Mechanism of superlubricity of a DLC/Si3N4 contact in the presence of castor oil and other green lubricants

To meet the surging needs in energy efficiency and eco-friendly lubricants, a novel superlubricious technology using a vegetable oil and ceramic materials is proposed. By coupling different hydrogen-free amorphous carbon coatings with varying fraction of sp2 and sp3 hybridized carbon in presence of a commercially available silicon nitride bulk ceramic, castor oil provides superlubricity although the liquid vegetable oil film in the contact is only a few nanometres thick at most. Besides a partial liquid film possibly separating surfaces in contact, local tribochemical reactions between asperities are essential to maintain superlubricity at low speeds. High local pressure activates chemical degradation of castor oil generating graphitic/graphenic-like species on top of asperities, thus helping both the chemical polishing of surface and its chemical passivation by H and OH species. Particularly, the formation of the formation of -(CH2-CH2)n- noligomers have been evidenced to have a major role in the friction reduction. Computer simulation unveils that formation of chemical degradation products of castor oil on friction surfaces are favoured by the quantity of sp2-hybridized carbon atoms in the amorphous carbon structure. Hence, tuning sp2-carbon content in hydrogen-free amorphous carbon, in particular, on the top layers of the coating, provides an alternative way to control superlubricity achieved with castor oil and other selected green lubricants.