silicon, and silicon carbide, reacted at 1650°C, and coated with SiC to create the carbon-carbon composite. To seal cracks caused by the thermal expansion difference between the C/C substrate and the SiC coating and prevent coating detachment, the material undergoes five TEOS (tetraethyl orthosilicate) impregnations, followed by a 315°C heat treatment, to create an oxidation-resistant coating.
C/C composites not only maintain the typical characteristics of carbon materials-high strength, heat resistance, thermal shock resistance, and thermal corrosion resistance-at high temperatures, as well as excellent thermal and electrical conductivity, but also offer advantages such as high strength, high rigidity, low density, high specific strength and rigidity, and excellent fracture toughness. However, like general carbon materials, they exhibit poor oxidation resistance in the presence of oxygen, necessitating a coating with an oxidation-resistant material such as SiC when used in oxidizing atmospheres.