Synergistic effect on spacetribology in the low earch orbit

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The purpose of this project is to clarify the synergistic effect of atomic oxygen (AO) and the other environmental factors, such as ultraviolet (UV) light or X-rays, in the tribology of satellites used in low earth orbital (LEO) environment. An ion beam type and a laser induced breakdown type AO sources are used in this study to generate energtic AO beam in the laboratory. The research results obtained in this project are as follows :1. The reaction efficiency of AO with the polyimide film formed by a spin-coating technique is influenced by UV irradiation. Only in the simultaneous irradiation of AO and UV,significant mass loss and CO_2 formation is detected. This research result suggests the photo-assisted chemical reaction of the functional groups such as carbonyl, which absorbs UV light, occurs at the AO bombarded polymer surfaces where carbonyl is formed by AO attacks.2. When AO hits MoS_2 (0001) surface, So is formed. This volatile product is detected by a quadrupole mass spectrometer during AO beam exposure on MoS_2 (0001). However, the rate of evaporation is not linear with time. This is probably due to the low diffusion rate of AO into the MoS_2 bulk, i.e., reaction is limited only at the very surface region where only limited amount of S is existed.3. Tribological properties of MoS_2 (0001) is not suffered by AO attack very much. We see maximum 40% increase of initial friction after AO exposure. However, similar effect is also observed even before AO exposure. Certain percentage of this effect is dur to wear track formation. This effect obviously depends on the sliding system and materials' constants. We need to know how much AO influences to the actual sliding component, because the effect of AO on the tribological property of sputtered MoS_2 may be different with single crystal. Further research on this point must be necessary.