Protection of carbon based composites

Based on their good mechanical properties, potential uses of structural carbon materials have been identified for future manned hypersonic vehicles and other aircraft applications. Carbon based composites are expected to be used for periods ranging from a few hours (at high temperature) to a few thousands of hours (at low temperature) in oxidizing environments. Unfortunately carbon reacts rapidly with oxygen at temperature as low as 500 OC. Therefore numerous researches have been conducted in an attempt to protect carbon-carbon composites against oxidation. The protection mechanism relies on different concepts depending on the temperature range. Two different basic approaches have been considered to protect carbonaceous materials: 1) a microscopic one (at low temperature) using poisoning agents which adsorb on the active sites responsible for the carbon gasification; and 2) a macroscopic one (at high temperature) using an oxygen barrier. But the difference in thermal behavior that exists between this layer and the carbon substrate results in coating cracks. Therefore other approaches have been developed including: a) transition layer; b) inhibited matrix; c) coating on fibers. Temperature, pressure and oxidizing atmospheres limitations of all theses concepts are discussed. Typically, protection for extended periods can be achieved in a limited range of pressure and temperature by the use of several concepts and multilayer coating.