Liquid Lubricants Research Articles

Terahertz time-domain spectroscopy of nanoadditive macroscale superlubricity: Quantitative hydration investigation and deeper insight into the solid-liquid synergistic lubrication

As a promising approach, terahertz time-domain spectroscopy (THz-TDs) provides the powerful assistance for quantitatively characterizing the solid-liquid synergistic lubrication. The hydration ratio in nanoadditive solutions was calculated by terahertz dielectric spectrum, and the exact threshold for superlubricity was found as 40%. The high hydration ratio also promoted the formation of high-quality tribofilm. Besides, not all hydrogen bonds, but the solid-liquid hydrogen bond was the true answer to the nanoadditive superlubricity. Finally, a synergistic lubrication model was proposed and it gave a detailed description of the molecular states with different lubrication. This work exerts the sensitivity of THz-TDs on hydration and liquid lubrication, providing avenues for the quick assessment of superlubricity and optimal design of nanoadditive superlubricity in the future.

Wear mechanism of alloy steel under dicationic liquid lubricant with IL-grafted graphene oxide as an additive

A dicationic liquid (DIL) was synthesized and used as a base lubricant. IL-grafted graphene oxides (MGO) were manufactured as an additive to the DIL lubricant. Then, the MGO/DIL was applied as a lubricant for the surface contact between a stainless steel ball and bearing steel. The tribological performance of MGO/DIL was measured and compared to those from a monocationic liquid (MIL) based lubricant. The MGO/DIL lubricant produced better friction and wear performance at room temperature but poor wear at 100 °C. Through novel instrumentation techniques, it could be found that when DIL is exposed to air at higher temperatures, more oxygen is adsorbed into DIL, which accordingly reacts with underlying bearing steel, resulting in critical surface oxidation and wear.

Wedge-shaped lyophilic pattern on superlyophobic surface for unidirectional liquid guidance and lubrication enhancement

Effective maintenance of lubricants within the frictional interface is of great importance for the long-life operation of mechanical components. This paper proposes a wedge-shaped lyophilic groove with secondary structures on superlyophobic surfaces to achieve unidirectional guidance of liquid droplets. The experimental studies demonstrate its rapid and long-distance unidirectional self-driven spreading on water and oil droplets. Then, wedge-shaped lyophilic grooves were designed on non-contact area of friction pair to guide lubricant. Relatively lower and more stable friction coefficients were achieved. Further analysis reveals that wedge-shaped grooves restrict the drainage of liquid lubricants. The results of bearing experiments show that designing lyophilic wedge-shaped grooves in the non-contact area of the rolling bearing can effectively reduce the temperature rise of the outer ring.

High temperature lubrication performance of chlorophenyl silicone oil

Most studies of liquid lubricants were carried out at temperatures below 200 °C. However, the service temperature of lubricants for aerospace and aeroengine has reached above 300 °C. In order to investigate the friction mechanism and provide data for high temperature lubrication, the friction and wear properties of chlorophenyl silicone oil (CPSO)-lubricated M50 steel and Si3N4 friction pairs were investigated herein. Ball-on-disk experimental results show that the lubrication performance of CPSO varies significantly with temperature. Below 150 °C, coefficient of friction (COF) remains at 0.13–0.15 after the short running-in stage (600 s), while the COF in the running-in stage is 0.2–0.3. At 200 °C and above, the running-in time is much longer (1,200 s), and the initial instantaneous maximum COF can reach 0.5. Under this condition, the COF gradually decreases and finally stabilizes at around 0.16–0.17 afterwards. This phenomenon is mainly due to the different thickness of boundary adsorption film. More importantly, the wear rate of M50 steel increases significantly with the temperature, while the wear rate barely changes at temperatures above 200 °C. The anti-wear mechanism is explained as tribochemical reactions are more likely to occur between CPSO and steel surface with the increased temperature, generating the FeCl2 protective film on the metal surface. Accordingly, FeCl2 tribochemical film improves the lubrication and anti-wear capacity of the system. At high temperatures (200–350 °C), FeCl2 film becomes thicker, and the contact region pressure becomes lower due to the larger wear scar size, so the wear rate growth of M50 steel is much smaller compared with that of low temperatures (22–150 °C). The main findings in this study demonstrate that CPSO lubricant has good anti-wear and lubrication capacity, which is capable of working under temperatures up to 350 °C.

Highly Interfacial Active Gemini Surfactants as Simple and Versatile Emulsifiers for Stabilizing, Lubricating and Structuring Liquids

AbstractTo date, locking the shape of liquids into non‐equilibrium states usually relies on jamming nanoparticle surfactants at an oil/water interface. Here we show that a synthetic water‐soluble zwitterionic Gemini surfactant can serve as an alternative to nanoparticle surfactants for stabilizing, structuring and additionally lubricating liquids. By having a high binding energy comparable to amphiphilic nanoparticles at the paraffin oil/water interface, the surfactant can attain near‐zero interfacial tensions and ultrahigh surface coverages after spontaneous adsorption. Owing to the strong association between neighboring surfactant molecules, closely packed monolayers with high mechanical elasticity can be generated at the oil/water interface, thus allowing the surfactant to produce not only ultra‐stable emulsions but also structured liquids with various geometries by using extrusion printing and 3D printing techniques. By undergoing tribochemical reactions at its sulfonic terminus, the surfactant can endow the resultant emulsions with favorable lubricity even under high load‐bearing conditions. Our study may provide new insights into creating complex liquid devices and new‐generation lubricants capable of combining the characteristics of both liquid and solid lubricants.

Highly Interfacial Active Gemini Surfactants as Simple and Versatile Emulsifiers for Stabilizing, Lubricating and Structuring Liquids.

To date, locking the shape of liquids into non-equilibrium states usually relies on jamming nanoparticle surfactants at an oil/water interface. Here we show that a synthetic water-soluble zwitterionic Gemini surfactant can serve as an alternative to nanoparticle surfactants for stabilizing, structuring and additionally lubricating liquids. By having a high binding energy comparable to amphiphilic nanoparticles at the paraffin oil/water interface, the surfactant can attain near-zero interfacial tensions and ultrahigh surface coverages after spontaneous adsorption. Owing to the strong association between neighboring surfactant molecules, closely packed monolayers with high mechanical elasticity can be generated at the oil/water interface, thus allowing the surfactant to produce not only ultra-stable emulsions but also structured liquids with various geometries by using extrusion printing and 3D printing techniques. By undergoing tribochemical reactions at its sulfonic terminus, the surfactant can endow the resultant emulsions with favorable lubricity even under high load-bearing conditions. Our study may provide new insights into creating complex liquid devices and new-generation lubricants capable of combining the characteristics of both liquid and solid lubricants.

Molecular Interactions between Ionic Liquid Lubricants and Silica Surfaces: An MD Simulation Study.

The unique physicochemical properties of ionic liquids (ILs) attracted interest in their application as lubricants of micro/nano-electromechanical systems. This work evaluates the feasibility of using the protic ionic liquids [4-picH][HSO4], [4-picH][CH3SO3], [MIMH][HSO4], and [MIMH][CH3SO3] and the aprotic ILs [C6mim][HSO4] and [C6mim][CH3SO3] as additives to model lubricant poly(ethylene glycol) (PEG200) to lubricate silicon surfaces. Additives based on the cation [4-picH]+ exhibited the best tribological performance, with the optimal value for 2% [4-picH][HSO4] in PEG200 (w/w). Molecular dynamics (MD) simulations of the first stages of adsorption of the ILs at the glass surface were performed to portray the molecular behavior of the ILs added to PEG200 and their interaction with the silica substrate. For the pure ILs at the solid substrates, the MD results indicated that weak specific interactions of the cation with the glass interface are lost to accommodate the larger anion in the first contact layer. For the PEG200 + 2% [4-picH][HSO4] system, the formation of a more compact protective film adsorbed at the glass surface is revealed by a larger trans population of the dihedral angle -O(R)-C-C-O(R)- in PEG200, in comparison to the same distribution for the pure model lubricant. Our findings suggest that the enhanced lubrication performance of PEG200 with [4-picH][HSO4] arises from synergistic interactions between the protic IL and PEG200 at the adsorbed layer.

Solid Lubricants Used in Extreme Conditions Experienced in Machining: A Comprehensive Review of Recent Developments and Applications

Contacting bodies in extreme environments are prone to severe wear and failure due to friction and seizure, which are associated with significant thermal and mechanical loads. This phenomenon greatly impacts the economy since most essential components encounter these challenges during machining, an unavoidable step in most manufacturing processes. In machining, stress can reach 4 GPa, and temperatures can exceed 1000 °C at the cutting zone. Severe seizure and friction are the primary causes of tool and workpiece failures. Liquid lubricants are popular in machining for combatting heat and friction; however, concerns about their environmental impact are growing, as two-thirds of the 40 million tons used annually are discarded and they produce other environmental and safety issues. Despite their overall efficacy, these lubricants also have limitations, including ineffectiveness in reducing seizure at the tool/chip interface and susceptibility to degradation at high temperatures. There is therefore a push towards solid lubricants, which promise a reduced environmental footprint, better friction management, and improved machining outcomes but also face challenges under extreme machining conditions. This review aims to provide a thorough insight into solid lubricant use in machining, discussing their mechanisms, effectiveness, constraints, and potential to boost productivity and environmental sustainability.

Highly Concentrated Electrolyte Superlubricants Enhanced by Interfacial Water Competition Around Chemically Active MgO Additives.

The low concentration of water-based lubricants and the high chemical inertness of the additives involved are often regarded as basic norms in the design of liquid lubricants. Herein, a novel liquid superlubricant of an aqueous solution containing a relatively high concentration of salt, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), is reported for the first time, and the superlubricity stability and load-bearing capacity of the optimized system (MgO0.10/LiTFSI10) are effectively strengthened by the addition of only trace (0.10 wt %) water-chemically active MgO additives. It demonstrates higher applicable loads, lower COF (∼0.004), and stability relative to the base solution. Only a trace amount of MgO additive is needed for the superlubricity, which makes up for the cost and environmental deficiencies of LiTFSI10. The weak interaction region between free water and the outer-layer water of Li+ hydration shells becomes a possible ultralow shear resistance sliding interface; the Mg(OH)2 layer, generated by the reaction of MgO with water, further creates additional weakly interacting interfaces, leading to the formation of an asymmetric contact between the clusters/particles within the hydrodynamic film by moderating the competition between interfacial water and free water, thus achieving high load-bearing macroscopic superlubricity. This study deepens the contribution of electrolyte concentration to ionic hydration and superlubricity due to the low shear slip layer formed by interfacial water competition with water-activated solid additives, providing new insights into the next generation of high load-bearing water-based liquid superlubricity systems.

Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication

Surface Curvature Enhances the Electrotunability of Ionic Liquid Lubrication

Modeling equilibrium and non-equilibrium thermophysical properties of liquid lubricants using semi-empirical approaches and neural network

Abstract This study explored the capability of semi-empirical and neural network approaches for correlating and predicting some equilibrium and non-equilibrium thermophysical properties of liquid lubricants. The equilibrium properties, including the densities and several thermodynamic coefficients for 12 liquid lubricants, were correlated and predicted through a perturbed hard-chain equation of state (PHC EoS) by an attractive term of Yukawa tail. The molecular parameters of PHC EoS were obtained by correlating them with 935 data points for the densities and isothermal compressibilities of studied systems in the 278–353 K range and pressure up to 70 MPa with the average absolute relative deviations (AARDs) of 0.36 % and 5.25 %, respectively. Then, that EoS was employed to predict the densities of other literature sources (with an AARD of 0.81 %) along with several thermodynamic coefficients, including isobaric expansivities (with an AARD of 12.92 %), thermal pressure coefficients (with the AARD of 12.93 %), and internal pressure (with the AARD of 13.67 %), for which the reference values were obtained from Tait-type equations and available in literature. Apart from the equilibrium mentioned above properties, the PHC EoS was combined with a rough hard-sphere-chain (RHSC) model to correlate and predict the 548 data points for the viscosities of 7 selected liquefied lubricants in 283–353 K range and pressures up to 100 MPa with the AARD of 11.85 %. The accuracy of the results from the RHSC-based model has also been compared with an empirical PηT equation of Tammann-Tait type and an artificial neural network (ANN), both of which were developed in this work. The ANN of one hidden layer and 13 neurons was trained using the back-propagation algorithm. The results acquired from this approach were very promising and demonstrated the potential of the ANN approach for predicting the viscosity of lubricants, reaching an AARD of 0.81 % for the entire dataset.

Stability Analysis of the Rotor-Journal Bearing System Considering Shear and Gaseous Cavitation

Part of the gas phase within the bearing emanates from the gaseous lubricating medium generated by the phase transition of the liquid lubricant under low pressure, while the remaining portion originates from the expansion of gases, such as air, present in the lubricant. This study delves into the impact of vapor and gas cavitation on the stability of the rotor-journal bearing system. Utilizing computational fluid dynamics (CFD), a 3D transient lubrication model is developed for the rotor-journal bearing system. This model integrates a combined cavitation approach, encompassing both vaporous and gaseous cavitation phenomena. Based on a new structured dynamic mesh method, the journal orbits are obtained when the journal moves in the rotor-journal bearing system. In vaporous and gaseous cavitation, shear stress and non-condensable gases (NCG) are incorporated successively. Compared with the combined cavitation model, the basic cavitation model journal orbit amplitude is significantly larger than the combined cavitation model. The carrying capacity of journal bearings under the basic cavitation model is overestimated, leading to a more conservative prediction for system stability.

Encapsulated Liquid Lubricants Incorporated in Metal Matrix Thermal Spraying Coatings

Encapsulated Liquid Lubricants Incorporated in Metal Matrix Thermal Spraying Coatings

Carbon Dots as Ligand Operons to Expand Cluster Size Distribution for High Load‐bearing Liquid Superlubricity

AbstractAdding additives (e.g., carbon dots, CDs) to liquid lubricants is an effective method for enhancing their load‐bearing capacity and friction reduction. However, it is challenging to simultaneously impart high load‐bearing capacity and superlubricity for them (e.g., ionic liquid analogs, ILAs) through this strategy due to strong Coulombic interactions. With CDs as a competing ligand operon, the cluster size distribution can be expanded, conferring superlubricity with ultrahigh load‐bearing capacity (>705 MPa, the highest value of CDs‐based superlubricious materials thus far) for CDs/ILAs. In particular, methodologies such as Raman, 1H‐NMR, fourier transform infrared spectroscopy, and laser particle size analysis are employed to characterize the evolution of their H‐bonding interactions and particle size distribution. CDs can act as competing ligands and enable CDs/ILAs to form systems with ultra‐low coefficient of friction and wear consisting of a mixture of large and small clusters. The mixed and expanded size clusters facilitate the reduction of the viscosity for CDs/ILAs under shear, thereby reducing the interfacial shear resistance during superlubricity, while the CDs within them efficiently transfer the normal bearing loads. The progress is made from the perspective of cluster particle size distribution rather than individual additive properties, providing a new avenue for designing ILAs‐based superlubricious materials.

Influence of 1-Ethyl-3-methylimidazolium Diethylphosphate Ionic Liquid on the Performance of Eu- and Gd-Doped Diamond-like Carbon Coatings

A composite lubricating system that combines solid and liquid lubrication can create a synergistic effect by leveraging the strengths of both types of lubricants. Solid lubrication coatings possess advantageous load-bearing abilities and exhibit low volatility. By adopting this approach, the system retains the merits of solid lubrication while simultaneously harnessing the advantages of liquid lubrication. The unique properties of diamond-like carbon coatings (DLCs) offer the potential to create binding locations for lubricant additives by introducing dopant elements that have a high affinity with additives. In the present work, the combined use of europium-doped diamond-like carbon (Eu-doped DLC) with varying atomic concentrations of the dopant element (1.7 at. % and 2.4 at. %) and gadolinium-doped diamond-like carbon (Gd-doped DLC) with different atomic concentrations of the dopant element (1.7 at. % and 2.3 at. %) was studied alongside a pure DLC coating and the incorporation of an ionic liquid (IL) additive in a tribological block-on-ring system. The focus was on the 1-Ethyl-3-methylimidazolium diethylphosphate ionic liquid with a concentration of 1 wt. % in polyalphaolefin (PAO) 8. Among the investigated pairs, the coefficient of friction (CoF) of 1.7 at. % Eu-doped DLC coupled with the IL was the smallest in boundary, mixed, and elastohydrodynamic lubrication regimes. Quantification of wear was challenging due to minimal and localized wear on the DLC coating surfaces. The decrease in friction within the boundary lubrication regime underscores the promise of mechanical systems that integrate 1.7 atomic percent Europium-doped diamond-like carbon coatings with ionic liquids (IL). This study presents a compelling avenue for future scholarly exploration and research efforts focused on reducing friction and improving the efficiency of moving components, particularly in situations where tribological properties exert a substantial influence

Microvesicle-Embedded Solid-liquid Composite Coating for the Tribological Behavior Regulation and Long-Acting Lubrication.

Friction interfaces in liquid-embedded composite lubrication coatings commonly comprise a combination of discontinuous fluid films and rough solid contact surfaces, which together ensure easy shearing and a prolonged wear life. However, achieving high efficacy in mixed lubrication poses a challenge due to the conflicting nature of enhanced migration freedom for the liquid lubricant and increased mechanical strength of the solid matrix. Recent efforts have focused on incorporating reinforcing fillers to develop multicomponent, multiphase composites that can address this paradox. Here, we describe a modified attapulgite (APT) with strong biphasic wettability via the oil decompressive osmosis treatment on APT nanocontainers grafted with long nonpolar alkyl chains. This modified APT enables control over the size, distribution, and mobility of lubricant droplets by constructing a Pickering emulsion and toughens the solid-phase matrix through dispersion strengthening. Additionally, the introduction of APT induces the formation of a solid tribofilm during friction, which possesses a higher oil adsorption capacity, as verified through first-principles calculations based on density functional theory (DFT). Consequently, the fluid films can be replenished by the fracture of nanocontainers and adsorption from the bulk phase; further comprehensive and effective regulation of the friction interface leads to low friction and wear.

Long range signature of liquid's inertia in nanoscale drainage flows.

In confinement, liquid flows are governed by a complex interplay of molecular, viscous and elastic forces. When a fluid is confined between two approaching surfaces, a transition is generally observed from a long range dynamical response dominated by viscous forces in the fluid to a short range elasto-hydrodynamic response due to the elastic deformation of the solid materials. This study investigates the behavior of fluids driven between oscillating solid surfaces using a dynamic Surface Force Apparatus. Our findings reveal that the dominant influence on fluid behavior arises from long-range inertial effects, superseding conventional elasto-hydrodynamic effects. Through systematic experimentation involving fluids of varied viscosities, diverse substrates, we identify key parameters and develop a comprehensive model which explains our measurements. Our findings not only provide insights into confined fluid dynamics but also offer practical implications for various applications in microfluidics, nanotechnology and liquid lubrication.

Molecular dynamics simulation and experimental study of the rheological performance of graphene lubricant oil

The microscopic molecular motion behavior between graphene and the liquid lubricant oil affects the rheological and frictional behavior of the nano-lubricant. However, the effect of graphene on the rheological behavior of industrial #3 lubricant oil has not been revealed. Thus, molecular dynamics (MD) and experimental study are used to investigate the effect of graphene concentration, size, temperature, and shear rate on the viscosity, mean square displacement (MSD), and density of graphene lubricant oil are investigated. The results show that the viscosity of GLO increases with the increase of graphene concentration and decreases with the increase of temperature (293 K–330 K). The MSD of graphene lubricant oil decreases with the increase of graphene size and increases with the increase of temperature. It is worth noting that graphene lubricant oil with 3G has the maximum value of MSD, which means the graphene has the best diffusion performance at this concentration. Compared to the base oil, the simulation and experiment viscosity values of the GOL with 3G increased by 22.9 % and 47.6 %, respectively. The GOL has the characteristics of a Newtonian fluid. This study can provide a reference for the application of graphene lubricant oil in industry.

МАСЛОУЛОВИТЕЛЬ ДЛЯ УНИФИЦИРОВАННЫХ ВАКУУМНЫХ УСТАНОВОК

A characteristic feature of plate-rotor vacuum pumps is the presence of liquid lubrication, in other words, a certain amount of oil is introduced into the working chamber to reduce friction and increase the sealing level between the plates and the pump housing (stator). In the designs of plate-rotor vacuum pumps of domestic and foreign production, two lubrication methods are used: fresh oil lubrication and circulating lubrication. (Research purpose) The research purpose is increasing operational reliability and service life of plate-rotor vacuum pumps, reducing lubricant consumption and reducing environmental pollution. (Materials and methods) Five prototypes and an experimental installation were manufactured to substantiate the most effective design of the oil trap. The efficiency of cleaning the exhaust air from oil was evaluated by the ratio of the amount of oil in the oil trap to the amount of oil consumed from the oil tank as a percentage. The oil trap was developed based on the information received at the Department of «Machines and Technologies of the Agroindustrial Complex» of the Stavropol State Agrarian University. (Results and discussion) It was determined that the most effective way to ensure the separation of oil from the air is two-sided disc oil traps. It was found that the completion of the oil trap with an additional filter increases the efficiency of the process by 8-10 percent, bringing it to 92 percent. (Conclusions) It has been shown that the design of the oil trap allows it to be used in vacuum installations with various pump lubrication systems (fresh oil or with circulating lubrication) and increases the operational reliability of the pump by eliminating dry friction of its working parts. It was stated that the economy of expensive lubricants and environmental safety also serve as the most important competitive advantage of vacuum installations equipped with the proposed oil trap.

Ready-to-use graphene-related material-added multi-grade oils: characterization and performance in car engine working conditions.

The need for energy efficiency is leading to the growing use of additives to enhance the performance of oil in automotive engines. Great interest is focused on nano-additives even if to date there is still no practical use in commercial liquid lubricants. Herein, the potential of industrially scalable and low-cost graphene-related materials (GRMs) as additives to enhance the performance of oil in automotive engines is explored. The use of polyalkylmethacrylate dispersants, the most common key additives to formulate "green technology" lubricant oils liquid-processed GRM, is explored, investigating the role of the lateral size and the chemical analysis in the stability of the lubricant GRM dispersions. Showing the maximum duration of stability and a production method that avoids the use of strong oxidants, rheological tests were then focused on multilayered graphene flakes with sub-micrometre lateral size mixed in two commercial oil grades (5W-30 and 5W-40) under conditions similar to those of engine operation. The addition of such a filler increases the viscosity without affecting the Newtonian fluid behavior, while four-ball tests show a reduction in wear, indicating improved lubrication performance. Finally, preliminary bench-test on a commercial car engine showed increased power output corresponding to enhanced engine efficiency. The results clearly indicate the effective improvement in lubricating commercial oils due to GRM additives.