Lubrication Mechanism Research Articles

Tribological considerations of threaded fastener friction and the importance of lubrication

The torque-tension relationship of threaded fasteners affects almost all engineering disciplines. Tribological processes at fastener interfaces manifest as the system's friction coefficient. Lubrication-related influences are usually described empirically using K or μ. The drive towards lightweight fastener materials in engineering systems and lubricants with reduced environmental impact is challenging existing knowledge and industrial practice in a range of applications, many safety critical. More comprehensive understanding is needed to achieve repeatable friction during assembly and re-assembly, resistance to loosening and fretting during operation, and effective anti-seize for disassembly with a growing range of materials and lubricants. The lubricants considered showed three predominant lubrication mechanisms: plastic deformation of metal powders; burnishing/alignment of molybdenum disulphide, MoS2; and adhering/embedding of non-metal particles. Multivariate analysis identified key sensitivities for these mechanisms. Assembly generated changes at fastener surfaces and in the lubricating materials. Re-assembly exhibited significant reductions in friction.

Layered Structure and Wear Mechanism of Concentrated Polymer Brushes.

Concentrated polymer brushes (CPBs), which are significantly denser and thicker than conventional semidilute polymer brushes, have received increasing attention in the field of tribology because of their superlow friction properties. However, despite numerous studies aimed at enhancing CPBs for mechanical applications, the relationship between the specific layered structure and lubrication mechanisms of CPBs is still not completely understood. In this study, to reveal the relationship, simultaneous time-resolved measurements of the interfacial gap, static mechanical response, and dynamic mechanical response of the CPB at the contact interface were conducted using optical interference and precise force measuring methods. Two types of tests (i.e., the "indentation" and "sliding" tests) were alternately performed on a glass substrate coated with the CPB against a steel ball immersed in an ionic liquid. The indentation tests measuring the time-resolved interfacial gap and changes in static and dynamic mechanical responses quantitatively confirmed the presence of dilute, middle, and concentrated layers in the CPB. In the sliding tests, the wear of the CPB was detected by observing a decrease in the interfacial gap at the contact interface. Moreover, the thickness of the dilute layer remained constant with sliding, whereas the thicknesses of the other layers decreased, indicating that the dilute layer was continuously formed due to sliding. Therefore, CPB wear occurs randomly at the friction interface alongside the formation of a dilute layer with low density and stiffness on the surface.

Influence of V doping on the microstructure, chemical stability, mechanical and tribological properties of MoS2 coatings

PurposeThis study aims to investigate the effect of V doping on the microstructure, chemical stability, mechanical and vacuum tribological behavior of sputtered MoS2 coatings.Design/methodology/approachThe MoS2-V coatings are fabricated via tuning V target current by magnetron sputtering technique. The structural characteristic and elemental content of the coatings are measured by field emission scanning electron microscopy, X-ray diffractometer, electron probe X-ray micro-analyzer, Raman, X-ray photoelectron spectroscopy, high resolution transmission electron microscope and energy dispersive spectrometer. The hardness of the deposited coatings are tested by a nanoindentation technique. The vacuum tribological properties of MoS2-V coatings are studied by a ball-on-disc tribometer.FindingsIntroducing V into the MoS2 coatings results in a more compact microstructure. The hardness of the coatings increases with the doping of V. The MoS2-V coating deposited at a current of 0.2 A obtains the lowest friction coefficient (0.043) under vacuum. As the amount of V doping increases, the wear rate of the coating decreases first and then increases, among which the coating deposited at a current of 0.5 A has the lowest wear rate of 2.2 × 10–6 mm3/N·m.Originality/valueThis work elucidates the role of V doping on the lubrication mechanism of MoS2 coatings in a vacuum environment, and the MoS2-V coating is expected to be applied as a solid lubricant in space environment.

Hydration lubrication modulated by water structure at TiO2-aqueous interfaces

The nature of solid–liquid interfaces is of great significance in lubrication. Remarkable advances have been made in lubrication based on hydration effects. However, a detailed molecular-level understanding is still lacking. Here, we investigated water molecule behaviors at the TiO2–aqueous interfaces by the sum-frequency generation vibrational spectroscopy (SFG-VS) and atomic force microscope (AFM) to elucidate the fundamental role of solid–liquid interfaces in lubrication. Combined contributions of water structures and hydration effects were revealed, where water structures played the dominant role in lubrication for TiO2 surfaces of varying hydrophilicity, while hydration effects dominated with the increasing of ion concentrations. Superior lubrication is observed on the initial TiO2 surfaces with strongly H-bonded water molecules compared to the hydrophilic TiO2 surfaces with more disordered water. The stable ordered water arrangement with strong hydrogen bonds and the shear plane occurring between the ordered water layer and subsequent water layer may play a significant role in achieving lower friction. More adsorbed hydrated molecules with the increasing ionic concentration perturb ordered water but lead to the enhancement of hydration effects, which is the main reason for the improved lubrication for both TiO2. This work provides more insights into the detailed molecular-level understanding of the mechanism of hydration lubrication.

Preparation and tribological properties of GO supported MoO3 composite nanomaterials

The MoO3/GO composites were synthesized via a hydrothermal method. The performance of these composites as lubricating oil additives was investigated by a multifunctional friction testing machine. And the lubrication mechanism of MoO3/GO in base oil was discussed based on SEM and EDS test data. The results demonstrate that MoO3/GO composites as additives exhibit excellent anti-friction and anti-wear properties. This is mainly due to the synergistic effect between the lubricating film formed by the composite material on the wear surface and the self-healing ability of nano-MoO3, which can effectively fill and repair wear scars while reducing friction and wear on the steel disc surface.

High-performance protic ionic liquids as additives in lubricating oils with varying polarity

Although ionic liquids (ILs) have proven to be multifunctional and universal lubricant additives, their solubility in different polar lubricating oils varies greatly. This variability restrains their widespread use as general-purpose lubricating additives. To address this issue, three protic ionic liquids (PILs) with high solubility in lubricating oils with differing polarities were selected as additives, and their tribological performances in polyethylene glycol (PEG) and synthetic pentaerythritol carboxylic ester oil (5750) were systematically investigated. These findings were interesting, as PILs effectively improved the lubrication properties of PEG but impaired those of 5750. Additionally, a variety of surface analysis techniques and MD simulation were employed to further explore the lubrication mechanisms of PILs, and results indicated that they exhibited distinct tribological performance.

The role of membrane components on the oleosome lubrication properties

HypothesisOleosomes are natural oil droplets with a unique phospholipid/protein membrane, abundant in plant seeds, from which they can be extracted and used in emulsion-based materials, such as foods, cosmetics and pharmaceutics. The lubrication properties of such materials are essential, on one hand, due to the importance of the in-mouth creaminess for the consumed products or the importance of spreading the topical creams. Therefore, here, we will evaluate the lubrication properties of oleosomes, and how these properties are affected by the components at the oleosome membrane. ExperimentOleosomes were extracted, and their oral lubricating properties were evaluated using tribology. To understand the influence of the oil droplet membrane composition, reconstituted oleosomes were also studied, with membranes that differed in protein/lecithin ratio. Additionally, whey protein- and lecithin-stabilised emulsions were used as reference samples. Confocal laser scattering microscopy was used to study the samples visually before and after tribological analysis. FindingsOleosomes followed a ball-bearing mechanism, which was probably related to their high physical stability due to the presence of membrane proteins. When the membrane protein concentration at the surface was reduced, the droplet stability weakened, leading to plating-out lubrication. Following our results, we elucidated the oleosome lubrication mechanism and showed their possible control by changing the membrane composition.

Study on the Lubricating Characteristics of Graphene Lubricants

Graphene is considered a good lubricant additive. The lubricating properties of graphene lubricant at different concentrations and temperatures are studied via a four-ball friction and wear-testing machine. The results show that the coefficient of friction (COF) and wear scar diameter (WSD) of the steel ball with 0.035 wt% graphene lubricant decreased by 40.8% and 50.4%, respectively. Finally, through surface analysis, the following lubrication mechanism is proposed: as the added graphene particles can easily fill and cover the pores of the friction surface, the contact pressure of the rough peak is reduced, resulting in a lower COF and smoother surface. Although the COF increases with temperature, graphene lubricants still exhibit good lubrication effects.

Bioinspired Hard–Soft Composite Scaffold with Excellent Lubrication and Osteogenic Properties for the Treatment of Osteochondral Defect

AbstractNatural articular cartilage is a typical self‐healing and superlubrication system capable of maintaining extremely low friction under physiological loadings. Cartilage wear and accidental trauma can cause irreversible defects to cartilage and subchondral bone with a significant decrease in intra‐articular lubrication, leading to the development of severe osteoarthritis and osteochondral defect. To address the important clinical problem of osteochondral defect, a bioinspired hard–soft (PEEK‐lubrication hydrogel) composite scaffold is designed and developed. The polymerization of polyethylene glycol diacrylamide (PEGDAA) and 2‐methacryloyloxyethyl phosphorylcholine (MPC) on polyetheretherketone (PEEK) substrate is achieved by UV initiation to form a strong interfacial bonding, and nano‐hydroxyapatite is deposited on porous PEEK substrate via polydopamine coating to improve osteogenic capability. Accordingly, the composite scaffold is successfully developed with lubrication and osteogenic activity. The tribological tests show that the lubrication performance of the composite scaffold is based on the hydration lubrication mechanism of the upper hydrogel layer, and the in vitro and in vivo experiments demonstrate that the composite scaffold is endowed with excellent biocompatibility and bioactivity. In conclusion, the bioinspired strategy for preparing a hard–soft composite scaffold shows a promising way in the treatment of osteochondral defect and provided a guideline for designing functional PEEK‐based biomaterials in tissue engineering scaffolds.

Accelerating Macroscale Superlubricity through Carbon Quantum Dots on Engineering Steel Surfaces

AbstractMacroscale superlubricity on engineering steel surfaces offers a promising solution for minimizing friction and wear in engineering applications. However, achieving superlubricity typically requires a long running‐in period, which may result in significant wear for the friction pair. Herein, a new lubricant with superlubricating properties is rationally designed by using polyethylene glycol (PEG) and critic acid (CA) under complexing effect with a running‐in period of about 800 s. Importantly, the introduction of carbon quantum dots (CQDs) obtained from the pyrolysis of CA into PEG aqueous solution shortens the running‐in period for achieving macroscale superlubricity (µ ≈ 0.005) between steel/steel contact to 44 s. The corresponding wear rate (1.15 × 10−7 mm3 N−1 m−1) on the steel disk is reduced by 77% due to the shorter running‐in time. Furthermore, the surface analysis combined with the molecular dynamics simulations demonstrates that CQDs easily adsorb on the surface of the friction pair, forming a carbon film that reduces interaction energy between the lubricant molecules and the substrate. This work provides new insights into the lubrication mechanism of CQDs and contributes to the design of liquid superlubricants with short running‐in periods and low wear rates on engineering steel surfaces.

Tuning fluorine content of fluorinated graphene by an ionothermal synthesis method for achieving excellent tribological behaviors

Fluorinated graphene (FG) has been an excellent lubricant additive due to its wider layer spacing and weaker interlayer forces, and FG with different fluorine content can exhibit different tribological performance. Herein, an ionothermal synthesis method was provided to prepare FG nanosheets with different fluorine content. Under the dual function of the ionic liquids ([C16C1im][Br]) and the temperature, the [C16C1im][Br] can work well as both an intercalation agent and polar catalyst, thus achieving tuning fluorine content of FG. Surprisingly, due to the chemical stability of [C16C1im][Br], the [C16C1im][Br] used in the exfoliated process of FG can be recycled and reused at least 5 times and still remain favorable exfoliation effect. Moreover, it can be obtained that friction coefficient and wear track width are reduced by 53.1 % and 52.8 % through adding FG nanosheets into PAO4 oil, the function generated by breaking of C–F bonds can form an adsorbed film on the friction surface, thus offering a synergistic lubricant mechanism with formation of FG tribo-film. Therefore, the FG dispersed into PAO4 oil can greatly enhance the tribological behaviors during the friction process.

Synergistic effect of sodium dodecyl sulfate (SDS) and oleic acid (OA) on gelation and lubrication performance

In this paper, it was found that with the addition of sodium dodecyl sulfate (SDS, a hydrophilic surfactant) and oleic acid (OA, a lipophilic surfactant) in a solvent with a proper composition ratio, a stable gel could be obtained via a simple ultrasonic treatment. The synergistic gelation of OA–SDS was verified in a series of solvents with different polarities, such as white oil (WO), LB-2000, n-hexane, polyalphaolefin (PAO10), and deionized water. The Fourier transform infrared (FTIR) analysis confirms that the gelation of OA–SDS is highly related to the ion exchange reaction between them. However, the xerogel of OA–SDS gelator in deionized water and WO (OSW) exhibited remarkably different morphologies, indicating that the gelation is based on different molecular assembly processes, which is governed by the orientation of the two surfactant molecules in liquid media. Further characterizations of OSW gel presented that the synergistic effect between OA and SDS not only existed in gelation, but also in enhancing the mechanical and thermal stability of WO, and endowing the OSW gel with reliable friction reduction and antiwear performances under variable working conditions. Subsequently, the lubrication mechanism was proposed based on rheological analysis and the morphological and compositional analysis of wear surfaces. This study provides a novel idea for the molecular design of lubricating gelators with wide adaptability to solvent polarity and working conditions.

Lean-water electrolyte to stabilize zinc anode and suppress manganese dissolution of cathode for ampere-hour zinc batteries

Rechargeable Zinc battery technologies are regarded as viable candidates for large-scale energy storage due to constitutional safety and inexpensive position. Nevertheless, the strong polarity of H2O molecules initiates the deterioration of both zinc metal anode and cathode. At anode side, radical H2O molecules cause side reactions of zinc corrosion, hydrogen evolution reaction (HER) and dendrite growth. At cathode side, the dissolution of transition metal brings about severe capacity fading of cathodes. Herein, we replace the 90% H2O sheath of Zn2+ solvation with 1,3-propanediol (PDO) molecules to form a lean-water electrolyte, in which the abundant PDO molecules can significantly suppress 99.9% HER and prohibit the Zn dendrite growth, while H2O molecules with a molecular lubrication mechanism enables fast ion transportation. Consequently, at anode side, Zn plating/stripping reversibly processes over 4000 cycles with high average Coulombic efficiency of 99.24% in ZnǀǀCu cell. At cathode side, the decreased water activity and scarce solute-water dissolving surfaces suppress 98.6% dissolution of manganese species. The ZnǀǀMnHCF full cell demonstrates high capacity of 140.7 mAh·g−1 at 0.1 A·g−1 and ultra-long lifespan of 3000 cycles with 85.4% initial capacity retained. More importantly, a 1.4 Ah ZnǀǀMnHCF pouch cell shows excellent lifespan over 150 cycles with charge/discharge run time > 68 days. We expect that this work will promote the practicability of aqueous Zn-ion batteries for grid-scale energy storage.

A hybrid data-driven approach for the analysis of hydrodynamic lubrication

The application of data mining technology has intensively advanced tribology research. While recent lubrication studies have highlighted the importance of data mining, researchers have not fully bridged the gap between massive lubrication data and intrinsic lubrication mechanisms. Thus, by revisiting lubrication modelling from the data-driven and physics-informed perspectives, we aim to construct a hybrid approach for hydrodynamic lubrication classification and prediction, where data-driven methods are combined with physics-informed approaches to achieve a fast and accurate prediction of the hydrodynamic lubrication scenario. Our approach will spur the application of data mining methods in lubrication studies.

Lubrication mechanics of broach with texture and fiber

Lubrication mechanics of broach with texture and fiber

Cellulose-armored “CNTs-soft metal” hybrid nanomaterials to improving the friction-reduction and anti-wear performance of bio-lubricants

Surface-inert carbon nanotubes (CNTs) exhibit poor interfacial compatibility with bio-oils and are prone to aggregation and precipitation which hinders their lubricity and anti-wear properties. Herein, polydopamine (PDA) was introduced to the surface of CNTs (CNTs-PDA) and constructed nanocellulose "armor" on the CNTs-PDA surface through hydrogen bonding effect (from amino and hydroxyl groups) to enhance the interfacial compatibility of carbon nanotubes with vegetable oils. The cellulose-armored “CNTs-soft metal” hybrid nanomaterial was used as a lubricant additive in sunflower oil, reducing the friction coefficient and wear rate of engineered steel surfaces by 61.9% and 79.5%, respectively. Furthermore, the tribo-chemical changes and lubrication mechanisms occurring at wear scars were revealed by the wear analysis, Raman spectrum, and lubrication theory calculations.

Correlation between viscoelastic response and frictional properties of hydrated zwitterionic polymer brush film in narrowing shear gap

HypothesisA hydrated 2-methacryloyloxyethyl phosphorylcholine (MPC) polymer brush exhibits exceptional lubricity. This lubrication mechanism has traditionally been attributed to either the inherent fluidity of the brush or the water film that forms owing to its hydrophilic nature. Given previous findings that the frictional properties of the MPC polymer brush film show load dependence, we hypothesize that the lubrication mechanism can be elucidated by examining the shear gap (varies owing to the load) dependence of the brush’s viscoelastic response. ExperimentsMPC polymer brush films with different thicknesses were prepared. Their viscoelastic responses were evaluated across different shear gap widths, and the frictional properties were subsequently compared across states with distinct viscoelastic behaviors. FindingsThe observed shear viscoelasticity demonstrated a clear gap dependence that correlated with frictional attributes. Our data suggests that the lubrication mechanism shifts based on the shear gap. Specifically, two states exhibited low coefficients of friction: one where the osmotic pressure supports the load while allowing flexible deformation of the brush film, and the other where the brush film undergoes compression and transitions to a fully elastic state.

Self-assembled hybrid phosphate nanoflowers as oil-based lubricant additive: Interfacial adsorption and lubrication mechanism

In this work, a self-assembled hybrid phosphate nanoflower (HPN) containing Ba3(PO4)2 and NaBaPO4 was synthesized via BaCO3 primary crystals inducement. A ball-on-disc reciprocating tribometer was used to explore the tribological behavior of HPN as an additive for polyalphaolefin 8 (PAO8) on titanium alloy. Compared with the neat PAO8, after being lubricated with PAO8 containing HPN, the friction coefficient and wear rate decreased by 74.98% and 99.89%, respectively. The tribofilm at the friction interface was characterized and conformed by SEM, EDS, XPS, and cross-sectional TEM. Asan additive toPAO8, HPN can participate in the formation of tribofilm and exhibit superior friction-reducing and anti-wear properties for titanium alloy. It was demonstrated that due to the P-O-Ti bonds, HPN can easily adsorb and deposit at the friction interface to form a tribofilm against wear. Besides, the simulation experiments showedthat the repulsive force at the solid-liquid interface between HPN and oil molecules is the key to friction reduction and lubrication, and the comparative tribological experiments of different types of base oils were performed to verify the results of molecular dynamics.

Recent Progress on the Tribological Applications of Solid Lubricants

Abstract Nonrenewable energy has produced abundant waste during tribological applications because a large portion of energy has been consumed to overcome friction and wear. Solid lubricants have recently aroused significant interest due to their defined friction and wear properties. Despite enormous efforts on solid lubricants, their important contributions to coatings, bulk materials, oil/grease, and super-lubricity have not yet been fully evaluated. This paper discusses in detail the present status of solid lubricants as effective reinforcements in tribology. It begins with the introduction of various descriptions and advanced structures of solid lubricants. Afterwards, it discussed their applications on improving friction properties in coatings and bulk materials. Additionally, lubrication mechanisms of solid lubricants in oil/grease are highlighted, followed by the detailed discussion of super-lubricity for solid lubricants. Finally, this review concludes final outlooks on the main challenges and future directions in this key area.

Numerical investigations on the lubrication characteristics of involute harmonic gears

PurposeHarmonic gears always work under different operating conditions and may usually break down due to lubrication failures, while its lubrication mechanism is still not clearly understood. This paper aims to present a lubrication model comprehensively considering the influence of contact geometry, lubrication properties and three-dimensional (3D) real surface roughness to analyze the lubrication performance under different conditions.Design/methodology/approachBased on the discrete convolution-fast Fourier transformation with duplicated padding and quasi-system numerical methods, the lubrication model for harmonic gears is developed, which is verified by comparing results with available lubrication data.FindingsThe effects of meshing process, working conditions and 3D roughness on the lubrication characteristics are discussed. From the calculated cases, the increase in rotational speed and decrease of applied torque may increase the film thickness, enhancing the lubrication performance of harmonic gears. It is also observed that proper surface roughness can be used for lubrication design.Originality/valueThe research results can provide theoretical guidance for improving lubrication performance and reducing friction/wear of the harmonic gear interfaces. This study can be promoted to various engineering scenarios of harmonic gears, such as industrial robots, space-driven agencies and precision measuring instruments.