Lubrication Mechanism Research Articles

The effects of medium and friction pair on the tribological behavior of Mo-doped DLC films

Diamond-like carbon (DLC) films are widely used to improve the tribological properties of key components in internal combustion engines. However, their performance can be significantly affected by the service environment, which may impact engine reliability. This study investigates the impact of various media and friction pairs on the tribological behavior of molybdenum-doped DLC (Mo-DLC) films. The results demonstrate that Mo-DLC films exhibit excellent tribological properties across different media, showcasing their adaptability to various conditions. The lubrication mechanism varies with media viscosity: in methanol, Mo-DLC films operate under boundary lubrication conditions, leading to a relatively high wear rate of approximately 5.2 × 10−8 mm3/N·m. Conversely, in diesel and polyalphaolefin-based oil (PAO), where fluid dynamic lubrication occurs, wear rates are significantly lower, at 4.4 × 10−8 mm3/N·m and 3.1 × 10−8 mm3/N·m, respectively. In addition, the friction pair significantly influences the tribological performance of the Mo-DLC film. When paired with D-GCr15 in methanol, Mo-DLC films exhibit a low friction coefficient of 0.09 and a wear rate of 1.6 × 10−8 mm3/N·m, respectively. However, when coupled with N-GCr15, the increased surface roughness extends the running-in period and raises the friction coefficient in methanol. These findings offer valuable theoretical insights and practical guidance for optimizing DLC films in internal combustion engines.

Synthesis and Performance Evaluation of High-Temperature-Resistant Extreme-Pressure Lubricants for a Water-Based Drilling Fluid Gel System.

Addressing the high friction and torque challenges encountered in drilling processes for high-displacement wells, horizontal wells, and directional wells, we successfully synthesized OAG, a high-temperature and high-salinity drilling fluid lubricant, using materials such as oleic acid and glycerol. OAG was characterized through Fourier-transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA). The research findings demonstrate the excellent lubricating performance of OAG under high-temperature and high-salinity conditions. After adding 1.0% OAG to a 4% freshwater-based slurry, the adhesion coefficient of the mud cake decreased to 0.0437, and at a dosage of 1.5%, the lubrication coefficient was 0.032, resulting in a reduction rate of 94.1% in the lubrication coefficient. After heating at 200 °C for 16 h, the reduction rate of the lubrication coefficient reached 93.6%. Even under 35% NaCl conditions, the reduction rate of the lubrication coefficient remained at 80.3%, indicating excellent lubrication retention performance. The lubricant OAG exhibits good compatibility with high-density drilling fluid gel systems, maintaining their rheological properties after heating at 200 °C and reducing filtration loss. The lubrication mechanism analysis indicates that OAG can effectively adsorb onto the surface of N80 steel sheets. The contact angle of the steel sheets increased from 41.9° to 83.3° before and after hot rolling, indicating a significant enhancement in hydrophobicity. This enhancement is primarily attributed to the formation of an extreme-pressure lubricating film through chemical reactions of OAG on the metal surface. Consequently, this film markedly reduces the friction between the drilling tools and the wellbore rocks, thereby enhancing lubrication performance and providing valuable guidance for constructing high-density water-based drilling fluid gel systems.

Tribological properties of oil-impregnated porous PTFE composites using CA as a novel pore-forming agent

PurposeThis paper aims to prepare an oil-impregnated porous polytetrafluoroethylene (PTFE) composite with advanced tribological properties using citric acid as a novel pore-forming agent.Design/methodology/approachCitric acid (CA) was used to form pores in PTFE, and then oil-impregnated PTFE composites were prepared. The pore-forming efficiency of CA was evaluated. The possible mechanism of lubrication was proposed according to the tribological properties.FindingsThe results show CA is an efficient pore-forming agent and completely removed, and the porosity of the PTFE increases with the increase of the CA content. The oil-impregnated porous PTFE exhibits an excellent tribological performance, an increased wear resistance of 77.29% was realized in comparison with neat PTFE.Originality/valueThis study enhances understanding of the lubrication mechanism of oil-impregnated porous polymers and guides for their tribological applications.

Enhancing the Range and Reliability of the Spacer Layer Imaging Method

The spacer layer imaging method (SLIM) is widely used to measure the thickness of additive and lubricant films, in lubricant development and evaluation, and for fundamental research into elastohydrodynamic lubrication and tribofilm formation mechanisms. The film thickness measurement, as implemented on several popular tribometers, provides powerful, non-destructive in-situ mapping of film topography with nanometre-scale height sensitivity. However, the results can be highly sensitive to experimental procedure, machine condition, and image analysis, in some cases reporting unphysical film thickness trends. The prevailing image analysis techniques make it challenging to interrogate these errors, often hiding their multivariate nonlinear behaviour from the user by spatial averaging. Herein, several common ‘silent errors’ in the SLIM measurement, including colour matching to incorrect fringe orders, and colour drift due to the optical properties of the system or film itself, are discussed, with examples. A robust suite of novel a priori and a posteriori methods to address these issues, and to improve the accuracy and reliability of the measurement, are also presented, including a novel, computationally inexpensive circle-finding algorithm for automated image processing. In combination, these methods allow reliable mapping of films up to at least 800 nm in thickness, representing a significant milestone for the utility of SLIM applied to elastohydrodynamic contact.Graphical abstract

Effect of aqueous layer thickness on nano-scratching of single-crystal γ-TiAl alloys

ABSTRACT The study of nano-scratching under water lubrication is helpful to understand the role of lubrication mechanism in the machining of single crystal γ-TiAl alloy. In this paper, through the comparative analysis of nano-scratching under different thicknesses of water layers, it was found that under a water layer thickness of 1 nm, nano-scratching not only improves the surface quality but also achieves multiple reductions in scratch force, stress, temperature and subsurface structural damage. In order to deeply reveal the internal mechanism of workpiece temperature variation under different thicknesses of water layer lubrication, a fitting formula was innovatively adopted to conduct quantitative analysis. In addition, with the increase of the thickness of the water layer, it was observed that the water film effect was continuously enhanced, and this change caused the matrix stress to gradually increase, which hindered the forward motion of the nanoscratch, ultimately resulting in an increase in the friction coefficient. These findings provide valuable insights into the nanoscratching mechanism of monocrystalline γ-TiAl alloy in the presence of water film, and provide a new perspective for understanding the complex mechanism of nanoscratch behaviour under water lubrication, thus promoting the understanding of nanoscale lubrication effects.

Tribological performance and mechanism of graphite, hBN and MoS2 as nano-additives in palm kernel oil-based lubricants: A comparative study

Layered nanoparticles have received considerable interest in tribological applications owing to their outstanding tribological performances, including in vegetable oil-based lubricants. Despite the advantages of vegetable oil-based lubricants over petroleum-based lubricants, the limited manufacturing scale of vegetable oil-based lubricants restricts their more comprehensive implementation in various industries. In order to gain further insights into the exquisite nano-additive properties of layered nanoparticles, this research was conducted to explore the tribological performance of palm kernel oil-based lubricants combined with three different layered nanoparticles, namely graphite, hexagonal boron nitride (hBN), and molybdenum disulfide (MoS2), using a four-ball tribotester. The four-ball tribotester analysis indicated that the wear scars lubricated with graphite, hBN, and MoS2 nanolubricants exhibited a lower coefficient of friction (COF) and wear scar diameter (WSD) than the base lubricant. The findings also showed that the MoS2 nanolubricant had the best friction reduction and anti-wear performance, with a 24.9% drop in COF and a 12.3% drop in WSD. It was followed by the hBN and graphite nanolubricants. To fully understand the lubricating mechanisms involved, the worn surfaces of the tested balls were characterised using various microscopic and spectroscopic methods. Accordingly, the structural changes that occurred during sliding were shown to affect the formation of protective tribofilm. In summary, the findings in this study suggest that the formulated layered nanolubricants possessed exceptional tribological performance, which was contributed by the inherent properties of the individual nanoparticles and the formation of effective tribofilms.

Synthesis and tribological assessment of oil-based nanolubricants blended with nano-zeolite for steel–steel contact

This study aims to synthesize zeolite with nano-sized morphology and investigate the tribological performance of nano-zeolite as a lubricating additive dispersed in an oil-based lubricant. Tribological tests were performed and morphologies of wear tracks were characterized to investigate the friction reduction and anti-wear properties of zeolite, respectively. The lubrication mechanism was discussed by illustrating the effects of concentration and particle size on the tribological behavior of zeolite. The optimal concentration of nano-zeolite to improve the tribological performance of the base oil was 2 wt.%, which exhibited a friction reduction of 34.3%. Zeolite particles with larger diameters showed better dispersion stability but worse lubrication performance, which could be concluded from the decrease in the friction reduction ratio. Moreover, the friction coefficient under the lubrication of zeolite at 200°C was 15.8% lower than that of the pure base oil, demonstrating the thermal stability of zeolite. Notably, the application of zeolite resulted in an increase in wear depth due to the occurrence of the three-body abrasion, while the surface asperities of the counterpart were also smoothed in the meantime. This work demonstrates the potential of nano-zeolite to enhance the tribological performance of oil-based lubricants in industrial applications, which could be attributed to their porous structure, lipophilicity, and chemical–thermal stability.

Synergistic Effect of Microconvex Texture and Particle Medium on the Tribological Property of the Rubber Sliding Interface.

In this study, we examined how surface topography and particle medium interact to affect the tribological performance of rubber sliding interfaces, uncovering the mechanisms of particle lubrication under various conditions. We found that microtextured surfaces, created using a mold transfer method, modestly reduced the friction coefficient of rubber under both dry and lubricated states, primarily by altering the real contact area. Additionally, the presence of different microconvex textures on the surface topography significantly influenced rubber's tribological properties. Our three-dimensional morphological analysis revealed that microtextured rubber surfaces with higher Sa, Sku, and Sal and lower Str values consistently showed lower friction coefficients during sliding. The friction mechanism was attributed to the combined effects of the material properties, surface topography, and contact area. With the addition of a particle medium, the dry friction coefficient of the rubber interface decreased but exhibited an initial increase, followed by a decrease with increasing particle diameter. When particles were mixed with a water-based cutting fluid, the concentration, diameter, and wettability of the particles significantly impacted the tribological properties due to the synergistic effects of surface topography and particle lubrication. This work enhances our understanding of tribological control for viscoelastic materials through surface design, providing a theoretical basis for the tribological optimization of rubber surfaces.

Synergistic lubricating effect for calcium phosphate modified calcium sulfonate grease with highly load bearing capacity

Calcium sulfonate complex grease exhibits different lubrication mechanisms due to its different combination of calcium carbonate soap and complex agents. In an effort to explore the synergistic lubricating effect of complex agents and find breakthroughs in the performance of calcium sulfonate grease, calcium phosphate modified calcium sulfonate grease was prepared and tested compared with borates. At high loads of 600 N, the introduction of P-containing complex improved load bearing capacity of calcium sulfonate grease. The advantage it exhibited in terms of lubrication was attributed to the synergistic effect of calcium carbonate and calcium phosphate in thickener under harsh condition. The synergistic films about 30 nm depth composed of iron oxide, calcium oxide and iron phosphate played a decisive role in lubrication.

Macroscopic liquid superlubric triboelectric nanogenerator: An in-depth understanding of solid-liquid interfacial charge behavior

Triboelectric nanogenerator (TENG) has been seen as one of the most promising energy harvesting technologies. However, there are an irreconcilable contradiction between low friction and high electrical output performance of the TENG. Here, we have devised a macroscale liquid superlubric triboelectric nanogenerator based on solid-liquid-solid structure, leveraging the concept of liquid superlubricity technology, resulting in a remarkable increase in both the open circuit voltage and short-circuit current by 53.0 % and 58.4 %, respectively. This huge increase in electrical output performance is accompanied by a 99.1 % reduction in friction coefficient (≈ 0.0025) and 99.993 % reduction in wear rate (≈ 0.76×10−7 mm3/N·m), when compared to those of a lubricant-free triboelectric nanogenerator. This work delves into the lubrication and charge transfer mechanisms. The liquid superlubricity technology achieves friction reduction through a hybrid mechanism combining boundary and hydrodynamic lubrication. And the high outputs arise from the charges transfer at solid-liquid interface. The triboelectric charges generated by the friction pair are transferred from solid to the lubricating liquid. Meanwhile, the lubricating liquid promotes electron transfer and contributes additional electrons. Our work provides profound insights into the lubrication and charge transfer mechanisms at solid-liquid interfaces, and addresses the paradox of high output and low friction in TENGs.

Alkyl Germanes: Active lubricant additives in PAO-10

Germanium-based functional materials have found wide application in the field of material industry and show promise as lubricant additives due to their good designability and unique chemical properties. In this work, seven examples of stable alkyl germanes were synthesized and employed lubricant additives in PAO-10. These alkyl germanes exhibit good solubility in PAO10 and demonstrate significant improvements in friction-reducing, anti-wear performances and extreme carrying capacity. Notably, the addition of 1 % Bu3GeC8 can reduce the average friction coefficients and wear volumes by about 57 % and 90 %, respectively. The lubrication mechanism was elucidated by contact angle, SEM, EDS, XPS and FIB-TEM. Results revealed that the protective tribofilm comprised a physical absorbed film (carbonaceous species) and a tribo-chemical film (ferrous oxides and GeO2).

Impact of tribofilm on the anti-wear and friction-reduction properties of interfaces

Zinc Dialkyl Dithiophosphate (ZDDP) is widely used in internal combustion engine lubricating oil, which forms tribofilm and effectively blocks the direct contact of the material interface. Tribofilm plays an important role in wear resistance and lubrication performance. This study analyses ZDDP additive lubricant performance and the tribofilm distribution under different concentrations and loads. Tribofilm formation and wear mechanism is characterized by Scanning electron microscope (SEM) and Energy dispersive x-ray spectrometer (EDS), and the lubrication performance is further explained by the Atomic Force Microscope (AFM). This study explored the anti-wear and friction-reducing properties of ZDDP tribofilm respectively, revealing that ZDDP tribofilm distribution plays a pivotal role in reducing wear, the wear amount can be reduced by 50%, but has a slight effect on friction-reducing, only 5.7%. In addition, the concentrations and loads significantly affect the growth of the tribofilm, and change the wear and lubrication characteristics. The tribofilm acts as a significant barrier, effectively protecting the surface from wear. However, excessive pressure may lead to the failure of the tribofilm, resulting in the loss of protection and subsequent severe wear of the surface. Furthermore, the mechanisms of lubrication are explained, wherein the tribofilm serves as micro-texture, reducing direct contact between asperities and thereby lowering the friction coefficient.

Influence of CePO4 on high temperature anti-friction and anti-wear behaviors of nickel-based h-BN composite coatings

Solid lubricating coatings which offer and maintain anti-friction and anti-wear are urging to develop in the advanced technology applications over a wide range of temperatures. The h-BN composite coatings on the Inconel X750 superalloy is prepared by spark plasma sintering (SPS) technology to obtain low friction and wear over a broad temperature ranging from room temperature (RT) to 800 °C using a high temperature rotary tribometer. The influence of the contents of h-BN and CePO4 powders of the tribological properties of the composite coatings was studied systemically under different temperatures. The experimental results show that the composite coatings with 20 wt% h-BN show excellent high temperature anti-friction behaviors. Coefficient of friction (CoF) of the composite coatings with 20 wt% h-BN with CePO4 powder is lowest about 0.12 among the composite coatings at 800 °C. The CePO4 powders are helpful to improve the sintering behaviors and reduce the wear rate of the composite coatings at elevated temperature. The synergistic lubrication mechanism of h-BN and CePO4 of the composite coatings are investigated systematically. The high temperature anti-friction and anti-wear mechanism of the composite coatings is due to the formation of a layered gradient lubrication film on the worn surface.

Zwitterionic Poly-Carboxybetaine Polymers Restore Lubrication of Inflamed Articular Cartilage.

Osteoarthritis is a degenerative joint disease that is associated with decreased synovial fluid viscosity and increased cartilage friction. Though viscosupplements are available for decades, their clinical efficacy is limited and there is ample need for more effective joint lubricants. This study first evaluates the tribological and biochemical properties of bovine articular cartilage explants after stimulation with the inflammatory cytokine interleukin-1β. This model is then used to investigate the tribological potential of carboxybetaine (CBAA)-based zwitterionic polymers of linear and bottlebrush architecture. Due to their affinity for cartilage tissue, these polymers form a highly hydrated surface layer that decreases friction under high load in the boundary lubrication regime. For linear pCBAA, these benefits are retained over several weeks and the relaxation time of cartilage explants under compression is furthermore decreased, thereby potentially boosting the weeping lubrication mechanism. Bottlebrush bb-pCBAA shows smaller benefits under boundary lubrication but is more viscous than linear pCBAA, therefore providing better lubrication under low load in the fluid-film regime and enabling a longer residence time to bind to the cartilage surface. Showing how CBAA-based polymers restore the lost lubrication mechanisms during inflammation can inspire the next steps toward more effective joint lubricants in the future.

Shear properties and dynamic responses of greases in a micrometer-order gap

Grease is used as a lubricant in a wide range of fields, including bearings, because it reduces friction, prevents harmful wear of components, protects against rust and corrosion, and acts as a seal to prevent the invasion of dirt and water. Although most of the research on grease has focused on the environment inside the bearing, there has been little research on the fundamental lubrication mechanism of grease. It is known that thickeners, which keep a complex three-dimensional structure in the grease, have a significant effect on the shear characteristics of grease, and it is assumed that this is due to the orientation of the thickener structure in the shear direction. In this study, the apparent viscosity of grease in a micro-order gap was measured using our original viscometer and compared with the apparent viscosity measured with a submillimeter-order gap rheometer because grease may show different rheological properties compared to conventional measurements. In addition, the dynamic response of viscous resistance that appeared when each grease was subjected to a change in the shear force was quantitatively evaluated using relaxation time. As a result, the apparent viscosity remarkably decreased in a micro-order gap compared to a submillimeter gap, and two types of shear thinning mechanisms were proposed based on the orientation of the thickener: one caused by the narrow gap and the other by the shear force. In addition, the behavior of viscous resistance due to changes in the shear force depended on the type of thickener. It was also confirmed that the relaxation time of each grease correlates with its oil film-forming ability and the entanglement level of the thickener’s structure. Furthermore, the mechanism of the dynamic response was proposed based on the reorientation of thickeners.

Influences of the spacing between nitrogen atoms on tribological performance of dual active site phosphorus-nitrogen (P-N) ionic liquid

Four dual active site ionic liquids (ILs): N2NP0, N4NP0, N6NP0, N10NP0 and three single active site ILs: N2P0, N3P0, N8P0 were synthesized successfully, furthermore, anti-wear and extreme pressure properties of the additives were tested with 0.5 wt% concentration in PAO10 base oil. The results showed that N6NP0 with carbon chain length of six between the nitrogen atoms exhibits the best tribological performance (The wear scar diameter is 0.39 mm, the values of PB (the last non-seizure loads) and PD (weld points) were 135 Kgf and 200 Kgf, respectively), which is mainly attributed to the formed thicker tribo-film (thickness of 50–60 nm) on the friction surface. Through analyzing the tribological morphology, chemical composition and tribo-film microstructure of wear scars surface by SEM-EDS, 3D profile, AFM, XPS and FIB-TEM, the lubrication mechanism of ILs was revealed. Furthermore, density functional theory (DFT) calculations confirmed that the tribological performance of dual active site ionic liquid additives depend strongly on their molecular configuration. It was found that rational adjustment of the length of the ILs cationic active site linkage group results in a reduction of the steric hindrance of the entire molecule. This study will help to guide the design of ILs with good tribological performance and low phosphorus content.

The relationship between the structure of stearic acid diamide lubricants and the lubrication properties in acrylonitrile-butadiene-styrene resins

Although amide lubricants are commonly used as lubricants for acrylonitrile-butadiene-styrene (ABS) resins, the relationship between their structure and lubricating properties remains unclear. In this paper, a series of stearic acid diamide lubricants for ABS resins were synthesized through an amidation reaction with the stearic acid and aliphatic diamines. The chemical structure was characterized by FTIR and 1H NMR. A detailed evaluation of the lubrication effect of the lubricant was carried out by dynamic mechanical analysis, rotational rheometry, contact angle measurements, and so on. The results showed that the complex viscosity, glass transition temperature and diamine's alkyl chain length of stearic acid diamides were positively correlated, while the elongation at break displayed a negative correlation. Compared to ethylenediamine bis stearate acid (EBA), 1,8-diaminooctane bis stearate acid (OBA) exhibited a 37.8 % increase in the complex viscosity at 0.1 rad/s, along with a 4.6 % rise in the glass transition temperature. It was indicated that the lubrication effect of stearic acid diamides deteriorated with the increase of the alkyl chain lengths of diamines. The lubrication mechanism was further validated by molecular dynamics simulations. As the alkyl chain lengths of diamines increased, the mean square displacement and interaction energy of the system exhibited a tendency to become lower and higher, respectively. This meant that the longer alkyl chains of the diamines in the lubricants were not advantageous to the free mobility of the matrix molecular chains. These findings provided insightful guidance for the sensible design of lubricants for ABS resins.

Acquisition of molecular rolling lubrication by self-curling of graphite nanosheet at cryogenic temperature

Friction as a fundamental physical phenomenon dominates nature and human civilization, among which the achievement of molecular rolling lubrication is desired to bring another breakthrough, like the macroscale design of wheel. Herein, an edge self-curling nanodeformation phenomenon of graphite nanosheets (GNSs) at cryogenic temperature is found, which is then used to promote the formation of graphite nanorollers in friction process towards molecular rolling lubrication. The observation of parallel nanorollers at the friction interface give the experimental evidence for the occurrence of molecular rolling lubrication, and the graphite exhibits abnormal lubrication performance in vacuum with ultra-low friction and wear at macroscale. The molecular rolling lubrication mechanism is elucidated from the electronic interaction perspective. Experiments and theoretical simulations indicate that the driving force of the self-curling is the uneven atomic shrinkage induced stress, and then the shear force promotes the intact nanoroller formation, while the constraint of atomic vibration decreases the dissipation of driving stress and favors the nanoroller formation therein. It will open up a new pathway for controlling friction at microscale and nanostructural manipulation.

Self-replenishing lubrication mechanism of nano-silver-reinforced AISI 4140 steel composites under dry sliding for mechanical applications

Wear is one of the most important factors limiting mechanical components' lifespan and durability. In this study, a novel, AISI 4140-Ag steel-based, self-lubricating nanocomposite was manufactured by the spark plasma sintering (SPS) technique. The high-level aim of this investigation is to obtain a deeper understanding of how silver (Ag) at the nanoscale can enhance the self-lubricating layer's replenishment, which aids in maintaining a steady supply of solid lubricant under dry conditions. Tribological experiments were conducted under various loads (30–150 N) at 5 Hz frequency and 300 °C temperature using a Rtec Universal Tribometer (ball-on-disc setup). After that, advanced surface characterization techniques were utilized to illustrate predominant wear modes, lubrication mechanisms, and the chemical identity of the self-lubrication layer. Noticeably, the AISI 4140-Ag nanocomposite reduced the friction coefficient and wear rate by 36.16–51.14 % and 93.40–97.36 %, respectively, compared to the AISI 4140 composite. The excellent self-lubricating performance is mainly attributed to the replenishment of a self-lubricating layer on the substrate material and a transfer layer on the counterbody, composed of AgO and Fe2O3. The results of our study hold considerable promise for the advancement of self-lubricating composites and the enhancement of their durability in mechanical applications.

Optimal lubricating protection and interfacial behavior for titanium alloy surface from phosphorus-based ionic liquids

The poor wear resistance of titanium alloy makes it challenging to develop high-performance lubricating materials. For the characteristic of highly designable structure, ionic liquids (ILs) are the potential lubricants with excellent physicochemical properties and lubricating performance. Herein, three protic phosphorus-based ionic liquids (PPILs) were designed and synthesized with the exploration of their interfacial lubrication behavior and interaction mechanism on the titanium substrate. The as-synthesized IL lubricants exhibit optimal tribological properties on steel-titanium friction pairs even at high load and temperature. The surface analysis showed that IL can form the lubricating protective film on the titanium-based metal surface, effectively reducing the adhesive wear. Combined with surface analysis techniques (SEM, XPS, FIB-TEM and TOF-SIMS), the lubrication mechanism was discussed systematically.