## 2 Degradation of Materials in Space--Stability Issues on Mirrors
### Atomic Oxygen
The loss in performance does not exclude the use of CVD-SiC for missions where oxygen is not present. Other than high-temperature CVD-SiC, the carbides deposited by sputtering at room temperature are a choice of moderate EUV-reflectance mirror that is attractive for optical coatings [53; 54]. [PERSON] et al. [55] tested the ability of ion-beam-sputtered deposited SiC and B\({}_{4}\)C to withstand the exposure to the LEO AO. For SiC, a severe reflectance decrease was observed when the coating was oriented in the ram direction, and it was measured that the presence of silicon oxide on the surface was three times larger than for the witness sample kept in the lab, which was attributed to the direct exposure to AO. A second SiC sample was exposed to LEO AO, but it was placed at 160\({}^{\circ}\) from the ram direction, so that it was protected from the effects related to direct AO bombardment. This sample displayed only a slight reflectance degradation, typical of an aged sample. Three B\({}_{4}\)C samples were also exposed to LEO AO at 0\({}^{\circ}\), 26\({}^{\circ}\), and 160\({}^{\circ}\) from the ram direction. All three samples experienced some EUV reflectance reduction, larger than the typical sample ageing, but the reduction was not as drastic as with SiC. The extra B\({}_{4}\)C reflectance reduction was mostly attributed to contamination. No roughness increase was observed for either SiC or B\({}_{4}\)C.  
[PERSON] et al. [50] flew Al/MgF\({}_{2}\) mirrors and exposed them to LEO environment. Even though some samples maintained their FUV reflectance, one sample experienced significant reflectance degradation at around 155 nm, and smaller degradation was observed at wavelengths of \(\sim\)120 nm and 200 nm. Degradation could be attributed to contamination, since the largest sensitivity to contamination was expected to be at \(\sim\)154 nm, where the intensity of the electric field on the coating surface is maximum for a 25 nm-thick MgF\({}_{2}\) protective layer. The change at 150 nm could be also attributed to the plasma resonance absorption in Al induced by surface roughness, even though no significant difference in roughness before and after orbit exposure was observed. To reduce or eliminate the atomic oxygen erosion in materials in space, the application of thin-film protective coatings made of durable dielectric materials is the most used approach [34; 35; 56; 57; 58; 59; 60]. As previously described, oxides and fluorides are materials that are relatively resistant to AO, making them suitable as capping layers in coatings for space optics. For example, [PERSON] et al. reported on the durability of protected silver surfaces in an AO environment [34]. In this case, the protective layer consisted of a thin Al\({}_{2}\)O\({}_{3}\) film, as alumina has been considered one of the suitable material choices to be applied as a protective coating because of its good adhesion to Ag and passivation properties. Interestingly, they applied two approaches to test AO resistance: radio-frequency (RF) oxygen plasma exposure and a laser detonation source of 5 eV AO. It was suggested that the RF plasma environment is too severe for a realistic simulation of the AO interaction, while a 5 eV AO exposure demonstrated that the protective coating was suitable for potential LEO applications. Silicon dioxide (SiO\({}_{2}\)) and magnesium fluoride (MgF\({}_{2}\)) are other commonly used protective coatings in the vacuum ultraviolet (VUV) spectral region because of their high transparency down to 110 nm. MgF\({}_{2}\) coating, for example, is used as a protective layer on Al in Hubble Space Telescope optics, covering the wavelength range from 110 nm to near infrared. Even though it is quite effective, MgF\({}_{2}\)-protected aluminum is a soft coating that scratches easily [3]. Therefore, optical components including a top layer of this material have to be handled carefully to avoid damage. Lithium fluoride (LiF) can extend the useful reflectance range of aluminum down to the LiF absorption cutoff of 102.5 nm.