This study proposes a numerical methodology based on the application of first, second, and third-order derivatives to analyze the evolution of the coefficient of friction (CoF) obtained from ball-on-disc (BoD) wear tests. The approach aims to provide an objective and quantitative identification of mechanisms, wear stages, and transition points, overcoming the subjectivity commonly associated with conventional friction curve interpretation. Before derivative computation, the CoF signal was smoothed to reduce experimental noise while preserving the morphological features of the friction curve. The methodology was applied to a newly developed multicomponent Al80Mg10Si5Cu5 HPDC alloy tested under dry sliding at room temperature (RT). The derivative-based analysis enabled the identification of successive wear stages, from the initial settling and running-in to transient and quasi-stationary regimes, and the determination of characteristic transition points, correlated with wear mechanisms through surface and microstructural analyses. The results demonstrate that the proposed methodology enables an objective determination of the duration and sequence of wear stages, reveals that the transition to stable sliding does not coincide with the maximum CoF value, and improves the identification of highly dynamic early wear regimes that are often underestimated by visual analysis. Due to its low computational cost and reliance on signals commonly available in tribological systems, the proposed derivative-based methodology shows strong potential for real-time friction and wear monitoring, predictive maintenance, and the automation of tribological control systems, although further validation under industrial operating conditions is required.

The vibration and noise issues of lightweight friction pairs in suburban train braking systems have become a critical bottleneck restricting their engineering application. This study investigated the lightweight friction pairs composed of three representative synthetic brake pads and an aluminum matrix composite brake disc. Utilizing tribological tests, interfacial wear analysis, and dynamic modelling, the study investigated the impact of interfacial wear and contact behaviors on vibration and noise, and elucidated the mechanisms by which pad material properties influence these responses. The experimental findings revealed that the pad material properties significantly affect the wear behavior and friction-induced vibration and noise responses of lightweight friction pairs. The pad enriched with lubricating phases (Pad A) readily established stable lubricating films, while the highly plastic pad (Pad C) effectively captured wear debris to build the third-body layers that cushioned loads. Both reduced friction fluctuations and contact stiffness, thereby attenuating vibration and noise. Conversely, the high-hardness pad (Pad B) failed to form continuous lubricating films, leading to intensified friction, higher contact stiffness, and pronounced vibration and noise. Numerical simulations further confirmed that the friction coefficient and normal contact stiffness synergistically regulated system stability, directly affecting the vibration and noise responses. Systems characterized by high friction and large contact stiffness (Pad B) were particularly susceptible to modal coupling, resulting in dynamic instability and elevated vibration and noise levels. Therefore, optimizing the pad material properties and regulating the behavior of wear debris to facilitate the stable formation of lubricating films or third-body layers can effectively suppress friction coefficient fluctuations, reduce normal contact stiffness, and enhance interfacial stability, thereby mitigating vibration and noise. The findings provide a theoretical foundation and engineering guidance for optimizing the design of low-noise lightweight braking systems and selecting appropriate friction materials.

Rubber products used in shoe soles and tires need high friction, especially under lubrication, to ensure safety in daily life. The frictional behavior of rubber under lubrication is influenced by various parameters, including the properties of the rubber tread, lubricant, mating surface, and sliding conditions. The effects of these parameters on friction have been investigated, but the mechanisms for enhancing friction under lubrication have not been studied systematically. This review is a summary of the methods used to enhance rubber friction under lubrication, including optimizing the tread groove geometry to improve drainage, texturing to increase the contact area, preventing liquid from flowing into the contact interface, increasing hysteresis friction, and controlling the surface free energy of rubber to promote dewetting. Additionally, suction-based attachment is emphasized as an effective mechanism for liquid-covered surfaces. Integrating these approaches may significantly advance the design of high-friction rubber tread, enabling the development of materials that maintain high friction regardless of lubrication conditions.

Self-dispersed graphene crumpled ball (GCB) demonstrates exceptional tribological performance as lubricant additive under elevated temperature. However, the critical relationship between its unique wrinkle architecture, internal porosity characteristic, and the resultant dispersion stability/friction-reduction mechanism remains insufficiently explored. Particularly, the synergistic effects arising from structural hierarchy and surface chemistry modulation in high-temperature lubrication systems require systematic investigation. Herein, we propose a wrinkle engineering strategy guided by Stokes' law to fabricate surface modifier-free GCB with programmable three-dimensional geometries. Systematic investigations reveal that the degree of wrinkling on the GCB critically dominates the dispersion characteristics and the interlayer shearing resistance. Upon the molybdenum disulfide quantum dots deposited on GCB, a more consistent and robust tribo-chemical reaction film can be formed on the friction interface and in response to protect from severe damage. This complex achieves over 2-fold enhancement in antifriction efficiency compared with commercial high-temperature chain oil (CH-27Q). Overall, this study establishes a structure-performance paradigm for developing autonomous lubrication systems under extreme thermal conditions.

Aluminium and other soft metal coated plain bearings are common in high-speed rotating machines due to their light weight and high performance. This makes the monitoring of bearing surface condition an important requirement to ensure healthy machine operation. In this paper, an ultrasound-based technique is investigated to measure the wear condition of thin aluminium bearing coatings. High-frequency 22 MHz piezoelectric sensors were selected based on both numerical simulations and experiments. Tests were performed on four groups of samples with artificial damage and three groups of samples worn by dry running. The results have shown the deviation between ultrasound measurement and microscope measurement are within 5 μm for artificially damaged samples and within 15 μm for samples worn under dry running conditions.

Graphene and fullerene nanoparticles exhibit remarkable tribological performance in solid-liquid composite lubrication systems. However, the atomic-scale understanding of how surface topography influences their tribological behavior and performance is still limited. Herein, the influence mechanisms of surface topography features (achieved by regulating asperity amplitude and frequency parameters) on system lubrication performance and nanoparticle friction behavior were systematically investigated through friction experiments and molecular simulations. The results indicate that, at the micro-nanoscale, the amplitude parameter predominantly governs the surface roughness features and frictional resistance. This is because an increased amplitude strengthens the boundary lubrication effect, exacerbates stress concentration and structural deformation of graphene, and makes fullerene more likely to fill grooves and difficult to bear normal loads, thereby exacerbating friction and wear (friction coefficient increased by 59%). In contrast, the frequency parameter primarily determines the surface kurtosis features and normal force. At low frequency, low kurtosis features intensify the normal squeezing effect of asperities, inducing the hydrodynamic pressure effect of the base oil, thus enhancing lubrication performance (friction coefficient decreased by 22%). Compared with frequency, the pronounced influence of amplitude on lubrication state and interface contact behavior dominates the tribological properties of the system and the lubrication mechanism of the nanoparticles. Lower surface roughness and kurtosis features are critical for achieving efficient lubrication. This study offers valuable insights into the design of surface topography and the optimization of lubrication performance.

Rolling contact fatigue (RCF) failures in critical components like precision gears and high-performance bearings, have become increasingly prominent under demanding conditions. Conventional lubricant additives struggle to simultaneously reduce friction, resist wear, and repair dynamic micropitting. To address this challenge, a composite material of ionic liquid-functionalized magnesium silicate hydroxide ([DDP][TOA]/MSH) was synthesized using hydrothermal synthesis and non-covalent modification. This composite exhibited remarkable dispersion stability and copper corrosion inhibition, as well as superior tribological properties including friction reduction, wear mitigation, and micropitting repair during rolling-sliding contact. Tribological evaluations revealed that 1.0 wt% [DDP][TOA]/MSH reduced friction coefficient by 17.2% and wear volume by 52.5%, demonstrating unprecedented load-bearing capacity and frequency adaptability. Notably, in rolling-sliding contact fatigue conditions, commercial gear oil exacerbated micro-pitting damage continuously, whereas the composite material could repair it, with a repair efficiency of 72.0%. Surface characterization reveals a three-stage mechanism for the dynamic repair of worn metal surfaces: (1) micro-asperities are removed through mechanical grinding, (2) micro-cracks are filled via tribochemical deposition of FeS/phosphate phases, and (3) a hybrid a-SiC/a-SiO_x repair layer is formed with improved mechanical strength, effectively preventing fatigue wear propagation. This work demonstrates the synergistic effect of ionic liquids and layered silicate additives on micropitting repair under rolling contact fatigue, expanding the application of MSH in the field of commercial lubricant additives.

Graphite has achieved widespread recognition as an effective solid lubricant due to its high functionality across diverse environmental conditions. Although superlubricity or ultra-low friction is readily observed at the graphite basal plane, it has been reported that certain adsorbates from the surrounding environment can deteriorate this friction regime. Here, we conducted a fundamental analysis of the effect of phenol, pentanol, and water adsorbates on the friction of graphite-silica interfaces using molecular dynamics simulations with the reactive force field ReaxFF. First, we evaluated three ReaxFF parameter sets optimized using friction-pertinent properties. It was observed that the force field optimization objective plays a major role in the obtained tribological properties. Secondly, parameters such as normal load and motion directionality were investigated. In addition to simulations of sliding friction, adsorption, and binding energy calculations were performed to expand upon the hypothesis that friction may be directly correlated to the interfacial molecular structure rather than binding energy and adsorbate commensuration with graphene. By quantitatively representing the interfacial roughness of each adsorbate, said hypothesis was confirmed by unequivocally explaining the calculated friction coefficients.

High-conductivity sliding electrical contact with low friction plays a significance role in the long life and high reliability of electro-mechanical systems. Reducing friction needs weak interfacial electronic coupling, contradictorily, enhancing conductivity requires the strong coupling, thus it is a serious challenge to achieve high-conductivity with low friction. Here, using our self-developed TAMET method, we achieved superlubricating electrical contact experimentally by establishing sliding electrical system between graphite layers incommensurately; the friction coefficient is as low as 0.0004 and the electric current density is as high as 510 A/cm². Compared to the commercial Ir AFM tip-Gr contact, the friction force of incommensurate graphene layers friction is an order of magnitude lower, yet it has similar high electrical conductivity. Based on the Electronic Property Fluctuation (EPF) model and first principles calculations, we revealed that the sliding energy barrier remains almost unchanged under applied current due to the negligible electron transfer variation during the sliding process. We offer a method for achieving superlubricating electrical contact with high conductivity and low friction, shedding light into improving the service life and reliability of sliding electrical contacts in a wide range of electromechanical systems.

Monolayer MoS₂ has garnered significant interest due to its exceptional optoelectronic and tribological properties and potential application as a lubrication layer in micro- and nano-electromechanical systems. Although the nanotribological performance of CVD-grown MoS₂ and the characteristics associated with CVD growth have been extensively studied, challenges remain in designing specific regions on the monolayer MoS₂ surface with reduced friction. Here, we develop nuclei with an onion-shell structure on CVD-grown monolayer MoS₂ to achieve remarkable friction and adhesion reduction. These nuclei, dispersed on high-quality and crystalline MoS₂, consist of an oxi-sulfide core surrounded by a multilayer MoS₂ shell. Lateral force microscopy results indicate that onion-shell nuclei create an ensemble effect that decreases friction and adhesion by up to 45% and 20%, respectively, compared to those of MoS₂ because of the multilayer structure and in-plane tensile strain, both of which minimize out-of-plane deformation. DMT model calculations and step-down load-friction correlations illustrate that the work of adhesion, shear strength, and coefficient of friction on the nucleus decrease by more than 22%, 19%, and 34%, respectively, compared with those on MoS₂. The onion-shell nucleus presents a novel lubrication strategy to mitigate friction and adhesion in CVD-grown 2D materials, with potential applications in lubricating nanoscale friction pairs.