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

研究了类金刚石薄膜在空间环境中的摩擦学性能。采用模拟地球大气环境的5 eV原子氧束辐照氢化类金刚石薄膜，利用同步辐射光电子能谱（SR-PES）、卢瑟福背散射谱（RBS）和弹性反冲探测分析（ERDA）等方法研究了氢化类金刚石薄膜表面性质的变化。经鉴定，与氧化学键合的碳原子即使在原子氧暴露之后也不是主要碳状态。这一实验结果可以解释为碳与超高温原子氧的氧化反应仅限于DLC表面的最上层。相反，RBS清楚地表明，DLC膜本身由于超高温原子氧暴露而损失。NEXFS测量结果显示，sp2/sp3比值随着原子氧暴露而降低。根据这些结果，推测DLC中的高反应性部分（非晶相）被原子氧选择性气化，稳定的碳骨架（金刚石相）留在表面上（选择性气化反应）。STS-8飞行使命的结果表明金刚石的侵蚀率小于非晶碳的5%，这一结论得到了支持。气化反应的能量依赖性也在这项研究中进行了调查。DLC与氧原子的气化反应产率随碰撞能的增加呈指数增长。该项目中获得的实验数据表明，未受保护的DLC在低地球轨道空间环境中无法存活。为了保护DLC免受这种高度氧化环境的影响，金属原子如Si原子或Ti原子的掺杂将是有效的。这一假设将在以下项目中进行评估。

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.