Tribology Letters最新文献

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.

Graphical Abstract

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.

Graphical Abstract

Dodecenylsuccinic anhydride was used as a surface modifier to further modify amino-functionalized silica nanoparticles, and the modified nanoparticles (RNS-1A-DDSA) showed good dispersion stability in both nonpolar PAO6 and polar DIOS base oils. The tribological properties of RNS-1A-DDSA were evaluated using an oscillating reciprocal friction and wear tester (SRV-5), and it was found to exhibit excellent tribological properties in both base oils with different polarities. EDS and SEM were used to characterize the wear surfaces, and combined with the results of AFM and QCM adsorption experiments, the reason for the applicability of RNS-1A-DDSA in both polar and non-polar base oils was deduced.

Atomistic simulations are performed to assess how the main characteristics of a pairwise interatomic potential function can affect the occurrence of wear. A Morse-like potential is tailored in its attractive part such as to vary independently the cut-off radius and the maximum value of the attractive (adhesive) force. An ideal numerical experiment is then performed where the interaction between a metal crystal and a probe changes, while their material properties are not affected, to isolate the behavior of the interface. Force functions with larger adhesive force can loosely be interpreted as describing dry contacts while those with smaller adhesive force can be interpreted as describing lubricated contacts. Results demonstrate that the occurrence of wear is strongly dependent on the shape of the interatomic force field, and more specifically on the combination of maximum adhesive force and effective length of the interatomic attraction. Wear can initiate also at small adhesive energy, provided that the maximum adhesive force between atoms is large. When the surface of the crystal is taken to be rough instead of flat, the effect of the interatomic potential function on friction and wear becomes smaller, as the atoms belonging to the roughness are weakly bound to the rest of the crystal and are easily dislodged with any of the force functions we used.

Due to tribocorrosion, metal ions are released from metallic components in hip implants and cause adverse reactions. The adverse reaction sensitivity to metal ions showed high dependency on individual patient and it has been recognized that adverse reactions even occur in patients with metal-on-polymer articulations. In this study, based on a tribocorrosion model for CoCrMo alloy, a lubricated wear accelerated corrosion model was developed for CoCrMo alloy–UHMWPE tribocorrosion contacts. The model was verified and calibrated using laboratory tribometer experimental results and was used to predict metal ion release from CoCrMo alloy heads in MoP hip joints. The results showed correspondence between model predicted wear accelerated corrosion and literature reported material loss of CoCrMo alloy heads in MoP hip joints tested using hip joint simulators. This model provides a tool to predict the level of metal ions released from MoP hip joints and has the potential to be used by medical doctors to evaluate the risk of adverse reactions for patients planned to receive a MoP hip implant.

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The rheological properties of lubricating greases are determined by the viscosity of the base oil, the interaction between base oil and thickener, and the interaction between thickener particles. The contribution of the oil–thickener interactions to the viscosity is well known, but the contribution of the thickener–thickener interactions has not yet been studied by employing theoretical or computational frameworks. In this paper, we use coarse-grained molecular dynamics to simulate a fibrous microstructure, and we show that the experimentally observed viscoelastic/plastic behaviour can be well reproduced. A parametric study shows that the apparent viscosity increases with increasing fibre length, fibre stiffness and thickener concentration. This is as expected, showing that this modelling approach is useful to study effects on grease rheology that are not accessible experimentally, such as impact of fibre entanglement or agglomeration.

Solid lubrication is a green manufacturing technology with high efficiency, which saves energy and material and thus it is suitable for extreme conditions in mechanical engineering fields such as aerospace and high temperature mold. In this study, a graphite layer of specified thickness was prepared on the end face of the upper specimen by the directional spray method. The effect of velocity and load on the friction characteristics of the graphite layer were investigated using a friction tester capable of real time observation of the friction interface. Subsequently, a 3D surface profilometer, SEM, and EDS were used to characterize the morphology and elemental composition of the worn surfaces. The results show that the lubrication performance of the graphite layer is most effective with a flatter worn surface (S_a and S_Z are smaller) and higher carbon content when the velocity is 12.5 mm/s and the load is 4N. Meanwhile, force chains are short, numerous and lasting for a long time, while being uniformly distributed in all directions and velocity fluctuates greatly, with slowly decreased coordination numbers. This study aims to provide a reasonable explanation for the mechanisms by which velocity and load influence the lubrication effect of the powder layer.