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Reducing friction and wear in moving mechanical systems is essential for their intended functionality. This is currently accomplished by using a large variety of anti-friction and anti-wear additives, that usually contain sulfur and phosphorous both of which cause harmful emission. Here, we introduce a series of diesters, typically dioctyl malate (DOM), as green and effective anti-friction and anti-wear additives which reduce wear by factors of 5-7 and friction by over 50% compared to base oil when tested under high pressures. Surface studies show that these impressive properties are primarily due to the formation of a 30 nm graphitic tribofilm that protects rubbing surfaces against wear and hence provides low shear stress at nanoscale. This graphitic tribofilm is prone to form from diesters derived from short-chain carboxylic acid due to their lone pair effect, which stabilizes the carbon free radicals. Furthermore, the formation of this tribofilm is catalyzed by nascent iron single atoms, which are in-situ generated due to the mechanochemical effects during sliding contact. Computational simulations provided additional insights into the steps involved in the catalytic decomposition of DOM by iron and the formation of a graphitic carbon tribofilm. Due to its anti-friction and wear properties, DOM holds promise to replace conventional additives, and thus provides a green and more effective alternative for next-generation lubricant formulations. Reducing friction and wear in mechanical systems can lower energy consumption and material loss. Here, a green lubricant additive is introduced, free of harmful elements, that effectively minimizes friction and wear, offering a sustainable solution.

Lubrication recovery has been observed in tribology research. This refers to the recovery of oil lubrication on a metal after failure or semi-failure by waiting for a period. Past studies attributed this recovery to metal oxidation. However, this article presented an alternative mechanism. We proposed that the accumulation of carbon-containing tribofilms from hydrocarbon oils was crucial for lubrication recovery. Changes in surface states, such as increased contact areas, also play a significant role. For studying wear mechanism maps, it is essential to consider both working conditions and surface states. Two new wear mechanism maps based on initial surfaces with or without pregenerated tribofilm were developed. These findings highlight the mechanochemical effects of base oils on lubrication and anti-wear performance.

Surfactant solutions are widely used in industry, and their steady-state lubrication properties have been comprehensively explored, while the “dynamic process” between steady states attracts much less attention. In this study, the lubrication behaviors of sodium dodecyl sulfate (SDS) and sodium bis (2–ethylhexyl) sulfosuccinate (Aerosol–OT, AOT) solutions were comparatively and extensively discussed. Experimental results showed that the duration of the dynamic process of AOT solution lubrication was significantly shorter than that of SDS. The essence of the dynamic process was revealed from the aspects of the running-in of solid surfaces and the adsorption process of surfactant molecules. Unlike the general recognition that the friction force evolution mainly corresponds to the running-in of surfaces, this study indicated that the dynamic adsorption behavior of surfactant molecules mainly contributes to this process. Various experiments and analyses showed that the smaller steric hindrance and lower orientation speed of SDS molecules led to longer diffusion into the confined contact zone and a longer duration of friction force decrease. This work enhances our understanding of the dynamic friction process in water-based lubrication, which could also have important implications for oil-based lubrication and its industrial applications.

In this work, a series of experiments on tribofilm formation of sulfur- and phosphorus-free organic molybdenum additive (SPFM) on bearing steel surfaces have been performed in a ZrO 2 ball-on-steel plate tester lubricated by gas-to-liquid (GTL), di-2-ethylhexylsebacate (Ester), or polyalphaolefin (PAO) synthetic oils under different temperature and external voltage conditions. The synergy effect of SPFM with 2,5-dimercapto-1,3,4-thiadiazole derivative (DMTD) and zinc dithiophosphate (ZDDP) additives has also been investigated. The results reveal that SPFM additive plays a major role in reduction of friction and wear by formation of MoO x tribofilm on steel surface lubricated with either GTL or PAO oil samples, which can be enhanced by an externally applied voltage. S element provided by DMTD additive can react with Mo element in SPFM to form MoS 2 , which also contributes friction reduction. Comparing with GTL and PAO base oils, the Ester oil is less effective in the tribofilm formation because of its low solubility for the additives. Higher temperatures (at 60 °C, 100 °C or 140 °C) and an initial running-in are beneficial to the tribofilm formation. A three-step mechanism, hydrolysis of SPFM, adsorption of the MoO 4 2− anions, and tribochemical reactions at the rubbing surface, is proposed to explain the observed voltage-assisted tribofilm formation results. Graphical Abstract

This study investigates the effect nano-silica particle additive with different concentrations and sizes on magneto-rheological behavior of carbonyl iron particle suspensions from the tribological point of view. The lubrication states between particle–particle contacts and particle-plate contacts affected the magnetorheological behaviors. The silica particle additive leads to a larger friction coefficient at boundary lubrication conditions of the base carrier fluid which results in solid–solid contact states between the particles and plate and improves the yield stress. The normal stress and plate gap reflect that the nano-silica particles prevent the end of the ferromagnetic particle from sliding at the plate under high magnetic field, which enhance the friction effect between the particle and plate. The normalization methods based on the concept of tribology disclosed the influence of silica particle additives on the structural evolution of iron particles. It provides an effective guidance for the formulation design of MRF considering the effect of additives on the lubrication performance of the base carrier fluid.