X‐ray photoelectron spectroscopic study of ion etching and electrical biasing of silicon nitride on a carbon fiber

Carbon fibers with a chemical vapor‐deposited silicon nitride surface film have been investigated with core and valence band x‐ray photoelectron spectroscopy (XPS). The ability of the films to protect the fibers from oxidation was evaluated by heating, by argon and oxygen ion etching and by using an electrical biasing potential. Oxygen ion etching appears to oxidize the Si 3 N 4 to SiO 2 , whereas argon etching removes the oxide and any hydrocarbon surface layer. The valence band data for Si 3 N 4 , Si 2 N 2 O, and SiO 2 are interpreted by multiple scattered wave Xα calculations. The films crack when an electrical potential is applied, and the cracking is reproducible and irreversible, occurring particularly with a negative potential bias. We find chemical shifts on the core levels consistent with differential charging of the silicon nitride coating, an observation that is supported by sample biasing experiments. The work suggests that the cracking of the silicon nitride coating will significantly limit its effectiveness in protecting carbon fibers and carbon–carbon composites from oxidation.