## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Atomic Oxygen
Among the most interesting results, coating materials such as fluorides (MgF\({}_{2}\), CaF\({}_{2}\), and LiF) and Ir and Pt showed no significant damage, but Ni mirrors showed oxide formation and the reflectivity of Au mirrors overcoated with Ni diminished because of the degradation of Ni. [PERSON] et al. [43] investigated the effect of space AO on the polarization contrast of polarization-modulated mirrors under different experimental doses using a terrestrial simulator. [PERSON] et al. [44] exposed Os, C, and bare Ag to ambient AO in a space shuttle flight. Post-flight laboratory analysis revealed that the unshielded C and Os films were totally removed, presumably by the formation of volatile oxides. Bare Ag was drastically modified to a nonconductor. Various attempts have been carried out in order to use metals as protective layers. An attempt to protect Os with a 6 nm-thick Pt film failed since the structure did not resist exposure to AO in the orbital direction and volatile Os oxide escaped through gaps in the Pt film, which resulted in poor UV reflectance measurements on both the unexposed as well as the exposed areas; on the other hand, a 10 nm-thick film of Pt provided almost complete protection [45]. However, such protection thickness would hinder the relatively large EUV reflectance of Os. Another experiment on protecting Os in order to avoid AO attack was carried out by [PERSON] et al. [46]. A 2 to 3 nm-thick Ir film was seen to protect an Os film, which had been deposited either on a Rh film or on a second Ir film. Such a 3-layer structure preserved the Os' high EUV reflectance characteristic at the grazing incidences used on gratings in the 9-26 nm spectral range. [PERSON] et al. [47] exposed films of various metals to a long LEO exposure. All materials--Cu, Ni, Pt, Au, Sn, Mo, and W--were somewhat affected by oxidation with AO, mostly in the ram direction of the spacecraft, although they were not affected as severely as had been found for Os, C, and Ag. Oxidation ranged between Au, the most stable, and Cu, the most affected. The effect of LEO AO on C was also analyzed by [PERSON] et al. [48], who exposed diamond-like C (along with 12 other materials) to an LEO environment and measured the total integrated scattering in situ over time. After several weeks, the C film was completely eroded away. [PERSON] et al. [49] exposed films of Os, Cr, Pt, and Ir to the LEO environment for a few days, and its effect on EUV reflectance was measured. Os was the most severely affected when exposed to the ram direction, in which case it was fully removed, whereas there was little change when it was masked. Cr, Pt, and Ir were much less affected. Ir underwent some reflectance decrease at wavelengths longer than 160 nm. Pt increased reflectance after exposure, which was attributed to the cleaning effect of AO on a sample that was assumed to be previously contaminated. As mentioned above, the presence of AO on the orbit may not only degrade the coating, but it also has the potential to remove contaminants from various types of coatings. [PERSON] et al. [50] also exposed transition-metal mirrors of Au, Ir, Os, and Pt to a LEO environment, close to the ram direction. As with the aforementioned experiments, Os was fully removed, whereas Pt and Ir behaved relatively well after exposure. Au suffered a severe reflectance decrease, but even though some outer monolayers may have been sputtered off the decrease was attributed to contamination from the surrounding areas. The same authors also exposed chemical vapor-deposited (CVD) SiC to an LEO environment and found that its Extreme Ultraviolet (EUV)-Far Ultraviolet (FUV) reflectance was severely affected, and the degradation was much larger for the exposed area than for a masked area. Degradation was attributed to surface oxidation to SiO\({}_{2}\). The effect of AO on the CVD-SiC EUV-FUV reflectance and the synergic effect of AO along with UV radiation on the CVD-SiC near-UV reflectance were reported by [PERSON] et al. [51] and [PERSON] et al. [52], respectively.