Lubricants

32 pages, 10740 KB

Open AccessArticle

Hydraulic Electromechanical Regenerative Damper in Vehicle–Track Dynamics: Power Regeneration and Wheel Wear for High-Speed Train

by Zifei He, Ruichen Wang, Zhonghui Yin, Tengchi Sun and Haotian Lyu

Lubricants 2025, 13(9), 424; https://doi.org/10.3390/lubricants13090424 - 22 Sep 2025

Viewed by 228

Abstract

A physics-based vehicle–track coupled dynamic model embedding a hydraulic electromechanical regenerative damper (HERD) is developed to quantify electrical power recovery and wear depth in high-speed service. The HERD subsystem resolves compressible hydraulics, hydraulic rectification, line losses, a hydraulic motor with a permanent-magnet generator, [...] Read more.

A physics-based vehicle–track coupled dynamic model embedding a hydraulic electromechanical regenerative damper (HERD) is developed to quantify electrical power recovery and wear depth in high-speed service. The HERD subsystem resolves compressible hydraulics, hydraulic rectification, line losses, a hydraulic motor with a permanent-magnet generator, an accumulator, and a controllable; co-simulation links SIMPACK with MATLAB/Simulink. Wheel–rail contact is computed with Hertz theory and FASTSIM, and wear depth is advanced with the Archard law using a pressure–velocity coefficient map. Both HERD power regeneration and wear depth predictions have been validated against independent measurements of regenerated power and wear degradation in previous studies. Parametric studies over speed, curve radius, mileage and braking show that increasing speed raises input and output power while recovery efficiency remains 49–50%, with instantaneous electrical peaks up to 425 W and weak sensitivity to curvature and mileage. Under braking from 350 to 150 km/h, force transients are bounded and do not change the lateral wear pattern. Installing HERD lowers peak wear in the wheel tread region; combining HERD with flexible wheelsets further reduces wear depth and slows down degradation relative to rigid wheelsets and matches measured wear more closely. The HERD electrical load provides a physically grounded tuning parameter that sets hydraulic back pressure and effective damping, which improves model accuracy and supports calibration and updating of digital twins for maintenance planning. Full article

(This article belongs to the Special Issue Tribological Challenges in Wheel-Rail Contact)

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18 pages, 4491 KB

Open AccessArticle

Wear and Friction Behavior of Cellulose Nanofibers-Based Biogreases

by Claudia Roman, Miguel Ángel Delgado Canto, María García-Pérez, Samuel D. Fernández-Silva, Ke Li and Moisés García-Morales

Lubricants 2025, 13(9), 423; https://doi.org/10.3390/lubricants13090423 - 20 Sep 2025

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Abstract

(1) Background: Developing fully bio-based lubricating greases requires eco-friendly alternatives to conventional harmful components. This study highlights unmodified nanocellulose as an effective structuring agent in vegetable oils, enabling 100% bio-based formulations. (2) Methods: Three bio-based greases were formulated using 1.4 wt.% cellulose nanofibers [...] Read more.

(1) Background: Developing fully bio-based lubricating greases requires eco-friendly alternatives to conventional harmful components. This study highlights unmodified nanocellulose as an effective structuring agent in vegetable oils, enabling 100% bio-based formulations. (2) Methods: Three bio-based greases were formulated using 1.4 wt.% cellulose nanofibers (CNFs), derived from elm wood pulp through mechanical and chemical pretreatment, as thickening agents in castor oil. Their tribological performance was evaluated under varying temperatures and contact loads and compared to a reference lithium-based grease (LBG) containing 14 wt.% thickener, also formulated with castor oil. (3) Results: Among the CNFs, the unbleached variant (CNF-U) which retained the highest lignin content exhibited the highest coefficient of friction (COF), ranging from 0.09 to 0.14 across test conditions, along with a wear scar diameter of approximately 615 µm at 60 °C. Notable differences in shear stress sensitivity were observed between mechanically and chemically treated nanofibers. The TEMPO-oxidized nanofiber (CNF-TO) grease demonstrated outstanding lubrication stability across contact loads of 10–40 N and temperatures from 25 to 100 °C, maintaining COF values below 0.1—comparable to the reference LBG at 40 N load. Wear scar analysis confirmed that CNF-based greases significantly reduced wear relative to the lithium reference: CNF-B produced the smallest scar diameter (188 µm at 25 °C) while CNF-TO yielded the lowest at 60 °C (457 µm). (4) Conclusions: Nanofiber type and pretreatment significantly impact the tribological performance of CNF-based biogreases. TEMPO-oxidized CNFs provided stable lubrication under varied loads and temperatures, while all CNFs showed strong thermal adaptability, supporting their use in sustainable lubrication. Full article

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24 pages, 1981 KB

Open AccessSystematic Review

Wear and Thermal Management in Cycloidal Reducers: A Patent Analysis

by Federico Rotini and Lorenzo Fiorineschi

Lubricants 2025, 13(9), 422; https://doi.org/10.3390/lubricants13090422 - 19 Sep 2025

Viewed by 263

Abstract

Wear and overheating in cycloidal systems significantly affect motion transmission efficiency, component lifespan, and maintenance costs. Numerous solutions have been proposed in the patent literature to address these issues. This paper identifies, analyzes, and classifies these solutions based on their working principles, strategies [...] Read more.

Wear and overheating in cycloidal systems significantly affect motion transmission efficiency, component lifespan, and maintenance costs. Numerous solutions have been proposed in the patent literature to address these issues. This paper identifies, analyzes, and classifies these solutions based on their working principles, strategies (Preventive, Compensative, Mitigative), and implementation methods. The results indicate that preventive strategies are predominant, mainly through friction reduction achieved via rolling elements. Compensative strategies rely on lubrication, while mitigative strategies, though rare, involve restoring contact surfaces through reshaping. This review serves as a comprehensive reference for the development of new solutions in the field. Full article

(This article belongs to the Special Issue Tribology of Cycloidal Reducers: Enhancing Efficiency, Durability and Reliability through Experimental and Numerical Methods)

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22 pages, 68111 KB

Open AccessArticle

Mechanistic Interpretation of Fretting Wear in Z10C13 Steel Under Displacement–Load Coupling

by Ruizhi Li, Bozhen Sun, Zhen Meng, Yigang Wang, Jing Ni and Haohan Zhang

Lubricants 2025, 13(9), 421; https://doi.org/10.3390/lubricants13090421 - 19 Sep 2025

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Abstract

Considering that the ferritic stainless steel Z10C13 support plate material in nuclear power equipment tends to undergo fretting wear during service, this paper systematically investigates the effect of varying normal loads (10–50 N) and displacement amplitudes (15–75 μm) on its fretting response and [...] Read more.

Considering that the ferritic stainless steel Z10C13 support plate material in nuclear power equipment tends to undergo fretting wear during service, this paper systematically investigates the effect of varying normal loads (10–50 N) and displacement amplitudes (15–75 μm) on its fretting response and wear mechanisms. Through ball-on-flat fretting wear experiments, together with macro- and micro-scale observations of wear scars, it is revealed that normal load primarily controls the contact intensity and the extent of adhesion, whereas displacement amplitude mainly affects the slip amplitude and features of fatigue damage. The results show that the fretting system’s dissipated energy increases nonlinearly with both load and amplitude, and their coupled effect significantly exacerbates interfacial damage. The wear scar morphology evolves from a shallow bowl shape to a structure characterized by multiple spalling pits and propagating fatigue cracks. An equivalent hardness-corrected Archard model is proposed based on the experimental data. The model captures the nonlinear dependence of equivalent material hardness on both load and amplitude. As a result, it accurately predicts wear volume (

R^{2} = 0.9838

), demonstrating its physical consistency and modeling reliability. Overall, this study elucidates the multi-scale damage evolution mechanism of Z10C13 under fretting conditions and provides a theoretical foundation and methodological support for wear-resistant design, life prediction, and safety evaluation of nuclear power support structures. Full article

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24 pages, 19579 KB

Open AccessArticle

Biomimetic Hexagonal Texture with Dual-Orientation Groove Interconnectivity Enhances Lubrication and Tribological Performance of Gear Tooth Surfaces

by Yan Wang, Shanming Luo, Tongwang Gao, Jingyu Mo, Dongfei Wang and Xuefeng Chang

Lubricants 2025, 13(9), 420; https://doi.org/10.3390/lubricants13090420 - 18 Sep 2025

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Abstract

Enhanced lubrication is critical for improving gear wear resistance. Current research on surface textures has overlooked the fundamental role of structural connectivity. Inspired by biological scales, a biomimetic hexagonal texture (BHT) was innovatively designed for tooth flanks, featuring dual-orientation grooves (perpendicular and inclined [...] Read more.

Enhanced lubrication is critical for improving gear wear resistance. Current research on surface textures has overlooked the fundamental role of structural connectivity. Inspired by biological scales, a biomimetic hexagonal texture (BHT) was innovatively designed for tooth flanks, featuring dual-orientation grooves (perpendicular and inclined to the rolling-sliding direction) with bidirectional interconnectivity. This design synergistically combines hydrodynamic effects and directional lubrication to achieve tribological breakthroughs. A lubrication model for line contact conditions was established. Subsequently, the texture parameters were then optimized using response surface methodology and numerical simulations. FZG gear tests demonstrated the superior performance of the optimized BHT, which achieved a substantial 82.83% reduction in the average wear area ratio and a 25.35% decrease in tooth profile deviation variation. This indicated that the biomimetic texture can effectively mitigate tooth surface wear, thereby extending the service life of gears. Furthermore, it significantly improves thermal management by enhancing convective heat transfer and lubricant distribution, as evidenced by a 7–11 °C rise in bulk lubricant temperature. This work elucidates the dual-mechanism coupling effect of bio-inspired textures in tribological enhancement, thus establishing a new paradigm for gear surface engineering. Full article

(This article belongs to the Special Issue Advanced Surface Treatments and Coatings for Friction and Wear Reduction)

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27 pages, 7975 KB

Open AccessArticle

Effect of Laser-Textured Groove Patterns on Friction Reduction and Stress Distribution in High-Speed Steel Surfaces

by Viboon Saetang, Ponthep Vengsungnle, Hao Zhu, Huan Qi, Haruetai Maskong and Witthaya Daodon

Lubricants 2025, 13(9), 419; https://doi.org/10.3390/lubricants13090419 - 18 Sep 2025

Viewed by 286

Abstract

Excessive surface friction encountered during metal-forming processes typically leads to die wear and seizure in part surfaces, which consequently shortens the die’s service lifespan and lowers the surface quality of the formed parts. To minimize surface friction, tool surface modification is required. This [...] Read more.

Excessive surface friction encountered during metal-forming processes typically leads to die wear and seizure in part surfaces, which consequently shortens the die’s service lifespan and lowers the surface quality of the formed parts. To minimize surface friction, tool surface modification is required. This study focuses on reducing the sliding friction of SKH51 high-speed steel by fabricating micro-grooves with various crosshatch angles using a nanosecond pulse laser. The effects of laser texturing parameters on achieving the groove aspect ratio of 0.1 were investigated. This aspect ratio facilitates lubricant retention and enhances lubrication performance on the contact surfaces. The influence of groove crosshatch angles (30°, 60°, and 90°) on the friction in the sliding contact between a textured high-speed steel disc and an AISI304 stainless steel pin was evaluated using a pin-on-disc test with a constant load. Moreover, the contact pressure distribution and stress concentration associated with each groove pattern were numerically analyzed using the finite element method. The results demonstrated that a laser power of 20 W effectively produced groove geometries with the desired aspect ratio. Among the tested patterns, the surface textured with a 60° crosshatch angle exhibited the lowest coefficient of friction of 0.111, compared to 0.148 for the untextured surface. Finite element analysis further revealed that the 60° crosshatch pattern provided the most balanced combination of load redistribution, reduced mean pressure, and average stress, which may reduce the friction under sliding conditions. These findings confirm that laser surface texturing, particularly with an optimized crosshatch angle, can significantly reduce sliding friction and enhance the tribological performance of high-speed steel tools. Full article

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15 pages, 2336 KB

Open AccessArticle

Tribo-Catalytic Degradation of Methyl Orange Dye via Cu/Al₂O₃ Nanoparticles

by Claudia Cirillo, Mariagrazia Iuliano, Sana Abrar, Elena Navarrete Astorga and Maria Sarno

Lubricants 2025, 13(9), 418; https://doi.org/10.3390/lubricants13090418 - 17 Sep 2025

Viewed by 309

Abstract

In this study, we report, for the first time, the tribo-catalytic degradation of methyl orange (MO) using Cu/Al₂O₃ nanoparticles under mechanical stirring conditions. The hybrid catalyst was synthesized via a wet impregnation method and characterized through different techniques, confirming structural [...] Read more.

In this study, we report, for the first time, the tribo-catalytic degradation of methyl orange (MO) using Cu/Al₂O₃ nanoparticles under mechanical stirring conditions. The hybrid catalyst was synthesized via a wet impregnation method and characterized through different techniques, confirming structural integrity and compositional uniformity. When subjected to friction generated by a PTFE-coated magnetic stir bar, Cu/Al₂O₃ nanoparticles exhibited high tribo-catalytic activity, achieving up to 95% MO degradation within 10 h under dark conditions. The observed activity surpasses that of alumina alone and is attributed to the synergistic effects between copper and alumina, facilitating charge separation and enhancing reactive oxygen species (ROS) formation. Tribo-catalytic efficiency was further influenced by stirring speed and contact area, confirming the key role of mechanical friction. Reusability tests demonstrated stable performance over five cycles, highlighting the material’s durability and potential for practical environmental remediation applications. Full article

(This article belongs to the Special Issue Tribo-Catalysis)

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23 pages, 5396 KB

Open AccessArticle

Biobased Lubricating Oil Prepared from Ethyl Cellulose/Montmorillonite Additives and Waste Cooking Oil

by Sha Wang, Haoyue Wang, Zhenpeng Wang, Tao Hou, Kai Zhang, Zhuoyi Lv, Gaole Zhao, Huimin Sun, Wenkai Li and Yinan Hao

Lubricants 2025, 13(9), 417; https://doi.org/10.3390/lubricants13090417 - 17 Sep 2025

Viewed by 302

Abstract

Mineral oil-based lubricants contain harmful elements, such as sulfur and phosphorus, pose significant harm to the environment. In current research on the application of waste oils and fats in bio-based lubricants, most studies focus on single pretreatment processes or additive preparation, lacking systematic [...] Read more.

Mineral oil-based lubricants contain harmful elements, such as sulfur and phosphorus, pose significant harm to the environment. In current research on the application of waste oils and fats in bio-based lubricants, most studies focus on single pretreatment processes or additive preparation, lacking systematic investigations into the combined use of composite pretreatment and additives on lubricant performance. Moreover, the decolorization efficiency of traditional physical adsorption methods for treating waste oils and fats is limited, making it difficult to meet the raw material requirements for bio-based lubricants. The purpose of this study is to conduct composite pretreatment processes on waste oils and fats, understand the impacts of parameters such as additive dosage and environmental factors on lubricant performance, establish an environmentally friendly and performance-compliant preparation process for bio-based lubricants, and provide a theoretical basis and technical support for its industrial application. Recent studies have shown that new decolorization processes for waste oil treatment significantly improve decolorization and recovery rates, as evidenced by research comparing new and traditional methods. Pretreatment with hydrogen peroxide, activated clay, and activated carbon significantly improved the color and odor of treated waste oil, meeting standards for bio-based lubricant production. The intercalation polymerization reaction between ethyl cellulose (EC) and montmorillonite (MMT) was employed to develop an additive (CTAB-MMT/KH560-EC). A thorough investigation was performed to analyze the impact of temperature, processing time, and additive concentration on the rheological behavior. The bio-based lubricant exhibited a kinematic viscosity of 200.3 mm²/s at 40 °C and 28.3 mm²/s at 100 °C, meeting the standard conditions as outlined in ASTM D2270-10e1. This lubricant achieved an improved low-temperature performance with a pour point of −22 °C, a friction coefficient of 0.081, and an average pitting diameter of 0.94 mm, indicating its suitability for a range of applications. These lubricants exhibit outstanding viscosity characteristics, meeting the relevant requirements for energy and environmental applications in green, eco-friendly, and biodegradable sustainable development strategies while expanding their application scope. Full article

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25 pages, 1345 KB

Open AccessArticle

Analysis of the MSD, ICF Function, G’ and G” Modulus and Raman and FTIR Spectroscopy Spectra to Explain Changes in the Microstructure of Vegetable Lubricants

by Rafal Kozdrach and Pawel Radulski

Lubricants 2025, 13(9), 416; https://doi.org/10.3390/lubricants13090416 - 16 Sep 2025

Viewed by 239

Abstract

This paper presents the results of a rheological and spectral study characterising the change in the microstructure of lubricants depending on the type of vegetable oil base. The three lubricating compositions were prepared based on vegetable oils (rapeseed, sunflower and abyssinian), where amorphous [...] Read more.

This paper presents the results of a rheological and spectral study characterising the change in the microstructure of lubricants depending on the type of vegetable oil base. The three lubricating compositions were prepared based on vegetable oils (rapeseed, sunflower and abyssinian), where amorphous silica of a specific particle size was used as a thickener. These three lubricating compositions were then modified by introducing the AW/EP additive (BCH 351) into their structure. Rheological tests were performed for the prepared lubricating compositions on a DWS diffusion spectrometer. Based on the tests, the dependence of ICF function values on time, MSD function values on time and G’ and G” modulus values on frequency were determined. From the collected data, rheological parameters such as the elasticity coefficient, MSD curve slope factor, diffusion coefficients and the value at which the G’ and G” curves intersect were determined, which characterise the microstructure of the tested lubricants. Raman and FTIR spectra were also performed to characterise the chemical structure of the compositions studied, and the intensity of integration of characteristic bands of vegetable greases was calculated. For vegetable greases made from different vegetable oils, a change in the value of the MSD function was observed, and the calculated value of the elasticity index indicates better viscoelastic properties for the grease made from rapeseed oil. Modification of vegetable greases with a multifunctional additive leads to a change in rheological parameters, indicating a change in the structure of the greases studied. The results of tests of diffusion coefficients for vegetable greases show a change in microstructure for greases made with different vegetable oils. Such results testify to moderately strong viscoelastic properties, leading to the conclusion that the produced greases are substances stable to changes in chemical structure depending on the base oil and modifying additive used. Raman and FTIR spectroscopy is a technique that enables changes in the chemical composition of vegetable oils to be assessed by analysing the degree of unsaturation of fatty acids in vegetable oils, making it a very good diagnostic method for quality control of lubricants based on vegetable oils. The results obtained make it possible to differentiate lubricants prepared with different vegetable oils and allow the chemical structure of the vegetable lubricants studied to be assessed on the basis of the intensity of integration of characteristic bands. Full article

(This article belongs to the Special Issue Condition Monitoring of Lubricating Oils)

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25 pages, 5420 KB

Open AccessArticle

Research and Optimization of Flow Characteristics in a Two-Stage Centrifugal Oil Supply Structure for Wet Clutches

by Guang Ye, Hongmei Wu, Xinyuan Yang, Yanyang Zhang, Xiang Zhu and Yu Dai

Lubricants 2025, 13(9), 415; https://doi.org/10.3390/lubricants13090415 - 16 Sep 2025

Viewed by 295

Abstract

In the tail rotor transmission system of a high-speed helicopter, the timely supply of lubricating oil to the wet friction clutch during frequent starts and stops has a significant impact on the performance of the transmission system. The oil flow requirements of clutches [...] Read more.

In the tail rotor transmission system of a high-speed helicopter, the timely supply of lubricating oil to the wet friction clutch during frequent starts and stops has a significant impact on the performance of the transmission system. The oil flow requirements of clutches vary across different operational stages, posing a challenge for traditional centrifugal oil supply methods to meet the demand for flow regulation under such dynamic conditions. This paper proposes a novel two-stage centrifugal oil supply structure capable of achieving superior flow control during various clutch operating phases. An experimentally validated two-phase oil–gas CFD model was established to analyze the effects of operational parameters, such as rotational speed and oil supply pressure difference, as well as structural parameters, on oil supply performance. To enhance oil supply flow rate and efficiency under high-speed conditions (rated speed of 4800 rpm and 85% speed) at a common supply pressure (0.45 MPa), while reducing the pressure at the input shaft interface, key structural parameters were determined and optimized using a combined approach of Taguchi orthogonal experiments and response surface methodology. The results demonstrate that the optimized structure achieves a 142.8% increase in the weighted oil supply flow rate, an 11.1% improvement in oil supply efficiency, and a 7.5% reduction in pressure at the input shaft interface. Full article

(This article belongs to the Special Issue Advances in Hydrodynamic Friction in Combustion Engines)

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17 pages, 3919 KB

Open AccessArticle

Boric Acid as an Effective Lubricant Additive in Glycerol Ethoxylate Aqueous Solution

by Shouzhi Huang, Zhongnan Wang and Zhongxian Hao

Lubricants 2025, 13(9), 414; https://doi.org/10.3390/lubricants13090414 - 16 Sep 2025

Viewed by 262

Abstract

Global temperature increases and more frequent extreme climate events have intensified the challenges faced by oil-based lubricants, including environmental impact and resource depletion. In recent years, glycerol ethoxylate (GE), a non-toxic and low-cost compound, has shown promise as a water-based lubricants capable of [...] Read more.

Global temperature increases and more frequent extreme climate events have intensified the challenges faced by oil-based lubricants, including environmental impact and resource depletion. In recent years, glycerol ethoxylate (GE), a non-toxic and low-cost compound, has shown promise as a water-based lubricants capable of replacing conventional oil-based systems. Boric acid (BA) is an effective additive that significantly improves the extreme pressure performance of water-based lubricants. This study demonstrates that adding BA to a GE aqueous solution decreases the friction coefficient by five times and increases extreme pressure by 12.5%. Higher concentrations of BA promote the formation of a lubricating film, enhancing the hydrodynamic pressure effect. The findings provide valuable insights into the formulation and tribological behavior of eco-friendly lubricants, promoting sustainable manufacturing, longer equipment life, and improved reliability. Full article

(This article belongs to the Special Issue Novel Lubricant Additives in 2025)

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15 pages, 3832 KB

Open AccessArticle

The Mechanism of a High Fluid Pressure Differential on the Sealing Performance of Rotary Lip Seals

by Bo He, Xia Li, Wenhao He, Zhiyu Dong, Kang Yang, Zhibin Lu and Qihua Wang

Lubricants 2025, 13(9), 413; https://doi.org/10.3390/lubricants13090413 - 15 Sep 2025

Viewed by 286

Abstract

Rotary lip seals serve as critical sealing components in industrial equipment, traditionally relying on the reverse pumping theory for their sealing mechanism. However, increasing operational demands characterized by high fluid pressure differentials, elevated speeds, and multi-physics coupling environments have revealed limitations in the [...] Read more.

Rotary lip seals serve as critical sealing components in industrial equipment, traditionally relying on the reverse pumping theory for their sealing mechanism. However, increasing operational demands characterized by high fluid pressure differentials, elevated speeds, and multi-physics coupling environments have revealed limitations in the applicability of the classical theory. This study aims to develop a comprehensive model to quantitatively characterize rotary lip seal performance, specifically frictional torque and reverse pumping rate, and to elucidate underlying mechanisms beyond classical theory. We developed a Mixed Thermo-Hydrodynamic Lubrication (MTHL) model that explicitly integrates fluid–solid–thermal coupling effects to simulate seal behavior under complex operating parameters. The simulations reveal that reverse pumping rate increases near-linearly with rotational speed from −8.54 mm³/s (0 m/s) to 122.82 mm³/s (3 m/s) and 220.27 mm³/s (6 m/s), validating classical theory, while under elevated fluid pressure differentials, a distinct non-monotonic trend emerges: rates evolve from 122.82 mm³/s (0.10 MPa) to 172.93 mm³/s (0.12 MPa), then decline to 52.67 mm³/s (0.18 MPa), and recover to 69.87 mm³/s (0.22 MPa), a phenomenon that cannot be explained by classical sealing mechanisms. Mechanistic analysis indicates that this anomaly stems from a competitive interaction between pressure-driven and shear-driven flow. This discovery not only enhances the reverse pumping theoretical system but also provides a theoretical foundation for optimizing sealing performance under diverse operational conditions. Full article

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32 pages, 10224 KB

Open AccessReview

Precision Machining of Different Metals by Plasma Electrolytic Polishing: A Review for Improving Surface Smoothness and Properties

by Tongtong Yan, Shuqi Wang, Weidi He, Rui Jin, Jiajun Zhao, Yongchun Zou, Jiahu Ouyang, Yaming Wang and Yu Zhou

Lubricants 2025, 13(9), 412; https://doi.org/10.3390/lubricants13090412 - 14 Sep 2025

Viewed by 609

Abstract

The surface quality of metal materials is closely related to their service life and performance. Appropriate polishing techniques can significantly reduce surface roughness and the coefficient of friction, thereby enhancing properties such as wear resistance and corrosion resistance. However, traditional polishing methods have [...] Read more.

The surface quality of metal materials is closely related to their service life and performance. Appropriate polishing techniques can significantly reduce surface roughness and the coefficient of friction, thereby enhancing properties such as wear resistance and corrosion resistance. However, traditional polishing methods have certain limitations. For instance, mechanical polishing has low processing efficiency and fails to ensure consistent product quality; chemical polishing can cause environmental pollution; and electrolytic polishing may result in severe corrosion. In contrast, plasma electrolytic polishing (PEP) has attracted considerable attention for its ability to achieve high-quality surface finishes, its use of environmentally friendly aqueous electrolytes, and its rapid processing speed. It has been successfully applied to the finishing of various metal materials. Hence, this review firstly introduces the basic principles of PEP from two perspectives of macroscopic structure and microscopic mechanism, and summarizes the typical features appearing in the polishing process. Secondly, the key parameters affecting the quality of the polished surface are discussed, including voltage, electrolyte composition and electrolyte temperature, and polishing time. Subsequently, the application of PEP on various metals was discussed, along with considerations regarding the polishing efficiency and removal characteristics of coatings and non-metallic substances. Finally, the challenges and potential future development prospects of PEP are summarized. Full article

(This article belongs to the Special Issue High Performance Machining and Surface Tribology)

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24 pages, 4736 KB

Open AccessArticle

Analysis of Gear System Dynamics Based on Thermal Elastohydrodynamic Lubrication Effects

by Zhaoxia He, Xiangjun Wang, Yinan Li and Yunfei Yang

Lubricants 2025, 13(9), 411; https://doi.org/10.3390/lubricants13090411 - 14 Sep 2025

Viewed by 375

Abstract

Lubrication plays a crucial role in reducing gear surface damage and defects such as pitting, wear, and scuffing; therefore, analyzing the influence of lubrication is essential for preventing such failures in gear transmission systems. To this end, the dynamic properties of gear systems [...] Read more.

Lubrication plays a crucial role in reducing gear surface damage and defects such as pitting, wear, and scuffing; therefore, analyzing the influence of lubrication is essential for preventing such failures in gear transmission systems. To this end, the dynamic properties of gear systems were examined, leading to the creation of a thermal elastohydrodynamic lubrication (TEHL) model for the line contact of involute spur gears. This model utilizes a multigrid method to calculate the oil film pressure and thickness. Subsequently, models for meshing stiffness, normal oil film stiffness, and overall normal stiffness were developed using energy methods and lubrication theory. Ultimately, a dynamic model of the spur gear system that incorporated lubrication effects was developed to examine how different operating conditions affect dynamic transmission error, vibration velocity, and dynamic meshing force. The findings revealed that when considering the TEHL effect, the dynamic transmission error along the gear meshing line increases, while both the vibration velocity and dynamic meshing force exhibit a decrease. Furthermore, as speed and load intensify, the amplitudes of dynamic transmission error, vibration velocity, and dynamic meshing force also rise. Notably, an increase in the initial viscosity of the lubricating oil correlates with a decrease in the fluctuation of dynamic transmission error, while the variations in vibration velocity and dynamic meshing force remain relatively insignificant. Full article

(This article belongs to the Special Issue Modeling and Simulation of Elastohydrodynamic Lubrication)

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18 pages, 3264 KB

Open AccessArticle

Road Performance Evaluation of Preventive Maintenance Techniques for Asphalt Pavements

by Fansheng Kong, Yalong Li, Ruilin Wang, Xing Hu, Miao Yu and Dongzhao Jin

Lubricants 2025, 13(9), 410; https://doi.org/10.3390/lubricants13090410 - 13 Sep 2025

Viewed by 333

Abstract

Preventive maintenance treatments are widely applied to asphalt pavements to mitigate deterioration and extend service life. This study evaluated four common technologies: a high-elasticity ultra-thin overlay, an Stone Mastic Asphalt (SMA)-10 thin overlay, micro-surfacing (MS-III), and a chip seal. Laboratory testing focused on [...] Read more.

Preventive maintenance treatments are widely applied to asphalt pavements to mitigate deterioration and extend service life. This study evaluated four common technologies: a high-elasticity ultra-thin overlay, an Stone Mastic Asphalt (SMA)-10 thin overlay, micro-surfacing (MS-III), and a chip seal. Laboratory testing focused on skid resistance, surface texture, and low-temperature cracking resistance. Skid resistance was measured with a tire–pavement dynamic friction analyzer under controlled load and speed, while surface macrotexture was assessed using a laser scanner. Low-temperature cracking resistance was determined through three-point bending beam tests at −10 °C. The results showed that chip seal achieved the highest initial friction and texture depth, immediately enhancing skid resistance but exhibiting rapid texture loss and gradual friction decay. Micro-surfacing also demonstrated good initial skid resistance but experienced a sharp reduction of over 30% due to fine aggregate polishing. By contrast, the high-elastic ultra-thin overlay and SMA thin overlay provided more stable skid resistance, lower long-term friction loss, and excellent crack resistance. The polymer-modified ultra-thin overlay achieved the highest low-temperature bending strain ≈40% higher than untreated pavement, indicating superior crack resistance, followed by the SMA thin overlay. Micro-surfacing with a chip seal layer only slightly improved low-temperature performance. Overall, the high-elastic ultra-thin overlay proved to be the most balanced preventive maintenance option under heavy-load traffic and cold climate conditions, combining durable skid resistance with enhanced crack resistance. Full article

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24 pages, 7646 KB

Open AccessArticle

The Influences of Surface Texture Topography and Orientation on Point-Contact Mixed Lubrication

by Chengjiao Yu, Rui Cheng, Hongwei Zhang, Chicheng Ma and Shuangcheng Yu

Lubricants 2025, 13(9), 409; https://doi.org/10.3390/lubricants13090409 - 12 Sep 2025

Viewed by 381

Abstract

Surface topography plays a critical role in determining the tribological performance of engineering surfaces. This study systematically investigates the lubrication film characteristics of bump array surfaces (isotropic and anisotropic), groove surfaces, and herringbone surfaces through point-contact elastohydrodynamic lubrication (EHL) analyses. Numerical simulations were [...] Read more.

Surface topography plays a critical role in determining the tribological performance of engineering surfaces. This study systematically investigates the lubrication film characteristics of bump array surfaces (isotropic and anisotropic), groove surfaces, and herringbone surfaces through point-contact elastohydrodynamic lubrication (EHL) analyses. Numerical simulations were conducted to evaluate the influences of surface topographical parameters on the lubrication performance, which is quantified by average film thickness and contact load ratio. The results indicate that transverse textures lead to thicker average film as compared with longitudinal textures. This is mainly because the transverse textures can generate more effective hydrodynamic pressures from the oil film behind the ridges due to micro-EHL. By analyzing the topographical parameters and their impacts on the average film thickness and contact load ratio, this study provides practical guidance for designing surface topographies that optimize average film thickness, applicable to a wide range of tribological systems. Full article

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17 pages, 5296 KB

Open AccessArticle

Modeling and Prediction of Tribological Performance of Surface Textures on Spherical Hydrostatic–Hydrodynamic Bearings

by Huanlin Xu, Chunxing Gu and Di Zhang

Lubricants 2025, 13(9), 408; https://doi.org/10.3390/lubricants13090408 - 12 Sep 2025

Viewed by 282

Abstract

Spherical hydrostatic-hydrodynamic bearings (SHHBs) combine the advantages of hydrostatic and hydrodynamic lubrication. Endowed with the characteristics of withstanding heavy loads, reducing friction, and self-aligning, they are widely applied in high-precision machinery and extreme environment systems. However, current research on the impact of surface [...] Read more.

Spherical hydrostatic-hydrodynamic bearings (SHHBs) combine the advantages of hydrostatic and hydrodynamic lubrication. Endowed with the characteristics of withstanding heavy loads, reducing friction, and self-aligning, they are widely applied in high-precision machinery and extreme environment systems. However, current research on the impact of surface textures on bearing lubrication performance has predominantly concentrated on journal bearings, while systematic studies concerning textures for spherical bearings remain relatively inadequate. Therefore, this study developed a lubrication model for SHHBs, aiming to analyze the impact of surface textures on such bearings, with a specific focus on investigating the effects of circular and rectangular surface textures on the load-carrying capacity and friction force of SHHBs. The research results demonstrate that both shapes of surface textures can effectively improve the performance of SHHBs. Specifically, for rectangular surface textures, when

r_{p} = 3.0 m m, {\bar{h}}_{p} = 2.4

, the load-carrying capacity of the bearing is increased by

19.01 %

. It is also revealed that for both shapes of surface textures, when

ε = 0.4, ω_{z} = 1800 d e g / s

, increasing the radial clearance leads to a reduction in the bearing’s load-carrying capacity. This work provides a theoretical foundation for designing advanced SHHBs with surface texturing. Full article

(This article belongs to the Special Issue Novel Tribology in Drivetrain Components)

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18 pages, 12804 KB

Open AccessArticle

Effects of WC Addition on Microstructure and Properties of Plasma-Cladded AlCoCrFeNi High-Entropy Alloy Coatings

by Xinbin Liu, Juangang Zhao, Tiansheng Li, Zhengbing Meng, Jinbiao Qing, Wen Xu, Youxuan Ouyang and Yuanyuan Zeng

Lubricants 2025, 13(9), 407; https://doi.org/10.3390/lubricants13090407 - 12 Sep 2025

Viewed by 343

Abstract

In order to enhance the performance of 20# steel, this study successfully fabricated AlCoCrFeNi high-entropy alloy coatings with different WC contents (x = 0, 10, 20, 30 wt%) on its surface using plasma cladding technology. The effects of WC content on the microstructure, [...] Read more.

In order to enhance the performance of 20# steel, this study successfully fabricated AlCoCrFeNi high-entropy alloy coatings with different WC contents (x = 0, 10, 20, 30 wt%) on its surface using plasma cladding technology. The effects of WC content on the microstructure, mechanical properties, and corrosion resistance of the coatings were systematically investigated. The results indicate that without WC addition, the coating consists of a dual-phase structure comprising BCC and FCC phases. With the incorporation of WC, the FCC phase disappears, and the coating evolves into a composite structure based on the BCC matrix, embedded with multiple carbide phases such as W₂C, M₇C₃, M_xC_γ, and Co₆W₆C. These carbides are predominantly distributed along grain boundaries. As the WC content increases, significant grain refinement occurs and the volume fraction of carbides rises. The coating exhibits a mixed microstructure of equiaxed and columnar crystals, with excellent metallurgical bonding to the substrate. The microhardness of the coating increases markedly with higher WC content; however, the rate of enhancement slows when WC exceeds 20 wt%. The hardness of 1066.36 HV is achieved at 30 wt% WC. Wear test results show that both the friction coefficient and wear rate first decrease and then increase with increasing WC content. The optimal wear resistance is observed at 20 wt% WC, with a friction coefficient of 0.549 and a wear mass loss of only 0.25 mg, representing an approximately 40% reduction compared to the WC-free coating. Electrochemical tests demonstrate that the coating with 20 wt% WC facilitates the formation of a dense and stable passive film in NaCl solution, effectively inhibiting Cl⁻ ion penetration. This coating exhibits the best corrosion resistance, characterized by the lowest corrosion current density of 1.349 × 10⁻⁶ A·cm⁻² and the highest passive film resistance of 2764 Ω·cm². Full article

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10 pages, 4421 KB

Open AccessArticle

Effect of Layer Exposure Time in SLA-LCD Printing on Surface Topography, Hardness and Chemical Structure of UV-Cured Photopolymer

by Bartosz Pszczółkowski and Magdalena Zaborowska

Lubricants 2025, 13(9), 406; https://doi.org/10.3390/lubricants13090406 - 11 Sep 2025

Viewed by 275

Abstract

The exposure parameters in stereolithography with liquid crystal display (SLA-LCD) influence the functional properties of photopolymers, which is particularly important for tribological applications. In this study, the influence of the exposure time of the layers (2–8 s) on the surface topography (ISO 25178), [...] Read more.

The exposure parameters in stereolithography with liquid crystal display (SLA-LCD) influence the functional properties of photopolymers, which is particularly important for tribological applications. In this study, the influence of the exposure time of the layers (2–8 s) on the surface topography (ISO 25178), Brinell hardness (HB) and chemical structure (FTIR spectroscopy) of UV-cured resin samples is investigated. Both insufficient and excessive UV irradiation led to undesirable effects ranging from incomplete cross-linking and surface irregularities to excessive curing, micro-cracking and increased surface kurtosis (high Sku values). The most balanced mechanical and topographical performance was observed at a layer exposure time of 6 s, characterised by low Spk values, uniform surface texture and high cohesion between layers. FTIR analysis confirmed the progressive cross-linking with increasing exposure time. The results show that precise control of irradiation parameters enables optimisation of the interrelationships between microstructure, mechanical properties and surface functionality, which is critical for improving the durability and performance of components operating under boundary or mixed lubrication. Full article

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15 pages, 1980 KB

Open AccessArticle

Optimizing the Artificial Aging Process of Lubricating Oils Contaminated by Alternative Fuel Using Design of Experiments Methodology

by Dominika Pintér and András Lajos Nagy

Lubricants 2025, 13(9), 405; https://doi.org/10.3390/lubricants13090405 - 11 Sep 2025

Viewed by 385

Abstract

This study aimed to develop an experimental method for producing artificially aged oil with properties—such as coefficient of friction, average wear scar diameter, and antiwear additive content—similar to those of used oil contaminated with alternative fuel, sampled after 129 h of engine test [...] Read more.

This study aimed to develop an experimental method for producing artificially aged oil with properties—such as coefficient of friction, average wear scar diameter, and antiwear additive content—similar to those of used oil contaminated with alternative fuel, sampled after 129 h of engine test bench operation. A design of experiment (DoE) methodology was applied to examine the effects of various parameters and identify optimal settings. Friction and wear tests were conducted using an Optimol SRV5 tribometer in a ball-on-disc configuration, while wear scars were analyzed with a Keyence VHX-1000 digital microscope. Oil analysis was conducted with an Anton Paar 3001 viscometer and a Bruker Invenio-S Fourier-transform infrared spectrometer. The DoE results showed that the heating duration had a negligible effect on oil degradation. Aging time primarily affected changes in the friction coefficient and average wear scar diameter, whereas aging temperature was the primary factor influencing the anti-wear additive content. Gaussian elimination identified the optimal aging parameters as 132.8 °C and 103.1 h. These results were confirmed through surface analysis using a ThermoFisher NexsaG2 X-ray photoelectron spectrometer, which showed that the tribofilm composition of the used oil most closely matched that of artificially aged oils prepared at 120 °C for 96 h and 140 °C for 120 h. The strong correlation between the predicted and experimentally confirmed conditions demonstrates the reliability of the proposed method for replicating realistic aging effects in lubricating oils. Full article

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14 pages, 4502 KB

Open AccessArticle

Synthesis and Performance Study of a New Ether-Polyalphaolefin Base Oil

by Lei Huang and Wumanjiang Eli

Lubricants 2025, 13(9), 404; https://doi.org/10.3390/lubricants13090404 - 11 Sep 2025

Viewed by 381

Abstract

This study reports the first synthesis of a new type of ether-polyalphaolefin (DVE-PAO) base oil via free radical bulk copolymerization using triethylene glycol divinyl ether (DVE-3) and α-olefin in drip-feed mode. The characteristic structure of DVE-PAO was characterized by Fourier Transform Infrared Spectroscopy [...] Read more.

This study reports the first synthesis of a new type of ether-polyalphaolefin (DVE-PAO) base oil via free radical bulk copolymerization using triethylene glycol divinyl ether (DVE-3) and α-olefin in drip-feed mode. The characteristic structure of DVE-PAO was characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance Spectroscopy (NMR). The relative molecular weight and molecular weight distribution of DVE-PAO were determined using gel permeation chromatography (GPC). Structurally, it is a new type of base oil that integrates both polyalkylene glycol (PAG) and polyalphaolefin (PAO) structural units. The research shows that the viscosity of DVE-PAO base oil, the conversion rate of α-olefin, and the pour point of the copolymer increase with rising copolymerization temperature. Additionally, results from the rotating oxygen bomb test indicate that the oxidation stability of DVE-PAO also improves with increasing viscosity. Based on the principles of free radical copolymerization, this study provides a preliminary elucidation of the copolymerization patterns between the aforementioned double-ended vinyl ethers and α-olefins. Furthermore, the DVE-PAO base oil exhibits excellent miscibility with both mineral oils and polyalphaolefin (PAO) base oils. As a result, this ether-based polyalphaolefin is expected to find broad applications in the field of lubricants. Full article

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15 pages, 3513 KB

Open AccessArticle

Study on the Formation Mechanism of Oil Sludge in Polyol Esters in Presence of High-Temperature Antioxidant N-Phenyl-α-naphthylamine

by Cheng Cao, Hanglin Li, Shichao Han and Jiusheng Li

Lubricants 2025, 13(9), 403; https://doi.org/10.3390/lubricants13090403 - 10 Sep 2025

Viewed by 370

Abstract

Compared with traditional lubricants, polyol ester lubricants exhibit superior oxidative stability and have been widely applied in extreme operating conditions such as aviation engines. However, under high-temperature conditions, polyol esters are still susceptible to oxidation and therefore require the addition of antioxidants. N-phenyl-α-naphthylamine [...] Read more.

Compared with traditional lubricants, polyol ester lubricants exhibit superior oxidative stability and have been widely applied in extreme operating conditions such as aviation engines. However, under high-temperature conditions, polyol esters are still susceptible to oxidation and therefore require the addition of antioxidants. N-phenyl-α-naphthylamine is an excellent high-temperature antioxidant used in polyol ester. However, a notable issue is that oil sludge may form when this antioxidant is used at high temperatures. Excessive sludge can lead to a series of problems such as oil circuit blockage, more severe mechanical wear, and poor heat dissipation performance. In this work, oil sludge formation from N-phenyl-α-naphthylamine was simulated via high-temperature oxidation experiments in a polyol ester base oil. The formed sludge was then characterized by various advanced techniques, such as FT-IR, GPC, TGA, MALDI-TOF MS, and XPS. The results showed that the oil sludge was mainly composed of derivatives of polyol esters and N-phenyl-α-naphthylamine, along with some metal components. Further analysis showed that polymerization reactions between antioxidant molecules are the key factors leading to sludge generation, and polycyclic aromatic compounds formed by polymerization are responsible for inducing sludge generation in polyol esters. Full article

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27 pages, 4453 KB

Open AccessArticle

Multi-Objective Optimization of Wet Clutch Groove Structures Based on Response Surface Methodology and Engagement Thermal–Flow Analysis

by Xiangping Liao, Langxin Sun, Ying Zhao and Xinyang Zhu

Lubricants 2025, 13(9), 402; https://doi.org/10.3390/lubricants13090402 - 10 Sep 2025

Viewed by 488

Abstract

This study addresses the thermal hazards that arise during the initial engagement stage of wet clutches, where rapid heat generation within the transient lubricating film may cause premature film rupture, torque instability, and accelerated wear. To overcome these challenges, a coupled thermo–fluid model [...] Read more.

This study addresses the thermal hazards that arise during the initial engagement stage of wet clutches, where rapid heat generation within the transient lubricating film may cause premature film rupture, torque instability, and accelerated wear. To overcome these challenges, a coupled thermo–fluid model was developed to capture oil film flow, heat transfer, and viscous torque behavior under varying groove structures. A novelty of this work is the first integration of computational fluid dynamics (CFD) with response surface methodology (RSM) to systematically reveal how groove geometry—cross-sectional shape, number, and inclination angle—collectively influences peak temperature and viscous torque during the lubricating film stage. Simulation results show that spiral semi-circular grooves provide superior thermal management, reducing the peak friction plate temperature to 75.5 °C, while the optimized design obtained via RSM (groove depth of 0.89 mm, 19 grooves, and a 5.28° inclination angle) further lowers the maximum temperature to 68.2 °C and sustains torque transmission above 18.5 N·m. These findings demonstrate that rational groove design, guided by multi-objective optimization, can mitigate thermal risks while maintaining torque stability, offering new insights for the high-performance design of wet clutches. Full article

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56 pages, 12556 KB

Open AccessReview

The Recent Advancements in Minimum Quantity Lubrication (MQL) and Its Application in Mechanical Machining—A State-of-the-Art Review

by Aqib Mashood Khan, MD Rahatuzzaman Rahat, Umayar Ahmed, Muhammad Jamil, Muhammad Asad Ali, Guolong Zhao and José V. Abellán-Nebot

Lubricants 2025, 13(9), 401; https://doi.org/10.3390/lubricants13090401 - 9 Sep 2025

Viewed by 693

Abstract

The move toward environmentally friendly methods in the global manufacturing sector has led to the use of minimum quantity lubrication (MQL) as an eco-friendly alternative to traditional flood cooling. However, the natural limits of MQL in high-performance settings have led to the use [...] Read more.

The move toward environmentally friendly methods in the global manufacturing sector has led to the use of minimum quantity lubrication (MQL) as an eco-friendly alternative to traditional flood cooling. However, the natural limits of MQL in high-performance settings have led to the use of nanotechnology, which has resulted in the creation of nanofluids, engineered colloidal suspensions that significantly improve the thermophysical and tribological properties of base fluids. This paper gives a complete overview of the latest developments in nanofluid technology for use in machining. It starts with the basics of MQL and the rules for making, describing, and keeping nanofluids stable. The review examines the application and effectiveness of single and hybrid nanofluids in various machining processes. It goes into detail about how they improve tool life, surface integrity, and overall efficiency. It also examines the benefits of integrating nanofluid-assisted MQL (NMQL) with more advanced and hybrid systems, including cryogenic cooling (cryo-NMQL), ultrasonic atomization, electrostatic–magnetic assistance, and multi-nozzle delivery systems. The paper also gives a critical look at the main problems that these technologies face, such as the long-term stability of nanoparticle suspensions, their environmental and economic viability as measured by life cycle assessment (LCA), and the important issues of safety, toxicology, and disposal. This review gives a full picture of the current state and future potential of nanofluid-assisted sustainable manufacturing by pointing out important research gaps, like the need for real-time LCA data, cost-effective scalability, and the use of artificial intelligence (AI) to improve processes, and by outlining future research directions. Full article

(This article belongs to the Special Issue Nanofluid Minimum Quantity Lubrication)

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13 pages, 2002 KB

Open AccessArticle

Thermal Elastohydrodynamic Lubrication Analysis of Grease in Tripod Sliding Universal Couplings

by Xinchen Chen, Xia Xiu, Ye Zhou, Chenxin Dong and Degong Chang

Lubricants 2025, 13(9), 400; https://doi.org/10.3390/lubricants13090400 - 9 Sep 2025

Viewed by 333

Abstract

The tripod sliding universal coupling (TSUC) is a novel type of coupling developed through independent research. This study theoretically investigates the effects of the grease flow index and initial viscosity on thermal elastohydrodynamic lubrication (TEHL) properties. Three common grease formulations were evaluated for [...] Read more.

The tripod sliding universal coupling (TSUC) is a novel type of coupling developed through independent research. This study theoretically investigates the effects of the grease flow index and initial viscosity on thermal elastohydrodynamic lubrication (TEHL) properties. Three common grease formulations were evaluated for TSUC lubrication. The numerical results yielded the following insights: a larger flow index increases film thickness and elevates the secondary pressure peak. A higher initial viscosity enhances film thickness yet significantly elevates the temperature distribution due to quadratic growth in viscous dissipation. It also intensifies the secondary pressure peak, which may exceed the central Hertzian pressure under heavy loads, thereby accelerating surface fatigue. The lubrication performance varies significantly across grease types. When pressure–viscosity coefficients and densities are similar, the initial viscosity becomes the dominant factor. These findings provide a theoretical basis for optimizing grease selection in TSUC systems to improve the efficiency and durability of lubrication. Full article

(This article belongs to the Special Issue Modeling and Simulation of Elastohydrodynamic Lubrication)

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13 pages, 3731 KB

Open AccessArticle

Improving the Wear Properties of Ductile Iron by Introducing Ultrafine Graphite Nodules

by Chen Liu, Yuzhou Du, Haohao Li, Caiyin You, Chao Yang, Na Tian and Bailing Jiang

Lubricants 2025, 13(9), 399; https://doi.org/10.3390/lubricants13090399 - 9 Sep 2025

Viewed by 508

Abstract

The tribological behavior of ferritic ductile iron without ultrafine graphite nodules (FDI) and ferritic ductile iron with ultrafine graphite nodules (FDI-UG) was investigated in the present study. Ultrafine graphite nodules with a count of 3400 nod/mm² were introduced by annealing treatment of [...] Read more.

The tribological behavior of ferritic ductile iron without ultrafine graphite nodules (FDI) and ferritic ductile iron with ultrafine graphite nodules (FDI-UG) was investigated in the present study. Ultrafine graphite nodules with a count of 3400 nod/mm² were introduced by annealing treatment of quenched ductile iron, which effectively reduced the friction coefficient of ferritic ductile iron from approximately 0.3 to 0.15. This improvement was attributed to the ultrafine graphite nodules, which, due to their small spacing, facilitated a more uniform distribution on the tribological surface. Additionally, the formation of ultrafine graphite nodules in ferritized ductile iron refined the grain size (15 μm) and enhanced the hardness of ferritic ductile iron (183 HV), thereby significantly reducing abrasive wear. The more uniform graphite lubrication on the tribosurface and high hardness of fine ferrite grains in FDI-UG further enhanced wear resistance between the frictional pairs, effectively suppressing adhesion wear at high loads (6 N). Consequently, the ferritic ductile iron containing ultrafine graphite nodules and fine ferrite grains exhibited a superior wear resistance (6.84 × 10⁻³ mm³ and 9.47 × 10⁻³ mm³) compared to its untreated counterpart (9.22 × 10⁻³ mm³ and 11.95 × 10⁻³ mm³). These findings suggest that the incorporation of ultrafine graphite nodules was an effective strategy to enhance the tribological properties of ductile iron. Full article

(This article belongs to the Special Issue Advances in Wear-Resistant Fe-Based Materials)

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15 pages, 4353 KB

Open AccessArticle

Simulation Study on the Effect of Molecular Structure Characteristics of Lubricant Base Oils on Lubrication Performance

by Boxi Tian, Yixi Shao, Feng Zhu, Chengzhi Hu, Tiedong Zhang, Jiaxin Liu, Honglin Xu, Chengyuan Cao, Hongliang Yu and Weiwei Wang

Lubricants 2025, 13(9), 398; https://doi.org/10.3390/lubricants13090398 - 8 Sep 2025

Viewed by 592

Abstract

The complex composition of lubricating base oils makes it difficult to analyze the influence of specific molecular structure on lubricating performance. To achieve this target, nine kinds of poly α-olefin molecules with different structure characteristics were designed, which prepared the lubricant models. Molecular [...] Read more.

The complex composition of lubricating base oils makes it difficult to analyze the influence of specific molecular structure on lubricating performance. To achieve this target, nine kinds of poly α-olefin molecules with different structure characteristics were designed, which prepared the lubricant models. Molecular dynamic simulation was used to analyze the tribological performance under pressure of 500 MPa, temperature 353 K, and shear velocity of 20 m/s; the volume compression and shear stress of lubricant films were obtained. Molecular volume, adsorption energies, radius of gyration, and mean square displacement were used to analyze the relationship between molecular structure and lubricant performance. Results show that the characteristic of the Iso and Mid type have the best friction reduction performance. The molecules of the Iso structure have the highest oil film thickness and the best load-bearing performance. The radius of gyration increases with the shear simulation for most of the molecules. The adsorption energy of End is the highest, and the Mid is the smallest. Among the nine molecules, C20Iso shows excellent performance both in load-bearing and friction reduction, which provides a reference for the molecular design of high-performance lubricant base oils. Full article

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24 pages, 1799 KB

Open AccessArticle

Lubricant Performance in Wind Turbines: A Data Study in Real-Use Conditions

by A. E. Jiménez, H. J. Barajas, M. D. Avilés, I. J. Martínez-Mateo and F. J. Carrión-Vilches

Lubricants 2025, 13(9), 397; https://doi.org/10.3390/lubricants13090397 - 7 Sep 2025

Viewed by 548

Abstract

An extensive statistical study was conducted on a 10-year dataset (2014–2023) containing the lubrication condition monitoring results from wind turbine gearboxes in the Iberian Peninsula. The dataset includes two mineral and two synthetic lubricants; all four were sampled and analyzed regularly in accordance [...] Read more.

An extensive statistical study was conducted on a 10-year dataset (2014–2023) containing the lubrication condition monitoring results from wind turbine gearboxes in the Iberian Peninsula. The dataset includes two mineral and two synthetic lubricants; all four were sampled and analyzed regularly in accordance with the ISO 14830-1 standard. This dataset comprises over 25,000 records across 24 distinct parameters, as defined by the standard maintenance procedure of lubricants in wind turbine gearboxes. To reduce dimensionality, the analysis begins with principal component analysis, followed by discussion of Spearman correlations and finishing with comparation of trends. Performance differences, correlations and typical trends will be evaluated and compared for mineral and synthetic lubricants. This data-driven study presents a significant contribution to supporting maintenance decision-making processes in wind farms. Despite the differences, P, Pb and K have been identified as having a major influence on the variance in the dataset for all the lubricants, although no significant correlations with these elements have been found. Mineral lubricants showed very few correlations between elements and lubricant parameters, with Fe as the major element to be considered. Meanwhile, synthetic lubricants showed several correlations between elements (Fe, Mn, Cu, Ba) and lubricant parameters denoting complex tribochemical reactions. Full article

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23 pages, 8724 KB

Open AccessArticle

Comparative Analysis of Emulsion, Cutting Oil, and Synthetic Oil-Free Fluids on Machining Temperatures and Performance in Side Milling of Ti-6Al-4V

by Hui Liu, Markus Meurer and Thomas Bergs

Lubricants 2025, 13(9), 396; https://doi.org/10.3390/lubricants13090396 - 6 Sep 2025

Viewed by 423

Abstract

During machining, most of the mechanical energy is converted into heat. A substantial part of this heat is transferred to the cutting tool, causing a rapid rise in tool temperature. Excessive thermal loads accelerate tool wear and lead to displacement of the tool [...] Read more.

During machining, most of the mechanical energy is converted into heat. A substantial part of this heat is transferred to the cutting tool, causing a rapid rise in tool temperature. Excessive thermal loads accelerate tool wear and lead to displacement of the tool center point, reducing machining accuracy and workpiece quality. This challenge is particularly pronounced when machining titanium alloys. Due to their low thermal conductivity, titanium alloys impose significantly higher thermal loads on the cutting tool compared to conventional carbon steels, making the process more difficult. To reduce temperatures in the cutting zone, cutting fluids are widely employed in titanium machining. They have been shown to significantly extend tool life. Cutting fluids are broadly categorized into cutting oils and water-based cutting fluids. Owing to their distinct thermophysical properties, these fluids exhibit notably different cooling and lubrication performance. However, current research lacks comprehensive cross-comparative studies of different cutting fluid types, which hinders the selection of optimal cutting fluids for process optimization. This study examines the influence of three cutting fluids—emulsion, cutting oil, and synthetic oil-free fluid—on tool wear, temperature, surface quality, and energy consumption during flood-cooled end milling of Ti-6Al-4V. A novel experimental setup incorporating embedded thermocouples enabled real-time temperature measurement near the cutting edge. Tool wear, torque, and surface roughness were recorded over defined feed lengths. Among the tested fluids, emulsion achieved the best balance of cooling and lubrication, resulting in the longest tool life with a feed travel path of 12.21 m. This corresponds to an increase of approximately 200% compared to cutting oil and oil-free fluid. Cutting oil offered superior lubrication but limited cooling capacity, resulting in localized thermal damage and edge chipping. Water-based cutting fluids reduced tool temperatures by over 300 °C compared to dry cutting but, in some cases, increased notch wear due to higher mechanical stress at the entry point. Power consumption analysis revealed that the cutting fluid supply system accounted for 60–70% of total energy use, particularly with high-viscosity fluids like cutting oil. Complementary thermal and CFD simulations were used to quantify heat partitioning and convective cooling efficiency. The results showed that water-based fluids achieved heat transfer coefficients up to 175 kW/m²·K, more than ten times higher than those of cutting oil. These findings emphasize the importance of selecting suitable cutting fluids and optimizing their supply to enhance tool performance and energy efficiency in Ti-6Al-4V machining. Full article

(This article belongs to the Special Issue Friction and Wear Mechanism Under Extreme Environments)

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12 pages, 4448 KB

Open AccessArticle

Effect of Low Concentrations of Organophosphorus Additives on Tribological Performance of Polyalkylene Glycol-Based Oils for Tin Bronze on Tungsten Carbide Applications

by Elena Torskaya, Vlada Petrova, Aleksey Morozov, Ivan Shkalei and Dmitrii Kozhevnikov

Lubricants 2025, 13(9), 395; https://doi.org/10.3390/lubricants13090395 - 5 Sep 2025

Viewed by 374

Abstract

Low concentrations of organophosphorus additives in PAG oil for friction units are under consideration. Concentrations of 0.1–0.5 percent maintain the environmental friendliness of the oil, but can improve its tribological properties. The friction pair (tin bronze versus tungsten carbide) was made from a [...] Read more.

Low concentrations of organophosphorus additives in PAG oil for friction units are under consideration. Concentrations of 0.1–0.5 percent maintain the environmental friendliness of the oil, but can improve its tribological properties. The friction pair (tin bronze versus tungsten carbide) was made from a plunger and compressor seal. It was tested in the reciprocating sliding mode under a load of 600 N (contact pressure ≈ 13 P), a frequency of 8.33 Hz, an amplitude of 15 mm and a temperature of +70 °C. It was found that the phosphate concentration of 0.2 percent provides the most stable values of the friction coefficient (0.04). The mechanism of action of the modifier was determined by analyzing the results of SEM and profilometry. Organophosphates make both interacting surfaces smoother; and there is an optimal concentration of additives that provides optimal roughness. Full article

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