Carbon-carbon composites: oxidation behavior and coatings protection

Based on their good mechanical performances, potential uses of structural carbon materials have been identified in high temperature applications. Many applications require extended periods ranging from a few hours (at high temperature) to a few thousands of hours (at low temperature) in oxidizing environments where unprotected carbon-carbon composites deteriorate rapidly. Therefore numerous investigations have been conducted in an attempt to protect carbon-carbon composites against oxidation. The main protection mechanism relies on the use of a barrier against oxygen diffusion. The problem, when applying this concept, is the thermal mismatch between coating and carbon substrate which results in cracking of the coatings. Thereby, oxygen diffuses through the cracks, causes oxidation of the underlying carbon which leads to the spallation of the coatings. Therefore other approaches have been developed including : (1) intermediate layers, (2) inhibited matrices and (3) fiber protections. The purpose of this work is to examine the oxidation of carbon-carbon composites and to evaluate the inherent limitations of the various current coatings. Recently due to these limitations, composite coatings have been studied. Moreover, a new concept has been developed. It is proposed that the introduction of an inert gas into the boundary layer at the carbon composite surface will provide some protection. (orig.)