Lubricants

30 pages, 5698 KB

Open AccessReview

Research Progress on Bionic Functional Surfaces for Friction Reduction, Wear Resistance, and Anti-Adhesion in Agricultural Machinery

by Honglei Zhang, Tiantian Jing, Jun Zhang, Dong Lv and Zhong Tang

Lubricants 2026, 14(6), 238; https://doi.org/10.3390/lubricants14060238 - 12 Jun 2026

Abstract

This review explicitly focuses on agricultural attachments and executing components that interact directly with soil and crops, rather than the tractor vehicle itself. Operating within complex and variable farmland media environments, the key components of agricultural machinery have long been constrained by bottlenecks [...] Read more.

This review explicitly focuses on agricultural attachments and executing components that interact directly with soil and crops, rather than the tractor vehicle itself. Operating within complex and variable farmland media environments, the key components of agricultural machinery have long been constrained by bottlenecks such as high-energy draught resistance, severe solid–liquid interfacial adhesion, and intense abrasive wear. Bionic functional surfaces, based on the coupling of micro-geometric morphology and surface-interface physical chemistry, provide a scientific approach to overcoming traditional tribological limitations by reconstructing the contact mechanics and fluid dynamics boundaries at the interface. This paper presents a comprehensive review of the latest research progress regarding bionic functional surfaces in the fields of friction reduction, wear resistance, and anti-adhesion in agricultural machinery. The article systematically categorises typical biological prototypes, such as soil-burrowing animals, aquatic organisms, and plant leaves, alongside their multidimensional feature extraction methods. It provides an in-depth analysis of core interaction mechanisms, ranging from static air cushion effects and dynamic wetting evolution to active electro-osmotic soil detachment, interfacial stress redistribution, and microscopic wear debris capture. Furthermore, it evaluates the efficacy of cross-scale coupled numerical simulation technologies in resolving interfacial interactions. At the engineering application level, this review extensively discusses the field performance of bionic structures in typical operational scenarios, including draught reduction in tillage and land preparation, blockage prevention in seed-metering channels, and low-damage harvesting in agricultural machinery. Finally, countermeasures are proposed to address the fatigue degradation of bionic surfaces under alternating field loads and the barriers to the large-scale fabrication of large-sized components. The paper further highlights the development trend towards the deep integration of bionic tribology with digital twins and intelligent wear-state perception technologies, aiming to provide systematic underlying theoretical and technical references for the research and development of the next generation of intelligent agricultural equipment characterised by low energy consumption and a prolonged service life. Full article

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50 pages, 16217 KB

Open AccessReview

Cavitation in Machine Elements: A Critical Review of Cavitation Damage, Experimental Methods, Standardization Challenges, and Applied Digital Technologies

by Pavle Ljubojević, Tatjana Lazović and Marina Dojčinović

Lubricants 2026, 14(6), 237; https://doi.org/10.3390/lubricants14060237 - 11 Jun 2026

Abstract

Cavitation in machine elements is often accompanied by surface degradation, material loss, and a reduction in functional performance and reliability. Despite extensive research on cavitation in hydraulic systems, its role in the behavior and durability of machine elements remains insufficiently addressed. This paper [...] Read more.

Cavitation in machine elements is often accompanied by surface degradation, material loss, and a reduction in functional performance and reliability. Despite extensive research on cavitation in hydraulic systems, its role in the behavior and durability of machine elements remains insufficiently addressed. This paper presents a critical review of cavitation and cavitation-induced erosion in machine elements, based on an analysis of relevant literature and standards. The study covers different types of components, including gears, plain and rolling bearings, and seals, with particular attention to the mechanisms of damage and the methods used for their investigation. The analysis shows that, although the fundamental mechanisms of cavitation are well understood and standardized testing methods are available, their application to machine elements is limited. Existing standards are not sufficiently adapted to specific components, while current numerical and experimental approaches rarely provide a direct link between cavitation phenomena and material degradation. The findings indicate the need for improved standardization, development of integrated modelling approaches, and a closer connection between cavitation mechanisms and the performance characteristics of machine elements. The presented analysis is relevant for design, reliability assessment, maintenance strategies, and the development of cavitation-resistant machine components in hydraulic and mechanical systems. Full article

(This article belongs to the Special Issue Machine Design and Tribology)

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17 pages, 22271 KB

Open AccessArticle

Analysis of Friction-Induced Vibration Behavior of Train Brake Systems Considering the Effect of Environmental Temperature

by Xiaocui Wang, Wanxin Li, Quan Wang, Zhiwei Wang and Jiliang Mo

Lubricants 2026, 14(6), 236; https://doi.org/10.3390/lubricants14060236 - 11 Jun 2026

Abstract

Train brake systems are characterized by strong friction and open-system features during the service process. Low environmental temperatures significantly affect the contact interface and the attrition characteristics of the braking frictional couple, thus intensifying friction-induced vibration and threatening operational safety. To elucidate the [...] Read more.

Train brake systems are characterized by strong friction and open-system features during the service process. Low environmental temperatures significantly affect the contact interface and the attrition characteristics of the braking frictional couple, thus intensifying friction-induced vibration and threatening operational safety. To elucidate the impact of environmental temperature on the frictional vibration characteristics of train brake systems, braking deceleration tests under different environmental temperatures were first conducted to obtain the evolution of vibration, noise, and friction coefficient with environmental temperature and brake disc rotational speed. Then, the Stribeck friction parameters under different environmental temperatures were identified using a genetic algorithm. On this basis, a brake system dynamic model was developed, incorporating disc–pad friction, wheel–rail adhesion, and the relative torsion between the brake disc and the wheelset, enabling accurate examination of the vibrational behaviour arising from friction under different environmental temperatures. And the dynamic relationship among environmental temperature, interface friction parameters, and vibration characteristics of the brake system during braking deceleration was elucidated. The findings indicate that as the environmental temperature decreases, the dynamic friction coefficient increases during the relatively high-speed braking phase, intensifying high-frequency unstable vibrations of the braking assembly. During the relatively low-speed braking phase, the friction coefficient exhibits an obvious negative-slope relationship with vehicle speed that means the friction coefficient increases as the speed decreases, and this negative slope effect is enhanced under low-temperature conditions. Consequently, it triggers intense stick–slip motion at the disc–pad interface and even severe vibrations of various components in the brake system, leading to a sudden increase in vibration intensity in the relatively low-speed range. Full article

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25 pages, 3524 KB

Open AccessArticle

Forecasting the Remaining Useful Life of Hydraulic Oils in Woodworking Equipment on Degradation of Key Properties

by Marián Kučera, Marek Svitok, Tatiana Hýrošová and Grzegorz Zajac

Lubricants 2026, 14(6), 235; https://doi.org/10.3390/lubricants14060235 - 10 Jun 2026

Abstract

In this article, the authors have experimentally investigated the changes in four key properties of six non-edible low-impact energy carries based on rapeseed oil quality grade HM and viscosity grade VG46, which were used as a filling in the hydraulic system of a [...] Read more.

In this article, the authors have experimentally investigated the changes in four key properties of six non-edible low-impact energy carries based on rapeseed oil quality grade HM and viscosity grade VG46, which were used as a filling in the hydraulic system of a round wood sorting and transporting trolley. These oils were enriched with thermo-oxidizing, extreme-pressure additives, anti-foaming, and lubricating additives to enhance performance. Three supervised machine learning prediction algorithms were used to predict key parameters essential for optimizing their performance and RUL (remaining useful life), namely support vector regression (SVR), generalized additive model (GAM), and Gaussian process regression (GPR). The model’s performance was scored from multiple perspectives using metrics such as root mean square error (RMSE), mean absolute error (MAE), mean absolute percentage error (MAPE), and coefficient of determination (R²) to state actual values, thereby demonstrating the validity of the models in predicting lubricant lifespan. Based on the collected data, this study demonstrated that it is possible to predict the degradation of hydraulic oil factors to the limit state, integrate these parameters into a comprehensive metric for more accurate remaining useful life (RUL) estimation, and obtain actual operating trends. A negative correlation was found between the remaining useful life (RUL) and parameters such as acid number, kinematic viscosity, peroxide number, and water content. The comparison of modeling algorithms showed that all three algorithms adequately described the degradation patterns. By using these performance criteria, we defined the most accurate and reliable soft-computing model for predicting hydraulic fluid parameters, providing valuable insights into optimizing machine learning models for practical applications. Full article

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

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21 pages, 24404 KB

Open AccessArticle

Research on Damage Mechanism of Ceramic Balls in Hybrid Rolling Friction Pairs

by Oleksandr Stelmakh, Yiqiao Guo, Anatoliy Maystrenko, Yansong Liu, Ruslan Kostunik, Alexsandr Vasylchuk, Dmytry Kustovskyi and Hao Zhang

Lubricants 2026, 14(6), 234; https://doi.org/10.3390/lubricants14060234 - 10 Jun 2026

Abstract

In hybrid rolling bearings operating under extreme high-temperature and high-load conditions, steel rolling elements are prone to early failure, which has accelerated the widespread adoption of ceramic materials. To address the limitations of conventional studies, which have focused mainly on macroscopic wear parameters [...] Read more.

In hybrid rolling bearings operating under extreme high-temperature and high-load conditions, steel rolling elements are prone to early failure, which has accelerated the widespread adoption of ceramic materials. To address the limitations of conventional studies, which have focused mainly on macroscopic wear parameters while neglecting subsurface failure mechanisms and the relationship among sintering process, microstructure, and fatigue performance, this work systematically compares the tribological behavior of Si₃N₄ ceramic balls fabricated by high-pressure electric resistance hot-pressing (REHP) and B₄C ceramic balls prepared by conventional hot pressing (HP) against 52100 steel counterparts. The central innovation of this study lies in clarifying, based on Hertzian contact theory and Lundberg-Palmgren life theory, that subsurface orthogonal shear stress, rather than surface compressive stress, is the fundamental driving force for contact fatigue failure of ceramic balls. In addition, two distinct damage evolution modes are revealed: B₄C exhibits early-stage brittle fracture and large-scale spalling, whereas REHP-Si₃N₄ is characterized by microcrack initiation and slow crack propagation. Moreover, the intrinsic mechanism by which the REHP process significantly enhances the contact fatigue life of ceramics is elucidated; namely, it refines grain size, eliminates residual porosity, and increases densification. The results show that, under the same high-load conditions, the mass loss of REHP-Si₃N₄ ceramic balls is only 35.7% of that of HP-B₄C, while the service life is extended by 20%. This work provides a key theoretical basis for ceramic material selection and sintering process optimization in high-performance hybrid bearings. Full article

(This article belongs to the Special Issue Tribological Characteristics of Bearing System, 4th Edition)

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28 pages, 6327 KB

Open AccessArticle

Effect of Complex Bio-Thickener Concentration on Thermal, Rheological, and Tribological Properties of a Novel Bio-Based Grease for Rolling Element Bearing Applications

by Rewan Abdelrahman, Mostafa El-Helaly, Florian Pape, Mohamed Abdelnaeem and Mohamed G. A. Nassef

Lubricants 2026, 14(6), 233; https://doi.org/10.3390/lubricants14060233 - 9 Jun 2026

Abstract

The recent motivation for sustainable lubrication has driven the development and advances of bio-based and potentially environmentally favorable alternatives to petroleum-based greases. Yet, their industrial adoption is largely hindered by the thickener weak network or inconsistency leading to grease unacceptable degradation under applied [...] Read more.

The recent motivation for sustainable lubrication has driven the development and advances of bio-based and potentially environmentally favorable alternatives to petroleum-based greases. Yet, their industrial adoption is largely hindered by the thickener weak network or inconsistency leading to grease unacceptable degradation under applied loads and operating temperatures in rotating machinery. This study investigates a novel grease formulated from 80% palm oil and a 20% complex thickener system from carnauba wax (CW) and glycerol monostearate (GMS). The effect of thickener composition on grease performance was investigated by testing their X-ray diffraction (XRD) spectra, Fourier transform infrared (FTIR) spectra, penetration level, oil separation percentage, viscosity, thermal properties, and tribological behavior. GMS-rich blends achieved up to 70% lower friction than lithium grease. However, they showed high wear rates and excessive oil separation ranging from 0.07% at room temperature for the 20% GMS blend to above 9% at 40 °C for softer formulations. The blend of 15% CW + 5% GMS showed only 0.113% and 3.145% oil bleed at room temperature and at 40 °C, respectively, with suitable consistency (NLGI 3) and acceptable dynamic viscosity rates. Regarding thermal behavior, CW-based samples revealed an enhanced melting point compared to GMS. For validation, investigations were conducted on rolling element bearings on a customized test setup operating at 1400 rpm under selected radial loads. The results demonstrate that CW/GMS bio-thickeners achieved lower vibration levels compared to the GMS thickener, approaching the performance of lithium grease. Full article

(This article belongs to the Special Issue Tribological Properties of Biolubricants)

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19 pages, 9733 KB

Open AccessArticle

Liquid Evolution Behavior in Soft Tribo-Contacts Featuring Bionic Surface Textures and Its Influence on Friction Under Wet Conditions

by Lirong Huang, Zhaoxiang Wang, Kunpeng Zhang and Binbin Su

Lubricants 2026, 14(6), 232; https://doi.org/10.3390/lubricants14060232 - 8 Jun 2026

Abstract

To elucidate the mechanisms responsible for high friction in micro-pillared soft tribo-contacts under wet conditions, this study investigates the liquid migration behavior across elasticity interfaces featuring bionic surface textures and examines the influence of this migration on interfacial friction properties. Micro-pillar bionic surface [...] Read more.

To elucidate the mechanisms responsible for high friction in micro-pillared soft tribo-contacts under wet conditions, this study investigates the liquid migration behavior across elasticity interfaces featuring bionic surface textures and examines the influence of this migration on interfacial friction properties. Micro-pillar bionic surface textures were fabricated on polydimethylsiloxane (PDMS) substrates. In situ observation of liquid migration and corresponding friction tests were systematically conducted using custom-built experimental setups on soft interfaces textured with micro-pillars of varying area densities. The results demonstrate that both geometrical shape and area density of surface textures play a critical role in regulating liquid migration behavior. Surface textures with circular and hexagonal geometries exhibit optimal migration rates, attributed to their smooth structural profiles, which reduce flow resistance within the microchannels. Liquid migration efficiency is effectively improved with increasing area density of the bionic surface texture owing to strengthened capillary forces. Correspondingly, bionic surface textures exhibiting superior liquid migration characteristics show the smallest relative reduction in friction force during transitions from dry to wet frictional states. This behavior is primarily attributed to the surface’s exceptionally rapid drainage capability, which effectively mitigates the adverse effects of interfacial liquid films on friction. Specifically, rapid liquid removal increases the effective solid–solid contact area and enhances mechanical interlocking at the interface. Consequently, these surfaces maintain outstanding frictional performance even under humid or wet conditions. These findings provide important theoretical support for the rational design of surface microstructures and the optimized regulation of friction and liquid film in wet contact conditions. Full article

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16 pages, 12980 KB

Open AccessArticle

Laser Cladding Fabrication of Self-Lubricating High-Wear-Resistance Coatings: Microstructural Evolution and Wear Mechanisms

by Linkai He, Xingqiang Wu, Zhenneng Chen, Kaiqiang Zhang, Qingnan Men, Yun Tian and Meilu Yu

Lubricants 2026, 14(6), 231; https://doi.org/10.3390/lubricants14060231 - 8 Jun 2026

Abstract

In pursuit of a composite coating for tunnel boring machine (TBM) disc cutters that offers both high wear resistance and self-lubricating functionality, we fabricated Fe-based composite coatings reinforced with WC and MoS₂ through laser cladding. Seven coating compositions with systematically tailored MoS [...] Read more.

In pursuit of a composite coating for tunnel boring machine (TBM) disc cutters that offers both high wear resistance and self-lubricating functionality, we fabricated Fe-based composite coatings reinforced with WC and MoS₂ through laser cladding. Seven coating compositions with systematically tailored MoS₂ contents were prepared to investigate the concentration-dependent effects of MoS₂ on microstructural evolution and tribological properties, and to evaluate their performance under various rock-contact conditions. XPS results reveal that MoS₂ decomposed during laser cladding, leading to the in situ formation of metal sulfides in the Fe-based matrix. These sulfides, characterized by low shear strength, readily form a continuous and stable lubricating tribofilm at the hob–rock interface. The tribofilm effectively lowers the coefficient of friction (COF), curtails friction-induced energy dissipation and surface degradation, and ultimately enhances the wear resistance of the disc cutter. Simultaneously, the rapid non-equilibrium solidification inherent in laser cladding stabilizes metastable phases, which refine the microstructure, improve densification, and bolster phase stability. Among the tested compositions, the coating containing 4 wt.% MoS₂ exhibited the most favorable dry-sliding tribological performance, as evidenced by an average coefficient of friction of 0.409, a hardness of 749.5 HV1, and consistently low wear mass losses below 2.1 × 10⁻³ g under different rock-contact conditions. Mechanistically, XRD and SEM analyses further attributed the superior performance of the 4 wt.% MoS₂ coating to concurrent strengthening mechanisms: grain refinement, dispersion strengthening from uniformly distributed second-phase particles, and increased dislocation density. Collectively, these effects substantially improve the wear resistance of the disc cutter, thereby extending its durability and service life under complex operating conditions. Full article

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22 pages, 4083 KB

Open AccessReview

Water-Induced Lubrication Challenges in Engine Oils: A Review with H₂-ICE as a Proxy for Alternative-Fuel Engines

by Le Ma, Yunfeng Zang, Zhancheng Dou, Lingyan Guo, Weimin Li, Qicheng Wang, Xinming Li and Haichao Liu

Lubricants 2026, 14(6), 230; https://doi.org/10.3390/lubricants14060230 - 5 Jun 2026

Abstract

Hydrogen-fueled internal combustion engines (H₂-ICEs) impose unique challenges on engine lubrication because water is an inevitable combustion product. This review summarizes the current understanding of water-induced degradation mechanisms in engine oils for H₂-ICEs, with emphasis on physicochemical property variation, [...] Read more.

Hydrogen-fueled internal combustion engines (H₂-ICEs) impose unique challenges on engine lubrication because water is an inevitable combustion product. This review summarizes the current understanding of water-induced degradation mechanisms in engine oils for H₂-ICEs, with emphasis on physicochemical property variation, additive depletion, tribofilm evolution, and tribological performance. Water present in dissolved, emulsified, or free states can significantly alter lubricant viscosity, destabilize additive systems, and accelerate oxidative aging. In particular, water promotes the depletion of zinc dialkyldithiophosphate (ZDDP) through tribofilm removal and competitive adsorption at rubbing interfaces, while also inducing additive hydrolysis that transforms long-chain phosphates into shorter-chain species with inferior film-forming capability. These processes inhibit tribofilm growth and reduce the mechanical integrity of protective films, thereby deteriorating anti-wear performance. Although substantial progress has been made in understanding the role of liquid water in lubrication, the tribochemical effects of high-temperature water vapor under realistic H₂-ICE operating conditions remain largely unexplored. Future research should therefore focus on water vapor-dominated lubrication environments representative of hydrogen combustion, aiming to elucidate the underlying tribochemical mechanisms and support the development of dedicated lubricants for durable and reliable H₂-ICE operation. Full article

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17 pages, 8792 KB

Open AccessArticle

Detection of Lubrication Condition in Hydrodynamic Journal Bearings Based on Dynamic Experimentation Using Acoustic Emission and Machine Learning

by Richard Heinlein, Markus Grebe and Christoph Herrmann

Lubricants 2026, 14(6), 229; https://doi.org/10.3390/lubricants14060229 - 3 Jun 2026

Abstract

Reliable detection of lubrication conditions in sliding bearings is crucial for condition monitoring and predictive maintenance. Despite advances in tribological research, there remains a need for accurate diagnostics that indicate worsening of lubricity in mixed and boundary lubrication states. In this study, a [...] Read more.

Reliable detection of lubrication conditions in sliding bearings is crucial for condition monitoring and predictive maintenance. Despite advances in tribological research, there remains a need for accurate diagnostics that indicate worsening of lubricity in mixed and boundary lubrication states. In this study, a dynamic test procedure is utilised to classify lubrication conditions with the help of a boosted tree classification algorithm. A radial journal bearing test rig is built and equipped with a high-frequency acoustic emission (AE) sensor on which experiments consisting of repeated dynamic speed and load alterations are conducted. AE signal features are extracted, compared and used to train an Extreme Gradient Boosting (XGBoost) classification model. The model achieves high accuracy (97.57%) in distinguishing adequate vs. starved lubrication conditions in mixed friction. Misclassifications are mainly observed at the lowest load or speed conditions, where residual lubrication effects make the classes less separable. The model’s generalisability is evaluated by applying it to tests with differing viscosity classes and alternative bearing materials without retraining, with the classifier retaining good performance. The model is also used to detect anomalies in a grease-lubricated system, where it successfully detects poor lubrication conditions. While it is known prior to this publication that AE is a good tool to detect anomalous behaviour in hydrodynamic journal bearings, the findings presented highlight the potential for the transferability of anomaly detection models trained in a laboratory setting and applied to different real-world applications to reduce life-cycle maintenance costs and increase uptime in industrial applications. Full article

(This article belongs to the Special Issue Experimental Modelling of Tribosystems)

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35 pages, 6667 KB

Open AccessArticle

Contact Mechanics Analysis of Main Rotor Shaft Bearings in a Helicopter Main Gearbox Under Flight Load Spectrum

by Feng Zhang, Hongjian Wu, Yanan Zhang, Hongbin Liu, Baolin Jia, Xinlong Wu, Kun Zhao, He Liu and Wenhu Zhang

Lubricants 2026, 14(6), 228; https://doi.org/10.3390/lubricants14060228 - 31 May 2026

Abstract

To investigate the contact mechanical performance of helicopter main gearbox rotor shaft bearings under a complex load spectrum, this study focuses on the contact stress and load-carrying characteristics of bearings operating under high-speed and heavy-load conditions. Based on the rotor shaft system of [...] Read more.

To investigate the contact mechanical performance of helicopter main gearbox rotor shaft bearings under a complex load spectrum, this study focuses on the contact stress and load-carrying characteristics of bearings operating under high-speed and heavy-load conditions. Based on the rotor shaft system of a helicopter main gearbox and Hertzian contact theory, quasi-static analyses were performed on four tapered roller bearings and one cylindrical roller bearing mounted on the shaft system conducted in Romax. The results indicate that the maximum contact stresses of the bearings do not exhibit sustained high-stress states under most operating conditions. The peak-stress conditions account for only extremely small time proportions in limited cases, namely 0.003429% and 0.025%. The contact stresses on both the inner and outer raceways exhibit a non-uniform distribution along the roller length, with local peak values appearing near the highly loaded roller-raceway contact regions. This suggests that during the design process of the helicopter main gearbox rotor shaft, special attention should be given to this region. The present results provide a theoretical basis for subsequent life-index verification and offer an effective analytical method for the design and validation of such critical components. Full article

(This article belongs to the Special Issue Machine Design and Tribology)

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27 pages, 4383 KB

Open AccessArticle

Classification of Tool Wear Condition During CNC Cutting Process from Spindle Motor Current Signal Monitoring

by Lloyd J. Augustine, Wani J. Morgan, Hsiao-Yeh Chu, Sheng-Jye Hwang and Hsin-Shu Peng

Lubricants 2026, 14(6), 227; https://doi.org/10.3390/lubricants14060227 - 31 May 2026

Abstract

Tool wear in CNC milling increases friction and torque demand at the tool-workpiece interface, which is reflected in spindle motor current. This study develops a non-intrusive tool wear condition classification method using spindle motor current monitoring during practical CNC milling of commercial medium-carbon [...] Read more.

Tool wear in CNC milling increases friction and torque demand at the tool-workpiece interface, which is reflected in spindle motor current. This study develops a non-intrusive tool wear condition classification method using spindle motor current monitoring during practical CNC milling of commercial medium-carbon steel workpieces (JIS S50C/AISI SAE 1050-equivalent; as-received and non-heat-treated; nominal laboratory hardness approximately 4.3 HRC). Experiments were performed on a Tongtai MDV-508 vertical machining center at fixed cutting conditions (3000 rpm spindle speed, 2 mm axial depth of cut, 5 mm cutting width, and 300 mm/min feed rate) using eight TiAlN-coated fine-grain WC–Co solid carbide end mills (10 mm diameter, four flutes; nominal Co binder approximately 10 wt%). An oil-based HS Highstart/HS-SSHS-BH10 cutting fluid was applied through the machine external coolant nozzle in flood mode at an estimated nominal flow rate of approximately 3 L/min and near-room coolant temperature (25 ± 2 °C), and was used as supplied without dilution. A clamp-type AC current sensor was installed on one phase line supplying the spindle motor, and current was acquired using an NI-9221 module at 20 kHz. Cutting intervals were isolated by envelope-based segmentation, concatenated, and divided into 1 s windows (0.5 s overlap) for feature extraction. Three feature sets were evaluated: time-domain statistics, frequency-domain statistics, and an FFT→PCA hybrid representation. Tool states (New, Mid-life, Old) were labeled using post-process surface roughness Ra thresholds supported by microscope observation. The PCA transformation was fitted only on training data and then applied to the held-out test data. A logistic regression classifier achieved 97.44% test accuracy (152/156 windows; 95% Wilson CI: 93.59–99.00%) with the PCA-hybrid features, outperforming time-domain (89.74%) and frequency-domain (94.87%) models. The results support spindle current monitoring as a low-cost approach for quality-aligned tool condition monitoring, while the external validity remains limited to the tested machine, material, tool, coolant, and cutting-parameter combination. Full article

(This article belongs to the Special Issue Monitoring and Remaining Useful Life (RUL) Technology of Tool Wear)

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32 pages, 11283 KB

Open AccessArticle

Multiscale Modeling of Micro-Textured Gear: Interface Enriched Lubrication and Anti-Scuffing Load-Bearing Capacity

by Weiqiang Zou, Xigui Wang, Yongmei Wang and Jiafu Ruan

Lubricants 2026, 14(6), 226; https://doi.org/10.3390/lubricants14060226 - 31 May 2026

Abstract

A multiscale contact model is developed for micro-textured gear interfaces incorporating Micro-Convex-Concave Asperity (MCCA) characteristics to elucidate the synergistic modulation between Interface Enriched Lubrication (IEL) performance and Anti-Scuffing Load-Bearing Capacity (ASLBC) of Micro-Textured Meshing Interfaces (MTMI) under transient Thermal Elastohydrodynamic Lubrication (TEHL) conditions. [...] Read more.

A multiscale contact model is developed for micro-textured gear interfaces incorporating Micro-Convex-Concave Asperity (MCCA) characteristics to elucidate the synergistic modulation between Interface Enriched Lubrication (IEL) performance and Anti-Scuffing Load-Bearing Capacity (ASLBC) of Micro-Textured Meshing Interfaces (MTMI) under transient Thermal Elastohydrodynamic Lubrication (TEHL) conditions. Homogenization theory is employed to quantify the effects of areal density and depth-to-diameter ratio on IEL characteristics. A time-resolved micro-elastohydrodynamic lubrication model, formulated through dimensionless discretization and adaptive mesh refinement, investigates the influences of autocorrelation length and MCCA amplitude on interfacial behavior. A correlation framework linking Micro-Element Texture (MET) geometric parameters to meshing ASLBC is established to identify optimal textures for simultaneous enhancement of IEL and ASLBC. Experimental observations demonstrate qualitative consistency with numerical predictions regarding the evolutionary trends of temperature fields and dynamic friction coefficients, providing preliminary physical validation for the proposed model. Univariate Sensitivity Analysis (USA) and Multiple Linear Regression (MLR) are further utilized to optimize microtexture parameters by elucidating the influences of MET sizes, area ratio, and configuration on meshing ASLBC and friction coefficients. Full article

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11 pages, 16497 KB

Open AccessArticle

The Influence of Ball Surface Properties on the Friction and Wear Results of the Four-Ball Tribo-Test

by Raimondas Kreivaitis, Irenijus Rastokas, Paulius Bendžiūnas, Adolfo Senatore, Audrius Žunda and Simona Tučkutė

Lubricants 2026, 14(6), 225; https://doi.org/10.3390/lubricants14060225 - 31 May 2026

Abstract

Lubricity is critical for maintaining the reliability of mechanical systems. Consistent monitoring of in-service lubricants and the development of new formulations require reliable test equipment and standardised procedures. The four-ball tribo-test is among the most widely used and acknowledged methods. Like other methods, [...] Read more.

Lubricity is critical for maintaining the reliability of mechanical systems. Consistent monitoring of in-service lubricants and the development of new formulations require reliable test equipment and standardised procedures. The four-ball tribo-test is among the most widely used and acknowledged methods. Like other methods, its effectiveness depends on the procedures and specimens used. Therefore, in this study, we aim to emphasise the influence of ball-specimen surface parameters on lubricity results, a factor that is often insufficiently acknowledged in reported experiments. Accordingly, lubricity tests of the same motor oil were performed using balls obtained from different suppliers. The experimental procedure was identical for all ball specimens. It was found that balls from different series exhibit distinct surface properties, leading to differences in lubricity even when the same lubricating oil was used. Based on the results obtained, it was concluded that the outcome is a multiparameter solution and not the result of a single surface parameter. On the other hand, if balls from the same series are used, the results will meet the standard requirements for repeatability regardless of their surface conditions. Full article

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21 pages, 23162 KB

Open AccessArticle

Effect of Nb Content on the Microstructure and Properties of Laser-Clad NiTi-Based Coatings

by Zhaowei Yang, Ying Zhang, Guoli Li, Kun Li, Long Jiang, Qingkai Fan and Kang Qi

Lubricants 2026, 14(6), 224; https://doi.org/10.3390/lubricants14060224 - 31 May 2026

Abstract

Laser cladding has attracted considerable attention for titanium alloy surface modification owing to its high energy density, rapid cooling rate, and excellent metallurgical bonding capability. To investigate the effect of Nb content on the microstructure and properties of NiTi-based coatings, composite coatings containing [...] Read more.

Laser cladding has attracted considerable attention for titanium alloy surface modification owing to its high energy density, rapid cooling rate, and excellent metallurgical bonding capability. To investigate the effect of Nb content on the microstructure and properties of NiTi-based coatings, composite coatings containing 10–40 wt% Nb were fabricated on a titanium alloy substrate via laser cladding. The effects of Nb content on phase constitution, microstructure evolution, mechanical properties, tribological performance, residual stress, and surface topography were systematically characterized using XRD, SEM, EDS, microhardness testing, wear testing, digital image correlation, and atomic force microscopy. The results show that increasing Nb content significantly affected the solidification behavior and phase evolution of the coatings. With increasing Nb addition, the dominant phase gradually evolved from NiTi to a coexistence structure of NbTi₄ and NiTi, while Ti dilution and elemental segregation became increasingly pronounced. The crystallite size increased from 19.63 nm to 25.91 nm, accompanied by intensified dendritic segregation and surface roughening. Among all samples, the coating containing 10 wt% Nb exhibited the best overall performance, characterized by the finest microstructure, the lowest surface roughness, the lowest residual stress, and the best wear resistance. The superior performance of the low-Nb coating was mainly associated with its finer and more homogeneous microstructure, reduced elemental segregation, lower stress concentration, and enhanced grain-boundary strengthening effect. Excessive Nb addition intensified Ti dilution, grain coarsening, residual stress accumulation, and microstructural heterogeneity, thereby degrading the overall coating performance. More importantly, this study reveals that Nb-regulated Ti dilution behavior governs the synergistic evolution of elemental segregation, surface roughening, residual stress accumulation, and tribological degradation during laser cladding. This work provides new insight into the process–structure–property relationship of NiTi-based composite coatings and offers theoretical guidance for the composition optimization and engineering application of high-performance laser-clad coatings on titanium alloys. Full article

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28 pages, 7476 KB

Open AccessArticle

Statistical and Neural Network-Based Prediction of Surface Roughness and Tool Wear in AISI 1040 Steel Machining Using SiO₂ Nanoparticle-Infused Pongamia pinnata Lubricant and Coolant

by Vishal Shenoy P, Vijay Kini M, Raghuvir Pai B, Srinivas Shenoy Heckadka, Raviraj Shetty, Supriya J P and Adithya Hegde

Lubricants 2026, 14(6), 223; https://doi.org/10.3390/lubricants14060223 - 30 May 2026

Abstract

AISI 1040 steel is extensively used in structural and automotive applications, where surface integrity plays a significant role in service performance and coating adhesion. Furthermore, the selected cutting fluids are expected to effectively reduce surface roughness and tool wear by improving lubrication at [...] Read more.

AISI 1040 steel is extensively used in structural and automotive applications, where surface integrity plays a significant role in service performance and coating adhesion. Furthermore, the selected cutting fluids are expected to effectively reduce surface roughness and tool wear by improving lubrication at the tool and workpiece interface. This study investigates the influence of SiO₂ nanoparticle-assisted Pongamia pinnata oil on surface roughness and tool wear during the machining of AISI 1040 steel using an uncoated tungsten carbide tool by varying nanoparticle concentration (Vol.%), cutting speed (m/min), depth of cut (mm), and feed rate (mm/rev). The incorporation of 0.5 (Vol.%) SiO₂ nanoparticles significantly enhances machining performance by improving surface finish and reducing tool wear. Further, a minimum surface roughness value of 1.95 microns and tool wear value of 0.047 mm were achieved at a cutting speed of 101 m/min, feed rate of 0.11 mm/rev, depth of cut of 0.25 mm and 0.5 (Vol.%) SiO₂ nanoparticle concentration. ANOVA results indicate that nanoparticle concentration is the most dominant parameter affecting both surface roughness and tool wear, contributing 85.35% to the variation in surface roughness and 82.2% to the total variation in tool wear. Cutting speed is the second most influential factor, accounting for 11.63% of surface roughness variation and 11.07% of tool wear variation, while feed rate and depth of cut exhibit minimal influence in both cases. A second-order RSM model was developed to predict surface roughness and tool wear, showing excellent agreement with experimental results. The model predicted surface roughness with an average error below 2.43%, while the second-order model for tool wear exhibited an average prediction error of 4.95%, confirming its statistical significance and predictive reliability. Desirability Function Method (DFM) analysis yielded a desirability value of 1.000, confirming the optimal combination of machining parameters at 0.5354 (Vol.%) nanoparticle concentration, a cutting speed of 45 m/min, a depth of cut of 0.50 mm, and a feed rate of 0.1298 mm/rev. Overall, this study demonstrates that 0.5 (Vol.%) SiO₂ nanoparticle-incorporated Pongamia pinnata oil is an effective and sustainable cutting fluid, significantly improving surface integrity and machining performance of AISI 1040 steel during machining. Under these settings, the predicted tool wear was 0.0614 mm, accompanied by a high composite desirability value of 0.92786, indicating excellent overall performance. Moreover, the close agreement between experimental, response surface model and BP-ANN-predicted tool wear and surface roughness confirms that the ANN model reliably and robustly captures the complex, nonlinear effects of machining parameters with minimal systematic error. Full article

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13 pages, 3242 KB

Open AccessArticle

Friction and Wear Properties of Spherical Methyl Silicone Resin as an Additive in Polyethylene Glycol Base Oil

by Haiyang Wang, Zhongyi He, Zongbin Wang, Haodi Zhang, Liping Xiong and Xiaogang Jiang

Lubricants 2026, 14(6), 222; https://doi.org/10.3390/lubricants14060222 - 29 May 2026

Abstract

This study investigates spherical methyl silicone resin, a potentially environmentally friendly additive free of sulfur, phosphorus, and chlorine, as a lubricant additive in polyethylene glycol 200 (PEG 200) base oils. We evaluated concentration-response characteristics and tribological performance across PEG base oils containing 0.01–0.05 [...] Read more.

This study investigates spherical methyl silicone resin, a potentially environmentally friendly additive free of sulfur, phosphorus, and chlorine, as a lubricant additive in polyethylene glycol 200 (PEG 200) base oils. We evaluated concentration-response characteristics and tribological performance across PEG base oils containing 0.01–0.05 wt% resin. Tribological testing was conducted with a four-ball wear tester at 98 N and 1450 rpm for 30 min. All tested concentrations demonstrated excellent friction-reduction and anti-wear performance, with an optimal efficacy observed at 0.02 wt%. Surface characterization was performed using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. This friction-reducing and anti-wear performance is attributed to the formation of silicon-oxygen species and graphene-like carbon structures, thereby effectively suppressing direct surface contact and mitigating wear. Consequently, spherical methyl silicone resin demonstrates considerable potential as a green lubricant additive for bearing steel applications. Full article

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22 pages, 5230 KB

Open AccessArticle

Visualization of Lubrication Conditions Using the Electrical Impedance Method Considering Surface Roughness

by Daichi Kosugi, Fumiaki Aikawa, Shunsuke Iwase, Taisuke Maruyama and Satoshi Momozono

Lubricants 2026, 14(6), 221; https://doi.org/10.3390/lubricants14060221 - 29 May 2026

Abstract

In this study, we developed an improved electrical impedance method for measuring oil film thickness with a correction for surface roughness effects. Statistical analysis of the oil film thickness distribution revealed that rough surfaces exhibit higher capacitance values than those predicted by the [...] Read more.

In this study, we developed an improved electrical impedance method for measuring oil film thickness with a correction for surface roughness effects. Statistical analysis of the oil film thickness distribution revealed that rough surfaces exhibit higher capacitance values than those predicted by the ideal parallel-plate model, despite having the same mean film thickness. Consequently, a corresponding roughness correction formula was derived. The accuracy of the method was verified in ball-on-disk type apparatus using balls with a rough surface. The corrected oil film thickness agreed more closely with the Hamrock–Dowson equation and with optical interferometry measurements than did the uncorrected result. These outcomes confirm that oil film thickness can be estimated considering surface roughness. The technique is therefore expected to facilitate the optimization of lubrication conditions and enable more reliable bearing-life prediction. Full article

(This article belongs to the Special Issue Mechanical Tribology and Surface Technology, 2nd Edition)

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30 pages, 3734 KB

Open AccessReview

Capacitance-Based Film Thickness Determination in Lubricated Machine Elements: From Dielectric-Gap Models to Constrained Electromechanical Inference

by Dan Bai, Jintao Zheng, Xiaohui Wang, Hang Wang, Yan Li and Hui Cen

Lubricants 2026, 14(6), 220; https://doi.org/10.3390/lubricants14060220 - 28 May 2026

Abstract

Capacitance-based methods are widely used to evaluate lubricant film thickness in machine elements where direct optical access is unavailable, especially in rolling bearings and other multi-contact components. This review examines the physical basis, historical development, and modern methodological routes of capacitance-based film thickness [...] Read more.

Capacitance-based methods are widely used to evaluate lubricant film thickness in machine elements where direct optical access is unavailable, especially in rolling bearings and other multi-contact components. This review examines the physical basis, historical development, and modern methodological routes of capacitance-based film thickness determination, with emphasis on four coupled interpretive layers: film geometry, dielectric response, electrical topology, and parasitic/background effects. The literature shows that the field has evolved from simple dielectric-gap conversion toward more strongly constrained interpretation using elastohydrodynamic lubrication priors, dielectric identification, network-aware reduction, and frequency-domain information, particularly under grease lubrication, starvation, and transient conditions. Across these studies, capacitance-derived film thickness is not a methodologically uniform quantity but an inferred result whose meaning depends on what is prescribed, what is estimated, and what ambiguity remains unresolved. The main unresolved challenges are geometry–dielectric non-uniqueness, parasitic and topology uncertainty, and limited validation under realistic operating conditions. Overall, capacitance-based film thickness determination in practical machine elements is best understood as a constrained electromechanical inference problem, and future progress will depend on stronger identifiability, more informative broadband measurements, and clearer reporting of assumptions, inference targets, and validation basis. Full article

(This article belongs to the Special Issue Oneness in Tribology of Mechanical Components)

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