Tribology Letters最新文献

MoS₂-based dry film lubricants are widely used in aerospace mechanisms that have different contact geometries. Standard tribotests for dry film lubricants simplify real geometries as either point or line contact to facilitate comparison of different materials or operating and environmental conditions. However, it remains unclear whether the results of a tribotest with one contact geometry can be generalized to tests or mechanisms with different contact geometries. To assess the effect of contact geometry, we measured friction and wear of three MoS₂-based dry film lubricants using reciprocating block-on-ring and pin-on-disk tests with the same initial Hertzian contact pressure and linear velocity. Friction and wear magnitudes, as well as some comparative trends between the dry film lubricants, differed for the two contact geometries. These findings encourage future studies to consider more than just point contact tribotests when evaluating MoS₂-based dry film lubricants.

Graphical Abstract

Ionic liquids have garnered significant interest from the tribological community due to their exceptional physico-chemical properties, such as high thermal stability, tuneable viscosity, and strong surface adsorption. These properties make them promising candidates for advanced lubrication systems, offering potential to reduce friction and wear under diverse operating conditions. Here, the tribological behaviour of two ionic liquids (ILs), [BMIM][PF₆] and [BMIM][TFSI], was investigated across macro-, micro-, and nano-scales, focusing on lubrication regimes and their influence on friction. At the macro-scale, mixed lubrication dominated, with friction decreasing with sliding velocity due to the formation of a fluid film that partially separates the two surfaces; the friction force in this case is related to the ability of the IL to form thicker films and therefore, the IL viscosity. Conversely, at the nano- and micro-scales, boundary lubrication prevailed under much lower velocities and higher contact pressures (at the nano-scale), with adsorbed ion layers mitigating friction. The different friction coefficients of the two ILs are attributed to their different interaction with the steel surface. To eliminate the influence of the hydrodynamic film formation, the friction force at the macro-scale was extrapolated to 1 µm/s, i.e. within the range of velocities probed at nano- and micro-scale. In this regime, a correlation between friction and load across 8 orders of magnitude for each IL was observed. The relevance of plastic deformation and scratching at the level of asperity contacts underlying the tribological performance at the three scales was highlighted and described using the Bowden & Tabor model. Differences between the correlations of the two ILs were attributed to variations in shear strength and the structure of the adsorbed boundary films, with [BMIM][TFSI] forming more cohesive and lubricious layers at low loads. The results emphasize the interplay between lubrication regimes, contact mechanics, and material properties in determining tribological performance across scales, offering insights for advanced lubricant design.

Graphical abstract

At engineering scales, Amontons–Coulomb friction (with coefficient of friction μ) and Archard wear (with wear coefficient K) can be treated as coupled under standard assumptions. Whether this coupling survives downscaling, however, remains unclear. We develop a composite diagnostic, K/μ, that algebraically links Archard’s and Amontons–Coulomb’s forms through K/μ ≈ VH/W, where V is wear volume, W is the work of friction, and H is hardness. The construct removes explicit normal-load scaling and enables a joint self-consistency check of the two laws at the nanoscale. We then test it using molecular dynamics simulations of aluminum films indented and abraded by three rigid diamond asperities, spanning asperity spacing (close, intermediate, and wide) and speed (2.5–100 m/s). The parameters respond differently to configuration and speed: apparent hardness increases with spacing and with speed; wear volume is weakly speed-dependent but peaks at intermediate spacing; and the work of friction grows with speed and spacing yet does not mirror the wear ordering. The friction coefficient remains in a narrow band (1.0–1.4) across conditions, while the wear coefficient varies by five times. The resulting K/μ magnitudes lie in the 0.2–1.0 range. Compared with laboratory-scale bands, these nanoscale K/μ values are considerably larger, suggesting that the classical friction–wear coupling does not carry over unchanged to multi-asperity nanoscale contacts.

The accurate determination of static friction at nanowire–substrate interfaces is critical for the design, fabrication, and reliability of nanowire-based micro/nano devices, as well as for advancing the fundamental understanding of tribology. The self-sensing test strategy, which infers friction from nanowire bending, offers a direct route for characterizing nanoscale friction. However, existing mechanical models often adopt simplified treatments, neglecting force components such as the axial friction component and introducing additional assumptions such as uniform stress distribution. Here, we develop a comprehensive mechanical model based on linear elasticity that explicitly incorporates both the tangential and axial components of static friction. This model enables the mapping of static friction force distribution from the bending profile of a nanowire under maximum static friction. The model is validated through optical microscopy–based nanomanipulation of bent SiC nanowires on Si substrates. Our results reveal that neglecting axial friction leads to profound errors in both the magnitude and direction of the inferred transverse friction. Furthermore, we measure a static-to-kinetic friction ratio of 1.68 ± 0.25 for SiC nanowires on Si substrate, slightly lower than the idealized value of ~ 2. This work provides a simple, intuitive, and accurate method for mapping static friction in nanowire–substrate systems, offering an important measurement platform for studying friction in low-dimensional materials.

Graphical Abstract

The wheel-rail interface is frequently subjected to the intrusion of external solid particles such as sand. These particles not only alter the adhesion characteristics at the contact zone but also contribute to rail surface damage. To investigate the damage mechanisms and mutual coupling effects of rail material and sand particle when sand is present at wheel-rail interface, this investigation employs an integrated methodology, combining laboratory experiments with computational simulations, to analyze the failure mechanisms of rail material, failure process of sand particle, and variation of stress in the contact zone under sandy condition. The results show that under sand-contaminated condition, the rail surface exhibits typical damage features such as flaking spallation, oblique and horizontal cracks, and embedded abrasive layer. The damage mechanism involves a coupled effect of rolling contact fatigue and abrasive wear. Additionally, the failure process of sand particle can be divided into four stages: stress concentration, crack propagation and fracture, fragmentation, and grinding-embedding. During the initial failure stage, the maximum von Mises stress inside the sand particle reaches 481 MPa, which gradually decreases after particle fragmentation. Upon intrusion into the contact zone, the von Mises stress on the rail surface first decreases from 444.8 to 196.3 MPa and then gradually rises to 1001 MPa, forming a localized damaged area in rail surface layer with a depth of 0.12 mm and a length of 3.52 mm, demonstrating clear wear-fatigue coupling damage phenomena. This research delineates the progressive failure mechanisms of sand particle and their damage mechanisms on rail material, providing a theoretical basis for rail maintenance, material selection, and service life assessment in sandy and windy environments.

This study focuses on the development of protective polymer coatings to reduce hydrogen diffusion (HD) in steel valve components used in hydrogen refueling stations (HRSs). Two low hydrogen permeability (HP) polymers, polytetrafluoroethylene (PTFE) and polyurethane (PU), were selected and deposited onto 316 stainless steel disks using a spray-coating technique. Tribological tests were conducted in atmosphere air, nitrogen (N₂), and hydrogen (H₂) at 0.2 MPa using a custom-built multi-environment pin-on-disk tribometer, with each test repeated twice to ensure reliability. PTFE consistently demonstrated superior tribological performance compared to PU across all environments. The coefficient of friction (CoF) for PTFE was lower by approximately 33% in atmosphere air, 33% in N₂, and 57% in H₂. Similarly, the specific wear rate (SWR) of PTFE was reduced by about 30% in atmosphere air, 16% in N₂, and 53% in H₂ relative to PU, confirming PTFE’s excellent suitability for H₂-exposed conditions. FESEM analysis showed that PTFE forms a fibrous coating structure, while PU exhibits denser morphology, with average coating thicknesses of 34 μm and 36 μm, respectively. CHNS analysis revealed major distinction in H₂ absorption. PU absorbed 5.61-wt% H₂, whereas PTFE absorbed only 0.87 wt%. The lower H₂ absorption in PTFE correlates strongly with its improved frictional stability, reduced wear, and enhanced hydrogen barrier properties. Additional chemical characterizations were performed to understand H₂ interactions and their influence on the observed tribological trends. Overall, PTFE exhibited excellent frictional behavior, wear resistance, and HP barrier capability, establishing it as a promising candidate for protecting steel components in HRSs.

Graphical Abstract

MoS₂ nanosheets as a quintessential layered solid lubricant has been demonstrated ultra-low friction in a dry environment due to the weak van der Waals forces arising from interlayer sliding. Nonetheless, MoS₂ nanosheets as lubricant additives are susceptible to agglomeration and complicating the attainment of ultra-low friction under air conditions. Herein, the modification and refining of MoS₂/CuS nanocomposites were achieved by a one-step liquid-phase laser irradiation technique in atmospheric conditions, which can result in an aqueous-based composite lubricant with excellent dispersion in water. Ball-on-disk rotational friction tests demonstrated that the optimized MoS₂/CuS composite aqueous lubricant exhibited excellent anti-wear and friction-reducing characteristics, achieving an ultra-low friction coefficient (COF) of ~ 0.06 and a wear scar diameter (WSD) reduction of 49.5% compared to pure deionized water. Such ultrafine nanocomposites can efficiently penetrate the tribological contact zone, thereby preventing direct contact between sliding interfaces. More importantly, the layered structures of both MoS₂ and CuS components facilitate interlayer sliding under shear stress, collectively mitigating the friction and wear. This study could resolve the challenges associated with the dispersion and aggregation of flaky MoS₂ in lubricants, while also tackling the limitations of low load-carrying capacity and inadequate lubrication performance encountered by aqueous lubricants in practical applications.

Graphical Abstract

The effect of frictional heat accumulation on the surface damage initiation at cyclic sliding contacts of M50 steel is investigated. The surface damage is detected by a sudden increase in the frictional coefficient during the ball-on-disk sliding contact test. The periodically thermal response of the rotating disk during tests is correspondingly calculated by using a numerical method based on Fourier transform. The results show that while the final failure mode is consistent under fixed pressure and velocity, the time to failure is controlled by the heat accumulation rate, which is highly sensitive to cyclic frequency. A smaller rotational radius increases contact frequency and reduces heat dissipation within a single cycle, accelerating temperature rise and surface damage initiation. The parametric analysis revealed the distinct effects of contact pressure, sliding velocity, and friction coefficient on frictional heat flux and heat dissipation. The study concludes that controlling heat accumulation behavior is critical for predicting and mitigating surface damage at cyclic sliding contact.

Rolling contact fatigue of M50 steel is investigated with consideration to the elasto-plastic behavior of materials. The multiaxial stress model is used for evaluating the influence of both the normal stress difference and shear stress on the fatigue evolution. The effect of material distortion induced by the normal stress difference is specifically analyzed during the initiation and propagation processes of spalling. Changes in the multiaxial stress on different surfaces, as well as their effect on the fatigue evolution are discussed. The results show that in addition to the shear stress, the normal stress difference has significant effect on fatigue evolution as well. Especially on certain rough surfaces, the stress concentration in near-surface layers generates obvious difference among the normal stresses in the orthogonal direction. The corresponding material distortion energy due to the increasing elastic strain accelerates the crack initiation in near-surface layers, which consequently reduces the rolling contact fatigue lifetime.

Lubrication in space mechanisms is governed by specific requirements arising from extreme conditions such as vacuum, radiation, thermal gradients, and the impossibility of in situ maintenance. This article firstly traces the development of space-grade lubricants, from early investigations with silicones and mineral oils to the current use of heritage lubricants such as perfluoropolyethers (PFPEs) and multiply alkylated cyclopentanes (MACs). While PFPEs have become the reference fluids thanks to their low vapor pressure and high thermal stability, increasing environmental restrictions on fluorinated compounds drive the search for sustainable alternatives. MACs are a modern alternative to PFPEs with improved boundary lubrication properties. However, these lubricants suffer from technical limitations such as dewettability and initial seizure-like high friction values. In this context, ionic liquids (ILs) have emerged as promising candidates due to their negligible volatility, high chemical, and thermal stability. The second part of the review compares the performance of these lubricant families, addressing challenges such as evaporation losses, tribochemical degradation, or atomic oxygen resistance, and concludes with recent advances on ILs as both primary lubricants and functional additives.