Space environment is completely different with atmospheric environment, such as ultra-high vacuum, alternate high/low temperature, high-energy particles irradiation, energetic atomic oxygen, and so forth. Diamond-like carbon (DLC) has high mechanical hardness, excellent anti-wear, good corrosion resistance and high chemical inertness because it is metastable form of amorphous carbon with most of sp3 bonds. Hence, it is regarded as one of the most promising solid lubricating materials for providing high hardness and low wear through minimizing abrasion, shear and adhesion. Kim et al. studied the effect of nano-scale surface texture on wear resistance of DLC in dry, humid and liquid water environments, found that nano-texturing significantly reduced the wear of DLC films in dry and humid nitrogen. Wang et al. synthesized fluorine and sulfur co-doped amorphous carbon (a-C:S:F) films with ordered carbon structure and a-C:S:F films with 2.0 at.% H show the ultra-low steady-state friction coefficient, which mainly depends on the concentrations of S and F in the films. Tagawa et al. investigated the effect of atomic oxygen beam exposure on the morphology and tribological behavior of MoS2 and DLC film, and found that DLC surface was intensively oxidized, probably causing the dramatic decrease of lifetime, but its friction behavior has not been affected. Liu et al. explored the evolution of the surface structure of DLC induced by space irradiation and the tribological performance of the composite coatings composed of DLC and lubricant under high vacuu. They found that the high-energy particles irradiation changes the structure of DLC films through the sp3-to-sp2 bonding transformation, and the wear was obviously increased due to the irradiation-induced damage to lubricating materials. Such excessive wear causes the short travelling distances of DLC film, namely rolling or sliding cycle of less than ten million under space environment, whereas it is of paramount importance and significance for DLC film to minimize the wear and ensure a long service life. For challenges in minimizing friction and wear of spatial mechanical systems, synergetic lubrication coatings were prepared by spinning liquid lubricants and hybrid greases on the diamond-like carbon (DLC) films, and were evaluated whether they could achieve a long-term safe and reliable operation under high vacuum. Under high vacuum conditions, liquid lubricants significantly reduce the friction and wear of DLC films. Hybrid greases not only show excellent lubrication performance, but also greatly enhance the tribological behavior of DLC films, especially the grease with optimal proportion. Such excellent tribological performance of DLC-based composite coatings at low applied loads depends on the synergy of DLC and fluid film, in reverse the tribo-chemical reaction film under harsh working conditions.
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