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Chemical-vapor-deposited silicon carbide mirrors were exposed to bombardment by 8-km/s oxygen atoms that simulated the effects of exposure in low Earth orbit for periods up to 7.5 yr. The reflectances of four mirrors were measured before and after exposure at five wavelengths (58.4, 73.6, 104.8, 121.6, and 161 nm) and at 11 angles of incidence from 5° to 80°. The oxygen exposure reduced the normal-incidence reflectances by factors of 1.5–4.5 in the wavelength rate of 58.4–121.6 nm but had no effect on the visual appearance. The optical constants and the thicknesses of the thin surface layers present on the SiC substrates were determined from the reflectance measurements. This analysis indicated that before exposure the surface layers were composed of SiOx (where x is ∼1.5) with thicknesses of 0.8–1.8 nm. After exposure to 8-km/s oxygen atoms, the surface layers were composed of SiOx with thicknesses of 3.5–4.5 nm. There were no systematic differences between the measured reflectances after simulated space exposures of 1.5, 4.5, and 7.5 years. This implied that most of the growth in thickness of the SiOx layers occurred early in the exposure and stabilized at thicknesses of 3.5–4.5 nm. The optical results were consistent with x-ray photoelectron spectroscopy of the four mirrors after oxygen exposure.
© 1993 Optical Society of America
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