Tang et al. [1] present a thorough literature review on the advances in liquid superlubricity and provide a comprehensive overview of the challenges associated with translating model testing findings into practical applications. The review has led to the formulation of the current requirements for achieving liquid superlubricity, namely (I) a low surface roughness, (II) low viscosity, (III) appropriate contact pressure, and (IV) appropriate velocity. While the review is extensive, we think it lacks a critical examination of the potential application of the reviewed tribosystems in the lubrication of machine elements such as roller bearings and gears. The transfer of superlubricity from model scale to machine elements requires the consideration of boundary conditions of industrial applications. These include material properties, lubricant characteristics, and geometric and kinematic constraints. This comment discusses the work of Tang et al. [1] in the context of superlubricity from the perspective of gears, which are typically characterized by rolling–sliding motion and non-conformal contact geometry. We think that it is imperative to refine the conclusions to avoid misinterpretation of the challenges and potential of superlubricity. We demonstrate that superlubricity can be easily achieved in elastohydrodynamically lubricated steel contacts on engineering surfaces, not limited by the four requirements brought up by Tang et al. [1].

Water-based lubricants (WBLs) have demonstrated promising frictional behaviour in component-level tribological studies; however, their performance under full-scale gearbox conditions remains largely unexplored. In this study, a modified commercial electric vehicle (EV) gearbox was employed to experimentally evaluate the influence of three fully formulated WBLs on drag losses, gearbox efficiency, thermal response, and post-test fluid stability, using a conventional gear oil as a reference. The results show that the WBLs achieved a relative efficiency improvement compared to the reference gear oil, with a maximum peak efficiency gain of 1.95% across the tested conditions. In addition, the WBLs exhibited enhanced thermal performance, reducing bearing temperature rise by up to 8 °C under equivalent load conditions. Post-test characterisation indicated no significant degradation or oxidative deterioration in the WBL samples, although minor water evaporation was detected in certain cases. Overall, the findings highlight a clear efficiency advantage of WBLs, while also identifying certain limitations that require consideration for the continued development of WBLs as potential e-fluids.

This study is an investigation of the impact of grinding-generated surface roughness on the wear and fatigue life of rails. The grinding process, essential for rail maintenance, creates a rough surface that significantly influences wheel/rail contact conditions. Using a combined experimental and numerical approach, this work replicates and integrates grinding-generated roughness into an elastoplastic rolling contact model. To evaluate the effects of surface roughness on rail degradation, Archard's wear equation is applied for wear assessment, while fatigue is analysed using the Jiang-Sehitoglu fatigue parameter. The results show that rough surfaces induce localised high stresses and plastic strains, accelerating material degradation, particularly in early rolling cycles. In contrast, smoother surfaces exhibit more stable plastic strain evolution over fewer cycles. Additionally, the grinding-generated roughness significantly increases the damage accumulation, highlighting its role in reducing rail life. These findings emphasize the need to incorporate surface roughness into predictive maintenance models and optimise grinding practices to ensure rail longevity and operational reliability.

Copper is a strategic raw material and an important component in electric motors, widely used across industries because of its excellent conductivity and recyclability. It plays an important role in the transformation from fossil fuel-based systems to green, electrified systems. However, substantial material losses continue throughout the lifecycle of electric motors, even with copper’s intrinsic capacity for circularity. Also, copper’s increasing demand, which is driven by the emergence of electric vehicles, industrial electrification, and renewable energy infrastructure, poses questions regarding its sustainable supply. The recovery of secondary copper sources from end-of-life (EoL) products is becoming more and more important in this context. However, it is still difficult to achieve circularity of copper, especially from industrial electric motors. This study investigates the challenges of closing the loop for copper during the lifecycle of motors in industrial applications. Based on an examination of EoL strategies, material flow insights, and practical investigation, the research pinpoints significant inefficiencies in the current processes. The widespread use of scraping as an approach of end-of-life management is one significant issue. Most of the electric motors are not built to separate their components, which makes both mechanical and manual disassembly difficult. The quality of recovered copper is thus compromised by the dominance of mixed metal shredding methods in the recycling step. This study highlights the need for systemic changes in addition to technical solutions to address copper circularity issues. It requires a focus on circularity in designing, giving disassembly and metal recovery a priority. This study focuses on circularity and its technological challenges in a value chain of copper. It not only identifies different processes such as supply chain disconnections and design constraints, but it also suggests workable solutions to close the copper flow loop in the electric motor sector. Copper quality and recovery is ultimately a problem involving design, technology, and cooperation, in addition to resources. This study supports the transition to a more sustainable and circular electric motor industry by offering a basis for directing such changes in industry practices and prospective EU regulations.

This study evaluates the tribological performance of fully formulated water-based lubricants (WBLs) under rolling/sliding conditions. The WBLs are composed of water-soluble glycols and glycerol with a functional proportion of water. They exhibited significantly lower friction across all lubrication regimes, with near superlubricity achieved under certain conditions. Unlike conventional oils, WBLs showed minimal shear heating effects and exhibited a low shear response with increasing slide-to-roll ratios. Long-duration tests confirmed consistently lower friction for the WBLs. Post-test analysis showed comparable wear between selected WBLs and the reference oil at 40 °C but greater surface modification for WBLs at higher temperatures. Observed wear mechanisms included abrasive and adhesive wear, with occasional instances of spalling after prolonged tests. X-ray photoelectron spectroscopy (XPS) analysis suggested the presence of chemical species on wear scars, attributed to additive-surface interactions. These findings highlight the potential of fully formulated WBLs in rolling/sliding contacts and their current state in comparison to a conventional gear oil.

Understanding elastohydrodynamic (EHL) film formation and the pressure-viscosity response of lubricants is necessary for designing rolling/sliding tribological contacts. This article investigates the EHL behaviour of four formulated water-based lubricants (glycerol-water, glycol-water, and ionic liquid-water) and one reference oil under moderately high pressures, typical in gears and bearings applications. A ball-on-disc tribometer with optical interferometry was employed to measure the film thickness of the water-based lubricants. The results highlight the sensitivity of film formation to entrainment speed, slide-to-roll ratio (SRR), temperature, and lubricant composition. Water loss due to evaporation significantly impacts film formation at high temperatures. Additionally, an unusual increase in film thickness was observed for the glycol-water solution, likely due to complex tribological conditions. The limitations of the classical Hamrock-Dowson film thickness equation for water-based lubricants are also discussed. Furthermore, pressure-viscosity coefficients of the water-based lubricants were estimated using both optical interferometry and high-pressure viscometer methods. The effect of water content on the pressure-viscosity coefficient was also examined, revealing that higher water content leads to reduced pressure and temperature dependence of viscosity.

Grinding is regularly conducted on railway tracks to prevent crack propagation and surface deterioration. However, grinding can introduce roughness on rail surfaces, potentially leading to stress and strain concentration that increase the likelihood of crack initiation. This paper proposes the utilization of surface roughness obtained by replicating the ground rail surface to assess its impact on train wheel-rail interactions. A novel approach which integrates the replicated roughness into an elastoplastic contact model, allows for a detailed assessment of its effects on contact pressure, and residual strain and stress distributions. The findings highlight the importance of considering surface roughness in predictive maintenance planning for railway infrastructure, as it can significantly influence the structural integrity and long-term performance of the track system.

Cross-country skiers employ various techniques, where the ski is exposed to different forces during the motion. This study utilized a novel sled tribometer to investigate the combined effects of load and positioning of the skier on the coefficient of friction (COF) between the skis and snow. Three different loads (40 kg, 80 kg and 120 kg) were applied to the sled, and the center of mass was systematically varied between three positions behind the binding position: 70mm (leaning forward), 140mm (centered) and 210mm (backward). A variety of skis were used, including different models of skate skis and one classic-style ski with grip wax. The results consistently demonstrated that increasing the load on the sled reduced the COF by up to 15% (from the lowest to highest load), regardless of the position of the center of mass. The position of the center of mass had a minimal effect on COF in most tests. An exception was observed when using grip wax, where a forward-leaning position combined with a heavy load significantly increased the COF (~ 8%) compared to what is expected without grip wax. This load-dependent reduction in the COF was observed across different skis and test sessions. The ski camber profile was measured for all skis in all configurations. In general, increasing the load increases the glide zone length but at the same time increasing the average pressure. The position of the center of mass has little to no effect on the rear glide zone but slightly alters the length and position of the front glide zone. While the mechanisms of friction are discussed, a complete understanding of these mechanisms has not yet been reached. 

This study evaluates the negative effects of grip wax application on the dynamic ski–snow coefficient of friction and subsequent performance during the double poling cycle in cross-country skiing. Utilising a linear ski tribometer, friction tests were performed on classic cross-country skiing skis prepared with no, thin, and thick grip wax under controlled laboratory conditions. The dynamic coefficient of friction was estimated under various load conditions, reflecting dynamic skiing motions. Results indicated a clear increase in coefficient of friction with the addition of grip wax, with significant differences observed between thin and thick applications. Specifically, compared to skis with no wax, the coefficient of friction for skis with thin and thick wax layers experienced a negative increased by 1.8% and 3.2% during double poling, and by 1.7% and 2.6% whilst gliding, respectively. These friction increases were associated with higher power requirements during skiing or a consequent time loss. This underscores the need for meticulous application of grip wax application, tailored to the snow conditions, ski camber profile and racecourse demands, to minimise impact on gliding performance whilst maintaining sufficient static coefficient of friction for effective use of the diagonal stride technique. Furthermore, the skier should utilise a skiing technique to minimise the risk of encountering load conditions that increase the coefficient of friction. Overall, this research provides quantitative insights into the trade-offs between grip enhancement and friction-related performance losses in cross-country skiing.

In the Olympic winter sports cross-country skiing and the biathlon, athletes aim to minimise resistive forces such as aerodynamic drag, gravity, and ski–snow friction to enhance performance. Ski–snow friction is complex, involving multiple friction mechanisms that vary depending on snow conditions. In cold environments, where the moisture and water content are minimal, friction is presumably influenced primarily by dry interactions between the ski and snow, particularly through adhesion and abrasion at the micro-scale. Here, we examined ski–snow friction under cold conditions using eight pairs of cross-country skis, with different apparent contact lengths and real contact areas. Our findings revealed that apparent contact length, a macro-scale parameter, had the greatest impact on friction, followed by total real contact area, which is a multi-scale parameter. For snow temperatures below approximately −10 °C, longer apparent contact lengths reduced friction, whereas shorter lengths are more effective above −10 °C. In addition, at −3 °C, minimising the total real contact area was advantageous for reducing friction, while this effect diminished at −8.5 °C. At the coldest tested temperature of −13 °C, a larger total real contact area resulted in the lowest friction. These findings highlight the importance of considering both macro- and micro-scale contact properties for optimising ski performance in different cold conditions.