Liquid phase coating process for protective ceramic layers on carbon fibers

The most outstanding feature of carbon fibers is their high tenacity at high temperatures; this property cannot be sustained if the unprotected fibers are exposed to an oxidative environment. Protecting the fiber surface by deposition of a ceramic layer shows the best prospects for introduction of high temperature oxidation stability. SiBCN and SiCN ceramics are the preferred coating materials due to their excellent oxidation resistance at elevated temperatures. The coating operation is carried out by a continuous liquid polymer impregnation (LPI) of carbon fibers with polyorgano(bora)silazanes. In order to reach an optimum behavior of the polymer in the LPI operation, detailed rheological studies were carried out. The viscosity of the polymer is adjusted by addition of organic solvents and other additives. In the pilot plant, the desizing and liquid phase coating of the fibers, polymerization and ceramization of the coating are subsequently carried out in a continuous process. Pilot plant construction was carried out in modular design in order to be able to modify parts of the equipment if necessary. In the first module, the fiber is desized in an organic solvent and dried. The second module is built as a vacuum chamber, the following operations of LPI, polymerization and ceramization are carried out under inert atmosphere. Before putting the plant into service, preliminary investigations were carried out. After fitting the rheological properties, the operations were optimized using a dip-coating process with subsequent batch polymerization and pyrolysis.