Tribology Letters最新文献

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.

Autoparametric resonance in a combined contact (tip apex)–driving-spring (cantilever) system, that is responsible for the appearance of multiple peaks in friction as a function of scanning velocity, is investigated in a wide range of the possible system damping. The role of cantilever damping, being practically inessential in conventional stick–slip regime at lower velocities, is shown to be crucial for the appearance of friction force peaks at the resonant and quasi-resonant velocities. With changing damping, the evolution of different peaks turns out to be nontrivial, that is related with an unusual manifestation of double slips of the tip and memory effects. Relative value of the main force peaks as functions of damping factor are non-monotonous with maximum, which can reach several tens percent, depending on the system parameters and temperature. Such a strong resonance enhancement of energy dissipation is likely to occur in practical systems, where damping of driving spring can be significant, in contrast to nearly ideal cantilevers used in AFM experiments.

Graphical Abstract

The friction behavior between soft and hard materials has long been a crucial research subject in diverse fields, such as artificial joints, human skin contact, and robotic grasping. This study combines theoretical analysis with experimental exploration. By analyzing the variation in friction force and the characteristics of the sliding traces of silicone rubber materials during the entire friction process, it aims to delve into the frictional characteristics of soft–hard material interfaces. The results show that the sliding behavior at the soft–hard interface occurs before the peak of static friction. Through the approximation of a frictional theoretical model, the relationship between the friction force and the relative sliding distance at the interface is verified. Moreover, a method for predicting the interface frictional contact state and the relative sliding distance at the interface based on the friction force–tangential displacement curve is proposed. This research enhances our understanding of the friction mechanism at soft–hard interfaces and offers both theoretical support and practical guidance for the development of fine tactile feedback and dexterous manipulation systems in robotic grasping.

A novel methodology using atomic force microscopy (AFM) has been developed for assessing the effect of surface oxidation on the adsorption and frictional properties of oiliness additives. This study focused on comparing the tribological behaviors of ester-based oiliness additives on oxidized versus pure Ti surfaces. The substrates were patterned to facilitate precise AFM-based friction testing. The oxide layer thickness was characterized by X-ray photoelectron spectroscopy and subsequently abraded using a diamond-coated AFM probe to expose the pure Ti surface. Tribological performance under atmospheric conditions was evaluated by comparing frictional properties using ester-based additives of varying ester functionalities, ranging from monoester to tetraester. Adsorption properties were characterized through neutron reflectometry and contact angle measurements. The results demonstrated a clear correlation between adsorption density and friction reduction, with tetraester showing superior performance, especially on pure Ti surfaces. These findings highlight the critical effect of oxidation states and additive molecular structures on frictional properties and adsorption behaviors at nanoscale resolution, providing valuable insights into boundary lubrication mechanisms and lubricant optimization.

This study examined and compared the tribological properties of a cold-sprayed CrMnCoFeNi high entropy alloy (Cantor alloy) coating under ambient and dry air conditions. Tribological testing was conducted using an in situ tribometer equipped with video microscopy, allowing real-time monitoring of the evolution of the sliding interfaces through a transparent sapphire counterface. This experimental setup provided the opportunity to observe phenomena that would otherwise remain concealed between the contacting bodies. The wear rate was 1.8 ± 0.5 × 10⁻⁴ mm³/Nm in ambient air and 7.5 ± 0.7 × 10⁻⁴ mm³/Nm in dry air. In dry air, the velocity accommodation mode was characterized by interfacial sliding of a static transfer film against the wear track, resulting in a stable steady-state coefficient of friction (CoF) of 0.5. In contrast, ambient air conditions led to an average CoF of 0.8, with fluctuations attributed to plastic shearing of the transfer film observed in situ. The higher humidity in ambient air inhibited cold welding of wear particles, resulting in a less stable transfer film that underwent removal or extrusion events, which were associated with sudden drops in CoF. Additionally, a “metal debris” oxide formation mechanism was observed in ambient air, contributing to the formation of a protective tribofilm and a reduction in the wear rate. In dry air, the “oxidation-scrape-reoxidation” mechanism dominated, facilitated by the absence of adsorbed water droplets. This resulted in an increased wear rate under dry conditions.

Graphical Abstract