Lubricant Film Research Articles

LUBRICATION OF OFFSHORE MECHANICAL COMPONENTS: TOWARDS SUSTAINABLE & RELIABLE POWER PRODUCTION

As different wave and tidal energy generators advance towards the commercial deployment phase, addressing potential issues related to the lubrication of the machine components integrating the Power Take Off system (PTO) due to the harsh operating conditions encountered in the marine environment becomes even more essential. Environmentally Acceptable Lubricants (EALs) based on water-soluble polymers are proposed as a way to reduce friction and wear of the mechanical components in wave and tidal energy generators. Although these fluids have the advantage of being biodegradable and non-toxic, they have not shown to be as effective as other synthetic fluids, or mineral oils in preventing corrosion, severe friction, and wear, thus increasing the risk of moving parts experiencing premature failure.
 This work explores the potential of different water-soluble polymers to be used in the formulation of EALs that can meet the strict environmental regulations while providing effective protection against wear, and corrosion in offshore operating conditions. To evaluate the potential of these lubricants as an alternative to replace conventional mineral oils, different polymers were analyzed from the point of view of their ability to form an effective full lubricant film that can keep separation between the contacting surfaces, mitigate wear, and prevent corrosion. The rheological properties of these polymers were also studied at different concentrations in order to optimize the performance in the application. The hydrodynamic film build-up properties of EALs formulated with water-soluble polymers with different molecular weight, concentration, and viscosity are reported. The corrosion resistance exhibited by steel components when exposed to the different formulations compared to seawater was an object of examination. The study also aimed to establish correlations between the lubricant film-build up properties, viscosity, and electrical impedance.
 The results showed that high molecular weight polymers can form a separating film at relative high pressure in the low-speed region even at low polymer concentrations. While with the increasing speed, the fluid viscosity becomes more important to sustain a full film between contacting surfaces. With the increasing concentration of polymer in the aqueous solution the open circuit potential (OCP) becomes more negative indicating the deterioration of the steel corrosion resistance.
 The results provide new insights into the design of EALs that can effectively protect the mechanical components of wave and tidal energy generators while minimizing environmental impact. The findings suggest that water-soluble polymers are a promising solution for offshore applications, as they can provide efficient full film lubrication, mitigate wear, and prevent corrosion. These polymers can help to improve the performance and lifespan of offshore power generators while minimizing the environmental impact.

Hydrodynamic analysis of journal bearing considering the effects of cavitation and minuscule particle

Journal bearing is the general element in the mechanical systems, which can provide the rotational motion of joints. The hydrodynamic analysis model of journal bearing is established and the effects of cavitation and minuscule particle are considered in the simulation model. The cavitation effects would be represented by the mixture model. Based on the CFD and DPM technology, the mixture lubrication medium of journal bearing is developed, which includes the minuscule element of solid in the lubrication film. And, the effects of structural parameters and operation conditions on the hydrodynamic characteristics of journal bearing is investigated. The simulation results could provide the suggestion for the design of structure and working condition.

Influence of hardness of nanoparticle additive in PTFE solid lubricant on tribological properties of GCr15 steel with bionic texture

The surface microstructure of biomimetic Procambarus clarkia claw was prepared on the surface of GCr15 steel, then PTFE solid lubricants dispersed with three different hardness nanoparticles (C, ZnO, Al2O3) were filled in. The results of the tribological experiment show the textured surface lubricated by PTFE with Al2O3 nanoparticles reflected the best performances. Because PTFE stored by the bionic texture could be supplied to the friction surface to form a solid lubrication film. Due to the addition of Al2O3 nanoparticles, the hardness and wear resistance of the film are improved, thus avoiding a large loss of PTFE. And the process of "rolling-embedding PTFE surface-exposing-rolling" occurs. As a final, the surface tribological properties of GCr15 steel can be significantly improved.

Microstructure, mechanical and tribological properties of multilayer graphene reinforced WC-10Ni3Al prepared by microwave sintering

The multilayer graphene (MLG) reinforced WC-10Ni3Al cemented carbide was prepared by microwave sintering. The effects of the sintering temperature and MLG content on the microstructure and mechanical properties of the WC-10Ni3Al cemented carbide were investigated. The promotion effect and strengthening mechanism of MLG on WC-10Ni3Al by microwave sintering were explored. The tribological properties of WC-10Ni3Al and Ti6Al4V(TC4) were studied, and the wear mechanism of WC-10Ni3Al was revealed. The experiment result show that the WC-10Ni3Al-0.5 wt.% MLG sample sintered at 1450 °C had its greatest mechanical properties with a relative density of 96.9%, hardness of 1430 HV, flexural strength of 1410 MPa and fracture toughness of 15.7 MPa mm1/2, which were 4.2%, 12.2%, 24.4%, and 29.6% greater than those of the sample without MLG added, respectively. The reinforcing and toughening mechanisms of adding MLG in WC-10Ni3Al samples consist of fine crystal strengthening, crack bridging and deflection, MLG pulling out and bridging. The results of friction experiment show that the friction coefficient and wear rate of the WC-10Ni3Al samples were lower than those of YG8 (WC-8 wt. % Co cemented carbide). The WC-10 Ni3Al-0.5 wt.% MLG sample obtained the lowest friction coefficient of 0.35 and the lowest wear rate of 1.69 × 10−5 mm3N−1m−1. A stable solid lubrication film was produced on the friction surface of the WC-10Ni3Al samples, which was conducive to reducing the breakage and wear of WC grains. MLG can enhance the stability of solid lubrication film and reduce its friction coefficient.

Thermal characteristics and distribution rule of lubrication film of full ceramic ball bearing under different service condition

PurposeThe study aims to the distribution rule of lubricating oil film of full ceramic ball bearing and improve its performance and life.Design/methodology/approachThe paper established an analysis model based on the fluid–solid conjugate heat transfer theory for full ceramic ball bearings. The distribution of flow, temperature and pressure field of bearings under variable working conditions is analyzed. Meanwhile, the mathematical model of elastohydrodynamic lubrication (EHL) of full ceramic ball bearings is established. The numerical analysis is used to study the influence of variable working conditions on the lubricant film thickness and pressure distribution of bearings. The temperature rise test of full ceramic ball bearing under oil lubrication was carried out to verify the correctness of simulation results.FindingsAs the speed increased, the oil volume fraction in full ceramic ball bearing decreased and the surface pressure of rolling element increased. The temperature rise of full ceramic ball bearings increases with increasing speed and load. The lubricant film thickness of full ceramic ball bearing is positively correlated with speed and negatively correlated with load. The pressure of lubricating film is positively correlated with speed and load. The test shows that the higher inner ring speed and radial load, the higher the steady-state temperature rise of full ceramic ball bearing. The test results are in high agreement with simulation results.Originality/valueBased on the fluid–solid conjugate heat transfer theory and combined with Reynolds equation, lubricating oil film thickness formula, viscosity temperature and viscosity pressure formula. The thermal analysis model and EHL mathematical model of ceramic ball bearings are established. The flow field, temperature field and pressure field distribution of the full ceramic ball bearing are determined. And the thickness and pressure distribution of lubricating oil film in the contact area of full ceramic ball bearing were determined.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-05-2023-0126/

Effect of droplet size on the film formation of O/W emulsion

PurposeThe purpose of this study is to reveal the underlying mechanism in film formation of oil-in-water (O/W) emulsion.Design/methodology/approachThis study focuses on the film forming characteristics of O/W emulsion between the surface of a steel ball and a glass disc coated with chromium. The lubricant film thicknesses of O/W emulsion with various mechanical stirring strength were discussed, which were observed by technique of relative optical interference intensity.FindingsThe authors directly observed the oil pool in the contact area, finding the size of oil pool was closely related to the film-forming ability of emulsion. Enrichment phenomenon occurs in oil pool, which was caused by phase inversion. Further investigations revealed that the emulsion is stable with strong stirring strength, resulting in a smaller oil pool size and worse film forming ability.Originality/valueWith the wide usage of O/W emulsion in both biological and industrial systems, the ability of emulsion film formation is considered as an important factor to evaluate the lubrication effectiveness.Peer reviewThe peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-12-2022-0354/

Comparative Study of the Tribological Properties of CFRPEEK, Stainless and Carbon Steel in High Water-Based Fluid

The advantageous physical and chemical properties of carbon-fiber-reinforced polyetheretherketone (CFRPEEK) make it a suitable friction pair for water hydraulic components. To explore the selection of friction pair materials, this paper investigated the tribological properties of CFRPEEK, stainless and carbon steel in water and high water-based fluids, and evaluated them using reciprocating friction and wear tester. After the test, the wear surface morphology of the contact surface was observed under an electron microscope. The results show that the degree of friction between specimens is dependent on the load and frequency. Meanwhile, both 316L and CFRPEEK have good tribological properties in both mediums. Microscope observation analysis showed that CFRPEEK surfaces easily absorb water to create a water film, thus improving its performance and reducing wear. Additionally, the friction between stainless and carbon steel in high water-base fluid will generate a lubricating film, the surface of which shows excellent tribological properties. The final conclusion was that the choice of material for plunger pair material depends on the medium, with 304-316L being more suitable for high water-based fluids, while CFRPEEK-316L is more suitable for water.

Tribological properties of high-speed steel surface with texture and vertical fibers

Inadequate lubrication of the two touching surfaces during friction can lead to severe wear, especially in metal cutting. Therefore, a surface with synergistic anti-friction effect of texture and solid lubricant was proposed to improve lubrication. A mesh texture with excellent wettability was prepared on the high-speed steel (HSS) surface by laser, and then nylon fibers were vertically implanted into the grooves of the texture using the electrostatic flocking technology. The friction and wear state of different surfaces (smooth, textured, flocking) under dry/oil-lubricated were studied by a linear reciprocating wear tester. The coefficient of friction (COF) under different working conditions was used to analyze the anti-friction properties, and the wear rate was used to evaluate the wear resistance of the surface. The results showed that the tribological properties of flocking surfaces were better than those of the other two surfaces. This is because the addition of nylon fibers eases shear at the edges of the texture. The broken fibers form a solid lubricating film on the specimen surface, which prevents the surface from being scratched by debris. In addition, it is found that COF decreases with increasing load. Finally, the rapid wettability of the oil droplets on the flocking surface shows the great potential of the surface for lubrication and anti-friction.

Tribological Performance and Model Establishment of Self-Compensating Lubrication Film Inspired by the Functional Surfaces of Scapharca subcrenata Shells

Composite surface structures inspired by the functional surface of Scapharca subcrenata shells can improve the tribological properties effectively, composed of the ordered “U”- shape micro-grooves and solid lubricant Sn-3.0Ag-0.5Cu (SAC305) alloys. A series of wear tests were conducted to further investigate the formation characteristics of the self-compensating lubrication film, and then the mathematical model of the spreading tribofilm could be proposed. The results showed that the appropriate surface texturing parameters (NBCSS-28) had a great effect on the formation of the self-compensating lubrication film, which exhibited a lower friction coefficient (0.386) and wear volume (0.682 mm3) than the other NBCSS samples. The tribofilm, with a thickness of a few microns, was deposited on the contact surface after the wear tests. The interfacial reactants (the Ni/Ni3Sn2 interface) of the SAC305 alloys, and Ni3Al alloys confirmed by the wetting experiments and the thermogravimetric analysis, could promote the deposition and diffusion of the tribofilm during the sliding process. Hence, distinguishable layered structures could be observed on the fractured surfaces of the NBCSS samples. Moreover, the formation process of the tribofilm exhibited an obvious relationship with the reduction in the dynamic friction coefficient. The tribofilm formation model was proposed by the accumulation behaviors of the spreading tribofilm randomly in the form of multiple discrete irregular film shapes on the worn surface, which could predict the formation characteristics of the self-compensating lubrication film to improve the optimization design of the parameters.

Assessing the fatigue wear effect on traction coefficient and dynamic performance of roller bearing

Rolling element bearings are commonly employed in rotating machinery to support rotating shafts. These machinery elements are vulnerable to failure due to increased friction and heat, which cause progressive wear on the rolling interaction surfaces of the bearing. The failure of bearings causes an unpredicted shutdown of equipment, leading to an increase in downtime and financial losses. Various condition monitoring techniques have been developed for periodic assessment of failure in roller-bearing systems. The focus of this work is to establish a relationship between surface fatigue wear, film thickness [Formula: see text], asperity load ratio [Formula: see text], lubricant film stiffness [Formula: see text] and traction coefficient [Formula: see text] parameters. A periodic assessment of the above-mentioned parameters was made on the test bearing, which was subjected to a fatigue test for 2000 h. The results acquired from the experimental study highlighted that, as surface fatigue wear on the bearing interaction surfaces and lubricant degradation occurred with an increase in fatigue load cycles, the asperity load ratio [Formula: see text] and traction coefficient [Formula: see text] parameters showed a gradual rise in trend which further resulted in a difference in the dynamic behaviour of the bearing system.

Lubricity and mechanism of catechol-based biomimetic lubricant in water-based drilling fluid

In this study, the lubrication performance and underlying mechanism of catechol-based biomimetic lubricants in water-based drilling fluids were investigated. A catechol-based biomimetic lubricant, L3,4, was synthesized by acid and oleyl alcohol utilizing the strong underwater adhesion of DOPA. Besides, the significant influence of phenolic hydroxyl group positioning on the adhesion performance, and consequently, the lubrication and wear resistance properties of the lubricants were revealed. Lubrication performance and wear resistance were evaluated through extreme pressure lubrication testers, four-ball friction machines, and scanning electron microscopy. In bentonite, L3,4 exhibited the best lubrication performance, with a low coefficient of friction (COF) of 0.06 at a 1% addition, a wear scar diameter of 0.365 mm, and a temperature resistance of 210 ℃. In contrast, L3,5 showed moderate lubricity with a COF of 0.16, while L2,5 showed no lubrication capabilities. X-ray photoelectron spectroscopy was used to analyze the composition and thickness of the surface lubrication film, revealing that the phenolic hydroxyl structure enhances the adhesion ability of lubricants on metal surfaces. L3,4, containing a catechol structure, formed a dense organic film of thickness exceeding 100 nm on the metal surface through bidentate metal-ligand bonds; L3,5, with a meta-hydroxy structure, formed a lubrication film of thickness lower than 80 nm through hydrogen bond adsorption; and L2,5, with a para-hydroxy structure, formed a lubrication film of thickness lower than 20 nm. L3,4 addressed the inadequate lubrication exhibited by ester-based lubricants in water, attributed to their insufficient adhesion, which prevented the formation of an effective lubrication film on the surface of drilling pipe.

Construction of the Stable Lubricant Film on One-Dimensional Cone-Structured Surfaces for Directional Liquid Transport.

Directional liquid transport along one-dimensional structures finds vast applications in fog harvest, liquid separation, sensing, chemical synthesis, and numerous other scenarios. Dynamically, the liquid transport speed is boosted by the driving force and retarded by the hysteresis from the liquid/substrate interface. A capillary force-relied lubricant film or a covalently attached polymer brush on the surface could increase liquid mobility temporarily by reducing the interfacial hysteresis. However, the easy depletion of the lubricant film and the stringent restriction of the substrate severely hamper droplet's directional transport in a long run. Herein, we report a feasible and durable hysteresis reduction design with the polymer-brush stabilized lubricating surface (PBSLS). The PBSLS is achieved through incorporating liquid-like polydimethylsiloxane brushes (b-PDMS) and the liquid PDMS oligomer (o-PDMS). The covalent attached b-PDMS "locks" the lubricant oil o-PDMS to the cone surface via strong intermolecular van der Waals force in between. The PBSLS on the cone surface can be sustained under constant shearing force from liquid transport for at least 6 h. A cone with such PBSLS shows increased ability of droplet transport and enhanced fog collection performance in the long run. This design supplies an effective way toward regulating macro-level interfacial performance through surface design on the molecular level.

Ultrasonic measurement for lubricant film thickness with consideration to the effect of the solid materials

An improved ultrasonic reflection coefficient phase model is proposed for lubricant film thickness measurement with consideration to the influence of the solid layers. In the model, the theoretical relation of the lubricant film thickness to ultrasonic reflection coefficient phase is established, which is used for effectively determining the lubricant film thickness. Furthermore, the proposed model is verified by both the finite element analysis and experiments. The results show that the lubricant film thickness ranges between 2 μm and 45 μm can be measured by using the proposed model, which covers both the spring model region and the blind region. In addition, at a small film thickness, the reflection coefficient phase is significantly affected by the acoustic impedances in both solid layers. However, the reflection coefficient phase is dominantly affected by the solid layer in the neighbourhood of the incidence source at a large film thickness.

Tribological performance and lubrication mechanism of carbon nitride nanosheets as novel and high-efficiency additives for water lubrication

Recently, water-based nano-additives have attracted more attention with the resurrection of water lubrication owing to the increasing awareness of environmental protection and energy-saving. Highly water-dispersible carbon nitride nanosheets (C3N4 nanosheets), obtained via a high-yield and low-cost hydrothermal approach, were first used as a novel and environmentally-friendly nano-additive for water-based lubrication. The tribotest results exhibited that the tribological performances of C3N4 nanosheets in the water-based lubricant are highly sensitive to their adding concentrations. It was found that a C3N4 nanosheet concentration of 0.2 wt% could decrease the friction and wear in steel-steel contacts by 49.0% and 54.2%, respectively. The lubrication performances of C3N4 nanosheets as additives were much better than those of C3N4, attributed to their regular shape, good water dispersibility, and high reactivity. In addition, C3N4 nanosheets as additives exhibited a long service life even under harsh lubrication conditions. In addition, the molecular dynamics (MD) simulation technique was first employed to study the lubrication performances and lubrication regimes of C3N4 nanosheets. By combining the results of MD simulations and wear track surface analyses, the formation of water-based lubricating film and tribochemical film (mainly composed of organic compounds, metal nitrides, iron oxides, metal carbonates, and C3N4 nanosheets) coupled with the interlayer sliding and polishing effects of C3N4 nanosheets were primarily responsible for the excellent tribological performances of C3N4 nanosheets.

Liquid superlubricity of lubricants containing hydroxyl groups and their aqueous solution under rolling/sliding conditions

Macroscale rolling/sliding conditions are in the superlubricity, a little-studied topic so far. The purpose of this paper is to examine the formation of elastohydrodynamic lubrication (EHL) films by water-based lubricants (glycerol and polyethylene glycol (PEG)), providing superlubricous friction. Experiments were carried out on an optical ball-on-disc tribometer under rolling/sliding conditions. The film thickness was measured by the thin film colorimetric interferometry, and the viscosity of liquids was measured by rotational and high-pressure falling body viscometers. The results show that tribochemical reactions are not the mandatory reason for friction to reach the superlubricity level when using the water-based lubricants. The studied liquids themselves are almost Newtonian. With the addition of water, the signs of shear thinning behavior disappear even more. Suitable conditions for this type of lubricant can be predicted using the known Hamrock–Dowson equations. An anomaly in the thickness of the lubricants was observed as an abrupt change at certain conditions. The more PEG there is in the lubricant, the higher the thickness at the beginning of the jump.

Friction reactions induced by selective hydrogenation of textured surface under lubricant conditions

The passivation of hydrogen atoms and the conformation of textured surfaces under oil-lubricated conditions are effective strategies to obtain amorphous carbon (a-C) films with extremely low friction. It is critical to understanding the influence mechanism of selective surface hydrogenation on the tribological behaviors of textured a-C film under oil-lubricated conditions. In particular, the interactions of hydrogen atoms and lubricants are confusing, which is enslaved to the in situ characterization technique. The reactive molecular dynamics (RMD) simulations were conducted to analyze the friction response of textured a-C films with selective hydrogenation surfaces under oil-lubricated conditions. The results indicate that the existence of hydrogen atoms on specific bump sites significantly decreases the friction coefficient (μ) of textured a-C film, which is highly dependent on the surface hydrogen content. The repulsion between hydrogen atoms and lubricant molecules prompts the formation of a dense lubricant film on the surface of the mating material. Interestingly, with the enhancement of the surface hydrogen content, the passivation of the friction interface and the repulsion between hydrogen atoms and lubricants play dominant roles in reducing the friction coefficient instead of hydrodynamic lubrication.

Atomic-scale insight into molecular adsorption of sodium dodecyl sulfate (SDS) on ideal bcc-Fe surface: A first-principles investigation

Adsorption is the most significant first step for boundary lubrication film formation, catalysis and chemical synthesis processes. Nevertheless, at present, the adsorption process and its mechanism of dodecyl sulfate (DS) molecules on solid surfaces are largely conjectured. Here, adsorption properties of DS on clean ideal bcc-Fe surfaces have been computational investigated via first principles method at atomic scale. At ground state, adsorption energies Eads are ranged from -11.75 eV to -3.09 eV. Adsorption Fe-O bond is a mixture of ionic bond and covalent one. Bond strength is affected by interfacial electron transfer and its stability is related to electron gain/loss of sub-interfacial atoms. Boltzmann distributions of adsorption conformations are calculated by partition functions that only depend on relative energies, and they are used as weight coefficients to estimate mixed-Eads. The temperature-induced Fermi-softness SF is calculated to reveal relationship between Eads and SF. Considered about solvent and temperature effects, the competitive adsorption and thermal activation can reduce the difference of adsorption energy barriers between conformations. Their adsorption probabilities become closer with the distribution proportion increase of the conformations with low Eads. This can provide theoretical foundations for further understanding of boundary lubrication film formation mechanism.

Predictions of friction and temperature at marine cam-tappet interface based on mixed lubrication analysis with real surface roughness

The working conditions of cam-tappet pairs in marine diesel engines are directly influenced by the engine output power, the operational speed, the temperature, as well as the components surface micro-morphology, etc., which cause the cam-tappet pairs work in the mixed lubrication state, thus the interfacial friction, pressure and temperature rise are vital to engine performance, efficiency, and durability. An interfacial friction–temperature prediction model for the cam-tappet pairs, considering the effects of transient working conditions and the real surface roughness, is developed in the present study, based on the theories of the transient mixed EHL and the heat transfer under the condition of a fast moving heat source. The numerical results of surface temperature rise are compared with those from the Blok formula, and a good agreement is found. The obtained results show that the presence of 3D roughness may lead to a decrease in the lubricant film thickness, and the surface temperature rise of tappet may exceed 700 K, which is close to the material scuffing temperature, causing the surface failure due to scuffing and wear. If increasing the cam speed and base circle radius within certain range, it may lead to the increment of film thickness and reduction of surface temperature rise, thus the lubrication effectiveness is increased. In addition, using cast aluminum bronze may significantly reduce the surface temperature rise and improve the interfacial characteristics.

Hydration layer structure modulates superlubrication by trivalent La 3+ electrolytes

Water-based lubricants provide lubrication of rubbing surfaces in many technical, biological, and physiological applications. The structure of hydrated ion layers adsorbed on solid surfaces that determine the lubricating properties of aqueous lubricants is thought to be invariable in hydration lubrication. However, we prove that the ion surface coverage dictates the roughness of the hydration layer and its lubricating properties, especially under subnanometer confinement. We characterize different hydration layer structures on surfaces lubricated by aqueous trivalent electrolytes. Two superlubrication regimes are observed with friction coefficients of 10 −4 and 10 −3 , depending on the structure and thickness of the hydration layer. Each regime exhibits a distinct energy dissipation pathway and a different dependence to the hydration layer structure. Our analysis supports the idea of an intimate relationship between the dynamic structure of a boundary lubricant film and its tribological properties and offers a framework to study such relationship at the molecular level.

Experimental investigations on the friction behavior of partially femtosecond laser-textured journal bearing shells

Journal bearings are the most commonly used load elements in rotating systems and have a long history in tribology research. However, the relatively new surface modification technique of femtosecond laser texturing has rarely been used to increase the efficiency of journal bearings. We used a 300-fs laser to create chevron-shaped microstructures in different circumferential texture locations on half-bearing shells to investigate the effect of partial texturing on the tribological performance of journal bearings. With a customized shaft-bearing shell setup, the coefficient of friction and lubrication film thickness was tracked in two load scenarios at varying rotational speeds. In fully film conditions, the findings demonstrate that textures in the high-load region of journal bearings provide the highest lubricant film thickness and lowest friction compared to textures in the pressure build-up, divergent zone, and unstructured reference. Moreover, surface textures in the high-load area also perform better in boundary and mixed lubrication. Experimental data have also shown that asymmetric texture designs have a detrimental effect on lubrication performance. In contrast to symmetric designs, the lubrication film build-up is hindered. The research results represent a step toward transferring laboratory findings on laser surface texturing for tribological optimization in real applications.