Frequency-Modulation Atomic Force Microscopic Observation for Ultralow Frictional Solid–Liquid Interface of Diamond-Like Carbon in an Environmentally Friendly Oil

The structure of the low friction solid–liquid interface of the diamond-like carbons (DLCs) in fatty acid was investigated by controlled frequency-modulation atomic force microscopy (FM-AFM) to elucidate the low frictional mechanism that arises under boundary lubrication. We focused on two factors in particular: the effect of the DLC type and the frictional energy in the structure of the adsorbents at the interface. The FM-AFM results indicate that the ultralow frictional tetrahedral amorphous carbon–fatty-acid interface was composed of a thick boundary film with a high molecular density, which may support the sliding surfaces and thereby reduce the friction.