## 3 Results and Discussion
### UV and Atomic Oxygen Test  
UV and AtOx aging characterization was carried out on the pure Radel R and on the hybrid coated R-HY. The effect of AtOx alone was first analyzed, and then the specimens were subjected to the blending UV+AtOx flux, as an attempt to discriminate different degradation phenomena and to appreciate the synergic erosion due to the combined action of two aging factors. Six samples (dimensions: 20 \(\times\) 20 \(\times\) 6 cm\({}^{3}\)) for each kind of material  
Figure 4: Flammability test: smoke length versus flame-time for Radel R material, experimental data and physical optimized modeling results.  
were analyzed in all the tests; the corresponding results about the erosion rate \(E_{Y}\) are given, by average, in Figure 5, recalling that this parameters is calculated from the measured mass loss, \(\Delta M\), by using the following relationship:  
\[E_{Y}=\frac{\Delta M}{\rho\cdot A\cdot F} \tag{4}\]  
where \(\rho\) is the material density (\(\sim\)1.33 g/cm\({}^{3}\)), \(A\) is the sample exposed area (\(\sim\)4.20 cm\({}^{2}\)) and \(F\) is the effective fluence obtained within the chamber (\(\sim\)1.25 \(\times\) 10\({}^{20}\) atom/cm\({}^{2}\)). The results obtained clearly show the ineffectiveness of the ceramic coating treatment, which gives rise to a sharp increase of the erosion rate in both AtOx and UV+AtOx exposure testing with respect to the naked material. Rather than to low anti-oxidation properties of the ceramic itself, such behavior has to be related to a weak adhesion to the base material, as well as to the presence of defects, pores and scratches on the coated surface. On the contrary, the Radel R itself demonstrates a good resistance to both AtOx and UV+AtOx irradiation. In particular, only a little increase of about 6 \(\div\) 8% of erosion is detected in the synergic conditions.  
The effect of UV exposure on the Radel R material is evident by the comparison between the appearance of the surface before and after the test (Figure 6). It is noticeable that the sample surface changed from light to dark yellow; thus, the transparency is, even though not dramatically, quite affected by UV radiation. That is the so-called \"darkening effect\", due to UV photon energy high enough to cause surface molecule activation and organic bond breakage, resulting in the degradation of the polymer optical properties [14; 29]. After AtOx alone exposure, on the contrary, the samples' transparency appeared almost unchanged, with no evident changes of the surface morphology. The erosion yield obtained in the adopted experimental setup is in good agreement to the results obtained for poly-sulfone materials in the most of other ground-based facilities, as well as in space-flight experiments [2; 13; 15]. The low surface recession shown by Radel R testified the good anti-oxidation properties of poly-sulfone materials, which are supposed to be due to residue formation after the exposure to LEO environment, acting as a barrier layer that slows down the material erosion. Such capability, on the other hand, is strictly  
Figure 5: UV/AtOx aging characterization: erosion rate estimated from the measured mass loss.  
related to the worsening of the material optical properties, as, for example, the degradation of the polymer original transparency.