Tribology Letters最新文献

Adhesive friction considering electrostatic interaction is an important problem in practical engineering. However, the friction mechanism considering electrostatic interaction and adhesion of elastoplastic materials on the contact interfaces remains poorly understood. A sliding friction model considering the van der Waals attraction, repulsive and electrostatic interactions is established in this work. The effect of charge density on the normal force, friction force, contact area and stress distribution is thoroughly investigated. And the repeated sliding friction process is also quantitatively analyzed. It is shown that adhesion enhances with the increase of surface charge density. Moreover, the contribution of electrostatic interaction to adhesion in the contact process is always greater than that to friction in the sliding friction process under the conditions studied. In the initial stage of friction, the friction will gradually increase and the normal force will gradually decrease. It is also found that a higher charge density results in a lower normal force in the friction process. Furthermore, the increase of charge density leads to a bigger contact diameter and the increased asymmetry of stress field, resulting in the increase of friction force, equivalent plastic strain and friction coefficient. In addition, both normal force and friction force arrive at a steady state after several repeated friction circles for elastoplastic materials, which is due to the fact that the contact diameter tends to be constant because of the accumulation of plastic deformation. This work reveals the contribution of electrostatic interaction to friction during adhesive sliding process and provides some insights into the adhesive friction problem considering electrostatic interaction.

Aiming at the phenomenon that nanosilver lubricants are prone to agglomeration and sedimentation, the influence of different anions and cations in ionic liquids on the adsorption capacity of nanosilver particles is studied, and three ionic liquids with different structures are selected to improve the agglomeration of nanosilver particles in lubricants and to improve the anti-wear performance of lubricants; the dispersive performance of different ionic liquids is studied using quantum chemical simulation methods to study the adsorption and dispersion mechanism of nanosilver. The tribological properties of the three different ionic liquids in combination with the nanosilver lubricant were investigated using a multifunctional friction and wear tester and a field emission scanning electron microscope. The results show that hexyltributylphosphine hexafluorophosphate ([P4446][PF6]) can stabilise the dispersion of nanosilver lubricant and solve the problem of agglomeration and sedimentation due to its strong adsorption cation, which is attributed to its high adsorption and the synergistic effect of friction chemistry, and its anionic intermixing is good, and the ionic liquids have excellent anti-friction and wear reduction properties. It is shown that the design and utilisation of functional ionic liquids with the compound effect of nanosilver particles can effectively improve the dispersion of nanosilver as a lubricant additive.

Graphic Abstract

Layered two-dimensional nanomaterials such as graphene and WS₂, possess superlubricity properties and thus offer a promising solution to mitigate friction and wear in micro-electromechanical systems. In this study, the atomic friction properties of graphene/graphene, WS₂/WS₂, and graphene/WS₂ bilayer heterostructure systems were examined through density functional theory simulations. Results indicated that the friction strength of the graphene/WS₂ bilayer heterostructure system was lower than that of the graphene/graphene and WS₂/WS₂ systems. Specifically, the graphene/WS₂ bilayer heterostructure system demonstrated ultra-low friction coefficients ranging from 0.0006 to 0.0096, resulting in friction strengths in the range of 10^⁻³ nN. Furthermore, the heightened electrostatic repulsion and smooth potential energy fluctuation helped reduce friction, validating the superlubricity performance of the graphene/WS₂ heterostructure system.

A vehicle start–stop system automatically shuts down and restarts the internal combustion engine to reduce the time the engine spends idling, thereby reducing fuel consumption and emissions. For the start–stop system to work, the engine must be at a certain temperature and conditions. If the engine is too hot, the system may not activate. This study explores the tribological characteristics of the start–stop system by applying principles of Continuum Damage Mechanics (CDM) to predict both the lifespan and wear volume subsequent to the start–stop cycles. A series of pin-on-disk tests were conducted to evaluate the efficacy of the modeling and predictions. The results from these tests were compared to the CDM predictions, demonstrating satisfactory accuracy. Additionally, a Finite Element Method (FEM) analysis was employed to model temperature variations during the start–stop cycles. Findings indicate that an increase in consecutive start–stop cycles impedes the system’s ability to sufficiently cool, thereby increasing wear. Conversely, extending the duration of the stop phase reduces wear and enhances the system’s lifespan.

Polyamide 66 (PA66) is one of the commonly used polymer gear materials. This paper focuses on the tribological properties of glass fiber reinforced PA66 composites in self-mated contact using a pin-on-disc tribometer. The effects of glass fiber content, PV (the product of the contact pressure and sliding speed), and lubrication on the tribological properties of the specimens are also investigated. The results show that the glass fiber reinforced PA66 exhibit higher coefficients of friction and specific wear rates than PA66 under dry sliding conditions. This is probably due to the peeled glass fibers during the sliding process acting as abrasive particles which have an aggressive effect on the surface. Under grease lubricated conditions, PA66 + 33% GF has the lowest coefficient of friction and specific wear rate due to its higher strength. Under dry sliding conditions, all specimens show the highest friction coefficient and specific wear rate at 30 MPa·m/s with the change of PV value. Under grease lubricated conditions, all specimens show the highest friction coefficient and specific wear rate at 4 MPa·m/s with the change of PV value. The addition of grease improves friction and wear of PA66 composites under most of the experimental conditions. However, the specific wear rates of PA66 and PA66 + 13% GF under grease lubrication are higher than those under dry sliding conditions at low PV values. This may be due to the fact that greases can reduce the surface mechanical strength of PA66 and PA66 + 13% GF.

Graphical abstract

Wear problem has become an important issue limiting the functionality and lifetime of sliding electrical contact components. Adding conductive solid lubricants is a potential means of improving the tribological performance of these devices. Graphene, a two-dimensional material with excellent electrical conductivity and lubrication property, has been proposed to be a promising candidate for such applications. However, the tribological performance graphene has been demonstrated to be very susceptible to humidity even under non-current-carrying conditions. In this work, we study the effect of humidity on the wear behavior of graphene in the sliding electrical contact interfaces. The tribological behaviors of graphene under 10%, 30%, 60%, and 90% relative humidity conditions and 1 A current are investigated. The results show that the humidity can effectively slow down the wear of graphene in the sliding electrical contact interface by two key mechanisms. Firstly, as revealed by the infrared temperature measurements, higher humidity can significantly reduce the Joule heating. Secondly, X-ray photoelectron spectroscopy shows that with the existence of the electric current, at high humidity water molecules can passivate the graphene carbon dangling bonds more readily thereby reducing oxidation and slowing down the wear process. At low humidity, Joule heating not only caused graphene to oxidize but also accelerated the evaporation of water molecules, which was not conducive to its passivation, resulting in severe wear of the graphene.

Core–shell structured silica nanoparticles with different sizes were successfully prepared by the reaction between tetramethoxysilane (TMOS) and the SiO₂ core under a mild condition. The obtained silica nanoparticles have a unique structure with tight cores and loose shells, which showed superior performance during tungsten (W) chemical mechanical planarization (CMP) process. The material removal rate (MRR) increased significantly from 763 to 1631 Å/min (with ~ 100 nm particles) while the surface roughness decreased from 1.802 to 1.252 nm. A series of characterization indicates that the superior performance of core–shell structured silica nanoparticles can be attributed to the formation of the irregular loose shell, increasing the mechanical friction during the W CMP process. Meanwhile, the loose shell structure can also contribute to the improvement of the wafer surface quality after CMP process. This work provides a new strategy for designing high-efficient abrasives for CMP process.

Graphical Abstract

Vegetable oil-based lubricants have a tendency to replace traditional petroleum-based lubricants due to their biodegradability, high flash point, low volatility, and low cost. However, polar molecules such as fatty acids in vegetable oil compete for adsorption with nanoparticles during rubbing process, resulting in imperfect tribological performance of nanoparticles. Magnetic nanoadditives can be adsorbed on the contact surface of iron-based friction materials through magnetic effects, which provides a new idea for solving competitive adsorption problems between additives and base oil. In this study, Ni nanoparticles with a particle size of approximately 15.6 nm were synthesized in-situ in olive oil using nickel acetylacetone as the nickel source and olive oil as the modifier and solvent required for the reaction, which is a simple, efficient, and environmentally friendly in-situ synthesis method. The as-synthesized Ni nanoparticles can significantly improve the antiwear capabilities of olive oil, reducing the wear scar diameter by 30%. The morphology and elemental analysis of wear scar indicated that a composite tribofilm including nickel, nickel oxide, iron oxide, carbon film, and polar fatty acid molecules in olive oil is formed on the rubbing surface, greatly improving the antiwear performance, which opens up an opportunity for the further application of new green nanolubricants.

In this study, the complementary lubricating function of the superficial area of the articular cartilage and synovial fluid (SF) constituents was examined. The cartilage specimens underwent two different degenerative treatments: gentle washing with detergent to remove lipids and proteins absorbed onto the cartilage surface and incubation in a NaCl solution to remove lubricin from the surface. Sliding experiments with a glass probe and cartilage specimens were conducted at various speeds and low contact loads using lubricants containing SF constituents, such as phospholipids, proteins, and hyaluronic acid (HA). The treated cartilage surface and protein adsorption were observed using a fluorescence microscope and water immersion objectives to explore the underlying mechanisms of the difference in friction. The results showed that fresh SF exhibited low friction even after degenerative treatment. HA and phospholipids had no boundary lubrication effect, whereas the lubricant containing albumin and γ-globulin maintained a consistently low coefficient of friction, even after degenerative treatment. The significance of the interaction between albumin and γ-globulin should be emphasized.

Graphical abstract

In this study, we investigate the tangential sliding of a rigid Hertzian indenter on a viscoelastic substrate, a problem of practical interest due to the crucial role that sliding contacts play in various applications involving soft materials. A finite element model is developed, where the substrate is modelled using a standard linear viscoelastic model with one relaxation time, and adhesion is incorporated using a Lennard–Jones potential law. We propose an innovative approach to model tangential sliding without imposing any lateral displacement, thereby enhancing the numerical efficiency. Our goal is to investigate the roles of adhesive regimes, boundary conditions (displacement and force-controlled conditions), and finite thickness of the substrate. Results indicate significant differences in the system’s behaviour depending on the boundary conditions and adhesion regime. In the short-range adhesion regime, the contact length (mathcal {L}) initially increases with sliding speed before decreasing, showing a maximum at intermediate speeds. This behaviour is consistent with experimental observations in rubber-like materials and is a result of the transition from small-scale to large-scale viscous dissipation regimes. For long-range adhesion, this behaviour disappears and (mathcal {L}) decreases monotonically with sliding speed. The viscoelastic friction coefficient (mu) exhibits a bell-shaped curve with its maximum value influenced by the applied load, both in long-range and short-range adhesion. However, under displacement control, (mu) can be unbounded near a specific sliding speed, correlating with the normal force crossing zero. Finally, a transition towards a long-range adhesive behaviour is observed when reducing the thickness t of the viscoelastic layer, which is assumed to be bonded to a rigid foundation. Moreover, the friction coefficient reduces when t tends to zero. These findings provide insights into the viscoelastic and adhesive interactions during sliding, highlighting the critical influence of boundary conditions on contact mechanics.