Micro/nano carbon spheres as liquid lubricant additive: Achievements and prospects

• Lubrication mechanisms of MNCS as lubricant additives have been elaborated in brief. • Pristine, modified and hybrid MNCS as oil-based lubricant additives have been evaluated in this review. • Pristine, hybrid and intelligent MNCS as water-based lubricant additives have been reported in detail. • Porous MNCS as lubricant additives has also been scrutinized in this review. • The prospects of MNCS have been proposed in the lubrication field. Friction and wear are the primary attributes of energy loss and failure of machine parts in mechanical systems. There is a great demand to develop novel and effective lubricants to reduce friction and wear in especial in new high technologies industries. As lubricant additives, carbon nanomaterials such as graphene, fullerene and micro/nano carbon spheres (MNCS) can significantly improve the friction reduction and wear resistance capacities of lubricants. MNCS has excellent lubricity due to its unique thermal, mechanical and chemical properties and rolling friction between two friction surfaces caused by the spherical structures. However, there is still a lack of a detailed review that outlines these research results. In this review, the category and lubrication mechanism of the widely used nanolubricant additives are summarized, and then recent advances in the applications of MNCS as oil-based and water-based lubricant additives, and porous MNCS as new lubricant additives are comprehensively introduced from the aspects of experimental content and lubrication performances. Finally, the review presents a brief summary and some future prospects of MNCS in the lubrication field.