LOW-EARTH-ORBIT EXPOSURE OF CARBON-BASED MATERIALS ABOARD SHUTTLE FLIGHT STS-46

Six different types of carbon and carbon-boron nitride composites were exposed to low Earth orbit (LEO) aboard Space Shuttle flight STS-46. The samples received a nominal atomic-oxygen fluence of 2.2 x 10(20) a toms/cm(2) in 42 h of exposure. Pyrolytic graphite and highly oriented pyrolytic graphite showed significant degradation, and the measured e rosion yield was within a factor of 2 of published values. The erosion yield of pyrolytic pyrolytic boron nitride was found to be 2.6 x 10(- 26) cm(3)/atom in plasma asher exposure, over 42 times lower than that of pyrolytic graphite. This low erosion yield makes graphite-boron ni tride mixtures quite resistant to LEO exposure. Evidence suggests that the graphitic component was preferentially etched,leaving the surface boron nitride rich. Atomic-oxygen resistance increases with boron nit ride composition. Carbon-fiber-carbon composites eroded in LEO, and th e carbon pitch binder was found to etch more easily than the graphite fibers, which have much higher atomic-oxygen resistance.