Lubrication Mechanism Research Articles

Superlubricity Achieved by a Transparent Poly(vinylpyrrolidone) Composite Hydrogel with Glycerol Ethoxylate in Ocular Conditions.

Efficient and stable ocular lubrication is pivotal in safeguarding eye tissues from wear, especially under repetitive strain due to frequent blinking. Hydrogels have been reported to possess adjustable mechanical properties, biocompatibility, durability, and elevated water content and extensive utilization in medical fields. In this work, a kind of visible photo-cross-linking poly(vinylpyrrolidone) (PVP) hydrogel was designed and synthesized using 1-vinyl-2-pyrrolidone (NVP) and poly(ethylene glycol) diacrylate (PEGDA). To optimize the structure and improve the lubrication performance of hydrogels, we prepared and investigated glycerol ethoxylate (GE)-introduced composite hydrogels (GE/PVP). The results show that the addition of 3 wt % GE helped the hydrogel to form a uniform and dense porous matrix and reduce the frictional coefficient (COF) by over 50%, achieving superlubricity (COF ≈ 0.005). However, with the excessive increase of GE (6 wt %), the structure of the hydrogel is destroyed, inducing pore walls to thin and expand. After that, a lubrication mechanism of the GE/PVP composite hydrogel was proposed, in which the addition of GE reduced the surface tension of the hydrogel, enhanced the hydration ability of the hydrogel, and thus decreased the friction between sliding surfaces. Besides, the cytotoxicity tests show that the composite hydrogels possess good biocompatibility. Overall, the as-synthesized hydrogels hold great potential as lubricating medium for use in ocular applications.

Constructing multi-layer Ti3C2Tx/rGO heterojunctions: Synergistic effect and unique lubrication mechanism

Two-dimensional materials, such as Ti3C2Tx and graphene oxide (GO), are considered candidates for a new generation of lubricating materials due to their unique physical and chemical properties. However, the dispersion stability of Ti3C2Tx and GO in non-polar oils is insufficient, hindering their potential as lubricating additives. Herein, 1,12-Dodecanediamine (DDM) was used as a bridge to connect Ti3C2Tx and GO, and to reduce graphene oxide (rGO) for forming the stacked two-dimensional multilayer Ti3C2Tx/rGO heterostructures. The multilayer Ti3C2Tx/rGO heterostructure exhibits excellent dispersion stability in PAO 20, ensuring its continuous integration into the tribo-interfaces throughout the friction process. The addition of 0.06 wt% multilayer Ti3C2Tx/rGO to the base oil reduced the average friction coefficient and wear volume by 35.3% and 41.6%. The enhanced lubrication performance is mainly due to the unique multilayer heterostructure of Ti3C2Tx/rGO. During friction, the disintegration of Ti3C2Tx into small fragments effectively safeguards the structural integrity of rGO, thereby facilitating the formation of a robust and smooth multilayer lubrication film. Additionally, Ti3C2Tx is adhered to the wear surface in smaller sizes or evolved into the form of Ti oxides. The emergence of this new composite material is not only expected to solve the problem of insufficient performance of traditional lubricating materials under extreme conditions, but also laid the foundation for expanding the application of two-dimensional materials in the field of lubrication.

Tribological performance of fatty acid ammonium ionic liquids as anti-wear additives in water-based fluids

In this study, we synthesized eight water-soluble ionic liquids (ILs) and investigated their tribological performance as additives in water-diethylene glycol (WDG) base fluids. The results of the tribological test suggested that these ILs additives demonstrate promising capabilities in friction reduction and anti-wear for water lubricated contacts. The ILs with the longest carboxylic chain length, namely C10MIPA and C10DIPA in this work, displayed superior tribological performances. Observations on the abraded surfaces showed that the IL forms a monolayer adsorption through self-assembly, and upon rubbing it creates a continuous and stable tribo-film with an uneven thickness of 30–60 nm through tribo-chemical reactions. The tribo-film mainly consists of iron oxides. The proposed lubrication mechanisms include adsorbed film and formed tribo-films, which work effectively to reduce the shear stress and thus sliding friction.

Study of low-toxicity copper pyrithione as a multifunctional additive for lithium grease

The feasibility of CuP as a multifunctional additive in lithium grease was evaluated for the first time. The experimental results indicated that the CuP significantly enhanced the friction reduction, antiwear properties, extreme pressure capabilities, and antimicrobial characteristics of lithium greases. The SRV test results suggested that the optimal tribological properties were obtained by adding 3 wt% of CuP to lithium greases, resulting in a decrease of the friction coefficient and wear volume by 28.25% and 34.98%, respectively. The four-ball results confirmed that the extreme pressure performance of the lithium grease with 3 wt% CuP was enhanced by a factor of 3.17. The bacterial inhibition rate of lithium grease with 3 wt% CuP was higher than 99.98%. The lubrication mechanism and antimicrobial mechanism was explained.

Nanostructured lubricant additives for titanium alloy: Lubrication by the solid-liquid interface with Coulomb repulsion

In this work, the advantage of Coulomb repulsion in the intermolecular forces experienced by molecules on the solid-liquid nanosized contact interface is taken, and the superior friction-reducing property of Cu3(PO4)2·3H2O (CuP) oil-based additives has been confirmed for titanium alloy. Three-dimensional (3D) CuP nanoflowers (CuP-Fs) with a strong capillary absorption effect are prepared to achieve the homogeneous mixing of solid CuP and lubricating oil. Lubrication by CuP-Fs additives for titanium alloy, friction coefficient (COF) can be reduced by 73.68%, and wear rate (WR) reduced by 99.69%. It is demonstrated that the extraordinary friction-reducing property is due to the repulsive solid-liquid interface with low viscous shear force originating from Coulomb repulsion between polar water molecules in CuP and non-polar oil molecules. However, any steric hindrance or connection between this repulsive solid-liquid interface will trigger the adhesion and increase the viscous shear force, for example, dispersant, hydrogen bondings, and shaky adsorbed water molecules. Besides, the lamellar thickness of CuP and the molecular size of lubricant both have a great influence on tribological properties. Here the lubrication mechanism based on interface Coulomb repulsion is proposed that may help broaden the scope of the exploration in low-friction nanomaterial design and new lubricant systems.

Study on friction reduction performance of granular flow lubrication during the milling of Inconel 718 superalloy

PurposeThe purpose of this study is to reveal the friction reduction performance and mechanism of granular flow lubrication during the milling of difficult-to-machining materials and provide a high-performance lubrication method for the precision cutting of nickel-based alloys.Design/methodology/approachThe milling tests for Inconel 718 superalloy under dry cutting, flood lubrication and granular flow lubrication were carried out, and the milling force and machined surface quality were used to evaluate their friction reduction effect. Furthermore, based on the energy dispersive spectrometer (EDS) spectrums and the topographical features of machined surface, the lubrication mechanism of different granular mediums was explored during granular flow lubrication.FindingsCompared with flood lubrication, the granular flow lubrication had a significant force reduction effect, and the maximum milling force was reduced by about 30%. At the same time, the granular flow lubrication was more conducive to reducing the tool trace size, repressing surface damage and thus achieving better surface quality. The soft particles had better friction reduction performance than the hard particles with the same particle size, and the friction reduction performance of nanoscale hard particles was superior to that of microscale hard particles. The friction reduction mechanism of MoS2 and WS2 soft particles is the mending effect and adsorption film effect, whereas that of SiO2 and Al2O3 hard particles is mainly manifested as the rolling and polishing effect.Originality/valueGranular flow lubrication was applied in the precision milling of Inconel 718 superalloy, and a comparative study was conducted on the friction reduction performance of soft particles (MoS2, WS2) and hard particles (SiO2, Al2O3). Based on the EDS spectrums and topographical features of machined surface, the friction reduction mechanism of soft and hard particles was explored.

Position of Carbonyl Group Affects Tribological Performance of Ester Friction Modifiers.

The tribological properties of lubricants can be effectively improved by the introduction of amphiphilic molecules, whose performance is largely affected by their polar head groups. In this work, the tribological performance in steel-steel contacts of two isomers, glycerol monostearate (GMS) and stearyl glycerate (SG), a glyceride and a glycerate, were investigated as organic friction modifiers (OFM) in hexadecane. SG exhibits a much lower friction coefficient and wear than GMS despite their similar structures. The same applies when comparing the performance of oleyl glycerate (OG) and its isomer, glycerol monooleate (GMO). Surface chemical analysis shows that SG forms a polar, carbon-based, tribofilm of around tens of nanometers thick, while GMS does not. This tribofilm shows low friction and robustness under nanotribology test, which may contribute to its superior performance at the macro-scale. The reason for this tribofilm formation can be due to the stronger adsorption of SG on the steel surface than that of GMS. The tribofilm formation can be stress-activated since lower friction and higher tribofilm coverage can be obtained under high load. This work offers insights into the lubrication mechanism of a novel OFM and provides strategies for OFM design.

Thermo-hydrodynamic lubrication and energy dissipation mechanism of a pump-turbine thrust bearing in load-rejection process

The dynamic behavior of the pump-turbine thrust bearing is important to the safety operation of the unit. This paper analyzed the lubrication and energy dissipation mechanism of pump-turbine thrust bearing during load-rejection based on the thermo-hydrodynamic model. The results show that the variation of the axial force and the maximum pad surface pressure is basically consistent with that of the inclination of the pad surface. The change of the friction loss is consistent with the change of rotational speed, while the change of pad surface temperature is affected by the combination of pad inclination and rotational speed. The chaotic flow in the oil tank is accompanied by different forms of vortices, such as Taylor vortices, vortex pairs, and Karman vortices, and results in a significant asymmetry in the pressure distribution. The flow in the bearing pad groove has an effect on the energy dissipation in the oil film. This paper provides a theoretical basis for the design and optimization of thrust bearings, and provides a reference for solving the problems of wear, oil mist, and other related problems of thrust bearings in engineering.

Research on maintenance-free design and low friction characteristics of industrial robot sliding components

With the continuous progress of industrial automation, the application of industrial robots in various fields has become increasingly common, and ensuring their efficient and stable operation and reducing maintenance costs is crucial. This study used powder metallurgy technology, combined with multiple sintering-rolling processes and oil-immersed vacuum assistance, to successfully prepare Fe-Cu-Ni-Sn-graphite oil-immersed self-lubricating composite materials to meet this demand. In-depth microstructure and wear surface studies revealed that this porous oil-immersed self-lubricating composite material exhibits long-term low friction and high wear resistance and reduces the friction coefficient by 40% compared to untreated samples after a specific composite process. In addition, the material exhibits excellent friction performance in the high-temperature pin-on-disc friction and wear test machine. Even after continuous sliding for 24 hours, its friction coefficient remains low and stable. The study also found that its lubrication mechanism may be attributed to solid-liquid synergistic lubrication, thanks to the appearance of oil-graphite mixtures around the wear track. This design ensures high rigidity and reduced frictional loss, providing a strong reference for the design and optimization of sliding components of industrial robots, and is highly suitable for the widespread application of industrial robots.

Investigating Lubrication Mechanisms for Silicon Nitride Sliding on Steel: A Multifaceted Study

Investigating Lubrication Mechanisms for Silicon Nitride Sliding on Steel: A Multifaceted Study

One-step preparation of high performance Ni-MoS2 self-lubricating coatings via jet electrodeposition on additive manufactured aluminum alloy

Composite coatings with solid lubricants can improve the tribological performance of aluminum alloy parts prepared by Laser Powder Bed Fusion (LPBF). In this study, Ni-MoS2 self-lubricating composite coatings were one-step prepared on additive manufactured aluminum alloy surface via jet electrodeposition. The properties and lubrication performance of the coatings prepared using composite electrolyte with different MoS2 particle diameters and concentrations were investigated. The lubrication mechanism of the composite coating was discussed. The results showed that 0.8 μm MoS2 had less agglomeration than 80 nm, resulting in a uniform surface. The lubrication performance of the composite coating was affected by MoS2 particles content and coating hardness. With the increase of MoS2 concentration in the composite electrolyte, the friction coefficient and wear rate decreased, and achieved the best lubrication performance of 0.12 friction coefficient and 0.11 mg/m wear rate when MoS2 composite electrolyte is 5 g/l and MoS2 is 0.8 μm. However, the addition of soft MoS2 decreased the coating hardness, lower hardness will cause lubrication failure.

Large-scale field test on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking: A case study

With the rapid advancement of urbanization, research on the shear behavior of concrete pipe-silty soil interfaces in pipe jacking has a high engineering application value. In this study, a large-scale field direct shear test apparatus and the corresponding test method were designed and proposed to investigate the shear properties of the concrete-silty soil interfaces in pipe jacking. A series of in-situ direct shear tests were conducted at five normal stress levels under four interface contact conditions. The results show that the shear stress-displacement relationships of the interfaces conform to the typical elastic-plastic constitutive relations, the shear strength-normal stress relationships of the interfaces can be well fitted by the Mohr-Coulomb failure criterion, and the interface shear properties are influenced by soil properties, normal stress, lubrication conditions and stagnation time. Without lubrication, the concrete-silty soil interfaces have a shear stress-displacement curve shape similar to that of the test results obtained from indoor direct shear tests, but their ductility and shear strengths are lower. Under lubrication conditions, the paraffin wax and thixotropic slurry can both effectively diminish the interface friction angle, thereby reducing the concrete-silty clay interface friction coefficient by 62.8% and 72.2% and reducing the concrete-silty sand interface friction coefficient by 60.8% and 59.2%, respectively. Based on the macroscopic and microscopic images observed in the tests, the lubrication mechanisms of the lubricants were analysed. The research results can provide a reference for the prediction of jacking force and construction disturbance in adjacent pipe jacking projects.

Elastohydrodynamic lubrication behaviors and mechanisms of gallium-based liquid metal/polydimethylsiloxane composite lubricant

Gallium-based liquid metal (GLM) has excellent extreme-pressure properties, but at low sliding speeds, it is typically in a boundary or mixed lubrication, causing poorer properties compared to traditional lubricants. In this paper, GLM/P composite lubricants composed of GLM particles and polydimethylsiloxane are prepared. Under a maximum-Hertz-stress of 0.69 GPa, GLM/P2–30% (viscosity of 1260 mPa∙s) can enter the elastohydrodynamic lubrication (EHL) regime at 0.2 m/s, has good EHL properties (friction coefficient of 0.06, wear rate in the order of 10−9 mm3/N·m). Meanwhile, it has high extreme-pressure properties (PB of 2300 N). The EHL film is formed mainly by polydimethylsiloxane, while GLM particles increase the thermal conductivity of lubricant and can be adsorbed on the frictional interfaces, thereby improving the EHL properties.

Synergistic Effects of Functionalized WS2 and SiO2 Nanoparticles and a Phosphonium Ionic Liquid as Hybrid Additives of Low-Viscosity Lubricants

This research shows the antifriction and antiwear synergies between a phosphonium ionic liquid (IL) and f-WS2 and f-SiO2 nanoparticles (NPs) as additives of a base oil with low viscosity (PAO6). Mass concentrations of 0.1 wt% nanoadditives and 1% IL were selected to formulate the nanolubricants. Pure sliding and rolling–sliding friction tests were performed at 120 °C, finding great friction reductions in comparison with the PAO6 base oil, specifically for the double hybrid nanolubricant (PAO6 + 1 wt% IL + 0.1 wt% f-WS2 + 0.1 wt% f-SiO2). Regarding the wear produced, the greatest antiwear behavior was also achieved for the double hybrid nanolubricant (width reduction of 48% and worn area decrease of 84%). Furthermore, by means of Raman microscopy and roughness examination of the worn surfaces, it can be proposed that the lubrication mechanism of doubled hybrid nanolubricants could be supported by the adsorbed tribofilm (IL and f-WS2) as well as the mending effects (f-WS2 and f-SiO2).

In Situ Lubrication in Forging of Pure Titanium Using Carbon Supersaturated Die Materials.

A new solid lubrication method was proposed for dry forging of pure titanium with high reduction in thickness. A free-carbon tribofilm was formed in situ at the hot spots on the contact interface to protect the die surfaces from severe adhesion of work materials. This film consisted of the free carbon, which isolated from the carbon supersaturated die substrate materials, diffused to the contact interface and agglomerated to a thin film. Two different routes of carbon supersaturation process were developed to prepare carbon supersaturated ceramic and metal dies for the dry forging of pure titanium wires. A pure titanium bar was utilized as an easy-to-adherent work material for upsetting in dry and cold. The round bar was upset up to 70% in reduction in thickness with a low friction coefficient from 0.05 to 0.1 in a single stroke. Work hardening was suppressed by this low friction. SEM-EDX, EBSD and Raman spectroscopy were utilized to analyze the contact interface and to understand the role of in situ formed free-carbon films on the low friction and low work hardening during forging. Precise nanostructure analyses were utilized to describe low friction forging behavior commonly observed in these two processes. The in situ solid lubrication mechanism is discussed based on the equivalence between the nitrogen and carbon supersaturation processes.

Influence of Non-Parallelism on the Micro-Interface Lubrication Mechanism of Water-Lubricated Bearings

Water-lubricated bearings can effectively solve the pollution problem caused by lubricant leakage and are used in offshore engineering equipment for this reason. Aiming at the problems of unclear and undefined micro-interface lubrication mechanisms of water-lubricated bearings, this paper investigates the influence of non-parallel micro-cavities on the micro-interface lubrication mechanism of bearings. Based on a single micro-cavity model, the lubrication mechanism of micro-cavities is studied in this paper. Lubrication models of the non-parallel contact friction pairs model are built, and the effect of the non-parallelism on the lubrication performance of the micro-cavities is obtained using the computational fluid dynamics method. The results show that, under the same Reynolds number and cavitation pressure, the wedge effect caused by the non-parallelism causes the pressure at the inlet to rise, thus increasing the load-carrying capacity. The existence of non-parallelism limits the rise of the high pressure of the inertia effect on the micro-cavities and reduces the load-carrying capacity. The presence of non-parallelism decreases the area of the negative pressure proportion and increases the proportion of the positive pressure zone inside the micro-cavities, thus increasing the load-carrying capacity.

The skating of drops impacting over gas or vapour layers

We report numerical simulations confirming the predictions in Gordillo & Riboux (J. Fluid Mech., vol. 941, 2022, A10), where we elucidated the lubrication mechanism by which a drop of a low-viscosity liquid impacting over a smooth solid substrate skates over a thin gas film that prevents contact with the wall. Moreover, with the purpose of explaining the so-called lift-off mechanism reported in Kolinski et al. (Phys. Rev. Lett., vol. 112, issue 13, 2014, 134501), we extend our previous findings and derive expressions for the time-varying thickness of the gas layer at the region where the distance to the wall is minimum, finding good agreement with the numerical results. In addition, we report that our predictions for the minimum thickness of the gas film separating a falling drop from a wall at room temperature follow closely the experimental values when gas kinetic effects are retained in the analysis, and also report that the analogous equation for the minimum thickness of the vapour layer formed after a drop impacts a superheated wall predicts well the experimental measurements.

Experimental study on boundary lubricity of superficial area of articular cartilage and synovial fluid

The boundary lubrication mechanism at the articulating surface of natural synovial joints has been the subject of much discussion in tribology. In this study, to elucidate the lubricating function of the superficial area of articular cartilage and synovial fluid (SF), cartilage specimens were processed with four different treatments: gentle and severe washing with detergent, incubation in NaCl solution, and trypsin digestion to selectively remove certain constituents from the cartilage surface. Subsequently, the frictional characteristics were examined in phosphate-buffered saline (PBS) and SF against glass. Angularly reciprocating sliding tests with a spherical glass probe and square articular cartilage specimens were performed at low contact loads in the mN range to extract the frictional behavior in the superficial area of the cartilage specimens. Meanwhile, the cartilage surface was observed to confirm the effects of treatments on the morphology of the cartilage surface using a fluorescence microscope and water-immersion methods. The coefficient of friction (COF) of the prepared cartilage specimens was varied from 0.05 to over 0.3 in PBS. However, a certain group of cartilage specimens exhibited a low COF of less than 0.1 with limited variation. For the low COF group of specimens, all four treatments increased the COF in PBS to different extents, and fluorescence microscopy revealed that the integrity of the cartilage surface was deteriorated by treatments. This means that the intact cartilage surface had lubricating constituents to maintain low friction, and the removal of such constituents resulted in the loss of the intrinsic boundary lubricity of the cartilage surface. The variation in the COF of the cartilage specimens was suppressed in SF because it had a clear boundary lubrication effect on the cartilage surface. The lubricating effect of SF could be confirmed even after degenerative treatment.

Boundary slip and lubrication mechanisms of organic friction modifiers with effect of surface moisture

Surface moisture or humidity impacting the lubrication property is a ubiquitous phenomenon in tribological systems, which is demonstrated by a combination of molecular dynamics (MD) simulation and experiment for the organic friction modifier (OFM)-containing lubricant. The stearic acid and poly-α-olefin 4cSt (PAO4) were chosen as the OFM and base oil molecules, respectively. The physical adsorption indicates that on the moist surface water molecules are preferentially adsorbed on friction surface, and even make OFM adsorption film thoroughly leave surface and mix with base oil. In shear process, the adsorption of water film and desorption OFM film are further enhanced, particularly under higher shear rate. The simulated friction coefficient (that is proportional to shear rate) increases firstly and then decreases with thickening water film, in good agreement with experiments, while the slip length shows a contrary change. The wear increases with humidity due to tribochemistry revealing the continuous formation and removal of Si-O-Si network. The tribological discrepancy of OFM-containing lubricant in dry and humid conditions is attributed to the slip plane’s transformation from the interface between OFM adsorption film and lubricant bulk to the interface between adsorbed water films. This work provides a new thought to understand the boundary lubrication and failure of lubricant in humid environments, likely water is not always harmful in oil lubrication systems.

Effects of metal element doping on the lubrication behaviors and mechanisms of gallium-based liquid metals

Gallium-based liquid metals (GLMs) are a novel high-end lubricant with low friction coefficient and excellent load-carrying capacity. This paper aims to improve the lubrication properties of GLMs by regulating the frictional interfaces. Six types of GLMs doped with Zn, Mg, Bi, Cu, Ti, or Sc elements are prepared respectively, and the effects of metal element doping on the lubrication behaviors and mechanisms for AISI 440C steel self-mated pairs are investigated in the mixed lubrication regime with a ball-on-three-plate setup. Doping 1 wt% Zn or Cu elements can improve the lubrication properties, with the friction coefficient reduced by 20.3 % and the wear rate reduced by 55.1 % compared with those under eutectic Ga/In/Sn lubrication. However, doping Ti, Sc, Mg, or Bi all reduce the lubrication properties. Doping Zn improves the lubrication properties of GLMs by promoting the adsorption of gallium on the frictional interfaces. Conversely, doping Bi inhibits the adsorption of gallium. The improvement in the lubrication properties of Cu-doped GLM is attributed to the formation of low-hardness CuGa2 particles. The doping of Ti or Sc generates high-hardness TiGa3 or ScGa3 particles, causing three-body abrasive wear. The doping of Mg generates a large number of Mg2Ga5 particles, seriously disrupting the stable lubrication.