Tribology Letters最新文献

Accurately predicting the interfacial gap and stiffness between rough solids is essential for understanding contact behavior in tribological, sealing, and thermal systems. This work proposes a novel model that accurately captures the evolution of mean interfacial gap and stiffness from initial finite contacts to accumulated statistical response. The obtained load-gap relationship is validated against boundary element simulations of elastic rough contact, showing excellent agreement across a wide range of normal loads. Unlike conventional models, the proposed approach captures finite-size effects at light loads, yielding a sublinear power-law scaling of stiffness with load, and transitions seamlessly to the linear regime at higher loads. The model provides a simple, parameter-free tool for quantitative analysis of rough surface contacts.

The Stribeck curve forms the backbone of our understanding of frictional behavior. It has traditionally been associated with lubricated journal bearings and later on with EHL lubricated rough contacts. Still later, this behavior appeared to manifest itself also in dry rough contacts. While in this latter case, the Stribeck behavior has been simulated theoretically, in the lubricated case, the models have been confined to a few restricted cases. Here, I show that the Stribeck behavior emanates directly from the most basic/generic type of hydrodynamic bearing; namely the Rayleigh-step bearing, without resort to the EHL/mixed-lubrication assumption. This bearing type can be taken in the macro sense, as a self-standing bearing; or in the micro sense, as an elemental roughness feature, which can be aggregated to understand rough lubricated sliding contacts. Furthermore, this simple model of a step bearing allows us to establish the dynamic behavior of the Stribeck curve; in particular, the friction lag phenomenon and the rate-state law. Main results show that the normalized CoF is a universal function of the normalized Sommerfeld number, reduced by the aspect ratio of the bearing, and that the dynamic Stribeck effect cannot be decomposed into time-dependent Stribect effect combined with time-independent (instantaneous) viscous effect.

Accurate in situ wear measurements of polymer-polymer wear combinations using techniques like ball-on-prism tribometers pose a significant challenge. Simultaneous wear of both counterparts complicates obtaining continuous wear data without interrupting experiments to perform slow, intermediate measurements. Building on Harden et al. (Tribol Lett 71(3):1–14, 2023), this work develops an accurate, low-noise measurement approach to enable reliable in situ measurements. The method employs a laser line scanner and a precision linear axis for accurate positioning within the test stand, complemented by an algorithm for debris detection and data smoothing to minimize noise from wear residue. This approach achieves up to a 98% reduction in peak-to-peak noise levels, enabling high-resolution measurements and supporting parallel experiments with various polymer-polymer wear combinations. Unlike existing non-in situ methods, this technique offers efficient and accurate in situ wear measurements, advancing tribological research and polymer material testing.

This study investigates the influence of the gas atmosphere on friction and wear characteristics of transition metal dichalcogenides synthesized via an in-operando method. MoSe₂ and WSe₂ layers are formed through tribochemical reactions between molybdenum or tungsten coatings and nanosized selenium powder. Their tribological behavior is evaluated under nitrogen and ambient air conditions using a ball-on-disc tribometer within a custom-built inert gas chamber. The results indicate that both MoSe₂ and WSe₂ layers exhibit consistently low friction coefficients (below 0.1) across different atmospheric conditions, with slightly lower friction coefficient observed in air compared to nitrogen. However, the durability of the transition metal dichalcogenide layers is significantly extended under nitrogen, particularly for MoSe₂, which demonstrates superior durability compared to WSe₂ in nitrogen. This disparity becomes less pronounced under ambient air conditions. The enhanced wear resistance of MoSe₂ in nitrogen may be attributed to easier in-operando formation, as confirmed by transmission electron microscopy analysis, which reveals a thicker and more well-defined layered tribofilm. These findings suggest that in-operando synthesized MoSe₂ and WSe₂ layers serve as effective solid lubricants, offering promising potential for a broader range of solid lubrication applications.

Graphical Abstract

The widespread use of retreaded tires has led to great concern about their uncertainty in terms of atmospheric pollution because their degraded tribological performance leads to emit more tire wear particles (TWPs) into the environment. This paper presented the features of TWPs from retreaded tire rubber via a self-developed rolling contact test rig. Normalized comparison, emission per unit worn mass and per kilometer were employed to compare the influence of the test parameters both on emission and on environmental impact quantitatively between the new tire rubber (N-TR) and tire rubber mixed with reclaimed rubber (RR-TR). The results showed that heavy load could induce retreaded tire emitting TWPs_{3.0 and 5.0} a maximum of 12–19% higher than N-TR. Meanwhile, N-TR could emit TWPs_3.0 1.2 times greater than RR-TR would when the same mass was worn, implying that N-TR could be 1.2 times more dangerous than RR₁₀-TR to the public respiratory health, which, however, must be treated dialectically and critically. Furthermore, in the same travel distance, the risk extent of retreaded tire to environmental pollution could enhance 7–9% than that of new tire according to the TWPs amount. Additionally, an interesting and practical comparison shown that negative filler, e.g., reclaimed rubber, which had inactive impacts on anti-wear performance, could induce more individual TWPs with pitted, and bumpy surfaces rather than uneven-surfaced TWPs with small bulges governed by positive filler, e.g., carbon nanotubes. Superior molecular-chain mobility and stronger interfacial adhesion in retreaded tires are the dominant mechanisms responsible for their higher TWPs emission compared with new tires. This study provides a scientific basis for improving and minimizing the emission of the TWPs emission from retreaded tire, balancing the cost economy and the air quality based on the contributions.

Graphical abstract

The penetration depth of tread rubber is an important parameter in the tire-road contact process, which is directly related to the evaluation of tire-road wear and friction. A new method for calculating tread rubber-pavement contact penetration depth, which combines theoretical derivation and simulation analysis, is proposed in this paper. Specifically, the equivalent contact model of two elastic bodies was constructed based on Persson theory, and the pavement was simplified to a rigid substrate. The self-affine fractal characteristics of pavement are verified by the height difference function, and the theoretical model of penetration depth was derived by combining the energy conservation principle. On this basis, ABAQUS was used to establish a finite element model, and MATLAB was used to process relevant data to realize numerical simulation calculation of penetration depth. By comparing the analytical data with Persson's theoretical model, the coefficient of determination R2 is greater than 60%, which proved the feasibility of this method. Based on the above method, the influence of rubber aspect ratio and material parameters on penetration depth was further explored. The results showed that the penetration depth fluctuated with the increase of aspect ratio when the aspect ratio of tread rubber was 16.67% ~ 33.33%, and the effect of aspect ratio on penetration depth was negligible when the aspect ratio was 33.33% ~ 100%. The elastic modulus of rubber is negatively correlated with penetration depth. Based on the above rules, a multiple linear regression prediction model was constructed. Three independent datasets were used for verification, with relative errors ranging from 0 to 5%, indicating that the model has good prediction accuracy. This study provides a reliable method for efficient calculation of tire-road contact penetration depth and has important reference value for optimizing tire design and improving driving safety.

We develop an analytical model to describe how an energetic model of friction (JKR-Griffith model) between nominally flat rough surfaces leads to an inception of slip which is governed by an elastic instability. By extending classical contact mechanics from Persson’s solution with a JKR approach, the model captures the transition from sticking to sliding under shear. The relation between mean shear and mean interfacial slip is derived. It reveals that static friction can exceed kinetic friction and that this enhancement depends on surface roughness and normal load. The model predicts a saturated enhancement in static friction at small pressure and diminishing value at high pressure. Such enhancement will be suppressed if the roughness amplitude of the surface is magnified. Comparisons with experimental data show good agreement, after considering that friction energy is time-dependent, offering insight into adhesion-driven friction in applications ranging from microscale to tectonic plate scales.

Organic friction modifier additives, such as fatty acids, are widely used in the boundary lubrication regime to reduce friction. In this study, we employed resonance shear measurements to study 0.1 wt% solutions of branched-chain fatty acids in a linear model base oil/hexadecane and branched base oil/poly(α-olefin) (PAO) confined between mica surfaces under various applied loads (L). At L = 3.3 mN, the viscosity parameter (b_s) values of isostearic acid and isostearic acid T in hexadecane were one-seventh and one-thirtieth of the value of pure hexadecane, respectively. Especially, isostearic acid T in hexadecane had a lower viscosity parameter b_s value, one-eighth that of pure hexadecane even at L = 22.7 mN. On the other hand, the b_s value at L = 3.3 mN of isostearic acid in PAO was half the value of pure PAO and this effect disappeared at L = 11.6 mN; isostearic acid T in PAO resulted in a behavior similar to that of pure PAO. The densely packed structure of linear lubricants as hexadecane confined between the surfaces is known to cause a high b_s value and such a structure could be disturbed in the presence of branched-chain fatty acid additives. In case of the branched base oil, such as PAO, the densely packed state was not formed under confinement, thus the effect of the branched-chain fatty  acid additive was not significant. This study demonstrated that a relatively small difference in molecular structure of an additive is important for efficiently reducing friction, especially in the boundary lubrication regime of linear base oils.

Graphical Abstract

The current paper analyses surface measurement results from the “The surface-topography challenge”. By dividing each surface in four sub-surfaces, statistical methods can be used on the surface roughness parameters, as well as on the contact-mechanics parameters. Concerning the A14 and A15 surfaces, the (S_a) and (S_q) values of surface A15 are roughly 10% higher than those of surface A14. The roughness parameters of the surfaces Q53 and Q94 are virtually identical. The contact stiffness of the two A surfaces are identical, the same is true for the two Q surfaces.

The transient behavior of rate-dependent adhesion in poro-viscoelastic contact is more complex than crack propagation in Mode I opening due to time-dependent material behavior, crack acceleration from nonlinear kinematics, and variation in contact radius. This study revisits our previous experiment, where a spherical glass probe is unloaded on flat gelatin, and investigates crack velocity ((V_text {c})) and energy release rate (ERR). For a given unloading rate, (V_text {c}) increases monotonically by one order of magnitude, and the wide range of unloading rates ensures that (V_text {c}) spans 3–4 orders of magnitude. ERR remains almost unchanged at 2–3 times the thermodynamic work of adhesion at slow rates. At fast rates, ERR initially increases to 4–8, then decreases until full separation. We hypothesize that the decreasing ERR trend is due to finite-size effects: the hysteretic energy dissipation zone grows with crack acceleration, while the material volume decreases during peeling. To explain these trends and the finite-size effect, we adapt de Gennes’ viscoelastic crack propagation model, modifying it to account for crack acceleration and the reduction in contact radius. Under the given time scales (peeling time and viscoelastic relaxation time) and length scales (crack tip radius and initial contact radius), we simulate the evolution of ERR as peeling proceeds and compare the results with experimental data. The model’s results show good qualitative agreement with the experiments. Finally, we discuss the model’s limitations, assumptions, and directions for future research.