| Summary: | The present paper describes the tribological properties of a mesh-like nanostructure of diamond-like carbon (DLC) in a formulated engine oil at DLC/steel contacts. This novel nanostructured DLC was characterized as a non-hydrogenated amorphous carbon (a-C) with a nano-mesh structure layer at the outermost surface, herein named NM-a-C. From the results of our friction tests, we observed that the NM-a-C/steel tribopair exhibited lower friction and higher wear-resistance than the a-C:H/steel tribopair, though the mechanical properties were nearly identical. The analytical result indicated that the tribofilm formation process and the chemical composition of the tribofilm varied depending on the types of the DLC. In particular, thicker MoS<sub>2</sub>-rich tribofilms formed on the NM-a-C surface. Hence, the NM-a-C structure promoted the formation of MoS<sub>2</sub> under the lubrication with the fully formulated oil, leading to lower friction and high wear-resistance at the DLC/steel contact under boundary lubrication conditions.
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