Tribology Letters最新文献

The impact of relative humidity on the formation of low-frictional interface in hydrogenated carbon nitride (CN_x:H) coatings sliding against Si₃N₄ balls and the formation continuity was elucidated through friction tests conducted in both air and nitrogen atmospheres with controlled relative humidity levels. In air atmosphere, a carbonaceous tribolayer with a transformed structure from the initial CN_x:H was formed on Si₃N₄ at less than the critical humidity that existed in 1.0–3.0% RH, resulting in low friction (μ < 0.05) and a low specific wear rate of the balls (< 2 × 10^–9 mm³/N·m). In contrast, this tribolayer failed to form above 3.0% RH. In nitrogen atmosphere, within the 0.25–1.0% RH range, the tribolayer continued to form concurrently with wear progression, maintaining low friction for over 50,000 cycles. However, in less than this humidity range, the lifetime of low friction was limited owing to the tribolayer’s structural alteration. Thus, relative humidity influences not only the formation of the low-frictional interface but also the formation continuity. On the CN_x:H friction surface, hydrogen, hydroxyl, and oxygen groups from environmental water and oxygen molecules continued to chemisorb owing to tribochemical reactions on the uppermost few nanometers during continuous low friction in a nitrogen atmosphere, while hydrogen content of CN_x:H desorbed. This study experimentally confirmed the critical role of controlling relative humidity in tribological systems using CN_x:H coatings to achieve low friction and improve its durability of low friction through the continuous formation of the low-frictional interface.

Shaft-surface friction pairs in marine equipment endure considerable wear and corrosion in seawater, particularly under cyclic loading. A friction and wear test bench was employed to replicate the conditions faced by ship stern shafts and bearings in seawater. The study aimed to assess the tribological performance of three types of polyphenylene sulfide (PPS) materials: pure PPS, PPS reinforced with 30% glass fiber (30% GF in PPS matrix), and PPS reinforced with 30% carbon fiber (30% CF in PPS matrix), against 316L stainless steel under varied lubrication conditions. Results demonstrate that all PPS variants exhibit minimal friction force and wear loss in seawater, with 30% CF in the PPS matrix displaying the least friction and wear characteristics. Friction force fluctuates within a 2.5 ~ 5 N range, and wear loss is 0.027 g. However, due to the uneven bonding of glass fiber (GF) within the PPS matrix, the addition of GF did not significantly enhance the materials’ anti-friction properties and wear resistance. The predominant wear mechanism involves severe three-body abrasive wear caused by GF detachment from the friction pairs surface. Additionally, the study explores carbon fiber’s resilience to cyclic loading, the lubricating effect of seawater mixed with carbon fiber debris, and the transfer mechanism of polymer films. These findings highlight synergistic anti-friction and wear-resisting effects between carbon fiber, seawater, and polymer transfer films, offering valuable insights for selecting effective shaft friction pairs materials in challenging seawater conditions.

On specific railway lines, trains are faced with the challenge of low-temperature environments. However, it is still unclear how the low-temperature service environment affects the friction-induced wear and vibration behavior at the train braking interface. Similarly, the impact of the brake pad friction block shape under low temperatures on this behavior is not well understood. This lack of knowledge adversely affects the operation, maintenance, and use of brake pads in low-temperature environments. Therefore, test samples of friction blocks were designed and processed based on several commonly used friction block shapes for trains (ellipse, triangle, and pentagon). Parking brake tests were conducted on a self-developed test bench with controllable temperature, exploring how different shapes affect friction-induced wear, vibration, and noise at the train braking interface under low temperatures (− 20 °C). The results showed that under the effects of low-temperature environments, the shape of the friction block significantly affects the friction-induced wear and vibration noise of the braking interface. Among them, the elliptical friction block has a smaller eccentric wear angle, a smaller and evenly distributed contact plateau area, relatively minor surface wear, and produces low-intensity vibration noise with small fluctuations. In contrast, the triangular and pentagonal friction blocks have larger eccentric wear angles, larger and concentrated contact plateau areas, relatively severe surface wear, and generate high-intensity vibration noise with large fluctuations. Analyzing the reasons, it is found that under the combined effects of low-temperature conditions and different shapes, the friction-induced wear on the surface of the friction block changes, thus exciting different intensities of vibration and noise.

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The wear state of mechanical seal friction pair directly determines the reliability of mechanical seal. In this paper, the mapping mechanism between contact wear response and acoustic emission (AE) signals of friction pair is indicatively proposed, and the relationship between wear frequency and time-averaged wear is explored. First of all, AE sensors were arranged on the Multi-function tribometer Rtec MFT-5000, static and dynamic friction tests were carried out on the contact form of M106K-WC (graphite-cemented carbide) and WC–WC, the AE signals are collected, and the wear amounts of the two groups of friction pairs were measured; then, the friction and wear signals are separated and reprocessed by time–frequency analysis. The results show that the static wear response frequency (SWRF) of M106K-WC is about 70 ± 10 kHz, the SWRF of WC–WC is about 90 ± 10 kHz, and the dynamic wear response frequency (DWRF) of WC–WC is about 175 ± 10 kHz; the root mean square (RMS) values of DWRF amplitudes is positively correlated with the wear amounts. According to the research results, it is inferred that there is a difference between the signal frequency in the quasi-static wear process and the dynamic wear process, there is a great correlation between the wear frequency and the material pair, and the working condition has little influence on the wear frequency. The mapping relationship between AE signal and time-averaged wear of friction pair is revealed.

Techniques preventing icing and ice accumulation on surfaces are required to solve snow- and ice-induced accidents and disasters. Recently, hydrophilic polymers have attracted attention as a passive anti-icing method. This study examined the ice-adhesion properties of the hydrophilic poly[poly(ethylene glycol) methyl ether methacrylate] (PPEGMA) concentrated polymer brush (CPB). A custom-built apparatus was developed to obtain the ice-adhesion strength and visualize the dynamics of the ice-adhesion interface under tangential loading. The ice-adhesion interface for a PPEGMA-CPB-coated glass substrate was investigated by comparing it with the bare glass substrate. As a result, the CPB exhibited a low ice-adhesion strength of less than 100 kPa, the dependencies of which on the drive speed and temperature indicate a high-viscous liquid-like layer at the interface, even below the melting point of water, leading to the smooth onset of sliding due to its self-lubricity without any rupture events (including precursory events) observed for the bare glass.

The spacer layer imaging method (SLIM) is widely used to measure the thickness of additive and lubricant films, in lubricant development and evaluation, and for fundamental research into elastohydrodynamic lubrication and tribofilm formation mechanisms. The film thickness measurement, as implemented on several popular tribometers, provides powerful, non-destructive in-situ mapping of film topography with nanometre-scale height sensitivity. However, the results can be highly sensitive to experimental procedure, machine condition, and image analysis, in some cases reporting unphysical film thickness trends. The prevailing image analysis techniques make it challenging to interrogate these errors, often hiding their multivariate nonlinear behaviour from the user by spatial averaging. Herein, several common ‘silent errors’ in the SLIM measurement, including colour matching to incorrect fringe orders, and colour drift due to the optical properties of the system or film itself, are discussed, with examples. A robust suite of novel a priori and a posteriori methods to address these issues, and to improve the accuracy and reliability of the measurement, are also presented, including a novel, computationally inexpensive circle-finding algorithm for automated image processing. In combination, these methods allow reliable mapping of films up to at least 800 nm in thickness, representing a significant milestone for the utility of SLIM applied to elastohydrodynamic contact.

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Using the molecular dynamics simulations we study sliding friction of two-dimensional systems with atom electronegative difference. We show that systems with large atom electronegative difference exhibit larger friction than systems with similar structures but less polarity. We demonstrate that the sliding friction along polar paths gives larger friction than along nonpolar paths, and exhibits stronger stick–slip behavior. Due to inertia and thermal effects the sliding path deviates from the minimum-energy path. We show that the electronegative friction is reduced by thermal fluctuations and that it depends linearly on the logarithm of the sliding velocity. Our findings will supply insight into the nature of the friction in low dimensional systems, which could facilitate the design of nanodevices.

Archard’s wear law encounters challenges in accurately predicting wear damage and volumes, particularly in complex situations like asperity–asperity collisions. A modified model is proposed and validated, showcasing its ability to predict wear in adhesive contacts with better accuracy than the original Archard’s wear law. The model introduces an improved wear coefficient linked to deformation energy, creating a spatially varying relationship between wear volume and load and imparting a non-linear characteristic to the problem. The improved wear model is coupled with the Boundary Element Method (BEM), assuming that the interacting surfaces are semi-infinite and flat. The deformation energy is calculated from the normal contact pressure and displacements, which are the common outputs of BEM. By relying solely on these outputs, the model can efficiently predict the correct shape and volume of the adhesive wear particle, without resorting to large and often slow models. An important observation is that the wear coefficient is expected to increase based on the accumulated deformation energy along the direction of frictional force. This approach enhances the model’s capability to capture complex wear mechanisms, providing a more accurate representation of real-world scenarios.

A renewed interest in elastohydrodynamic lubrication (EHL) phenomena at high speeds, for which thermal effects strongly influence both traction and film thickness, has grown out of the challenges presented by high-speed geared transmissions in electric vehicles. This study uses a new ball-on-disc set-up employing the well-known ultra-thin-film interferometry technique to simultaneously measure EHL film thickness and traction at entrainment speeds up to 20 m/s and slide-roll ratios up to 100%. The effect of fluid composition is examined for Group I, II and III mineral oils, for two polyalphaolefins in Group IV, and for the traction fluid Santotrac 50. The effect of viscosity in the range 4–180 mPa.s is investigated by varying bulk fluid temperature. At high speeds, both film thickness and traction are considerably lower than predicted by conventional EHL theory. The contact is seen to be fully-flooded for all conditions tested. The widely-used thermal EHL correction of Gupta is shown to overcorrect for the film thickness reduction even at modest SRRs. Finally, the influence of the sliding direction on traction and film thickness is discussed for this set-up, and a thermal model is proposed to explain the observed behaviour.

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Top-of-rail (TOR) lubricants are commonly used for friction control in railway operations. They aim to lower friction and reduce noise and wear while ensuring sufficient transmission of traction/braking forces. However, the wheel–rail interface is an open system, so the conditions may suddenly change due to the weather, and different contaminants may enter the contact and influence the performance of these lubricants. Thus, this study examined the effect of humidity and dew on two commercial products, as these conditions often occur on the track. A methodology based on a creep curves measurement approach was used to assess product performance under various scenarios. All measurements were conducted on a universal tribometer in the ball-on-disc configuration covered with a climate chamber. The results show a strong influence of dew on the tested products, as dew lowered their performance parameters and caused low adhesion problems. Possible mechanisms of water–oil interaction and formation of oxidic third body layers were discussed. The main findings indicate that TOR lubricants may cause traction/braking problems if used in dew conditions. The present study may be helpful in optimising friction management methods in the future.

Graphical Abstract