Abstract
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 atoms/cm in 42 h of exposure. Pyrolytic graphite and highly oriented pyrolytic graphite showed significant degradation, and the measured erosion yield was within a factor of 2 of published values. The erosion yield of pyrolytic boron nitride was found to be 2.6 X10 cnrVatom in plasma asher exposure, over 42 times lower than that of pyrolytic graphite. This low erosion yield makes graphite-boron nitride 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 nitride composition. Carbon-fiber-carbon composites eroded in LEO, and the carbon pitch binder was found to etch more easily than the graphite fibers, which have much higher atomic-oxygen resistance.
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