Effect of Electric Potential and Chain Length on Tribological Performances of Ionic Liquids as Additives for Aqueous Systems and Molecular Dynamics Simulations

Synthesis of Poly(ionic liquid)s-Functionalized Carbon Dots with Superior Tribological Stability and Potential-Controlled Friction Effect under Oil Lubrication

Potential-controlled friction provides the possibility for the active and dynamic regulation of the tribological behaviors of nanoadditives. However, the major challenge in achieving this goal is how to ensure the outstanding dispersibility and conductivity of nanoadditives in lubricants. Herein, the poly(ionic liquid)-functionalized carbon dots (CDs-PILs) with good dispersion stability in PEG200 were designed and synthesized from the polyethylenimine-grafted CDs (CDs-PEI). Compared with PEG200, the CDs-PILs (5.0 wt %)/PEG200 dispersion not only realized 38.2% and 52.3% reductions in friction and wear, respectively, but also showed superior tribological stability in heavy load (∼2.93 GPa), high frequency (10 Hz), long duration (300 min), and various start-stop modes due to the robust adsorption and nanolubrication effects of CDs-PILs at friction surfaces. Noticeably, the lubricity of dispersion with higher ionic conductivity (158.2 μS/cm) than PEG200 (5.7 μS/cm) presented an obvious response to the voltage range of -10 to +18 V, supporting the eminent potential-controlled friction characteristics of CDs-PILs. In addition, the deposition of uniform tribofilm with about 76 nm thickness was observed on the rubbed surfaces, which contributed to the ideal tribological properties of the CDs-PILs. This study proposed a simple approach for the CDs-PILs synthesis, enabling the high-efficiency potential-controlled friction of nanoadditives under oil lubrication.

Peculiar Effect of Water on Tribological Properties of Natural Deep Eutectic Solvent

Deep eutectic solvents (DESs) have emerged as promising green liquid lubricants for solving tribological problems due to their economic and excellent physicochemical and lubrication properties. However, a trace amount of water would affect their structure, physicochemical properties, and tribological properties. The effect of water on the tribological properties of DES is still unclear and needs further investigate. Herein, we carried out a systematic investigation into the chemical structure, rheological properties, and tribological performance of DES-water (DES-W) binary systems constructed by combining DES with varying contents of water. The results revealed that low levels of water in DES had a minimal impact on its chemical structure but affected its fluidity and viscosity. Frictional experiments demonstrated that DES-W binary systems displayed a reduced coefficient of friction from 0.094 to 0.025 compared to pure DESs and manifested outstanding antiwear properties under a high-load condition. This was attributed to the formation of hydration layers, adsorption layers, and tribochemical films at the tribointerface through physicochemical adsorption and tribochemical reactions. Our findings not only foster the design and development of green lubricating materials but also expand the engineering applications of DESs to solve wear-related mechanical failures in practical application.

Carbon Dots with Spatially-Mediated-N/S-Co-Doping Enabling One-Year Stable Lubricant with Oil Leakage Detection Capability

The dispersion stability of nano-lubricating additives is crucial for the shelf life of lubricant and its practical applications. Nitrogen-sulfur co-doped carbon dots (N,S@CDs) via a one-step hydrothermal method with nitropyrene and thiourea as raw materials are hereby presented. The N and S elements are selectively distributed throughout the entire carbon skeleton with a doping amount of 22.6 at%. The as-synthesized N,S@CDs exhibit excellent dispersion stability in PEG200 and maintain stability for over one year. The experiment results indicate that N,S@CDs significantly improve the anti-wear and friction reduction properties of PEG200, while the friction coefficient is reduced from 0.25 to 0.09 with 1.5 wt% N,S@CDs addition, and the wear volume, depth, and width are reduced by 68%, 52%, and 57%, respectively. The good lubrication performance is attributed to N,S@CDs excellent dispersion stability, enhanced filling and polishing effects, and complex tribochemical reactions caused by heteroatom doping to form a stable protective film on the worn surface. Furthermore, the as-prepared N,S@CDs exhibit intrinsic fluorescence intensity in PEG200 with the photoluminescence quantum yield (PLQY) of 12.5% and remain fluorescent stable during the long-term friction process, therefore the N,S@CDs have a potential application prospect in non-destructive detection of oil leakage via fluorescence labeling method.

Pulsed Laser Manufactured Heteroatom Doped Carbon Dots via Heterocyclic Aromatic Hydrocarbons for Improved Tribology Performance

Traditional laser-assisted method (top-down synthesis strategy) is applied in the preparation of carbon dots (CDs) by cutting larger carbon materials, which requires harsh conditions, and the size distribution of the CDs is seldom monodisperse. In this work, heteroatom-doped CDs, represented by N,S co-doped CDs (N,S-CDs), can be prepared successfully by pulsed laser irradiation of heterocyclic aromatic hydrocarbons-based small molecule compound solution. The friction coefficient (COF) of base oil PAO decreases from 0.650 to 0.093, and the wear volume reduces by 92.0% accompanied by 1 wt.% N,S-CDs addition, while the load-bearing capacity is improved from 100 to 950 N. The excellent lubrication performance is mainly attributed to the formation of a robust tribofilm via a tribochemical reaction between N,S-CDs and friction pairs, and the N,S-CDs can play a mending effect and polishing effect for worn surfaces. Furthermore, the lubricant containing heteroatom doped CDs are capable of being prepared in situ via pulsed laser irradiation of heterocyclic aromatic hydrocarbons in base oil, which can avoid the redispersed problem of nano-additive in base oil to maintain long-term dispersion, with COF of 0.103 and low wear volume ≈1.99 × 105 µm3 (76.9% reduction) even after standing for 9 months.

Towards outstanding lubricity performance of proton-type ionic liquids or synergistic effects with friction modifiers used as oil additives at the steel/steel interface

Anti-wear (AW) additives and friction modifiers (FMs) and their interactions in lubricants are critical to tribological performance. This research investigates the compatibility and synergism of three oil-soluble alkylamine-phosphate ionic liquids with friction modifiers, organomolybdenum compounds. Three proton-based ionic liquids (PILs) were synthesized using a simple, low-cost, and unadulterated procedure as well as the chain lengths of the PILs affected the effectiveness of friction reduction and anti-wear. For example, the effect of a short-chain PIL alone as an additive on friction and wear behavior was not significant, whereas a long-chain PIL was more effective. In addition, PILs appeared to be able to coexist with organic molybdenum compounds and worked synergistically with dialkyl dithiophosphate oxygen molybdenum (MoDDP) to produce a sustained low coefficient of boundary friction (the coefficient of friction approaching 0.042). We proposed a three-stage tribochemical process to explain this interaction of PILs + MoDDP with contact surfaces to form physically adsorbed friction-reducing films and chemically reactive wear-protective films. This study reveals the compatibility and synergistic effects of two common lubricant components, which can be used to guide lubricant development in the future.

Study on the Superlubricity Behavior of Ions under External Electric Fields at Steel Interfaces

Realizing macroscopic superlubricity in the presence of external electric fields (EEFs) at the steel interfaces is still challenging. In this work, macroscopic superlubricity with a coefficient of friction value of approximately 0.008 was realized under EEFs with the lubrication of LiPF6-based ionic liquids at steel interfaces. The roles of cations and anions in the superlubricity realization under EEFs were studied. Based on the experimental results, the macroscopic superlubricity behavior of Li(PEG)PF6 under EEFs at steel interfaces is attributed to the strong hydration effect of Li+ cations and the complete reactions of anions that contributed to the formation of a boundary film on the appropriate surface. Moreover, the reduction in the number of iron oxides in the boundary film on the disc was beneficial for friction reduction. We also provide a calculation model to describe the relationship between the hydration effect and the optimal voltage position, at which the lowest friction might occur. Ultimately, this work proves that macroscopic superlubricity can be realized under EEFs at steel interfaces and provides a foundation for engineering applications of superlubricity in an electrical environment.

Anti-corrosive non-aqueous DBSA/MEA lamellar liquid crystal lubrication system

HYPOTHESIS

Since lamellar liquid crystals (LLCS) could be used for lubrication, many LLCS systems have been constructed to improve lubrication performance. However, most studies focused on the LLCS of the water system, and its corrosiveness brought some limitations to its application. Therefore, it is necessary to construct a non-aqueous LLCS system with good lubrication and anti-corrosion properties to improve its applicability.

EXPERIMENTS

Anionic surfactant dodecyl benzene sulfonic acid (DBSA) was used to construct non-aqueous LLCS in different solvents, including monoethanolamine (MEA) and diethanolamine (DEA). DBSA/H₂O LLCS system was constructed for comparison. The LLCS was characterized by polarizing microscope (POM), small-angle X-ray scattering (SAXS), and rheology. Its microstructure was discussed. Meanwhile, we evaluated the lubrication and anti-corrosion performance of LLCS. Its lubrication mechanism was explained through tribology tests and X-ray photoelectron spectrometer (XPS) analysis of the wear scar surface. Its anti-corrosion mechanism was investigated by using the weightlessness method, electrochemical test method, and quantum chemical theoretical calculations.

FINDINGS

The DBSA/MEA non-aqueous LLCS system showed better lubrication performance than DBSA/DEA and DBSA/H₂O LLCS. It can adsorb on the surface of the friction pair to form a lubrication friction film, which plays a better role in reducing friction and wear. The DBSA/MEA LLCS is less corrosive to metals because it can effectively isolate oxygen and water in the air between friction pairs. Furthermore, the lone pair electrons in the 2p orbital of the N atom in the MEA molecule could coordinate with the 3d empty orbital of the Fe atom, forming a protective film on the metal surface, which plays a good anti-corrosion effect. This work not only enriched the study of non-aqueous LLCS but also expanded its potential applications in the field of lubrication.

Tween-80 on Water/Oil Interface: Structure and Interfacial Tension by Molecular Dynamics Simulations

Surfactants are used in many fields of the chemical industry in a wide range of applications. Generally, surfactants on water/oil interfaces reduce interfacial tension, enabling the formation of emulsions or providing greater stability to the emulsion formed. Although molecular dynamics has been extensively used and has achieved remarkable success in describing thermodynamic and molecular properties of systems with surface-active compounds, the traditional molecular simulation force fields considerably constrain the system size and the time scale of simulations. Here, we propose a coarse-grained model of polysorbate 80, a nonionic surfactant commercially known as Tween-80. Based on the proposed coarse-grained model, we evaluate the influence of the more internal ethylene oxide chain on the properties of the water/Tween-80/decane system. We verify with the simulation results that the model can reproduce the expected decrease in interfacial tension as the surfactant quantity increases in the simulations. Furthermore, we observe changes in the surfactant orientation as their quantity in the interface increases, indicating a preferential orientation for these molecules in the adsorption layer. We also assessed the partition coefficient of the surfactant between the two bulk phases by performing free energy calculations, which showed a higher affinity of Tween-80 surfactants with the water phase.

Interactions Balancing Competition and Cooperation between Covalent-Organic Framework Additives and PEG Base Oil toward Advanced Lubrication

Severe competition between nanolubricant additives and polar lubricating oil molecules for the formation of lubricant films has been hindering the progress of green and advanced lubricants. In this work, based on the tautomerism of cyanuric acid molecule (trione and triol configurations), two kinds of triazine-based covalent-organic frameworks (COFs), that is, Ton-COFs and Tol-COFs, were synthesized as additives of the polar PEG 400 oil, realizing compromise between them by providing delicate interactions. The triazine matrix bonding with intense polar groups in the framework of additives offers more powerful interactions to competitively form the adsorbed lubricant film on the surface of the metal substrate over PEG 400 oil and also bolts PEG 400 oil molecules by the hydrogen bonding inside the pore of the framework to cooperatively bear against the load. Molecular quantum chemical calculations further confirm that Ton-COFs can produce a more intense interaction with Fe atoms in the form of coordination and ions···π than Tol-COFs, far beyond PEG 400, and the cross-sectional profile of the worn surface definitely exhibits a protective lubricant film only composed of Ton-COFs. Consequently, at the low concentration of 0.3 wt %, the excellent friction reduction (41.2%) and antiwear property (97.4%) are achieved for the Ton-COFs compared to pure PEG 400 oil; moreover, 28.6% and 79.0% for Tol-COFs at the essential concentration of 0.7 wt % are achieved. This finding provides a novel insight from molecules to materials into guiding the development of additives for advanced lubricants.

Synthetic Kilogram Carbon Dots for Superior Friction Reduction and Antiwear Additives

Because of the high synthesis cost, strong chemical inertness, complex process, and easy to endanger environment of traditional carbon-based nanolubricant additives, the development of its application in lubrication is limited. Therefore, a new type of lubricant additive with low cost, high yield, high performance, and environmental protection is urgently needed. Herein, a kilogram-scale carbon dots (CDs) lubricant additive was prepared by a simple and green one-step reaction of aldol condensation, which showed excellent lubricating properties in water and sunflower oil. The tribological properties of the CDs lubricant additive at different concentrations, loads, and speeds were systematically studied. The results show that the average friction coefficient of water is significantly reduced by 75% by a CDs lubricant additive. In particular, CDs not only exhibited excellent service life and lubrication stability during friction but also kept the friction coefficient change rate of sunflower seed oil close to 0 within 500 min. According to the tribological evaluation and wear surface analysis, the lubrication mechanism of CDs was attributed to their own morphological characteristics and abundant oxygen-containing functional groups on the surface. In the friction process, the charge adsorption effect, the adsorption protective film, and the hydrogen bonding layer are generated, which play an essential role in obvious antiwear and friction reduction. Therefore, this work provides a reference for the preparation of high-performance and high-yield lubricant additives.

The superior lubricating performance and unique mechanism of oil-soluble protic ionic liquids with short alkyl chains

HYPOTHESIS

Ionic liquids (ILs), as lubricant additives, can greatly improve the lubricating behavior of the frictional interfaces. However, it is urgent to explore ILs with good oil solubility in nonpolar oils, and it is necessary to further study and verify the lubrication mechanism of ILs from the perspective of alkyl chain length.

EXPERIMENTS

Five protic ILs (PILs) with varying alkyl chain lengths were synthesized by proton transfer method. As additives in PAO oil, their tribological properties were investigated on SRV-V tester. Through molecular dynamics simulation, the adsorption behavior of PILs at the frictional interface was illustrated.

FINDINGS

The tribological properties of base oil could be significantly improved by adding PIL additives, but interestingly, PILs with short-chain anions showed better lubricating performance, which contradicted most of the early findings. Further analyses revealed that PILs achieved effective lubrication by the tribochemical interaction between anions and frictional interface, and the formation of cationic protective layer. However, PILs with shorter-chain anions form a denser protective layer that can better support the interfacial anions to participate in tribochemical reactions and thus abnormally exhibit superior lubricating performance than those with longer-chain anions.

An Amino Acid Functionalized Ionic Liquid as A Multifunctional Lubricant Additive in Water-Glycerol

Water-glycerol as one promising alternative for mineral oil can be applied as a green lubricant but has poor lubricity and strong corrosivity. It is desirable to design multifunctional water soluble lubricant additives. Protic ionic liquids (PILs) show considerable potential due to their facile preparation and environmental friendliness. Herein, an eco-friendly PIL ([osar][mea]) was facilely prepared from an amino acid derivate and investigated its anticorrosion and lubrication performances in the glycerol solution. Furthermore, the wear traces were measured using SEM/EDS and XPS for exploring the lubrication mechanism. The prepared PIL can rapidly increase the corrosion inhibition ability of water-glycerol as its concentration increases over CMC, and the anion playing a key role in the light of DFT calculations. Furthermore, [osar][mea] can greatly enhance the lubrication capability especially of water-glycerol while its concentration in the glycerol solution exceed 3%. The lubricity reduced with the increasing load. By the means of using SEM/EDS and XPS evaluation of the wear traces, we can speculate the possible lubrication mechanism may be the presence of the [osar][mea] adsorption film and the tribofilm containing complex nitrogen compounds.