Tribology Letters最新文献

Pitting is a typical failure behavior in rolling contact fatigue. Generally, there are two different pitting cases. One is the corrosion-induced pitting, and the other is the dent-induced pitting. This study focused on the dent-induced pitting. The surface dent is caused by the particle compression on the ring surface. During the compression process, grain distribution affects the dent geometry and the damage evolution behavior. However, researchers have almost ignored the effects of grain distribution on surface defect-induced RCF. Therefore, the authors proposed a crystal plasticity continuum damage method (CP-CDM) model by combining crystal plasticity constitutive equations with continuum damage equations to study the grain distribution effects on the damage evolution of the surface defect-induced RCF. The results show that the proposed model can simulate the crack propagation characteristics of the RCF. Grain distribution has effects on the damage propagation behavior randomly. However, the lead cracks have little difference for different grain distribution microstructures. The damage evolution behavior of pitting is affected by the interaction between stress concentration from the shoulder and the crack tip and strain localization from the grain boundaries. Moreover, the cracks in the surface pitting and the subsurface spalling evolve simultaneously.

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Hydrogels are soft and wet materials with remarkable properties. Among such properties, low friction has attracted much attention from scientists and engineers. While many studies investigated the correlation between material properties and frictional behavior, little attention has been paid to other effects, such as sample sizes. In this study, we developed a theoretical model based on the biphasic lubrication theory to describe the frictional behavior of hydrogels against a glass substrate in the rotational shear test. Consequently, we obtained analytical solutions for the time evolutions of friction coefficient, hydrostatic pressure, and elastic stresses. To validate our model, we prepared cylindrical PVA (Polyvinyl alcohol) hydrogels with different radii and thicknesses and conducted friction experiments. We confirmed reasonable agreements with theoretical predictions for the transient responses and their size dependences. Interestingly, we found that the friction coefficient at the initial phase decreased drastically with decreasing thickness. Our results indicate the fundamental importance of bulk transport in surface friction of hydrogels, and the controllability of friction by varying the geometry of hydrogels.

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Under lubrication conditions such as water and anti-wear fluids, the wheel–rail interface typically demonstrates a low-adhesion state. More and more evidence suggests that the adhesion coefficient is not static but dynamically coupled with the rolling and sliding behaviors of the wheels. When the slip ratio between the wheel and rail is high, strong friction can greatly improve the rail surface condition, resulting in adhesion recovery. Nevertheless, experimental research on wheel–rail adhesion under high-speed and high slip ratios is lacking, and especially the critical point of adhesion recovery has not been clarified. This study investigates the adhesion characteristics at a speed of 400 km/h and analyzes the relationship between changes in adhesion and wheel surface temperature under high slip ratio conditions. The results suggest that there is a strong correlation between the wheel surface temperature and the critical point for adhesion recovery. Lubricants such as water and anti-wear fluids have a critical lubrication failure temperature, and once this temperature is exceeded, the lubrication state will be damaged, leading to an adhesion recovery phenomenon.

The phenomenon of inter-yarn friction plays a crucial role in determining the behavior of woven materials. However, understanding and modeling this phenomenon remain complex due to the numerous parameters involved-such as normal force, velocity, angles, and sizing agents. To better analyze the influence of these various parameters on friction, a friction test bench without force sensors was developed, along with an associated finite element model. Motor control is used to vary carriage speed. An automatic image analysis method is used to measure the slippage between fiber tows. The model relies on an Arbitrary Lagrangian Eulerian (ALE) formulation, which provides simplified handling of inter-yarn contacts and slippage, while offering enhanced computational speed and robustness compared to a model based on a Lagrangian formulation. By comparing simulation results with experimental curves, a contact law was formulated, and its parameters were identified.

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Current research concerning Fe₃O₄ nanoparticles (NPs) as lubricant additives primarily focuses on the macroscale tribological performance, with insufficient exploration of anti-wear mechanisms at the microscopic scale. In this study, the formation and growth behavior of Fe₃O₄-based tribofilms was investigated. Oleic acid-modified Fe₃O₄ NPs dispersed in a polyalphaolefin base oil were tested between ZrO₂ balls and GCr15 steel substrates utilizing a reciprocating micro-tribometer. The morphology, microstructure, and chemical composition of tribofilms were meticulously characterized. Results revealed that the tribofilm primarily consisted of cubic Fe₃O₄ nanocrystals, consistent with initial particles. The organic modification layers of NPs were removed, facilitating direct inter-particle bonding. Growth mechanisms of tribofilms involving tribosintering of Fe₃O₄ NPs and shear-induced removal were proposed, demonstrating strong dependence on sliding cycles and contact pressure. Under an initial contact pressure of 1.15 GPa, the volume of the tribofilm increased with the number of sliding cycles, eventually reaching a state of saturation. While stress-dependent growth was observed, excessive stress led to wear on the substrate.

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The adhesion and damage behavior of the wheel-rail contact is crucial for the safety of railway operations. To investigate the effects of oxygen content and humidity of airflow on the adhesion and damage behavior of the wheel-rail interface, tribological testing of the wheel-rail interface is conducted using a rolling-sliding contact in the laboratory. Results show that the adhesion coefficient of the wheel-rail interface decreases with the increase of relative humidity (RH). The content of Fe²⁺ increases with the increase of humidity of the airflow, which promotes hydration reactions. Specifically, under the condition of RH = 50%, the presence of an appropriate amount of water molecules in the environment, results in a higher degree of oxidation on the worn surface of the wheels and the formation of a large amount of oxides. Furthermore, Fe³⁺ and Fe²⁺ are easily generated on the surface when the oxygen content around the environment is sufficient or insufficient, respectively. In the oxygen enrichment condition, a higher amount of Fe₂O₃ is observed, whereas for the hypoxic condition, the oxide film cannot be quickly formed again after being damaged, resulting in more severe wear on the wheel-rail interface because of the lower oxygen content in the surrounding environment. This work provides critical insights into the friction properties as well as wear mechanism in response to humidity and oxygen content when the wheel-rail interface encounters the humid and warm airflow with different humidity levels in the tunnel, which is crucial for proposing effective measures to improve the adhesion of wheel-rail interface and avoid the occurrence of the low adhesion problems.

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Low-speed pre-ignition (LSPI), where hot spots in the combustion chamber can cause fuel ignition to occur, can result in increased pressure in the combustion chamber, causing damage to the system and negatively affecting emissions. These hot spots have been identified as calcium (Ca) deposits, causing a push to reduce Ca in engine oils. For this study, tribofilms were generated with an amount of Ca so that LSPI did not occur. Zinc dialkyldithiophosphate (ZDDP), Ca, and magnesium (Mg) detergents were kept constant, with the metal in the tribofilm adjusted by varying the surfactants (friction modifiers) in the engine oil, ensuring enough ZDDP and detergent to be able to protect the engines. Energy dispersive x-ray spectroscopy (EDS) and atomic force microscopy (AFM) revealed three distinct tribofilm components: smooth, streaky films, deeper worn steel, and a mixed region of rougher tribofilm growth. Tribofilms with higher Ca had lower bulk friction than those with higher Zn, while AFM friction maps of the different sections showed that the friction in the mixed region was similar in magnitude to macro-scale friction, but film and substrate COF were much lower. Similarly, AFM modulus mapping of the mixed region showed higher Ca contributing to increased modulus when compared to Zn. Nanoindentation across the entire tribofilm revealed that higher modulus in the film and mixed regions contributed to more effective tribofilm coverage. Overall, Ca was shown to be critical to the wear and friction performance of tribofilms and their mechanical properties even when reduced to avoid LSPI.

Molybdenum dialkyldithiocarbamate is a highly effective friction modifier lubricant additive in boundary lubrication, owing to the formation of a MoS₂ nanosheet lattice structure that significantly reduces friction. The friction reduction behaviour is linked to the MoS₂ amount and coverage buildup at the contacting interface, however, accurately predicting friction reduction based on a semi-deterministic model incorporating MoS₂ formation and removal remains challenging. In this study, a Raman map collection methodology was developed for accurate quantitative analysis of MoS₂ tribofilms. The growth rate of MoS₂ tribofilms was determined by coupling tribochemical experimental data with sophisticated numerical models. A full numerical procedure was implemented under rubbing of two rough surfaces at different temperatures. The results demonstrated localised MoS₂ tribofilms buildup. The friction coefficients show a close agreement with the measurements. The developed model can be adapted to diverse experimental setups and surface geometries.

Hydrogen internal combustion engines are transitional power devices for cleaner and low-carbon energy in transportation. However, high-temperature engine lubricants are prone to contact with hydrogen and oxygen, flowing through metal components during operation. The corrosive properties of the lubricants containing these gases and their tribological performance after prolonged contact with gases and engine metal parts have yet to be studied. This paper investigates the corrosive properties of fully formulated lubricants contacted with steel and metal coupons with air and varying flow rates of hydrogen introduced, as well as the tribological performance of the post-corrosion lubricants. The results show that as the hydrogen flow rate increases from 0 to 12 lph, the weight loss rates of copper and lead coupons decrease by 74.4% and 79.7%. Energy-dispersive spectroscopy and gas chromatography–mass spectrometry confirmed the reduction in oxygen content on the metal surfaces and the degradation of additives such as diphenylamine by hydrogen during corrosion. Compared to those without hydrogen, the friction coefficients of the lubricants after exposure to metal and steel coupons and 12 lph of hydrogen decreased by 50.4% and 46.6%. This significant improvement in lubrication performance is attributed to the reduced degradation of ZDDP and the formation of more zinc phosphate and zinc sulfide during friction in the hydrogen-introduced post-corrosion lubricants, compared to post-corrosion lubricants without hydrogen exposure. The research of corrosive and tribological performance characteristics can be applied to enhance the design of reliable engine tribo-pairs and improve the lubrication requirements of engine oil in hydrogen environments.

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Shaft voltages in electric vehicle drivetrains are believed to be capable of causing significant damage to rolling bearing surfaces. While surface damage due to high current discharge in full-film lubrication is relatively well understood, the effects of electric potentials on surface damage and tribofilm formation in thin-film, mixed lubrication are rarely studied and remain poorly understood. This paper investigates the influence of relatively low DC potentials (1 V, 5 V) and currents (<10 mA) on wear and tribofilm growth in rolling–sliding contacts in operating mixed lubrication. A suitably modified ball-on-disc MTM-SLIM rig is used to apply an electric potential across a lubricated ball-on-disc contact. Tests were conducted with bearing steel specimens and three oils: PAO base oil, PAO with antiwear additive ZDDP, and a commercial automatic transmission fluid (ATF). The effect of electric polarity was an integral part of the study. The results reveal that even small electric potentials and currents significantly affect wear. For the oil containing ZDDP, electric potential both suppressed the formation of tribofilm and significantly increased the wear on the anodic surface, be it the ball or the disc. The wear was localised in discrete bands within the rubbing track. ZDDP film was shown to be electrically resistive and it is postulated that the non-uniform nature of the ZDDP film leads to the concentration of current within the thin-film regions and this high current density causes wear in these local areas (wear bands) via electric discharge. In contrast, with the ATF, electric potential promoted tribofilm on the anode but resulted in higher wear on the cathode disc. The study highlights a complex interplay between the electric potential, tribofilm formation, the consequent distribution of electrical resistance within the contact, and the resulting wear evolution.

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