Achieving superlubricity using selected tribo-pairs lubricated by castor oil and unsaturated fatty acids

Systematic friction tests have been performed to unveil the key factors for reaching superlubricity (CoF < 0.01) in the presence of unsaturated fatty acid-based lubricants under boundary lubrication regime. The impact of different tribo-pairs and lubricants has been investigated. When the counterpart is steel, results show that a hydrogen-free DLC coating is indispensable to achieve superlubricity in castor oil. Among pure unsaturated fatty acids as lubricants, oleic and ricinoleic acid gave superlow friction but linoleic acid is not recommended to lubricate steel/a-C contact due to the occurrence of high friction and wear. X-ray photoelectron spectroscopy (XPS) and X-Ray-Excited Auger Electron Spectroscopy (XAES) analyses reveal that hydrogen-free carbon can activate two pathways to passivate surface and to achieve superlubricity with fatty acid-based lubricants (i) carbon re-hybridization by forming a high-density sp2 carbon network and (ii) -(CH2-CH2)n- oligomers formation on top surface. At the opposite, linoleic acid hinders the formation of -(CH2-CH2)n- oligomers and oxidizes carbon surfaces.