Controllable friction regulation has drawn much interest in both scientific and industrial fields. And there have been many researches on friction regulation by many physical fields and chemical factors. Photo-sensitive materials are promising because it is relatively easy to change their properties compared with others. Here, a new kind of photo-induced grease/oil switched lubricant is designed and shows great reversibility under ultraviolet (UV) and visible (Vis) irradiation. Its viscosity can change more than 50 times under different irradiation, and the coefficients of friction (COFs) obviously increase under Vis irradiation and decrease under UV irradiation, which performs better than those of the common grease. According to the experimental results, the phenomena are contributed to the break and reconstruction of the three-dimensional network inside the lubricant. With a switchable grease/oil state under different irradiation, this work provides a new principle for designing a smart lubricant with controllable friction regulation.

Superlubricity control is of great interest in both industry and scientific research, and several methods have been proposed to achieve this goal. In this work, ultraviolet (UV) light was introduced into titanium dioxide (TiO₂) and silicon nitride (Si₃N₄) tribosystems to accomplish photoinduced superlubricity. The friction coefficients (COFs) between Si₃N₄ balls and TiO₂ plates in the mixtures of sulfuric acid (H₂SO₄) solution and glycerol solution were obviously reduced, and the system entered the superlubricity region (COF < 0.01) after UV illumination at a speed of 56 mm/s. However, the COF was much larger without UV treatment than that with UV treatment. The formation of silica (SiO₂) layers on the surfaces of Si₃N₄ balls and the elastohydrodynamic effects were determined to be fundamental to the low friction in this experiment, and the enhancement of the combination between the TiO₂ surface and the hydroxy group of glycerol by UV illumination was the key to the photoinduced superlubricity in this system. These findings showed one method for achieving superlubricity by introducing a light field that could be further applied to special working conditions.

Extensive efforts have been made to pursue a low-friction state with promising applications in many fields, such as mechanical and biomedical engineering. Among which, the load capacity of the low-friction state has been considered to be crucial for industrial applications. Here, we report a low friction under ultrahigh contact pressure by building a novel self-assembled fluorinated azobenzene layer on an atomically smooth highly-oriented pyrolytic graphite (HOPG) surface. Sliding friction coefficients could be as low as 0.0005 or even lower under a contact pressure of up to 4 GPa. It demonstrates that the low friction under ultrahigh contact pressure is attributed to molecular fluorination. The fluorination leads to effective and robust lubrication between the tip and the self-assembled layer and enhances tighter rigidity which can reduce the stress concentration in the substrate, which was verified by density functional theory (DFT) and molecular dynamics (MD) simulation. This work provides a new approach to avoid the failure of ultralow friction coefficient under relatively high contact pressure, which has promising potential application value in the future.

Very recently, two-dimensional quantum dots (2D QDs) have been pioneeringly investigated as lubricant additives, which exhibit superior friction-reducing and wear resistance. Compared with 2D nanoparticles, 2D QDs possess small size (~10 nm) and abundant active groups. These distinguished advantages enable them to quickly disperse into common lube mediums and maintain long-term storage stability. The good dispersion stability of 2D QDs not only effectively improves their embedding capacity, but also enables continuous supplements of lubricants during the sliding process. Therefore, 2D QDs are attracting increasing research interest as efficient lubricants with desirable service life. In this review, we focus on the latest studies of 2D QDs as liquid lubricant additives (both in polar and nonpolar mediums), self-lubricating solid coatings and gels, etc. Various advanced strategies for synthesis and modification of 2D QDs are summarized. A comprehensive insight into the tribological behavior of a variety of 2D QDs together with the associated mechanism is reviewed in detail. The superior lubricating performances of 2D QDs are attributed to various mechanisms, including rolling effect, self-mending performance, polishing effect, tribofilm formation, nanostructure transfer and synergistic effects, etc. Strategies for friction modulation of 2D QDs, including internal factors (surface modification, elemental doping) and extrinsic factors (counter surfaces, test conditions) are discussed, special attentions for achieving intelligent tribology toward superlubricity and bio-engineering, are also included. Finally, the future challenges and research directions regarding QDs as lubricants conforming to the concept of "green tribology" toward a sustainable society are discussed.

Cavitation is common and important in hydrodynamic lubrication that generally causes the rupture of lubrication film and affects the performance of the lubrication, which even leads to cavitation erosion in some situations. This is especially important in connecting-rod bearings in diesel engines. Thus, accurately characterizing cavitation is of great significance. Herein, a multi-body dynamic model of an engine considering elastohydrodynamic lubrication (EHL) was established based on the AVL-EXCITE software platform. According to the EHL results, a finger-like cavitation region in the big-end bearing of the connecting rod was distributed in the lubrication outlet during the work stroke. Meanwhile, the cavitation bubbles in the cavitation region might collapse and result in cavitation erosion when the oil supply bore passed through. This provides a potential method to predict the occurrence of cavitation erosion. Furthermore, the influences of lubricating-oil viscosity and oil-supply pressure on cavitation characteristics were investigated. An appropriate increase in these two parameters can slow down the cavitation effect.