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Lubricants
Volume 11
Issue 8
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Lubricants, Volume 11, Issue 8 (August 2023) – 42 articles

Cover Story (view full-size image): Surface texture is a widely used strategy for improving the tribological property of friction pairs. The internal flow field of surface texture significantly impacts its performance. This research establishes a three-dimensional CFD model to comprehensively analyze the internal flow behavior of straight-groove texture in the thrust bearing under hydrodynamic lubrication. The continuity and momentum equations are solved using the software FLUENT, considering the cavitation effect in the lubricating oil. The influences of operating parameters on tribological performance are analyzed. A prediction method for determining the cavitation position within the texture is presented. More importantly, the mechanism of groove texture on lubrication is explored from the perspective of flow behavior. View this paper
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13 pages, 10665 KiB  
Article
Anti-Wear Property of Laser Textured 42CrMo Steel Surface
by Hua Zhang, Xinfeng Pei and Xindong Jiang
Lubricants 2023, 11(8), 353; https://doi.org/10.3390/lubricants11080353 - 21 Aug 2023
Cited by 1 | Viewed by 858
Abstract
In this work, laser processing technology was utilized to fabricate micro-textures on the surface of 42CrMo steel to improve its wear resistance under high load conditions and provide an effective method to solve the wear of tooth plates in oil drilling wellhead machinery. [...] Read more.
In this work, laser processing technology was utilized to fabricate micro-textures on the surface of 42CrMo steel to improve its wear resistance under high load conditions and provide an effective method to solve the wear of tooth plates in oil drilling wellhead machinery. Firstly, the friction process of the textured components was conducted by finite element analysis. Additionally, various forms of textures were compared and measured by this method to optimize the shape and parameters of the patterns. Secondly, three types of texture shapes, such as micro-dimples, micro-grooves, and reticular grooves, were created on the surface of 42CrMo steel. Lastly, the tribological characteristics of the micro-textures were analyzed in the dry friction experiments. Compared with the untextured surface, the wear resistance of the textured 42CrMo steel has been improved, and the anti-wear property of the micro-dimples was better than micro-grooves and reticular grooves. Along the direction of friction sliding, the wear of the front end is more worn than the rear end. Micro-dimples with a diameter of 0.8 mm, a spacing of 1.2 mm, and an area occupancy of 34.8% were fabricated at an output power of 200 W and a frequency of 5 Hz. The wear of the textured surface has been reduced by more than 80% in the process of ring-block dry friction with a load of 50 N, a rotation speed of 35 r/min, and a time of 15 min. The wear mechanism is mainly abrasive wear. The results showed that the hardness of the surface could be improved by laser hardening. In addition, micro-dimples on 42CrMo steel can store abrasive particles, mitigate the formation of furrows and reduce the abrasive wear of tooth plates. Full article
(This article belongs to the Special Issue Laser Surface Engineering for Tribology)
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14 pages, 8120 KiB  
Article
Influence of Concentration of Sodium Metasilicate and Descaling on the High Temperature Lubricating Effects Evaluated by Hot Rolling Mill
by Hongliang Liu, Xun Wu, Jiaxuan Huang, Xibo Shao, Pei Wang, Guanyu Deng and Long Wang
Lubricants 2023, 11(8), 352; https://doi.org/10.3390/lubricants11080352 - 18 Aug 2023
Viewed by 1117
Abstract
Lubricant is vital to improve energy efficiency and workpiece durability for the moving counterpart. High-temperature lubricants are important for the hot rolling process to reduce the rolling force and protect the roller and the strips. The current paper concerns eco-friendly sodium metasilicate as [...] Read more.
Lubricant is vital to improve energy efficiency and workpiece durability for the moving counterpart. High-temperature lubricants are important for the hot rolling process to reduce the rolling force and protect the roller and the strips. The current paper concerns eco-friendly sodium metasilicate as a high-temperature lubricant. A hot rolling mill is employed to evaluate the lubrication effect of sodium metasilicate. The influence of crucial factors of concentration of lubricant and descaling is discussed; the rolled surface was analyzed by scanning electron microscopy, energy dispersive spectroscopy, and 3D profilometer. The results depict that the sodium metasilicate can reduce the rolling force by about 7.8% when the concentration of sodium metasilicate is 18% and above, and descaling of the hot stripe makes the lubrication effect more effective, which can reach a 12.7% reduction in the rolling force. This lubrication is attributed to the formed melts of the sodium silicate layer that offers an easy shearing interface. For the un-descaled samples, the lubricant will be compacted and mixed with the oxide scale, and weakens the lubrication effect. This work suggests that sodium metasilicate can be a high-temperature lubricant for hot rolling; descaling is vital, not only for the quality of the product but also for the efficiency of the lubricant. This work will also be useful for the concentration selection of glass lubricant. Full article
(This article belongs to the Special Issue Recent Advances in High Temperature Tribology)
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20 pages, 9304 KiB  
Article
Study on a Novel Strategy for High-Quality Grinding Surface Based on the Coefficient of Friction
by Yang Li, Li Jiao, Yanhou Liu, Yebing Tian, Tianyang Qiu, Tianfeng Zhou, Xibin Wang and Bin Zhao
Lubricants 2023, 11(8), 351; https://doi.org/10.3390/lubricants11080351 - 17 Aug 2023
Viewed by 883
Abstract
Surface quality has a significant impact on the service life of machine parts. Grinding is often the last process to ensure surface quality and accuracy of material formation. In this study, a high-quality surface was developed by determining the coefficient of friction in [...] Read more.
Surface quality has a significant impact on the service life of machine parts. Grinding is often the last process to ensure surface quality and accuracy of material formation. In this study, a high-quality surface was developed by determining the coefficient of friction in grinding a quartz fiber-reinforced silica ceramic composite. By processing the physical signals in the grinding process, a multi-objective function was established by considering grinding parameters, i.e., surface roughness, coefficient of friction, active energy consumption, and effective grinding time. The weight vector coefficients of the sub-objective functions were optimized through a multi-objective evolutionary algorithm based on the decomposition (MOEA/D) algorithm. The genetic algorithm was used to optimize the process parameters of the multi-objective function, and the optimal range for the coefficient of friction was determined to be 0.197~0.216. The experimental results indicated that when the coefficient of friction tends to 0.197, the distribution distance of the microscopic data points on the surface profile is small and the distribution uniformity is good. When the coefficient of friction tends to 0.216, the surface profile shows a good periodic characteristic. The quality of a grinding surface depends on the uniformity and periodicity of the surface’s topography. The coefficient of friction explained the typical physical characteristics of high-quality grinding surfaces. The multi-objective optimization function was even more important for the subsequent high-quality machining of mechanical parts to provide guidance and reference significance. Full article
(This article belongs to the Special Issue Friction and Wear of Cutting Tools and Cutting Tool Materials)
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13 pages, 4297 KiB  
Article
Comparative Micro-Scale Abrasive Wear Testing of Thermally Sprayed and Hard Chromium Coatings
by Georgiana Chișiu, Roxana-Alexandra Gheța, Alina-Maria Stoica and Nicolae-Alexandru Stoica
Lubricants 2023, 11(8), 350; https://doi.org/10.3390/lubricants11080350 - 17 Aug 2023
Cited by 1 | Viewed by 866
Abstract
Nowadays, due to the carcinogenic effects of chrome, replacing the hard chromium used for hydraulic components like rods and cylinders is becoming increasingly requested. Thermally sprayed coatings are a solution to the problem; however, proper understanding and characterisation of their tribological behaviour are [...] Read more.
Nowadays, due to the carcinogenic effects of chrome, replacing the hard chromium used for hydraulic components like rods and cylinders is becoming increasingly requested. Thermally sprayed coatings are a solution to the problem; however, proper understanding and characterisation of their tribological behaviour are essential for the successful exploitation of surface engineering. Thus, the main aim of this study is to evaluate the abrasive wear characteristics of two metal sprayed layers, tungsten carbide (WC) deposited through the high-velocity oxygen fuel coating (HVOF) method and Fe alloy coating deposited through thermal spraying with an electric arc with a wire-electrode G3Si1, and compare the results with those of an electrochemically deposited hard chromium layer. Their wear resistance is then related to their hardness. The results highlight the tribological performances of the thermally sprayed coatings. The HVOF WC10Co4Cr coating has a wear coefficient and a material wear volume that are hundreds of times lower than those of the other two coatings. Full article
(This article belongs to the Special Issue Friction and Wear of Alloys)
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15 pages, 11012 KiB  
Article
Machining Temperature, Surface Integrity and Burr Size Investigation during Coolant-Free Hole Milling in Ti6Al4V Titanium Alloy
by Ragavanantham Shanmugam, Satish Shenoy Baloor, Ugur Koklu, Ashwin Polishetty and Gururaj Bolar
Lubricants 2023, 11(8), 349; https://doi.org/10.3390/lubricants11080349 - 15 Aug 2023
Cited by 3 | Viewed by 1032
Abstract
Modern Aircraft structures use titanium alloys where the processing of holes becomes essential to assemble aerospace parts. Considering the limitations of drilling, the study evaluates the helical milling for hole processing in Ti6Al4V. The experimental evaluation was conducted by considering burr size, surface [...] Read more.
Modern Aircraft structures use titanium alloys where the processing of holes becomes essential to assemble aerospace parts. Considering the limitations of drilling, the study evaluates the helical milling for hole processing in Ti6Al4V. The experimental evaluation was conducted by considering burr size, surface roughness, machining temperature, and microhardness under coolant-free conditions. The axial feed and cutting speed were varied at three levels, and nine experiments were conducted. The results exhibit a lower machining temperature during helical milling than during drilling. In addition, the helical milling helped to lower the surface roughness and size of the exit burrs. However, helical-milled holes showed higher subsurface microhardness than conventionally drilled holes. The process variables were influential on machining temperature magnitude. The highest recorded temperature of 234.7 °C was observed at 60 m/min of cutting speed and 0.6 mm/rev feed. However, the temperature rise did not affect the microhardness. Strain hardening associated with mechanical deformation was the primary mechanism driving the increase in microhardness. Helical-milled holes exhibited an excellent surface finish at lower axial feeds, while chatter due to tool deformation at higher feeds (0.6 mm/rev) diminished the surface finish. The surface roughness increased by 98% when the cutting speed increased to 60 m/min from 20 m/min, while a moderate increment of 28% was observed when the axial feed increased to 0.6 mm/rev from 0.2 mm/rev. Furthermore, the formation of relatively smaller burrs was noted due to significantly lower thrust load and temperature produced during helical milling. Full article
(This article belongs to the Special Issue Advances in Sustainable Machining)
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15 pages, 13033 KiB  
Article
Analyzing the Fretting Fatigue of Bolt Joints by Experiments and Finite Element Analysis
by Robert Szlosarek, Paul Holzmüller and Matthias Kröger
Lubricants 2023, 11(8), 348; https://doi.org/10.3390/lubricants11080348 - 15 Aug 2023
Cited by 1 | Viewed by 1081
Abstract
The appearance of fretting fatigue cracks in bolted sheets limits their lifetime. Furthermore, repairing these failures requires much effort due to needing to replace the components instead of replacing just the bolt. To prevent such failures, the purpose of this study is to [...] Read more.
The appearance of fretting fatigue cracks in bolted sheets limits their lifetime. Furthermore, repairing these failures requires much effort due to needing to replace the components instead of replacing just the bolt. To prevent such failures, the purpose of this study is to understand the failure mechanism and to identify the major influencing parameters. Therefore, a representative joint of a bolt of size M22 and sheet material were investigated by experiments and a finite element analysis. The experiments were conducted over a wide range of preloads from zero to maximum preload. It turned out that the failure mode changes at 50 kN. For this preload, the influence of the surface and the use of a lubricant was observed. A grinded surface as well as the use of lubricant showed a change in the failure mode. The accompanying simulation showed that an analysis of the stresses delivers no proper explanation for the observed effects in the experiment. Therefore, the contact status was analyzed for various preloads and friction coefficients. The results correlate with the change in the failure mode. The conclusion is that both the stress state and the tribological behavior influence the failure mode and have to be considered in a numerical analysis. Full article
(This article belongs to the Special Issue Fretting Fatigue in Mechanical Joints)
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10 pages, 2767 KiB  
Article
Influence of an Oxygen-Free Atmosphere on Process Forces and Workpiece Quality during the Surface Grinding of Ti-6Al-4V
by Berend Denkena, Benjamin Bergmann, Nils Hansen and Roman Lang
Lubricants 2023, 11(8), 347; https://doi.org/10.3390/lubricants11080347 - 14 Aug 2023
Viewed by 735
Abstract
Most manufacturing processes, such as grinding, are usually conducted in a standard air atmosphere. The oxygen within this atmosphere leads to oxidation effects on tools and workpieces when machining metal components. This is primarily a factor in the processing of titanium due to [...] Read more.
Most manufacturing processes, such as grinding, are usually conducted in a standard air atmosphere. The oxygen within this atmosphere leads to oxidation effects on tools and workpieces when machining metal components. This is primarily a factor in the processing of titanium due to its high affinity for oxygen. The oxidation of the surface increases tool wear and reduces surface quality. Hence, this paper investigates the influence of the atmosphere on process forces and workpiece quality when grinding titanium (Ti6Al4V) with metal-bonded diamond grinding tools. To generate oxygen-free conditions in production processes, a novel approach allows an atmosphere with a very low oxygen partial pressure. Using a silane gas, which reacts with oxygen, the oxygen partial pressure, pO2, can be reduced below 10−12 mbar, equal to the oxygen partial pressure in an extremely high vacuum (XHV). The results show a significant influence of the atmosphere on the process forces. When grinding in XHV-adequate conditions, the grinding forces are reduced by 16% in the tangential direction and 50% in the normal direction on average, while the quality of the ground titanium surfaces is consistent (both atmospheres: Rz = 13–21 µm). Phase analysis via XRD revealed a distinct amount of titanium nitride (TiN) on the ground surfaces independently of the atmospheric conditions. Full article
(This article belongs to the Special Issue Assessment of Adhesive Wear)
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10 pages, 4334 KiB  
Article
DFT Calculation of the Mechanism of the Acid-Catalyzed Aldol Condensation of a Lubricant Base Stock
by Yan Li, Lei Xia and Rendong Liu
Lubricants 2023, 11(8), 346; https://doi.org/10.3390/lubricants11080346 - 13 Aug 2023
Viewed by 912
Abstract
Aldehyde condensation is a reaction step in the oxidization of a lubricant base stock into high-molecular-weight products, forming sludge and a paint film, which lead to the failure of lubricating oil. Calculations on the basis of the density functional theory (DFT) were employed [...] Read more.
Aldehyde condensation is a reaction step in the oxidization of a lubricant base stock into high-molecular-weight products, forming sludge and a paint film, which lead to the failure of lubricating oil. Calculations on the basis of the density functional theory (DFT) were employed to investigate the reaction mechanism of the acid-catalyzed aldol condensation of a lubricant base stock. Carbonyl compounds could be converted into their resonant enol structures. However, the activation energy of the process was relatively high, and it was difficult to initiate. The existence of the acid could obviously decrease the activation energy of the reaction from 269.17–287.82 kJ/mol to 177.10–177.63 kJ/mol, and it significantly reduced the difficulty of initiating this reaction. The carbocation formed by the carbonyl compounds and acid could further react with the enol and produce an intermediate reaction product in which the chain of molecules grew longer. This process was not difficult to initiate, with a reaction activation energy of 65.10 kJ/mol. The intermediate product with a larger molecular weight could be converted into carbonyl compounds containing a β-hydroxy by removing a hydrogen proton from it. The energy barrier for this process was 193.15 kJ/mol, and it was not easy to initiate the reaction. Full article
(This article belongs to the Special Issue Additives for Lubricating Oil and Grease: Mechanism, Properties and Applications)
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17 pages, 2262 KiB  
Article
Characterization of Base Oil and Additive Oxidation Products from Formulated Lubricant by Ultra-High Resolution Mass Spectrometry
by Oscar Lacroix-Andrivet, Marie Hubert-Roux, Corinne Loutelier Bourhis, Samira Moualdi, Anna Luiza Mendes Siqueira and Carlos Afonso
Lubricants 2023, 11(8), 345; https://doi.org/10.3390/lubricants11080345 - 12 Aug 2023
Cited by 1 | Viewed by 1235
Abstract
Automotive formulated lubricants are high value products composed of 80% base oil and 20% various additives. During their life service, lubricants are exposed to several factors that will cause degradation over time, such as high temperature, shear, and oxidation. Base oil is a [...] Read more.
Automotive formulated lubricants are high value products composed of 80% base oil and 20% various additives. During their life service, lubricants are exposed to several factors that will cause degradation over time, such as high temperature, shear, and oxidation. Base oil is a complex combination of hydrocarbons that are relatively sensitive to oxidation. During the initiation phase of oxidation, free radicals are formed, leading to the production of hydroperoxide ROOH and an alkyl radical R•. These compounds will react with the base oil molecules to form aldehydes, ketones, and carboxylic acids in the termination phase. Owing to the molecular complexity of these mixtures, Fourier transform mass spectrometry seems to be the most appropriate tool to cover their wide range of compounds due to its ultra-high resolving power and mass accuracy. In this study, a native formulated lubricant and its different oxidized states at 140 °C under air flow (3, 5, 7, 8, and 9 days of oxidation) were analyzed by FTICR MS. The combination of atmospheric pressure chemical ionization (APCI) was used to achieve a non-selective ionization of molecules, including base oils, while Electrospray ionization (ESI) was used to selectively ionize acidic molecules. Apparent Kendrick mass defect (aKMD) plots were used to separate homologous series of molecules on different horizontal lines on the basis of the CH2 repetition unit. Aging of lubricants was mainly characterized by a rapid consumption of certain additive families, such as molybdenum dithiocarbamates (MoDTCs) and zinc dithiophosphate (ZnDTPs), but also by the emergence of many oxidation products. Thus, the presence of aldehydes, ketones, and acids was characterized in the early stage of aging while larger products from polymerization were observed in a more advanced stage of aging. Interaction products between peroxy radicals and hindered phenols/alkyl diphenylamines (ADPAs) antioxidations were elucidated toward the high m/z. The formation of such products can be explained by trapping mechanisms of these additives at high temperature (>120 °C). Other types of interaction products were observed with the formation of antioxidant complexes. Additive degradation products were also characterized. For instance, polyisobutenyl succinimide dispersant oxidation products were clearly evidenced on the aKMD plots due to the gaps of 56 Da between each point. Overall, this study demonstrated the efficiency of the aKMD approach, and the use of ESI/APCI to characterize base oil and additive oxidation products. Full article
(This article belongs to the Special Issue Science and Technology in Nanotribology)
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17 pages, 16686 KiB  
Article
A Multi-Scale Investigation to Predict the Dynamic Instabilities Induced by Frictional Contact
by Farouk Maaboudallah and Noureddine Atalla
Lubricants 2023, 11(8), 344; https://doi.org/10.3390/lubricants11080344 - 11 Aug 2023
Viewed by 739
Abstract
We propose a new variational formulation to model and predict friction-induced vibrations. The multi-scale computational framework exploits the results of (i) the roughness measurements and (ii) the micro-scale contact simulations, using the boundary element method, to enrich the contact zone of the macroscopic [...] Read more.
We propose a new variational formulation to model and predict friction-induced vibrations. The multi-scale computational framework exploits the results of (i) the roughness measurements and (ii) the micro-scale contact simulations, using the boundary element method, to enrich the contact zone of the macroscopic finite element model of rubbing systems with nominally flat contact boundaries. The resulting finite elements at the contact interface of the macroscopic model include (i) a modified normal gap and (ii) a micro-scale description of the contact law (i.e., pressure gap) derived by solving the frictionless contact problem on a rough surface indenting a rigid half-plane. The method is applied to a disc brake system to show its robustness in comparison with classical deterministic formulations. With respect to the traditional complex eigenvalues analysis, the proposed multi-scale approach shows that the inclusion of roughness significantly improves the results at low frequencies. In this panorama, any improvement of dynamic instabilities predictions should be based on an uncertainty analysis incorporating roughness combined with other parameters such as friction coefficient and shear moduli of the pads, rather than on roughness itself. Full article
(This article belongs to the Special Issue Advances in Contact Mechanics)
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15 pages, 2354 KiB  
Article
Dynamic Temperature Prediction on High-Speed Angular Contact Ball Bearings of Machine Tool Spindles Based on CNN and Informer
by Hongyu Li, Chunyang Liu, Fang Yang, Xiqiang Ma, Nan Guo, Xin Sui and Xiao Wang
Lubricants 2023, 11(8), 343; https://doi.org/10.3390/lubricants11080343 - 11 Aug 2023
Cited by 1 | Viewed by 1180
Abstract
This study addressed the issues related to the difficulty of determining the operating status of machine tool spindle bearings due to the high rotational speeds and rapid temperature fluctuations. This paper presents an optimized model that combines Convolutional Neural Networks (CNNs) and Informer [...] Read more.
This study addressed the issues related to the difficulty of determining the operating status of machine tool spindle bearings due to the high rotational speeds and rapid temperature fluctuations. This paper presents an optimized model that combines Convolutional Neural Networks (CNNs) and Informer to dynamically predict the temperature rise process of bearings. Taking the H7006C angular contact ball bearing as the research object, a combination of experimental data and simulations was used to obtain the training dataset. Next, a model for predicting the temperature rise of the bearing was constructed using CNN + Informer and the structural parameters were optimized. Finally, the model’s generalization ability was then verified by predicting the bearing temperature rise process under various working conditions. The results show that the error of the simulation data source model was less than 1 °C at steady state; the temperature error of the bearing temperature rise prediction model was less than 0.5 °C at both the temperature rise and steady-state stages under variable rotational speeds and variable load conditions compared to Informer and Long Short Term Memory (LSTM) models; the maximum prediction error of the operating conditions outside the dataset was less than 0.5 °C, and the temperature rise prediction model has a high accuracy, robustness, and generalization capability. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermodynamics 2023)
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10 pages, 3092 KiB  
Article
Revisiting the Influence of Contact Length and Surrounding Medium on Metal Cutting Tribology
by Carlos E. H. Ventura, Afonso V. L. Gregório, Lara S. M. Fernandes, Alexandre M. Abrão and Pedro A. R. C. Rosa
Lubricants 2023, 11(8), 342; https://doi.org/10.3390/lubricants11080342 - 11 Aug 2023
Viewed by 725
Abstract
Metal cutting processes involve severe frictional conditions at the tool-chip contact interface, physico-chemical phenomena that are not fully understood and theoretical models with a strong empirical basis. In the literature, it is common to find values for the friction coefficient which are higher [...] Read more.
Metal cutting processes involve severe frictional conditions at the tool-chip contact interface, physico-chemical phenomena that are not fully understood and theoretical models with a strong empirical basis. In the literature, it is common to find values for the friction coefficient which are higher than the unit, despite the inconsistency of these findings with the mathematical theory of plasticity used for accurate modelling of the chip formation mechanics. Thus, the present work seeks to contribute to a better understanding of metal cutting tribology based on well-controlled and specially designed experimental conditions. To enable the stress state at the contact interface to be determined and to provide a known real contact area, polished cutting inserts with prepared rake faces were used to restrict the contact length in orthogonal cutting tests under controlled atmospheres. These conditions also allowed the contribution of the sticking mechanisms to be minimized. Cutting tools of restricted contact lengths have been found to reduce process forces and the friction coefficient, while industrial standard inserts make cutting operation sensitive to the chemical composition of the surrounding atmosphere. Full article
(This article belongs to the Special Issue New Technologies to Improve Machining Performance from a Tribological Viewpoint)
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18 pages, 20818 KiB  
Article
Influence of Ball Burnishing on Lubricated Fretting of the Titanium Alloy Ti6Al4V
by Slawomir Swirad
Lubricants 2023, 11(8), 341; https://doi.org/10.3390/lubricants11080341 - 10 Aug 2023
Viewed by 777
Abstract
Ball burnishing is a very promising alternative to grinding because of it produces little environmental pollution. It can cause improvement of the functional properties of machine parts, such as friction and wear. The connection between the ball burnishing and the lubricated fretting has [...] Read more.
Ball burnishing is a very promising alternative to grinding because of it produces little environmental pollution. It can cause improvement of the functional properties of machine parts, such as friction and wear. The connection between the ball burnishing and the lubricated fretting has not been analysed yet. In this study, it was found that ball burnishing discs from titanium alloy Ti6Al4V caused a decrease in the height of the roughness up to 84% and an increase in the microhardness up to 26% compared to the turned surface. Tribological experiments were carried out under lubricated fretting conditions. Ceramic balls from WC material co-acted with the burnished discs. Ball burnishing resulted in significant improvement in the tribological behaviour of the ball-on-disc sliding pair. Due to ball burnishing, the friction coefficient decreased up to 45% and the volumetric wear of the disc decreased up to 50% compared to the turned disc. The smallest friction and disc wear were achieved for the sample burnished with a pressure of 30 MPa; this sample was characterised by a low roughness height and great microhardness. The turned disc sample corresponded to high friction and wear. Wear losses of the balls were negligible due to the large difference between the hardness values of the balls and discs. Full article
(This article belongs to the Special Issue Friction and Wear of Alloys)
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29 pages, 9354 KiB  
Article
The Effect of Dry Friction upon the Dynamics of a Short Eccentric Rotor: An Analytical and Experimental Study
by Stelian Alaci, Ilie Musca, Carmen Bujoreanu, Ionut-Cristian Romanu, Nicolae-Adrian Nitu and Florina-Carmen Ciornei
Lubricants 2023, 11(8), 340; https://doi.org/10.3390/lubricants11080340 - 09 Aug 2023
Viewed by 812
Abstract
The paper proposes the study of dry friction by means of a short eccentric rotor. The rotor was designed and realised in the laboratory. In an ideal frictionless situation, a rotor actuated by gravity will have an angular velocity which increases indefinitely. The [...] Read more.
The paper proposes the study of dry friction by means of a short eccentric rotor. The rotor was designed and realised in the laboratory. In an ideal frictionless situation, a rotor actuated by gravity will have an angular velocity which increases indefinitely. The analytical model assumes dry friction in the bushing of the rotor and the main result reveals that the angular velocity either stabilizes around a certain value or drops to zero. Two situations of friction were considered for the theoretical model: first only dynamic friction and secondly, both static and dynamic friction are present. The analytical model of the dynamics of the rotor presents the advantage that it can be applied for diverse friction cases, from dry friction to complex dry friction and wet friction. Experimental tests were carried out on the designed and constructed device; they are in very good agreement with the results of the theoretical model. Full article
(This article belongs to the Special Issue Wear Prediction in Aeroengine Rotor System)
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19 pages, 3912 KiB  
Article
Polymer Dispersion Effects on Drag, Heat Transfer, and Mass Transfer in Non-Newtonian Based Nanofluids
by Ayesha Sahreen, Adeel Ahmad, Razi Khan and Rab Nawaz
Lubricants 2023, 11(8), 339; https://doi.org/10.3390/lubricants11080339 - 09 Aug 2023
Cited by 2 | Viewed by 812
Abstract
This article explores the influence of polymers on the boundary layer flow, heat transfer, and mass transfer control of non-Newtonian-based nanofluids flowing past a stretching surface. The mathematical model incorporates the Oldroyd-B model to analyze the effects of polymers, while the Powell–Eyring and [...] Read more.
This article explores the influence of polymers on the boundary layer flow, heat transfer, and mass transfer control of non-Newtonian-based nanofluids flowing past a stretching surface. The mathematical model incorporates the Oldroyd-B model to analyze the effects of polymers, while the Powell–Eyring and Reiner–Philippoff viscosity models are employed to study the behavior of non-Newtonian fluids. The dispersion model is adopted to account for nanofluid characteristics. Appropriate transformations yield governing equations with similar forms, which are solved numerically to investigate the impact of polymer inclusion on skin friction, Nusselt number, and Sherwood number. The study’s findings reveal that the addition of polymers to the non-Newtonian-based nanofluids leads to a reduction in heat and mass transport while enhancing skin drag. Detailed analysis of these effects sheds light on the underlying physical mechanisms. Full article
(This article belongs to the Special Issue Tribology of Polymer-Based Composites)
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32 pages, 60160 KiB  
Article
Research on Internal Flow Field Characteristics of Straight-Groove Texture Using Three-Dimensional Modeling
by Yulong Li, Zhehao Zhang, Yongyong He and Jianbin Luo
Lubricants 2023, 11(8), 338; https://doi.org/10.3390/lubricants11080338 - 09 Aug 2023
Cited by 1 | Viewed by 916
Abstract
Surface texture modification is a reasonable strategy for improving the tribological property of friction pairs. The internal flow behavior of the surface texture significantly impacts its performance. In this study, a three-dimensional computational fluid dynamics (CFD) model is constructed to explore the internal [...] Read more.
Surface texture modification is a reasonable strategy for improving the tribological property of friction pairs. The internal flow behavior of the surface texture significantly impacts its performance. In this study, a three-dimensional computational fluid dynamics (CFD) model is constructed to explore the internal flow behavior of the straight-groove texture in the thrust bearing. The influences of the Reynolds number, depth ratio, and area ratio of the straight-groove texture on the internal flow behavior are systematically investigated. Furthermore, the streamline and tribological performance parameters are checked to reveal the mechanism of the groove texture influencing the tribological properties. It is found that the vortex and cavitation significantly affect the tribological performance of textured surfaces under hydrodynamic lubrication. The cavitation and upstream vortex areas increase with the Reynolds number, while the downstream vortex area shows a reverse trend. The increase in depth ratio strengthens the upstream and downstream vortexes while reducing the cavitation area. Additionally, a method is proposed to determine the location of the cavitation within the groove texture. Certain operating conditions create the optimal texture depth ratio and area ratio, which could maximize the load-carrying capacity (LCC) of the oil film, and the friction coefficient is relatively small. Full article
(This article belongs to the Special Issue Hydrodynamic Lubrication of Textured Surfaces)
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19 pages, 18801 KiB  
Article
Tribological Investigation of the Effect of Nanosized Transition Metal Oxides on a Base Oil Containing Overbased Calcium Sulfonate
by Álmos Dávid Tóth, Hajnalka Hargitai and Ádám István Szabó
Lubricants 2023, 11(8), 337; https://doi.org/10.3390/lubricants11080337 - 08 Aug 2023
Cited by 1 | Viewed by 1011
Abstract
In this study, copper(II) oxide, titanium dioxide and yttrium(III) oxide nanoparticles were added to Group III-type base oil formulated with overbased calcium sulfonate. The nanosized oxides were treated with ethyl oleate surface modification. The tribological properties of the homogenized oil samples were tested [...] Read more.
In this study, copper(II) oxide, titanium dioxide and yttrium(III) oxide nanoparticles were added to Group III-type base oil formulated with overbased calcium sulfonate. The nanosized oxides were treated with ethyl oleate surface modification. The tribological properties of the homogenized oil samples were tested on a linear oscillating tribometer. Friction was continuously monitored during the tribological tests. A surface analysis was performed on the worn samples: the amount of wear was determined using a digital optical and confocal microscope. The type of wear was examined with a scanning electron microscope, while the additives adhered to the surface were examined with energy-dispersive X-ray spectroscopy. From the results of the measurements, it can be concluded that the surface-modified nanoparticles worked well with the overbased calcium sulfonate and significantly reduced both wear and friction. In the present tribology system, the optimal concentration of all three oxide ceramic nanoadditives is 0.4 wt%. By using oxide nanoparticles, friction can be reduced by up to 15% and the wear volume by up to 77%. Overbased calcium sulfonate and oxide ceramic nanoparticles together form a lower friction anti-wear boundary layer on the worn surfaces. The results of the tests represent another step toward the applicability of these nanoparticles in commercial engine lubricants. It is advisable to further investigate the possibility of formulating nanoparticles into the oil. Full article
(This article belongs to the Special Issue Lubricant Additives and Ash: Do We Know Enough?)
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15 pages, 7090 KiB  
Article
Enhancing Lubrication Performance of Calcium Sulfonate Complex Grease Dispersed with Two-Dimensional MoS2 Nanosheets
by Shuo Xiang, Xufei Long, Qinhui Zhang, Pengfei Ma, Xin Yang, Hui Xu, Peng Lu, Peng Su, Weihua Yang and Yan He
Lubricants 2023, 11(8), 336; https://doi.org/10.3390/lubricants11080336 - 08 Aug 2023
Cited by 2 | Viewed by 1726
Abstract
Calcium sulfonate complex greases (CSCG) have proven to be a sustainable alternative to lithium complex greases, which still require appropriate additives to deliver lubrication performance benefits under extreme working conditions such as heavy load, high speed, and high temperature. The anti-wear and friction [...] Read more.
Calcium sulfonate complex greases (CSCG) have proven to be a sustainable alternative to lithium complex greases, which still require appropriate additives to deliver lubrication performance benefits under extreme working conditions such as heavy load, high speed, and high temperature. The anti-wear and friction reducing properties of CSCG enhanced by two-dimensional MoS2 nanosheets (2D MoS2) with a narrow lateral size and thickness distributions were evaluated by a four-ball tribometer. The results showed that the CSCG with 0.6 wt.% 2D MoS2 performs best, with a 56.4% decrease in average friction coefficient (AFC), 16.5% reduction in wear scar diameter (WSD), 14.3% decrease in surface roughness, and a 59.4% reduction in average wear depth. Combining SEM-EDS images, Raman, and X-ray photoelectron spectra, it is illustrated that the physical transferred film and tribo-chemical film consisting of MoS2, Fe2O3, FeSO4, CaCO3, CaO, and MoO3 were generated on the worn surface, which improves the lubrication performance of CSCG considerably. Full article
(This article belongs to the Special Issue Applied Nanotribology, 3rd Edition)
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14 pages, 6232 KiB  
Article
Effects of Nitrogen Modification of Porous PVD–MoS2 Coatings on the Tribological Behavior under Rolling–Sliding Conditions in Vacuum
by Armin Seynstahl, Markus Polzer, Marcel Bartz, Sandro Wartzack and Stephan Tremmel
Lubricants 2023, 11(8), 335; https://doi.org/10.3390/lubricants11080335 - 08 Aug 2023
Viewed by 900
Abstract
In order to improve the tribological performance of PVD–MoS2 coatings, which are frequently used as a solid lubricant for operating in challenging environments, e.g., in a vacuum, they can be modified with nitrogen. This work evaluates the tribological behavior and a possible [...] Read more.
In order to improve the tribological performance of PVD–MoS2 coatings, which are frequently used as a solid lubricant for operating in challenging environments, e.g., in a vacuum, they can be modified with nitrogen. This work evaluates the tribological behavior and a possible compaction occurring during the initial tribological load in the rolling contact for pure and nitrogen-modified PVD–MoS2 coatings in a vacuum. Short-running tests (1000 cycles) of coated steel discs paired with uncoated steel discs made from 100Cr6 (1.3505, AISI 52100) were conducted on a two-disc tribometer. The slide-to-roll ratio of 10.5% was kept constant, while the load was varied in two steps from 1.1 GPa to 1.6 GPa. Subsequently, a comparison was made between the worn and the pristine coatings by means of nanoindentation and an optical analysis of the wear track. The formation of a load-bearing solid lubrication was achieved for both MoS2-variants. The main differences affected the material transfer and wear mechanisms. The worn coatings reached a similar wear coefficient of 4 × 10−6 mm3N−1m−1 and a possible compaction of the coatings was found, indicated through an increased indentation hardness (for MoS2 1158% and MoS2:N 96% at a 1.1 GPa load). The assumed tribological mechanism changed with nitrogen modification, but scales with increasing load. The nitrogen-modified MoS2 coating showed less compaction than pure MoS2, while the frictional behavior was improved by a 17% reduction of the coefficient of friction. Full article
(This article belongs to the Special Issue Coatings and Lubrication in Extreme Environments)
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23 pages, 1284 KiB  
Review
Abrasion Evaluation of Moon and Mars Simulants on Rotating Shaft/Sealing Materials: Simulants and Structural Materials Review and Selection
by György Barkó, Gábor Kalácska, Róbert Keresztes, László Zsidai, Hailemariam Shegawu and Ádám Kalácska
Lubricants 2023, 11(8), 334; https://doi.org/10.3390/lubricants11080334 - 07 Aug 2023
Cited by 1 | Viewed by 1288
Abstract
Tribological testing of moving shaft/sealing pairs in complex environments is at the frontline of research. Machines working in abrasive conditions are subject to different wear effects. It is not only valid on Earth but especially valid for rovers and future robots used in [...] Read more.
Tribological testing of moving shaft/sealing pairs in complex environments is at the frontline of research. Machines working in abrasive conditions are subject to different wear effects. It is not only valid on Earth but especially valid for rovers and future robots used in Mars and Moon missions. The aim of our joint research with the European Space Agency is to study the abrasion phenomena of moving machine elements on Mars and the Moon by using artificial soil samples (“simulants”). This review details mainly the available simulant sources and recommend a selection of the most suitable ones for tribological testing. Moreover, the potential mating structural materials subjected to abrasive space applications are reviewed briefly. The tribological tests are exploring the features of the rotary shaft/seal relationship that is subject to dry friction and intense abrasion. By using the simulants, measurements are performed under laboratory conditions with both a sample test and a real shaft/seal connection. Parameters of the selection criteria were defined, and classification of the simulant sources were made. It was found that simulant particle size distribution and chemical substance content are detailed enough only for a limited type of available artificial Moon and Mars soil samples. Four simulants were identified and applied later in the tribological testing. For the shaft materials, based on a detailed case study of polymers, steel, and aluminum alloys, a high-strength aluminum alloy with a hard anodized surface and a stainless steel were selected for further abrasion tests. Full article
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13 pages, 3350 KiB  
Article
Effect of Non-Linear Properties of Intermediate Layer on Sliding Contact of Homogeneous and Coated Elastic Solids
by Elena V. Torskaya and Fedor I. Stepanov
Lubricants 2023, 11(8), 333; https://doi.org/10.3390/lubricants11080333 - 06 Aug 2023
Viewed by 601
Abstract
Lubrication in the contact area can be considered as a viscoelastic layer, especially in the presence of particles in it, or under conditions of low temperatures. The properties of this layer are often non-linear, in particular, they depend on local pressure. The paper [...] Read more.
Lubrication in the contact area can be considered as a viscoelastic layer, especially in the presence of particles in it, or under conditions of low temperatures. The properties of this layer are often non-linear, in particular, they depend on local pressure. The paper presents a formulation and numerical-analytical method for solving the contact problem in the presence of viscoelastic layers, the compliance of which depends on the applied pressure and is included in the formulation as the corresponding operator. The layer is homogeneous or coated elastic half-space. For the selected type of operator, the influence of parameters, which characterize the nonlinearity of the model, on the distribution of contact pressure and the coefficient of friction due to hysteresis losses was analyzed. It is shown that for the nonlinear model, the maximum contact pressures are higher, and the friction coefficient is lower than for the linear model with constant compliance. The effect of non-linearity for a wide range of sliding velocities is considered. An analysis of principal shear and tensile-compressive stresses for a homogeneous elastic half-space and for a coating, in particular, for a coating-substrate interface, was also carried out. Full article
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15 pages, 9052 KiB  
Article
Effects of Machining Parameters of C45 Steel Applying Vegetable Lubricant with Minimum Quantity Cooling Lubrication (MQCL)
by Mayur A. Makhesana, Prashant J. Bagga, Kaushik M. Patel and Jose J. Taha-Tijerina
Lubricants 2023, 11(8), 332; https://doi.org/10.3390/lubricants11080332 - 05 Aug 2023
Cited by 1 | Viewed by 1325
Abstract
One of the most significant performance indicators for measuring the machinability of materials is tool wear and surface roughness. Choosing the best combination of cutting parameters can help reduce production costs, which is what the manufacturing industry is interested in. At the same [...] Read more.
One of the most significant performance indicators for measuring the machinability of materials is tool wear and surface roughness. Choosing the best combination of cutting parameters can help reduce production costs, which is what the manufacturing industry is interested in. At the same time, industries are always looking for an alternative to conventional flood cooling since its use creates an environmental burden and health concerns for the operators. Therefore, vegetable oil-based minimum quantity cooling lubrication (MQCL) is considered a cutting environment. Sunflower oil is utilized as base fluid in MQCL and applied to the cutting zone through a nozzle. The turning experiments are conducted on C45 material which is widely used in various industrial applications, including numerous automotive components. Since flood cooling is widely utilized in machining C45, it is the present-day need to assess alternative cooling and lubricating approaches to avoid the adverse effects of flood cooling. The Taguchi method was used in the present work to minimize surface roughness and tool wear. L9 orthogonal array was constructed, and experiments were performed on C45 steel using coated carbide cutting tools. The statistical approach is utilized to evaluate the effect of cutting parameters on output responses. The optimal cutting settings for cutting speed, feed, and depth of cut to minimize surface roughness are 100 m/min, 0.18 mm/rev, 0.150 mm, and 80 m/min, 0.18 mm/rev, and 0.150 mm for tool wear. According to the findings, cutting speed, feed rate, and depth of cut varied surface roughness by 1.9%, 78.3%, and 14.04%, and tool wear by around 43.8%, 37.9%, and 6.3%, respectively. The outcomes can be useful to metal-cutting industries to identify the combination of machining parameters with vegetable oil-based MQCL. Full article
(This article belongs to the Special Issue Minimum Quantity Lubrication: Environmental Alternatives in Processing)
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33 pages, 2740 KiB  
Review
Recent Progress in Electrically Conductive and Thermally Conductive Lubricants: A Critical Review
by Bayazid Bustami, Md Mahfuzur Rahman, Mst. Jeba Shazida, Mohaiminul Islam, Mahmudul Hasan Rohan, Shakhawat Hossain, Alam S. M. Nur and Hammad Younes
Lubricants 2023, 11(8), 331; https://doi.org/10.3390/lubricants11080331 - 04 Aug 2023
Cited by 4 | Viewed by 2829
Abstract
Electrically as well as thermally conductive lubricants have drawn considerable attention and are an emerging research topic because they have unique advantages and advanced lubrication performance over traditional lubricants such as corrosion protection and efficient heat dissipation. For instance, some components of electric [...] Read more.
Electrically as well as thermally conductive lubricants have drawn considerable attention and are an emerging research topic because they have unique advantages and advanced lubrication performance over traditional lubricants such as corrosion protection and efficient heat dissipation. For instance, some components of electric vehicles (EVs) such as bearings, seals, pads and gears require conductive lubricants to avoid premature failure and electromagnetic interference (EMI) problems due to induced shaft voltages and currents. This review provides a comprehensive overview of the recent developments in conductive lubricants. The review focuses on the important aspects to enhance the thermal and electrical conductivities as well as the tribological behavior (COF, and wear rate) of conductive solid, semisolid, and liquid lubricants. The lubricants that are electrically and thermally conductive with superior tribological performances have been identified through extensive literature review and presented in tabular form. This review summarizes the effect of various additives used to improve the conductive properties of the lubricants, such as polyalphaolefin oil, hydraulic oil, paraffin oil, and mineral oil. Furthermore, the review discusses the lubricating mechanism of conductive solid and liquid lubricants to facilitate a deeper understanding. Finally, the future perspectives and the research directions for conductive lubricants are also addressed. Full article
(This article belongs to the Special Issue Heat Dissipation and Electron Transport Phenomena in the Tribomechanical Systems with Conductive Lubricants)
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13 pages, 5898 KiB  
Article
Research on Rolling Contact Fatigue Failure of the Bearing Used in High-Speed Electric Multiple Units’ Axle Box Based on a Damage-Coupled Elastic–Plastic Constitutive Model
by Ling Ma, Junyi Liu, Feng Guo, Xinming Li and Xiaohan Zhang
Lubricants 2023, 11(8), 330; https://doi.org/10.3390/lubricants11080330 - 04 Aug 2023
Viewed by 956
Abstract
The axle box bearing is a crucial component of high-speed electric multiple units (EMU) and is exposed to harsh working conditions, making it susceptible to subsurface-induced rolling contact fatigue (RCF) under long-term alternating stress. The objective of this paper is to develop a [...] Read more.
The axle box bearing is a crucial component of high-speed electric multiple units (EMU) and is exposed to harsh working conditions, making it susceptible to subsurface-induced rolling contact fatigue (RCF) under long-term alternating stress. The objective of this paper is to develop a damage-coupled elastic–plastic constitutive model that can accurately predict the RCF life of EMU axle box bearings made from AISI 52100 bearing steel. The total damage is divided into elastic damage related to the shear stress range and plastic damage associated with plastic deformation. Material parameters are determined based on experimental data from the literature, and validation is conducted to ensure the validity of the model. Finally, the RCF behavior of the EMU axle box bearing, including crack initiation, crack propagation, and spalling, is simulated, and reasonable results are obtained. This study provides valuable insights into the RCF behavior of EMU axle box bearings and contributes to the accurate prediction of the fatigue life. Full article
(This article belongs to the Special Issue Friction and Wear of Rolling-Element Bearings)
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13 pages, 6553 KiB  
Article
Lubricating Ability of Protic Ionic Liquids as Additives to a Biodegradable Oil for Aluminum-Steel Contact: Effect of Alkyl Chain Length and Propensity to Hydrogen Bonding
by Hong Guo, Brandon Stoyanovich, Junru Pang and Patricia Iglesias
Lubricants 2023, 11(8), 329; https://doi.org/10.3390/lubricants11080329 - 03 Aug 2023
Cited by 2 | Viewed by 1583
Abstract
Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are [...] Read more.
Although aluminum alloys are widely used in the automotive and aerospace industries due to their excellent strength-to-weight ratio and good corrosion resistance, the poor tribological performance and low compatibility of these materials with lubricant anti-wear and anti-friction additives in conventional mineral oils are major limitations. In addition, environmental awareness has increased the need for more environmentally friendly lubricants. Ionic Liquids (ILs) have exhibited significant potential as lubricants and lubricant additives. One of the more interesting properties of ILs is that they can form physically-adsorbed or chemically-reacted layers that reduce friction and wear of the surfaces in contact. Among ILs, Protic Ionic Liquids (PILs) have received more attention recently because of their simple and economic synthesis route. Furthermore, the anions and cations of PILs can be selected to be considered environmentally benign. In this article, the tribological behavior of a family of six PILs are studied as additives to a biodegradable oil (BO), under aluminum-steel contact. Al2024 disks slid against AISI52100 steel balls under a normal load of 3 N and a frequency of 5 Hz at room temperature and using a ball-on-flat reciprocating tribometer. PILs used in this study, were synthesized using two strong acids, with short and long hydrocarbon chains, and three weak bases with different propensities to hydrogen bonds. Results show that, although adding just 1 wt.% of any PIL to BO reduced friction and wear, the alkyl chain length influenced the lubricating ability of these ordered fluids. Wear mechanisms and surface interaction are discussed on the basis of 3D profilometry, SEM-EDX and RAMAN spectroscopy. Full article
(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)
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20 pages, 4434 KiB  
Article
Application of Machine Learning Models to the Analysis of Skid Resistance Data
by Aboubakar Koné, Ahmed Es-Sabar and Minh-Tan Do
Lubricants 2023, 11(8), 328; https://doi.org/10.3390/lubricants11080328 - 01 Aug 2023
Cited by 2 | Viewed by 920
Abstract
This paper evaluates the ability of some state-of-the-art Machine Learning models, namely SVM (support vector machines), DT (decision tree) and MLR (multiple linear regression), to predict pavement skid resistance. The study encompasses both regression and classification tasks. In the regression task, the aim [...] Read more.
This paper evaluates the ability of some state-of-the-art Machine Learning models, namely SVM (support vector machines), DT (decision tree) and MLR (multiple linear regression), to predict pavement skid resistance. The study encompasses both regression and classification tasks. In the regression task, the aim is to predict the coefficient of friction values, while the classification task seeks to identify three classes of skid resistance: good, intermediate and bad. The dataset used in this work was gathered through an extensive test campaign that involved a fifth-wheel device to measure the coefficient of friction at different slip ratios on different road surfaces, vehicle speeds, tire tread depths and water depths. It was found that the RBF-SVM model, due to its ability to capture non-linear relationships between the features and the target for a relatively small dataset, is the most adapted tool compared with, on one side, MLR, linear SVM and DT models for the regression task and, on the other side, linear SVM and DT models for the classification task. The paper also discusses the strengths and weaknesses of the investigated models based on the underlying physical phenomena related to skid resistance. Full article
(This article belongs to the Special Issue Friction Assessment in Pavement Engineering)
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33 pages, 7132 KiB  
Review
Tribocatalysis Induced Carbon-Based Tribofilms—An Emerging Tribological Approach for Sustainable Lubrications
by Khai K. Huynh, Sang T. Pham, Kiet A. Tieu and Shanhong Wan
Lubricants 2023, 11(8), 327; https://doi.org/10.3390/lubricants11080327 - 01 Aug 2023
Cited by 2 | Viewed by 1769
Abstract
To comply with the high demand for efficient and sustainable lubrications, carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the [...] Read more.
To comply with the high demand for efficient and sustainable lubrications, carbon-based tribofilms and/or nanomaterials have emerged as a potential solution that can resolve the current major shortcomings of phosphorus- and sulphur-rich tribofilms and protective coatings. Although their employment is still in the early stages of realization and research, these tribofilms receive significant interest due to their capability to continuously and in situ repair/replenish themselves during sliding, which has been an ultimate goal of all moving mechanical systems. Structurally, these tribofilms are complex and predominantly amorphous or disordered with/without graphitic domains (e.g., graphene/graphite, onion-like carbon, etc.). Chemically, the compositions of these tribofilms vary significantly with environments, conditions, and material precursors. Yet, the structural properties of carbon-based tribofilms remain largely ambiguous, which precludes a full understanding of the mechanisms underlying the formation and lubrication performance. This review will summarize the current state-of-art research about the in situ carbon-based tribofilms that have been published since the pioneering works. Particularly, this work will highlight the recent approaches to generate these tribofilms, their associated lubrication performance, current understanding of the formation mechanics, common analytical approaches for these tribofilms, and the compatibility of these tribofilms with other additives. Together, the overall outlooks will be drawn, demonstrating the knowledge gaps and proposing further investigation tactics to tackle these emerging issues. Full article
(This article belongs to the Special Issue Green Tribology: New Insights toward a Sustainable World 2023)
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25 pages, 719 KiB  
Article
On the Numerical Modeling of Friction Hysteresis of Conformal Rough Contacts
by Kristof Driesen, Sylvie Castagne, Bert Lauwers and Dieter Fauconnier
Lubricants 2023, 11(8), 326; https://doi.org/10.3390/lubricants11080326 - 01 Aug 2023
Viewed by 866
Abstract
In this work, a numerical model simulating friction hysteresis for lubricated rough and textured surfaces in contact is presented. Friction hysteresis occurs in sliding contacts that are subjected to a non-constant (e.g., sinusoidal) motion. It refers to the phenomenon where the observed friction [...] Read more.
In this work, a numerical model simulating friction hysteresis for lubricated rough and textured surfaces in contact is presented. Friction hysteresis occurs in sliding contacts that are subjected to a non-constant (e.g., sinusoidal) motion. It refers to the phenomenon where the observed friction force during acceleration differs from that during deceleration. Besides the dynamics of the sliding system, a classic mixed friction model is adopted, in which the transient Reynolds equation for the description of the thin lubricant film is combined with a statistical Greenwood–Williamson model for the description of rough surface asperity contacts. The model enables the prediction of the friction hysteresis for predefined contact descriptions (i.e., surface profile and roughness, lubricant, etc.) and allows the study of the physics and parametric influences of dynamically sliding contacts. In this paper, it is shown that (i) friction hysteresis is captured by classic transient models for mixed lubrication; (ii) system parameters, such as roughness, applied load, viscosity and velocity, including the offset, amplitude and motion reversal, influence the shape and area of friction hysteresis; and (iii) the selection of the aforementioned parameters may minimize friction hysteresis. Full article
(This article belongs to the Special Issue Multiscale and Modern Solutions in the Simulation of Lubricated Contacts)
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18 pages, 8396 KiB  
Article
The Effect of Steel Electropolishing on the Tribological Behavior of a Steel–Bronze Pair in the Mixed and Boundary Lubrication Regimes
by Robert Mašović, Daniel Miler, Ivan Čular, Suzana Jakovljević, Mario Šercer and Dragan Žeželj
Lubricants 2023, 11(8), 325; https://doi.org/10.3390/lubricants11080325 - 01 Aug 2023
Viewed by 730
Abstract
Electropolishing at high current densities without agitation of the electrolyte results in a pitting phenomenon that produces dimple-like surface features. Although pitting is unfavorable in the electropolishing process, its effect on surface modification, such as surface texturing, has not been thoroughly investigated. Surface [...] Read more.
Electropolishing at high current densities without agitation of the electrolyte results in a pitting phenomenon that produces dimple-like surface features. Although pitting is unfavorable in the electropolishing process, its effect on surface modification, such as surface texturing, has not been thoroughly investigated. Surface topography and chemical composition analyses of electropolished steel revealed surface pits and an oxide surface layer, indicating the presence of surface texture and coating. The resulting surface is characterized by negative skewness and high kurtosis values. The tribological behavior of the electropolished steel-bronze pair is investigated by evaluating coefficients of friction and bronze wear using sliding tests conducted in mixed and boundary lubrication regimes. The results are compared to those of the ground steel-bronze pair. In the mixed and upper range of the boundary lubrication regime, coefficients of friction reduction up to 30% and shorter running-in phases are observed for electropolished steel (electropolished steel μavg = 0.019 vs. ground steel μavg = 0.028). In contrast, the coefficient of friction increased in the lower range of boundary lubrication regime by 50% (electropolished steel μavg = 0.098 vs. ground steel μavg = 0.065). Electropolishing, as a cost- and time-effective method applicable to complex geometries, presents an alternative method for achieving surface modifications aimed at friction reduction and improved tribological behavior for non-conformal contacts in the boundary and mixed lubrication regimes. Full article
(This article belongs to the Special Issue Advances in Surface Engineering and Tribology)
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37 pages, 33553 KiB  
Review
Effect of Material Selection and Surface Texture on Tribological Properties of Key Friction Pairs in Water Hydraulic Axial Piston Pumps: A Review
by Yingna Liang, Wei Wang, Zhepeng Zhang, Hao Xing, Cunyuan Wang, Zongyi Zhang, Tianyuan Guan and Dianrong Gao
Lubricants 2023, 11(8), 324; https://doi.org/10.3390/lubricants11080324 - 31 Jul 2023
Viewed by 1542
Abstract
A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, [...] Read more.
A water hydraulic axial piston pump has become the preferred power component of environmentally friendly water hydraulic transmission systems, due to its advantages of a compact structure, high power density, and so on. The poor friction and wear performance in the water medium, especially under extreme conditions of high speed and high pressure, limit the engineering application of the water hydraulic axial piston pump. In this review, the research progress for key friction pair materials (such as special corrosion-resistant alloys, engineering plastics, and engineering ceramics) for water hydraulic axial piston pumps is, firstly, summarized. Secondly, inspired by nature, the processing methods, lubrication drag-reduction mechanism, and tribological properties of the biomimetic surface textures are discussed. The effects of the surface texture shape, equivalent diameter, depth, and arrangement on the pump’s tribological properties are reviewed in detail. Finally, the application status of, and problems with, surface texture technology in water hydraulic axial piston pumps are summarized. It is suggested that future studies should focus on the multi-field coupling lubrication anti-friction mechanism of the multi-type composite texture under extreme conditions and mixed lubrication; and the anti-wear performance of the texture coupled with a coating modification, to further promote the surface texture in the field of lubrication antifriction engineering applications. Full article
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