Tribology Letters最新文献

Friction coefficient depends on various factors or surface characteristics during tribological testing, and this friction coefficient can be modified by altering the properties of one of the two contacting surfaces. It is crucial to monitor the friction coefficient continuously, not only at the conclusion of the test. This research examined the evolution of friction coefficient of silicon nitride (Si₃N₄) coating and H13 steel over different sliding distances (250, 500, 750, 1000 m). The study assessed surface wear and oxidation through three-dimensional profilometry and SEM/EDX. The findings indicated a reduction in friction coefficient by 22%, a decrease in wear rate by 88%, and a reduction in wear volume by 87% when comparing the silicon nitride coated steel to the uncoated steel. Furthermore, the changes in friction coefficient provided insights into the timing of the complete fracture of the hard coating.

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Ambient hydrocarbons adsorbed on the contact surface of nanoelectromechanical (NEM) switches would impact its performance. In this study, we utilized reactive molecular dynamics simulations to investigate the cyclic contact/separation process of Pt(111)/C₂H₂/Pt(111) systems. Our results demonstrate that substrate damage decreases as acetylene coverage increases from sub-monolayer to multilayer. This suppression occurs due to the presence of acetylene molecules, which can suppress direct (Pt–Pt connection) and indirect (Pt–(C_x)–Pt-like connection) interfacial bonding between the two substrates, depending on their coverage. Moreover, we observed the formation of chain-like oligomers after multiple contact/separation simulations in the monolayer model, much more significantly compared with the sub-monolayer and multilayer models. These oligomers arise from polymerization reactions among fragmented acetylene molecules, primarily formed through acetylene dehydrogenation. In the sub-monolayer model, numerous transferred Pt atoms at the interface hinder bonding between acetylene fragments, whereas in the multilayer model, only a few acetylene fragments form during the contact process, due to the well-organized and dense acetylene layer adsorbed on the substrate surfaces. These insights shed light on the atomic-scale mechanisms underlying substrate damage and chain-like oligomers formation in metal NEM switches.

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As railway transportation advances towards higher speeds and increased axle loads, the fatigue damage between wheels and rails has become more severe, significantly limiting the service life and safety of trains. Therefore, developing upgrade wheel-rail materials with enhanced contact fatigue properties has been considered an effective approach to avoid damage. This study reports a newly developed heavy-haul wheel steel with a superior rolling contact fatigue performance and the fatigue damage of wheel was studied by a novel RCF tester with a vision system. The results indicate that the newly developed heavy-haul wheel steel (NW) consists of smaller pearlite layer spacing and reduced proeutectoid ferrite. The NW steel demonstrates outstanding fatigue resistance in both oil and dry conditions, with a fatigue life 2.7 times longer than CL65 wheel steel and superior performance compared to most typical wheel steels. With increasing in pearlite content and decreasing in pearlitic interlamellar spacing, the fatigue damage degree of wheels under oil or dry contact conditions decreases obviously, leading to a significant enhancement in fatigue life. Properly controlling the pearlite content and the interlamellar spacing can optimize the fatigue properties of wheel materials. The vision system observed that the average area and perimeter of the defects gradually increased on the sample surface. The shape of the defect became more rounded under oil contact conditions but showed the opposite result in dry contact. When subjected to cyclic loading, surface cracks propagated along various paths after initiation, eventually forming different morphologies of peeling. The results will not only help optimize wheel materials for heavy-haul railways, but also offer an effective means for analyzing damage evolution in wheel-rail contact.

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In this study, we developed and parametrized a friction model for finite element (FE) cutting simulations of AISI4140 steel, combining experimental data and numerical simulations at various scales. Given the severe thermomechanical loads during cutting, parametrization of friction models based on analogous experiments has been proven difficult, such that the cutting process itself is often used for calibration. Instead, our model is based on the real area of contact between rough surfaces and the stress required to shear adhesive micro contacts. We utilized microtextured cutting tools and their negative imprint on chips to orient chip and tool surfaces, enabling the determination of a combined surface roughness. This effective roughness was then applied in contact mechanics calculations using a penetration hardness model informed by indentation hardness measurements. Consistent with Bowden and Tabor theory, we observed that the fractional contact area increased linearly with the applied normal load, and the effective roughness remained insensitive to cutting fluid application. Additionally, we calculated the required shear stress as a function of normal load using DFT-based molecular dynamics simulations for a tribofilm formed at the interface, with its composition inferred from ex-situ XPS depth profiling of the cutting tools. Our friction model demonstrated good agreement with experimental results in two-dimensional FE chip forming simulations of orthogonal cutting processes, evaluated by means of cutting force, passive force, and contact length prediction. This work presents a proof of concept for a physics-based approach to calibrate constitutive models in metal cutting, potentially advancing the use of multiscale and multiphysical simulations in machining.

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Constructing van der Waals (vdW) heterostructures offers a feasible approach for enhancing the electronic properties and obtaining superlubricity of various electromechanical devices. Here, the electronic and frictional properties of GeSe/SnS heterostructures were investigated through first-principle calculations based on density function theory. The GeSe/SnS bilayer with AC stacking configuration revealed a direct band structure with a gap value of about 0.95 eV and a standard typical type-II band alignment. Within the GeSe/SnS vdW heterostructure, AC stacking setup demonstrates a more uniform potential energy surface (PES) than AB stacking arrangement, verified by a lower friction barrier. The peak PES value of the GeSe/SnS heterostructure in AC stacking is merely 0.0016 eV. The weak vdW interaction between the adjacent layers in vdW heterostructure and smooth PES are responsible for reducing potential energy fluctuations and friction. The investigation on the friction characteristics of GeSe/SnS vdW heterostructure with AC stacking configuration provides valuable insights for understanding the atomic-scale friction behavior in two-dimensional (2D) materials.

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The friction and wear characteristics of a-C:H:Si and a-C:H coatings in an ethanol environment at 120 °C and 10 MPa focusing on the effects of water and dissolved oxygen in ethanol were investigated. The friction tests were conducted using an autoclave chamber, with gases (oxygen or nitrogen) and ethanol–water mixtures (0 and 6 vol.%). The wear acceleration of a-C:H:Si took place when it slid in ethanol with 6 vol.% water and pressurized by oxygen, thus the specific wear rate was the highest, approximately 1.8 × 10⁻⁷ mm³/Nm. The reason of this wear acceleration was assumed the effect of hardness reduction due to oxidation of the a-C:H:Si, so the O/C ratio by AES and the hardness of topmost surface by AFM nano-scratch were investigated. As a result, the higher O/C ratio showed higher specific wear rate due to the hardness reduction from 13.3 to 6.0 GPa. These findings highlight the role of water and dissolved oxygen in accelerating wear of a-C:H:Si coatings.

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There is an ongoing debate concerning the best rheological model for fluids sheared in elastohydrodynamic lubrication (EHL) where high pressures lead to a dramatic rise in Newtonian viscosity and high strain rates lead to pronounced shear thinning. Two classes of phenomenological models at the center of this debate are based on the work of Eyring and Carreau. We use intermediate scaling plots and a justification criterion proposed by Gao and Müser in a recent article (Tribol Lett 72(1):16, 2024) to evaluate the fitting of Eyring, Carreau, and Carreau–Yasuda (CY) models to rheological data on the shear thinning of squalane, a model EHL fluid, at 293 K over a wide range of pressures (P in (0.1,955)) MPa that change the Newtonian viscosity (eta _0) of squalane from (sim 10) to (sim 10^7) mPa s. The justification criterion enables a fair comparison of Eyring, Carreau, and CY models that have two, three, and four fitting parameters, respectively. We find that the use of Eyring model to describe shear thinning of squalane is justified over Carreau model and CY model with crossover parameter (a>1) for (eta _0 > 10^2) mPa s and (eta _0 > 10^3) mPa s, respectively. The unrestricted CY model produces (a<1) in all fits and fares better but is not justified over Eyring for squalane sheared at high Newtonian viscosities (eta _0 > 10^4) mPa s. More importantly, CY models fitted with (a<1) fail to produce a physical low-rate asymptotic behavior and an appropriate crossover to Newtonian flow. Our findings show that CY models with (a > 1) guarantee a proper description of the crossover from shear thinning to Newtonian zone, and should be the phenomenological model of choice for shear thinning at low pressures. As pressure rises and (eta _0) becomes large (e.g., (> 10^3) mPa s for squalane), the use of Eyring model to describe shear thinning is justified.

To delve into the mechanisms of lubricating additives in electrically charged environments, this study utilizes a non-covalent modification method combining N-butylpyridinium tetrafluoroborate ([BPy]BF₄) with multilayer graphene (MG) to create graphene/ionic liquid (G/IL) composites. These composites were tested as lubricating additives in polyalphaolefin 40 (PAO40) using the UMT-2 experimental platform to assess their performance and electrical regulation mechanisms. Results demonstrated that G/IL composites significantly enhance lubrication and electrical stability. The study discovered that varying the current’s intensity and polarity substantially influences ion concentration and Zeta potential at the interface, reducing the electroviscous effect and facilitating the formation of an interfacial adsorption film. The interplay of these mechanisms greatly optimizes the interface condition. Additionally, real-time contact resistance data indicated a correlation between friction coefficient and contact resistance, validating the synergistic effect’s impact. This research not only clarifies the complex action mechanisms of lubricating additives in charged conditions but also offers critical insights for designing highly efficient lubricating materials.

While ZDDP tribofilm formation has been widely studied, the mechanism of ZDDP tribofilm removal during rubbing is still unclear. The study employs a ball on disc tribometer to monitor ZDDP tribofilm development in rolling-sliding, mixed lubrication conditions. It is found that when ZDDP tribofilms are formed very rapidly, as is the case with short alkyl chain, secondary ZDDPs, a large proportion of the initially-formed tribofilm is suddenly lost during rubbing. By contrast, the tribofilms that form more slowly from primary ZDDPs and longer chain secondaries are not partially lost during rubbing. XPS analysis showed that a rapidly-formed tribofilm before its partial removal has a very small Zn/O ratio, and a high BO/NBO. This suggests that such tribofilm contains a significant proportion of ultraphosphate, which is likely to have a relatively weak structure due to lack of stabilising cations. This may result in the tribofilm being partially removed when it reaches a certain thickness. By comparison, the remaining tribofilm, and also tribofilms that form slowly, have high Zn/O and low BO/NBO. This suggests that they consist of short chain polyphosphates and are thus stronger and more durable.

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A sealed reciprocating tribometer has been used to study the influence of different gaseous environments on the friction and wear properties of AISI52100 bearing steel at atmospheric pressure and 25 °C. Helium, argon, hydrogen, carbon dioxide and nitrogen all give high friction and wear, suggestive of very little, if any tribofilm formation under the conditions studied. Dry air and oxygen also give high friction, slightly lower than the inert gases, but produce extremely high wear, much higher than the inert gases. This is characteristic of the phenomenon of “oxidational wear”. The two gases ammonia and carbon monoxide give relatively low friction and wear, and XPS analysis indicates that this is due to the formation of adsorbed ammonia/nitride and carbonate films respectively. For the hydrocarbon gases studied, two factors appear to control friction and wear, degree of unsaturation and molecular weight. For the saturated hydrocarbons, methane and ethane give high friction and wear but propane and butane give low friction after a period of rubbing that decreases with molecular weight. The unsaturated hydrocarbons all give an immediate reduction in friction with correspondingly low wear. Raman analysis shows that all the hydrocarbons that reduce friction and wear form a carbonaceous tribofilm on the rubbed surfaces.

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