Surface Studies of Potentially Oxidation Protective Si−B−N−C Films for Carbon Fibers

Abstract

The paper reports a study of CVD deposited thin films (1−5 μm) 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 °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.

Amorphous Multinary Ceramics in the Si-B-N-C System

“…Since, moreover, the coefficients of thermal expansion of SiBN 3 C and carbon are comparable, and thus only low thermal tension between the coating and the carbon substrate is to be expected, SiBN 3 C coatings are particularly suitable for carbon containing materials. The potential of SiBN 3 C as protective layers has been demonstrated by experimental coatings of, e.g., graphite [20], carbon fiber reinforced composite materials (C/C and C/SiC) [105], carbon fibers [106], and some metals [107,108]. Here, different methods for the fabrication of the ceramic coatings have been used.…”

“…The CVD process has been used for coating carbon fibers using PBS-Me as precursor [106]. For this purpose a PBS-Me fraction was prepared, which can be evaporated at moderate temperatures and at low pressure.…”

Silicon Boron Nitrides: Hypothetical Polymorphs of Si 3 B 3 N 7

“…[3,4] Recent reviews on their synthesis by polymer-to-ceramic transformations are given by Riedel et al and by Baldus and Jansen. [5,6] In particular, Baldus and Jansen concentrated on the synthesis of two stoichiometric compounds, Si 3 B 3 N 7 and SiBCN 3 .…”

“…First we concentrated on the structures derived from a-and b-Si 3 N 4 . To broaden our investigation we also viewed structures derived from willemite-II, [14] sphalerite, g-Si, the boron subnet in CrB 4 , and MAPO-39 structures. [15] The ground-state properties of the structures were calculated with use of standard density functional theory methods with full geometry optimization.…”

“…Cation ± p interactions have been studied both experimentally and theoretically, and have been recently reviewed. [4] In gas-phase calculations, Li forms both 1:1 and 1:2 complexes with benzene. These are more strongly bound than analogous complexes of the other alkali metal cations.…”