Lubricants are widely employed in mechanical systems to reduce energy loss and the wear induced by friction. With increasing concern for environmental protection, the development of eco-friendly lubricants has become increasingly significant. Nanocellulose, a natural material derived from cellulose, has attracted increasing research attention in the field of lubrication due to its renewable, nontoxic, and biodegradable properties. Focusing on this emerging research topic, this state-of-the-art review first analyzes the theoretical basis of applying nanocellulose as a thickening agent in eco-friendly lubricating formulations. Subsequently, it presents an overview of research advances in cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs), categorized by their applications as additives or thickeners in aqueous, oil, and grease lubricants. Moreover, this review also highlights the challenges associated with eco-friendly lubricants based on nanocellulose and offers suggestions for future investigations in this field. It is hoped that this review will guide the research direction of eco-friendly lubricants and promote the development of nanocellulose materials for industrial lubrication applications.

The clarification of the critical operating conditions and the failure mechanism of superlubricity systems is of great significance for seeking appropriate applications in industry. In this work, the superlubricity region of 1,3-diketone oil EPND (1-(4-ethyl phenyl) nonane-1,3-dione) on steel surfaces was identified by performing a series of ball-on-disk rotation friction tests under various normal loads (3.5–64 N) and sliding velocities (100–600 mm/s). The result shows that beyond certain loads or velocities superlubricity failed to be reached due to the following negative effects: (1) Under low load (≤ 3.5 N), insufficient running-in could not ensure good asperity level conformity between the upper and lower surfaces; (2) the high load (≥ 64 N) produced excessive wear and big debris; (3) at low velocity (≤ 100 mm/s), the weak hydrodynamic effect and the generated debris deteriorated the lubrication performance; (4) at high velocity (≥ 500 mm/s), oil migration occurred and resulted in oil starvation. In order to expand the load and velocity boundaries of the superlubricity region, an optimized running-in method was proposed to avoid the above negative effects. By initially operating a running-in process under a suitable combination of load and velocity (e.g. 16 N and 300 mm/s) and then switching to the target certain higher or lower load/velocity (e.g. 100 N), the superlubricity region could break through its original boundaries. The result of this work suggests that oil-based superlubricity of 1,3-diketone is a promising solution to friction reduction under suitable operating conditions especially using a well-designed running-in strategy.