Carbon nanomaterials as additives in lubricating oils for tribological applications: A review

Friction and wear are inevitable in mechanical operation, and the development of advanced lubricants is essential for the pursuit of energy efficiency and sustainable development. Great progress has been made in the application of carbon nanomaterials as lubricant additives, which is of great significance for controlling friction and wear. This review mainly introduces the types of carbon nanomaterials (including zero-dimensional, one-dimensional, and two-dimensional carbon nanomaterials) for lubricants, the lubrication mechanism of nanoadditives (including the formation of tribofilm, ball bearing effect, mending or self-repairing effect, and polishing effect), and discusses the dispersion and film-forming ability of carbon nanomaterials in lubricants. Finally, the drawbacks of current research are discussed from three aspects: high production costs, insufficient dispersion, and unclear mechanism elucidation. Further discussions were held on the solutions for the drawbacks of current research and future research directions (including machine learning and molecular simulation in design of lubricant additives and optimizing carbon nanomaterial additives for electric vehicle lubricants) in the outlook. Therefore, this review will promote a better fundamental understanding of carbon nanomaterials in lubricant applications and help design carbon nanomaterials with excellent tribological properties.