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The adhesion and damage behaviour of wheel–rail systems play an important role in the safety of railway operations. To verify the low adhesion behaviour of wheel and rail contact during the train entering tunnels in alpine regions caused by intermittent humid and warm airflow, tribology testing using a rolling–sliding contact in the laboratory was conducted to investigate the effect of humid and warm airflow with various ambient temperatures (− 55 to 60 °C) and airflow with different humidities (relative humidity from 5 to 99%) on the instantaneous low adhesion and damage failure between the wheel–rail interface. Results indicate that the instantaneous low adhesion phenomenon and multifarious damage are affected by the humidity of the airflow and the ambient temperature. In the temperature range of − 55 to − 20 °C, the wheel–rail interface will be in a low adhesion state once it encounters a humid and warm airflow attack. As a result, the adhesion coefficient curve exhibits a ‘comb-shaped’ feature. In this temperature range, the wear surface is rougher under the action of high moisture flow, and the main damage mechanism is fatigue wear. In particular, the moisture in the airflow will liquefy into water or even condense directly into frost and ice film on the wheel when the humid and warm airflow encounters the cold wheel surface. Consequently, the adhesion coefficient exhibits a ‘comb-shaped’ feature. In the meantime, the tribological oxidation reaction is more active at humid airflow conditions of 40–60 °C. The high humidity environment is conducive to the formation of water paste during friction, that is, a mixture of oxidized debris and water molecules that acts as friction-reducing layers, which also reduces the adhesion coefficient. These results will help deeply understand that, the wheel–rail adhesion coefficient drops sharply in an instant once the wheel–rail interface encounters the humid and warm airflow in the tunnel. Thus, railway operations department have to be very cognizant of the low adhesion problem, and take effective measures to improve the adhesion of wheel-rail interface to avoid the occurrence this kind of problem. Graphical Abstract

Rolling–sliding contacts are found in a variety of systems, such as gears, drum brakes, and tire pavements. Such systems inherently have multiple nonlinearities such as kinematic, contact, and friction nonlinearities. Further, most of these systems have clearance between components which causes excessive vibration and noise. The combination of clearance with other nonlinearities makes the system dynamically interesting. It is important to understand the dynamic behavior of such systems under various operating conditions. Hence, this article focuses on theoretically investigating the transient and steady-state responses of a cam–follower system with rolling–sliding contact and clearance as an exemplary case. A contact mechanics-based model for the same has been developed for this purpose. The transient behavior of the system is examined based on energy and time-varying frequency contents. The domain of attractors, frequency response plots, and phase portraits are deployed to analyze the effect of initial conditions and excitation speed on the steady-state behavior, quantitatively and qualitatively. The steady-state solutions were classified into various branches based on periodicity and phase portraits. In addition, parametric analyses of the effects of loading, damping, and friction on the response have been conducted. Finally, the system is examined for start-up and shut-down characteristics with a constant rate of change in excitation frequency.

This research aims to examine the relationship between roughness, film thickness, and friction coefficient in relation to pitting wear resistance during the rolling-sliding contact fatigue of forged and plasma-nitrided gears. In the present work, forged gears in continuous cooling bainitic steel were manufactured and plasma nitrided with three sets of N2-H2 gas mixtures, containing 5 vol%, 24 vol%, and 76 vol% N2. The present investigation evaluated the contact analysis between gears from the macroscopic point of view and the microscopic. At the same time, the evolution of roughness and pitting damage area after each test stage was monitored. The study determined the stress field after each loading cycle and correlated the regions of higher stresses with surface and sub-surface fatigue. Weibull’s statistical approach showed that nitrided gears with 24 vol% N2 exhibit the most reduced pitting wear rates among the gears tested. This outcome is attributed to the optimal balance of surface hardness, fracture toughness, compound layer depth, and surface phases. The most pitting damage occurred in the dedendum and pitch line regions, being the regions that reported the greatest Hertz contact pressure. This is due to the rolling direction being opposed to the friction force in the dedendum region. This paper shows that pitting wear intensifies with increasing roughness, but this same behavior was not observed between the wear evolution and the maximum shear stress field in the sub-surface. Another interesting fact is that nitrided gears with 24 vol% N2 (best condition) have a greater film thickness at the end of the rolling-sliding contact fatigue, which gives it greater protection, less friction, and pitting wear. In general, the cracks were observed in areas very close to the surface and, with the course of the propagation stage, reached the surface, causing the formation of damage by pitting or spalling.

The magnetic track brake is a mechanical contact (with friction) based braking system that is typically actuated electromagnetically and used as an emergency brake in railway transport. Within this paper, the practically relevant task of predicting the effective local and global forces of the contacting bodies and the respective deformations during the quasi-static braking process is addressed. Therefore, a simplified, yet efficient and accurate numerical contact model is developed to treat the frictional sliding contact problem. In order to verify and validate the model a couple of numerical experiments are carried out. The proposed model and algorithm are first tested against an analytic benchmark problem of a parabolic indenter indenting an elastic half-space. The developed model is then compared against a reference Abaqus finite element simulation in application-oriented braking simulations that treat the contact problem between a single braking element (pole shoe) and the rail. The results demonstrate and highlight the applicability and efficiency of the proposed model but also show the current limitations and shortcomings that hint at possible future augmentations.

This paper presents a comprehensive study on the vibration-based wear evolution characterisation by coupling the dynamic behaviour, surface roughness, and lubrication effects under rolling-sliding contact. Initially, a dynamic model is developed to examine the contact vibration characteristics induced by a randomly rough surface. A contact resonance frequency (CRF) can be obtained, and it is only positively correlated with the load, while the amplitude of CRF (CRFA) negatively correlates with the load and positively correlates with the velocity and surface profile height. Thereafter, a mixed-EHL model is employed to simulate the wear and lubrication progression of rolling-sliding contact. The surface roughness, contact load ratio (CLR), and contact area ratio (CAR) within this process are assigned specific physical interpretations and incorporated into the dynamic model. Given the nonlinear and coupled interactions of these three factors, the CRF and CRFA can distinguish the normal wear and severe wear stages. When the tribo-pair is in a normal wear stage, the CRF and CRFA show an increasing trend with the increase in surface roughness. Upon reaching a severe wear stage, the CRF gradually stabilises while the CRFA exhibits noticeable irregular fluctuations as the roughness increases. Finally, experiments are conducted to demonstrate the effectiveness of this method.

A magnetic ionic liquid (abridged as MIL) [C 6 mim] 5 [Dy(SCN) 8 ] was prepared and used as the magnetic lubricant of a steel-steel sliding pair. The tribological properties of the as-prepared MIL were evaluated with a commercially obtained magnetic fluid lubricant (abridged as MF; the mixture of dioctyl sebacate and Fe 3 O 4 , denoted as DIOS-Fe 3 O 4 ) as a control. The lubrication mechanisms of the two types of magnetic lubricants were discussed in relation to worn surface analyses by SEM-EDS, XPS, and profilometry, as well as measurement of the electric contact resistance of the rubbed steel surfaces. The results revealed that the MIL exhibits better friction-reducing and antiwear performances than the as-received MF under varying test temperatures and loads. This is because the MIL participates in tribochemical reactions during the sliding process, and forms a boundary lubrication film composed of Dy 2 O 3 , FeS, FeSO 4 , nitrogen-containing organics, and thioether on the rubbed disk surface, thereby reducing the friction and wear of the frictional pair. However, the MF is unable to form a lubricating film on the surface of the rubbed steel at 25 °C, though it can form a boundary film consisting of Fe 3 O 4 and a small amount of organics under high temperature. Furthermore, the excessive Fe 3 O 4 particulates that accumulate in the sliding zone may lead to enhanced abrasive wear of the sliding pair.

In this paper, we investigate the thermal friction sliding contact of a functionally graded piezoelectric material (FGPM)-coated half-plane subjected to a rigid conductive cylindrical punch. This study considers the effect of the thermal convection term in heat conduction. The thermo-electro-elastic material parameters of the coating vary exponentially along its thickness direction. Utilizing thermoelastic theory and Fourier integral transforms, the problem is formulated into Cauchy singular integral equations of the first and second kinds with surface stress, contact width, and electric displacement as the unknown variables. The numerical solutions for the contact stress, electric displacement, and temperature field of the graded coating surface are obtained using the least-squares method and iterative techniques. It can be observed that the thermo-electro-elastic contact behavior of the coating surface undergoes significant changes as the graded index varies from −0.5 to 0.5, the friction coefficient ranges from 0.1 to 0.5, and the sliding velocity changes from 0.01 m/s to 0.05 m/s. The results indicate that adjusting the graded index of the coating, the sliding speed of the punch, and the friction coefficient can improve the thermo-electro-elastic contact damage of the material’s surface.

Electrical brushes are essential parts of electrical circuits that can transmit electric power and signal between the moving and stationary parts by sliding, so they are exposed to both mechanical and electrical loading. In this study Cu-graphite brush was prepared by the powder metallurgy process, and was tested in rotating sliding electrical contact under different currents. The effect of electrical current on the tribological property of the Cu-graphite brush was investigated by evaluating the friction, wear, contact resistance and temperature. Results illustrate that the static contact resistance of Cu-graphite brush decreases sharply with an increase in the contact force. The friction coefficient and sliding contact resistance decrease gradually with increasing current, which is because of the Joule heat resulting in the reduction of the shear strength and the increase of real contact area. The formation of a continuous transfer layer on steel disc also contributes to the decrease of contact resistance. Oxidation of the wear debris and formation of hard particles causes abrasive wear. The dominant wear mechanism of the Cu-graphite brush with current is electrical wear. Graphical Abstract

This paper’s aim is to develop the tribological repair method in PEEK (Poly-ether-ether-ketone) polymer mechanical parts with surface damage. The radial rolling sliding contact tests were carried out by using a machined PEEK shaft with an artificial defect as surface damage. An alumina ball and a PTFE composite pin contacted a PEEK shaft specimen under cyclic compressive stress with friction. The particle as solid lubricant from a PTFE composite pin filled up the crevice of the artificial defect. After that, the artificial defect was covered with the deformed microgrooves on the PEEK shaft surface. Due to adhesion by compressive stress and friction, the crevice of the artificial defect was closed. In a running-in process, the artificial defect could be reduced by the adhesion repair.

Stresses due to frictional sliding contact between a rigid cylinder and laminated glass fiber composites are calculated in this study. A novel analytical formulation based on Cholesky decomposition, Fourier transforms, and singular integral equation is presented to solve the stress and displacement fields both at the contact patch and sub-surface, which can take into account transition of laminae with different fiber angle. An augmented finite element algorithm having an adaptive mesh refinement algorithm is also developed to verify the results of the analytical formulation. A perfect match between these results reveals the success of the new analytical formulation. Then, the formulation is implemented for various loading types and composite-related parameters to reveal the effects of those on the surface and sub-surface stresses. The results and discussions presented can be beneficial for the structural design of laminated composites under severe contact conditions.