The synthesis of SiC and TiC protective coatings for carbon fibers by the reactive replica process

Abstract

The possibility to use the reactive replica technique to synthesize mono- and multi-layer coatings make it possible to cover carbon materials of complex shape by a ceramic layer. The reactive replica synthesis requires two steps: the deposition of an oxide layer on the carbon material by a sol–gel process, and a heat treatment above 1400 °C to form the carbide by reaction between the carbon material and the oxide layer. Several multi-layer coatings based on SiC and on SiC–TiC were synthesized by this process by using different carbon fibers. In this study, mono- and multi-layer carbide coatings were prepared on four different carbon fibers by the reactive replica technique to improve their resistance to oxidation between 500 and 1000 °C. The characteristics (surface area, thickness, …) of the carbide layers were studied in relation to the synthesis parameters such as the carbon surface properties, the nature of the carbide and the oxide thickness. The resistance to oxidation of the ceramic layers which can act as protective layers was studied between 500 and 1000 °C. The oxidation kinetics of the pristine and coated carbon fibers were studied in isothermal conditions by thermogravimetric analysis. It was observed that the surface of the pristine carbon fiber has a great influence on the efficiency of the protective coating. The thermogravimetric study showed that below 800 °C, the oxidation is controlled by the rate constant of the reaction either for non-coated or coated carbon fibers. Above this temperature, the oxidation rate is limited by the diffusion of oxygen through the coating layer. Moreover, the activation energy of the kinetic constant of the oxidation is not changed by the addition of the protective layer. This behaviour is an indication that the ceramic layer does not act as a diffusion barrier, the protection efficiency may be more related to the fact that the ceramic formation proceeds on the active sites of the carbon fibers, and therefore reduces its reactivity toward oxygen. The most effective protection against oxidation was obtained with a double layer of SiC. The addition of TiC in the protective layer either as a multi-layer of SiC and TiC or as a mono-layer of a mixture of the two carbides does not lead to any improvement of the protection efficiency against carbon oxidation.