Tribology Letters最新文献

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.

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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.

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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.

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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.

As the quintessential representation of graphene derivatives, graphene oxide (GO) has demonstrated unparalleled potential in micro/nano electronic mechanical systems, which visibly enhances the efficiency and accuracy of moving mechanical devices. However, GO has always been subject to the problem of insufficient wear lifetime, and the subsequent improvement is still a challenge, especially under high contact stress. In this paper, making use of the strong charge interactions between positively charged poly(acrylamide-co-diallyldimethylammonium chloride) (Brand: PQ-7) and negatively charged GO, both were alternately spin-coated on the silicon substrates modified by 3-aminopropyltriethoxysilane as an adhesive layer to form (GO/PQ-7)_n composite multilayer film. The service life of (GO/PQ-7)₅ multilayer film exceeds 27000 s under high load of 4N, which is 20 times longer than that of the GO film. The superior friction performance is ascribed to the distinctive structure of (GO/PQ-7)_n composite multilayers, that is, an elastic 3-dimensional stack composed of rigid GO and flexible polymer. This soft and hard interbeded formation film not only integrates the interface well, but also effectively prevents the crack expansion. It also leverages the advantages of soft layers providing stress relief and hard layers providing load-bearing capacity. What's more, friction-induced conversion of partial GO to graphene ensures low friction at the sliding interface. This strategy provides an open platform for the design and fabrication of lubricating films for micro/nano electronic mechanical systems and other microdevices.

The superlubricity of sputtered MoS₂ film under dry air environment was achieved by low-energy proton irradiation of 25 keV for the first time. We found that proton (H⁺) irradiation is able to break the Mo-S covalent bonding of as-deposited MoS₂ film and leads to the formation of MoS₂ nanocrystalline domains. The dangling bonds at edge planes or newly exposed edges could be passivated with hydrogen ions by bonding interaction under proton irradiation, forming hydric MoS₂ nanocrystalline domains with stable S–H bind, resulting in superior antioxidant capacity of proton-irradiated MoS₂ film compared to its non-irradiated counterpart. Importantly, proton irradiation can penetrate the interior of sputtered MoS₂ film with thickness over 1 µm and restructure the as-deposited MoS₂ film into nanocrystalline MoS₂ domains to achieve superlubricity and life extension under dry air conditions.

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