Surface studies of potentially oxidation protective Si-B-N-C films for carbon fibers

The paper reports a study of CVD deposited thin films (1-5 mu m) of an Si-B-N-C ceramic polymer coating onto a tow of carbon fibers. The coating process was developed with the goal of good adhesion and good oxidation protection for the fibers when exposed to a reactive oxygen environment. Core and valence band photoemission was used to monitor the surface chemistry of the films. An ab initio calculation of a ceramic fragment indicated that the valence band spectra was consistent with a surface mixture of ceramic Si-B-N-C and SiO2. To prevent cracking of the coating on the fibers, it was necessary to electrochemically oxidize the carbon fibers before applying the CVD deposited ceramic film. Upon exposure to reactive oxygen, the coating rapidly formed an oxide scale, which may offer a very stable diffusion barrier to further oxidation. Thermogravimetric analysis suggests that the coating retarded gasification of the fiber up to high temperatures, with a weight loss of less than 25% when compared to an unprotected fiber which was completely gasified at 950 degrees C. Most of the weight loss was probably due to oxidation down the unprotected fiber ends rather than oxidation of the protected fiber. We believe that this ceramic has considerable potential as an oxidation barrier for carbon fibers.