Tribology Letters最新文献

Top-of-rail (TOR) lubricants are commonly used for friction control in railway operations. They aim to lower friction and reduce noise and wear while ensuring sufficient transmission of traction/braking forces. However, the wheel–rail interface is an open system, so the conditions may suddenly change due to the weather, and different contaminants may enter the contact and influence the performance of these lubricants. Thus, this study examined the effect of humidity and dew on two commercial products, as these conditions often occur on the track. A methodology based on a creep curves measurement approach was used to assess product performance under various scenarios. All measurements were conducted on a universal tribometer in the ball-on-disc configuration covered with a climate chamber. The results show a strong influence of dew on the tested products, as dew lowered their performance parameters and caused low adhesion problems. Possible mechanisms of water–oil interaction and formation of oxidic third body layers were discussed. The main findings indicate that TOR lubricants may cause traction/braking problems if used in dew conditions. The present study may be helpful in optimising friction management methods in the future.

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Atomic force microscopy (AFM) provides the opportunity to perform fundamental and mechanistic observations of complex, dynamic, and transient systems and ultimately link material microstructure and its evolution during tribological interactions. This investigation focuses on the evolution of a dynamic fluoropolymer tribofilm formed during sliding of polytetrafluoroethylene (PTFE) mixed with 5 wt% alpha-phase alumina particles against 304L stainless steel. Sliding was periodically interrupted for AFM topography scans. The average film roughness, the average friction coefficient, and polymer wear rate based on sample height recession were recorded as a function of increasing sliding cycles. Topographical maps suggested tribofilm nucleates in grooves of the steel countersample, spreads, and develops into a uniform film through sliding. Prominent nanoscale features were visible around 10,000 sliding cycles and thereafter. Scanning electron microscopy and energy-dispersive X-ray spectroscopy showed good correlations between these features and aluminum-rich domains, suggesting the presence of alumina particles on the surface.

Adhesive contact with the effect of tangential force may have important implications in friction and wear performances of small-sized devices and joining technologies. In the present work, adhesive contact involving tangential loading but before gross slip between spherical objects has been simulated through molecular dynamics (MD) to reveal the interaction between adhesion and the applied forces. When only the normal force is present, the results on force–displacement relationship and interfacial traction have been presented to compare with the predictions of Johnson–Kendall–Roberts (JKR), Maugis–Dugdale (M–D) and the Double–Hertz (D–H) models with the purpose of evaluating their applicability. In the presence of additional tangential forces, their interaction with adhesion has been studied in depth through loading and unloading. Distribution of the shear traction at the interface which is different from that in the existent models has been obtained. Those altogether may help to develop reasonable continuum models for adhesive contact under inclined forces.

This study investigates the capability of WC/C coatings to protect bearing bores against fretting wear when there is a lack of lubrication, a typical failure mechanism in bearings subjected to vibrations. Linear pin-on-disc and rotary oscillating block-on-ring tribological tests were carried out reproducing fretting conditions to evaluate the tribological performance and fretting resistance of the developed WC/C coating in comparison with those of the two common coatings currently employed to prevent this type of bearing failure, i.e. thin-dense chrome and fluoropolymer coatings. The friction forces generated under fretting conditions were evaluated, and a deep surface analysis of the tribosystems was carried out through different microscopic techniques in order to identify the wear mechanisms prevailing in each coating. Results suggested that WC/C coatings could be promising candidates to mitigate fretting damage, as they combine a low friction coefficient with good wear resistance. Furthermore, WC/C coatings constitute a more sustainable solution compared to the currently employed PTFE and chrome-based coatings, paving the way to a greener society.

N-doped graphene oxide quantum dots (NGOQDs) with MoS₂ and Al₂O₃ nanocomposites were prepared by solvothermal method. The morphology and the composition and structure of the prepared composites were characterized by TEM, XRD, Raman, ATR-FTIR, and XPS. Tribological behavior of NGOQDs-MoS₂ and NGOQDs-Al₂O₃ nanocomposites as lubricant additive in aqueous glycerol were studied. Through experiments and MD simulations, the tribochemistry-induced lubrication mechanism was disclosed. The results shows that the combination of NGOQDs and hydrated glycerol can significantly improve lubrication performance, and the addition of NGOQDs-MoS₂ and NGOQDs-Al₂O₃ nanoparticles can further improve tribological properties. The formation of a tribofilm through tribochemical induced lubrication mechanism improves the wear resistance of metal surfaces.

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PA66 is a commonly used material for plastic gears due to its excellent high-temperature resistance, high strength, self-lubrication, and friction resistance. In this study, the effect of different humidity levels on the tribological properties of PA66 materials in self-mated contacts are investigated using a pin-on-disk test rig. The results show that the friction coefficient and wear rate of the PA66-PA66 sliding combination increase drastically after humidity treatment mainly due to the surface plasticization caused by water absorption and the decrease of cohesive strength and glass transition temperature. Moreover, the limiting PV value of PA66 materials decreased significantly after moisture absorption, and when the actual PV value exceeds this reduced material limit, the degree of friction and wear increases drastically. The wear mechanism of the PA66-PA66 sliding combination is mainly adhesive wear without humidity treatment. The wear mechanism is adhesive wear combined with abrasive wear after humidity treatment (50%, 70%, 90%, immersion in water) and abrasive wear is most significant at 50% humidity. Abrasive wear decreases with the increase of the moisture content, while adhesive wear increases.

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Mechanical contacts in dry conditions are often characterized by an interfacial layer called “third body”, which generally originates from the degradations of the surfaces, but which can exhibit strongly different material properties. This layer is a direct consequence of past wear, but also exerts a control on the rate at which surfaces in contact will keep getting worn. A comprehensive understanding of mechanical contacts therefore relies on a theory describing the interplay between this sheared layer and the moving surfaces which confine it. In this paper, we make a step towards such a theory by quantitatively investigating the link between the flow regime of the third body and the mechanical loading it applies to the surfaces. For that purpose, a previously developed local model of solid flow based on the Multibody Meshfree Approach is employed, in order to simulate characteristic flow regimes identified in experiments. Typical stress concentration patterns endured by the surfaces are then described and quantified, and a simple damage model is used to demonstrate how such a model could lead to wear prediction. We demonstrate that agglomerated flow regimes are prone to enhance large and deep damaging of surfaces, while granular third body flows have a more limited and shallow damaging effect.

Hydrogen embrittlement (HE) can cause catastrophic failure of stainless steel valve and related components in hydrogen refueling stations (HRSs), reducing reliability, safety and increasing the cost. Here, in this study, the ability of chemically modified polytetrafluoroethylene (CM-PTFE) coatings on steel substrates in reducing the HE susceptibility and, the friction and wear of valve parts, are explored due to its low hydrogen (H₂) permeability and excellent solid lubrication. The solid lubrication properties of CM-PTFE-coated steel samples were investigated before and after H₂ charging at a pressure of 7 × 10⁵ Pascals. After H₂ charging, the samples were subjected to CHNS and X-ray diffraction (XRD) analysis to quantify the percentage of H₂ absorption and its effect on crystallinity of the samples, respectively, and interesting insights were obtained from both CHNS and XRD analysis. Furthermore, the effect of H₂ charging on uncoated steel discs and CM-PTFE-coated discs were thoroughly investigated by hardness measurements, tribological characterization, wear behavior analysis of discs and pins and chemical elemental mapping. All test results are harmoniously suggesting that the H₂ charging indeed softened the material significantly. The developed double function CM-PTFE coatings can minimize H₂ permeability and also reduce friction, and wear of the components in HRSs.

Adsorptive polymer additives have been reported to improve the retention capacity of oil films under hydrodynamic lubrication and to reduce friction under boundary lubrication. These effects are believed to result from the formation of a polymer adsorption film on the surface that acts as a lubricious coating. Polymer adsorption films have become dominant in nanometer-order microscopic gaps. However, their mechanical properties are difficult to quantify. This hinders the development of polymer additives. In our previous study, we successfully measured the shear viscoelasticity of lubricants (base oils) sheared in nanogaps using an originally developed measurement method called the fiber wobbling method (FWM). In this study, we measured the shear viscoelasticity of polymer-added lubricants in nanogaps by using FWM. In addition, we developed a heating stage in the FWM to quantify the temperature dependence of shear viscoelasticity in nanogaps. As a result, the viscosity index improved and elasticity was observed in the nanogap, where the polymer adsorption film was dominant. Furthermore, our results indicated that the elasticity of the adsorbed polymer film originated from entropic elasticity.

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