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The effect of irradiation by a hyperthermal-atomic-oxygen beam on hydrogenated titanium-doped diamond-like carbon (hydrogenated Ti-DLC) films, applied as a solid lubricant for equipment used in low-earth orbit was investigated. Unlike the film thickness of hydrogenated non-doped DLC films, that of hydrogenated Ti-DLC films was found to be constant after the films were exposed to atomic oxygen. In addition, bulk composition of the hydrogenated Ti-DLC film stayed constant, and in particular, hydrogen content in the film did not decrease. These results indicate that a hydrogenated Ti-DLC film can keep its low friction properties under vacuum. Surface chemical analysis showed that a titanium-oxide layer is form on the film by exposure to atomic oxygen. The thickness of the titanium oxide layer was estimated to be about 5 nm from the element distribution in the depth direction of the hydrogenated Ti-DLC films. The titanium-oxide OPEN ACCESSMetals 2015, 5 1958 layer was interpreted to protect the bulk film from erosion by hyperthermal atomic oxygen.

We investigated the effect of hyperthermal atomic oxygen beam irradiation on hydrogenated Si-doped Diamond-Like Carbon (hydrogenated Si-DLC) films for the purpose of use as a solid lubrication material in space. We found that film thickness of hydrogenated Si-DLC was constant after the exposure to atomic oxygen beam. From this result, the hydrogenated Si-DLC films have resistance to etching by the irradiation of atomic oxygen, unlike hydrogenated non-dope DLC films. In addition bulk composition of hydrogenated Si-DLC film kept constant. Especially, hydrogen content in Si-DLC film did not decrease. Therefore, hydrogenated Si-DLC film is expected to keep low fiction properties in a vacuum. Furthermore, the atomic oxygen beam fluence dependence of X-ray Photoelectron Spectroscopy (XPS) and Near Edge X-Ray Absorption Fine Structure (NEXAFS) spectra of hydrogenated Si-DLC films were measured. From these studies, it was found that the C atoms on the hydrogenated Si-DLC surface were desorbed, but Si atoms were remained on the hydrogenated Si-DLC surface as the SiO x by the collision of atomic oxygen. This SiO x layer was considerable to disturb the erosion of bulk film against atomic oxygen. From the element distribution in depth direction, it was found that the thickness of SiO x was about 5 nm.