SYNERGISTIC EFFECT OF CARBON-BASED ULF-SOLID LUBRICANTS IN AN SPACE ENVIRONMENT

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The purpose of this research is to clarify the tribological properties of diamond-like carbon (DLC) films in a space environment. Hydrogenated-DLC samples were exposed to the 5 eV atomic oxygen beam, which simulated the environment at the upper atmospheric condition of Earth, and the change in surface properties were measured by synchrotron radiation photoemission spectroscopy (SR-PES), Rutherford backscattering spectroscopy (RBS) and elastic recoil detection analysis (ERDA). It was identified that the carbon atom, which is chemically bonded with oxygen, is not a major carbon state even after atomic oxygen exposure. This experimental result can be explained by the fact that the oxidation reaction of carbon with hyperthermal atomic oxygen restricted at the topmost layer of the DLC surface. In contrast, it was clearly indicated by RBS that the DLC film itself was lost by the hyperthermal atomic oxygen exposure. NEXFS measurement revealed that the sp2/sp3 ratio decreased with atomic oxygen exposures. From these results, it was estimated that the highly reactive part in DLC (amorphous phase) is selectively gasified by atomic oxygen and the stable carbon frame (diamond phase) is left on the surface (selective gasification reaction). This conclusion is supported by the results of STS-8 flight mission which indicates that the erosion rate of diamond is less than 5 % of amorphous carbon. The energy dependence of the gasification reaction was also investigated in this study. It was also demonstrated that the reaction yield of gasification reaction of DLC with oxygen atom increased exponentially with its collision energy. The experimental data obtained in this project indicated that the non-protected DLC would not be survived in the low Earth orbit space environment. For protecting DLC from such a highly oxidative environment, doping of metallic atoms such as Si- or Ti-atom would be effective. This hypothesis will be evaluated in the following project.