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Lubricants, Volume 10, Issue 8 (August 2022) – 33 articles

Cover Story (view full-size image): High-speed trains require a stringent brake disc that could efficiently work under severe service conditions, especially during emergency braking. Braking performance depends on the tribological properties of braking components, particularly at the disc and pad interface. Brake disc material is proposed for high-speed trains over 350 Km/h and its frictional properties and thermomechanical repercussions, along with the evolutions of wear debris were particularly investigated through an experimental trial. A comparative performance analysis was performed with commercially available brake disc material in terms of COF (coefficient of friction), contact surface topography, and wear debris evolution. Results demonstrated the excellent performance of newly developed material from a service viewpoint. View this paper
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Article
Nano- and Micro-Tribological Investigations of Boundary Layers on Axial Bearing Washers Tested under WEC Critical Conditions
Lubricants 2022, 10(8), 198; https://doi.org/10.3390/lubricants10080198 - 22 Aug 2022
Viewed by 651
Abstract
The formation of boundary layers on bearing surfaces due to the operational conditions has a significant influence on bearing lifetimes and frictional properties. Zinc dialkyldithiophosphate is an anti-wear additive widely used in oil and grease formulations that forms beneficial surface boundary layers. Under [...] Read more.
The formation of boundary layers on bearing surfaces due to the operational conditions has a significant influence on bearing lifetimes and frictional properties. Zinc dialkyldithiophosphate is an anti-wear additive widely used in oil and grease formulations that forms beneficial surface boundary layers. Under certain circumstances, this additive can cause early bearing failure due to white etching cracks (WEC) formation. By influencing chemical reactions and diffusion processes, the boundary films are suspected to be a reason for the emergence of WECs. The properties of these layers under WEC critical and uncritical conditions are of interest. To gain knowledge of these layers, nano- and micro-tribological tests were performed. One possibility is to measure the hardness by nanoindentation and scratching on and into the layers by nano scratch tests. Another way is to perform local resolved micro-pin-on-disk tests. Additionally, ToF-SIMS (Time-of-Flight Secondary Ion Mass Spectrometry) was performed on the bearing surfaces to conclude the chemical compositions of the boundary layers. In the focus was, if the measured properties of the layers can be correlated to the bearing’s early failures due to WEC, frictional properties of the whole system, and the wear on the surfaces. Full article
(This article belongs to the Special Issue Rolling Contact Fatigue and White Etching Cracks of Bearings)
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Article
Effect of the Applied Voltages on the Corrosion–Wear Behavior of Thermal Spray Al Coating
Lubricants 2022, 10(8), 197; https://doi.org/10.3390/lubricants10080197 - 22 Aug 2022
Viewed by 396
Abstract
In this work, an Al coating prepared using the arc spray process was carried out with a corrosion–wear analysis by a block-on-ring system. The interaction of corrosion and wear of coatings in seawater was also investigated. The effect of different corrosion potentials on [...] Read more.
In this work, an Al coating prepared using the arc spray process was carried out with a corrosion–wear analysis by a block-on-ring system. The interaction of corrosion and wear of coatings in seawater was also investigated. The effect of different corrosion potentials on the corrosion and wear of the coating was discussed, and the structure and corrosion products of the coating were observed by SEM. The results of dynamic potential polarization curves and friction coefficient measurements were used to clarify the corrosion and wear behavior of aluminum coatings in seawater. After quantitative analysis of a corrosion and wear test, it was found that with the increase in polarization potential, the total weight loss of corrosion and wear (Wtotal) of the aluminum coating increased significantly. This means that the corrosion–wear interaction accounts for most of the weight loss of the coating. Full article
(This article belongs to the Special Issue Advances in Surface Engineering and Tribology)
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Review
Research Progress Regarding the Use of Metal and Metal Oxide Nanoparticles as Lubricant Additives
Lubricants 2022, 10(8), 196; https://doi.org/10.3390/lubricants10080196 - 22 Aug 2022
Viewed by 326
Abstract
Lubricating oil can effectively reduce friction between mechanical parts, thereby reducing energy consumption and improving service life and reliability. Due to the development of science and technology, it is necessary to improve the performance of lubricating oil to fulfill the higher tribological requirements [...] Read more.
Lubricating oil can effectively reduce friction between mechanical parts, thereby reducing energy consumption and improving service life and reliability. Due to the development of science and technology, it is necessary to improve the performance of lubricating oil to fulfill the higher tribological requirements for countering wear and providing lubrication. Nanolubricant additives have the four lubrication mechanisms of micro-bearing, protective film, polishing, and repair effects. A nanolubricant additive can often demonstrate a variety of lubrication mechanisms at the same time. As lubricating additives, metal and metal oxide nanoparticles have outstanding effects which improve the tribological properties of lubricating oil and have been widely studied in the field of tribology. This paper introduces the lubrication mechanism of nanoadditives and the latest research results for metal and metal-oxide nanoparticle lubrication additives. Full article
(This article belongs to the Special Issue Matching Ability and Anti-Wear Properties of Lubricants)
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Article
Research on Simulation Method for Wear for Dynamic Seal in Axial Flux Hub Motor
Lubricants 2022, 10(8), 195; https://doi.org/10.3390/lubricants10080195 - 22 Aug 2022
Viewed by 289
Abstract
A simulation method for the wear of a dynamic seal in an axial flux hub motor is proposed in this work. A quasi-3D magnetic model without deflection between the axes of a stator and a rotor is built. An analytical model for unbalanced [...] Read more.
A simulation method for the wear of a dynamic seal in an axial flux hub motor is proposed in this work. A quasi-3D magnetic model without deflection between the axes of a stator and a rotor is built. An analytical model for unbalanced magnetic force considering the cogging effect and axial deflection is presented based on the quasi-3D magnetic model. Boundary conditions of the dynamic seal are obtained through solving the FE model of a hub motor. Both the structural and thermal FE model of a dynamic seal are built and the thermal–structural coupling method is given. A dynamic wear simulation method is displayed based on the mesh reconstruction method proposed previously. It is proved through the contrast with the experimental results that the presented method is feasible. Full article
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Review
Electrical Bearing Damage, A Problem in the Nano- and Macro-Range
Lubricants 2022, 10(8), 194; https://doi.org/10.3390/lubricants10080194 - 21 Aug 2022
Cited by 1 | Viewed by 464
Abstract
Rolling bearings face different damaging effects: Besides mechanical effects, current-induced bearing damage occurs in electrical drive systems. Therefore, it is of increasing interest to understand the differences leading to known electrical damage patterns. It is of utmost importance not to consider the harmful [...] Read more.
Rolling bearings face different damaging effects: Besides mechanical effects, current-induced bearing damage occurs in electrical drive systems. Therefore, it is of increasing interest to understand the differences leading to known electrical damage patterns. It is of utmost importance not to consider the harmful current passage in the machine element as an isolated phenomenon but to take into account the whole drive system consisting of the machine elements, the electric motor and the connected power electronics. This publication works toward providing an overview of the state-of-the-art of research regarding electrical bearing currents. Full article
(This article belongs to the Special Issue Behavior of Lubricated Bearings in Electric Circuits)
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Article
Influence of Graphene Nanosheets on Thermo-Physical and Tribological Properties of Sustainable Cutting Fluids for MQL Application in Machining Processes
Lubricants 2022, 10(8), 193; https://doi.org/10.3390/lubricants10080193 - 21 Aug 2022
Viewed by 406
Abstract
The growing need to increase productivity and pressures for more sustainable manufacturing processes lead to a shift to less harmful lubrication systems that are less harmful to nature and the people involved. The minimal quantity lubrication system (MQL) stands out in this respect, [...] Read more.
The growing need to increase productivity and pressures for more sustainable manufacturing processes lead to a shift to less harmful lubrication systems that are less harmful to nature and the people involved. The minimal quantity lubrication system (MQL) stands out in this respect, especially in interrupted cutting processes such as milling, due to the cutting interface’s highly dynamic and chaotic nature. Using graphene sheets in cutting fluids also increases the efficiency of machining processes. This work investigates the influence on thermophysical and tribological properties of concentrations of 0.05 wt% and 0.1 wt% of graphene sheets in two vegetable-based and one mineral-based cutting fluids. The fluids are first characterized (viscosity, thermal conductivity, diffusivity, and wettability) and tested in reciprocating and ramp milling tests; all experiments are based on norms. The results show that the experiments with cutting fluids (with and without graphene) showed better tribological behavior than those in dry conditions. The graphene sheets alter the thermo-physical and tribological properties of the cutting fluids. The MQL15 vegetable-based fluid showed better lubricating properties in the milling tests, with better conditions for tribosystem chip–tool–workpiece interfaces, which makes the friction coefficient, and wear rate stable. Vegetable-based cutting fluids, even in minimum quantities and with graphene nanoparticles, have a high potential for increasing the efficiency and sustainability of the milling process. Full article
(This article belongs to the Special Issue Methods of Application of Cutting Fluids in Machining)
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Article
Ramification of Hall and Mixed Convective Radiative Flow towards a Stagnation Point into the Motion of Water Conveying Alumina Nanoparticles Past a Flat Vertical Plate with a Convective Boundary Condition: The Case of Non-Newtonian Williamson Fluid
Lubricants 2022, 10(8), 192; https://doi.org/10.3390/lubricants10080192 - 19 Aug 2022
Cited by 1 | Viewed by 413
Abstract
Heat transfer technologies are experiencing rapid expansion as a result of the demand for efficient heating and cooling systems in the automotive, chemical, and aerospace industries. Therefore, the current study peruses an inspection of mixed convective radiative Williamson flow close to a stagnation [...] Read more.
Heat transfer technologies are experiencing rapid expansion as a result of the demand for efficient heating and cooling systems in the automotive, chemical, and aerospace industries. Therefore, the current study peruses an inspection of mixed convective radiative Williamson flow close to a stagnation point aggravated by a single nanoparticle (alumina) from a vertical flat plate with the impact of Hall. The convective heating of water conveying alumina (Al2O3) nanoparticles, as appropriate in engineering or industry, is investigated. Using pertinent similarity variables, the dominating equations are non-dimensionalized, and after that, via the bvp4c solver, they are numerically solved. We extensively explore the effects of many relevant parameters on axial velocity, transverse velocity, temperature profile, heat transfer, and drag force. In the opposing flow, there are two solutions seen; in the aiding flow, just one solution is found. In addition, the results designate that, due to nanofluid, the thickness of the velocity boundary layer decreases, and the thermal boundary layer width upsurges. The gradients for the branch of stable outcome escalate due to a higher Weissenberg parameter, while they decline for the branch of lower outcomes. Moreover, a magnetic field can be used to influence the flow and the properties of heat transfer. Full article
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Article
Regulation Mechanism of Trivalent Cations on Friction Coefficient of a Poly(Vinylphosphonic Acid) (PVPA) Superlubricity System
Lubricants 2022, 10(8), 191; https://doi.org/10.3390/lubricants10080191 - 19 Aug 2022
Viewed by 417
Abstract
The application range of superlubricity systems can be extended effectively by realizing an adjustable friction coefficient. In this study, a stable poly(vinylphosphonic acid) (PVPA) superlubricity system was developed using sodium chloride (NaCl) solution as the lubricant. A sudden increase in the friction coefficient [...] Read more.
The application range of superlubricity systems can be extended effectively by realizing an adjustable friction coefficient. In this study, a stable poly(vinylphosphonic acid) (PVPA) superlubricity system was developed using sodium chloride (NaCl) solution as the lubricant. A sudden increase in the friction coefficient occurred when a trivalent salt solution was introduced to the base lubricant during the friction process. The changes in surface microstructure and interfacial molecular behavior induced by trivalent cations were studied by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and molecular dynamics simulation. The regulation mechanism of trivalent cations with respect to the friction coefficient of the PVPA superlubricity system was explored. Although La3+ and Fe3+ in solutions exist in two forms (La3+ and Fe(OH)3), both can destroy the stable coating structure through a bridging effect, resulting in a sudden increase in the friction coefficient. The ability of various cations to attract the surrounding water molecules is an important reason for the varying degree of change of the friction coefficient. In addition, the degree of sudden increase in the friction coefficient is dependent on the concentration of trivalent cations. There is an extreme concentration at which the maximum sudden increase degree in friction coefficient can be obtained. This study provides insights into the realization of oil-based superlubricity through interface regulation. Full article
(This article belongs to the Special Issue Superlubricity: From Nanoscale to Macroscale)
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Article
Modified Graphene/Muscovite Nanocomposite as a Lubricant Additive: Tribological Performance and Mechanism
Lubricants 2022, 10(8), 190; https://doi.org/10.3390/lubricants10080190 - 19 Aug 2022
Viewed by 302
Abstract
Modified graphene/muscovite (MGMu) nanocomposite was synthesized with muscovite (Mu) and silane coupling agent modified graphene oxide through a simple hydrothermal method that exhibited excellent dispersion stability in oil. Compared with the base oil sample, the average friction coefficient and wear scar diameter of [...] Read more.
Modified graphene/muscovite (MGMu) nanocomposite was synthesized with muscovite (Mu) and silane coupling agent modified graphene oxide through a simple hydrothermal method that exhibited excellent dispersion stability in oil. Compared with the base oil sample, the average friction coefficient and wear scar diameter of the MGMu oil sample decreased by 64.4 and 20.0%, respectively, and the microhardness of its wear scar was increased by 16.1%. The MGMu showed better tribological performance than its individual component due to the synergetic effect between the two components. The lubrication mechanism was proposed according to the morphology, chemical composition, and microhardness of the surface of wear scars. MGMu as an oil additive could fill between the friction pairs, cling to some asperities, and occur relative sliding between unit layers, thus playing a role in lubrication. It was found that MGMu would react with the surface of the friction pair during the friction process to generate Fe2O3, SiO2, SiC, and new aluminosilicate, which formed a self-repairing layer with high hardness. This chemically reactive film exhibited a lower shear strength, which made the oil sample containing MGMu have a lower coefficient of friction. Full article
(This article belongs to the Special Issue State-of-the-Art of Tribology in China)
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Article
Lubrication Mechanisms of a Nanocutting Fluid with Carbon Nanotubes and Sulfurized Isobutylene ([email protected]) Composites as Additives
Lubricants 2022, 10(8), 189; https://doi.org/10.3390/lubricants10080189 - 19 Aug 2022
Viewed by 278
Abstract
Developing high-efficiency lubricant additives and high-performance green cutting fluids has universal significance for maximizing processing efficiency, lowering manufacturing cost, and more importantly reducing environmental concerns caused by the use of conventional mineral oil-based cutting fluids. In this study, a nanocomposite is synthesized by [...] Read more.
Developing high-efficiency lubricant additives and high-performance green cutting fluids has universal significance for maximizing processing efficiency, lowering manufacturing cost, and more importantly reducing environmental concerns caused by the use of conventional mineral oil-based cutting fluids. In this study, a nanocomposite is synthesized by filling sulfurized isobutylene (T321) into acid-treated carbon nanotubes (CNTs) with a liquid-phase wet chemical method. The milling performance of a nanocutting fluid containing [email protected] composites is assessed using a micro-lubrication technology in terms of cutting temperature, cutting force, tool wear, and surface roughness. The composite nanofluid performs better than an individual CNT nanofluid regarding milling performance, with 12%, 20%, and 15% reductions in the cutting force, machining temperature, and surface roughness, respectively. The addition of [email protected] nanocomposites improves the thermal conductivity and wetting performance of the nanofluid, as well as produces a complex lubricating film by releasing T321 during machining. The synergistic effect improves the cutting state at the tool–chip interface, thereby resulting in improved machining performance. Full article
(This article belongs to the Special Issue Advances in Water-Based Nanolubricants)
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Article
The Impact of Water on the Tribological Behavior of Lubricating Grease Based on Calcium Carbonate Polymorphs
Lubricants 2022, 10(8), 188; https://doi.org/10.3390/lubricants10080188 - 19 Aug 2022
Viewed by 303
Abstract
In this article, calcium sulfonate complex grease with different polymorphs was acquired. With reference to the improved rolling stability test method, 5% and 10% water were added to the calcium sulfonate composite grease respectively. The influence of water on the flow and deformation [...] Read more.
In this article, calcium sulfonate complex grease with different polymorphs was acquired. With reference to the improved rolling stability test method, 5% and 10% water were added to the calcium sulfonate composite grease respectively. The influence of water on the flow and deformation characteristics of grease under external force was characterized by rheological analysis. Moreover, the influence of water on the tribological behaviors of grease was evaluated at 30 and 80 °C. The results indicate that water could affect the flow and deformation characteristics of calcium sulfonate complex grease in vaterite polymorph (CSCG-V) more than calcium sulfonate complex grease in calcite polymorph (CSCG-C), and CSCG-V could combine with water to form a more stable grease under external force. Tribological results also clearly show that CSCG-C with water shows better tribological properties than CSCG-V with water under harsh conditions. There was no significant difference in the components of the films that consisted of CaCO3 and CaO regardless of grease within or without water. The improvement of tribological properties of CSCG-C with 5% and 10% water benefited from the influence of water on the film forming capacity of calcium carbonate on the steel surface. Full article
(This article belongs to the Special Issue Grease II)
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Article
A Comparative Study of Hardfacing Deposits Using a Modified Tribological Testing Strategy
Lubricants 2022, 10(8), 187; https://doi.org/10.3390/lubricants10080187 - 18 Aug 2022
Viewed by 343
Abstract
In this study, hardfacing deposits using materials of different surface hardness are investigated using an innovative strategy for tribological testing. The abrasive wear behaviour of AISI 316L stainless steel is compared to the Cr–Ni–Mn alloy (OK Autrod 16.95) and the Cr-Mo alloy (Fluxofil [...] Read more.
In this study, hardfacing deposits using materials of different surface hardness are investigated using an innovative strategy for tribological testing. The abrasive wear behaviour of AISI 316L stainless steel is compared to the Cr–Ni–Mn alloy (OK Autrod 16.95) and the Cr-Mo alloy (Fluxofil 58), deposited on a substrate of S355JR steel. A modified three-body abrasion test and a modified scratch test were used to evaluate the tribological behaviour and wear mechanisms of these materials. The modified double-pass scratch test on the abraded surfaces is analysed using the geometrical parameters of grooves to aid in predicting the lifetime of machinery parts in abrasive working conditions. This leads to a shortening of the resistance to abrasion wear time of the evaluation of the abrasion wear resistance of materials. The validation of the results obtained in the double-pass scratch tests was carried out using three-body abrasion tests, according to the ASTM G65 standard. Wear mechanism investigations were carried out by scanning electron microscopy and three-dimensional surface topography and was analysed using an optical microscope. The results obtained from experimental research show that double-pass scratch tests demonstrated that it is possible to shorten the time needed to predict the abrasive behaviour of materials using this method. Full article
(This article belongs to the Special Issue Assessment of Abrasive Wear)
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Article
Lubricated Impact Dynamics and Pressure Spike Generation: Expanding on Contributions of Dr. R. Gohar
Lubricants 2022, 10(8), 186; https://doi.org/10.3390/lubricants10080186 - 16 Aug 2022
Viewed by 306
Abstract
The current paper focuses on the research of Dr. R. Gohar and in particular on his impacting ball and pressure spike measurement work. Its scope then expands to discuss contributions from other researchers concerning these two fields. The authors combined the two themes [...] Read more.
The current paper focuses on the research of Dr. R. Gohar and in particular on his impacting ball and pressure spike measurement work. Its scope then expands to discuss contributions from other researchers concerning these two fields. The authors combined the two themes in a numerical study of an impacting contact. This study shows the detailed position of the pressure spike as a function of time. Then, the pressure spike position velocity is derived, and it is demonstrated that this velocity varies with time. As such, the paper concludes that the pressure spike shape itself must vary with time. Full article
(This article belongs to the Special Issue Special Issue in Elastohydrodynamics: Remembering Ramsey Gohar)
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Article
Research on the Bearing Sliding Loss Based on Time-Varying Contact Angle between Ball and Raceway
Lubricants 2022, 10(8), 185; https://doi.org/10.3390/lubricants10080185 - 15 Aug 2022
Viewed by 341
Abstract
Based on the mechanical model, the friction loss between the ball and the raceway along the major axis of the contact ellipse is analyzed. The result shows that this part of the loss accounts for about 13.67% of the overall loss, which is [...] Read more.
Based on the mechanical model, the friction loss between the ball and the raceway along the major axis of the contact ellipse is analyzed. The result shows that this part of the loss accounts for about 13.67% of the overall loss, which is mainly determined by the ball sliding length and cannot be ignored. The effects of the radial force, torque, rotational speed and groove curvature ratio on the sliding are all studied. Compared with other factors, radial force has the greatest influence on the sliding loss. As bearing speed gradually grows, the sliding on the inner raceway gradually increases while it gradually decreases on the outer raceway. Compared to the outer raceway curvature ratio, the sliding length is less sensitive to changes in the curvature ratio of the inner raceway. The paper provides theoretical guidance for the design and application of low-friction bearings. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermal Sciences)
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Article
CFD Investigation on Oil Injection Lubrication of Meshing Spur Gears via Lattice Boltzmann Method
Lubricants 2022, 10(8), 184; https://doi.org/10.3390/lubricants10080184 - 11 Aug 2022
Viewed by 330
Abstract
The meshless Lattice Boltzmann Method (LBM) is introduced and employed to solve the complex two-phase flow problem of jet lubrication of meshing spur gears. Computational fluid dynamics (CFD) simulations based on LBM are carried out using the model of an oil jet impacting [...] Read more.
The meshless Lattice Boltzmann Method (LBM) is introduced and employed to solve the complex two-phase flow problem of jet lubrication of meshing spur gears. Computational fluid dynamics (CFD) simulations based on LBM are carried out using the model of an oil jet impacting rotating gear presented by available experiments, which reveals how the liquid column is broken throughout the tooth tip cutting off the oil jet. Typical oil flow phenomena obtained by simulations are compared with experiments, demonstrating good qualitative agreement, which validates the feasibility of LBM to simulate the air–oil–structure interaction problems involved in the jet lubrication of spur gears. A three-dimensional (3D) simulation model of a spur gear pair lubricated by an oil jet is established, and simulations with different operating conditions are conducted. The evolution process of the oil jet while injecting into the meshing zone is captured, and the effects of jet velocities, jet heights and jet angles on the lubrication performance are investigated. Full article
(This article belongs to the Special Issue Gear Load-Independent Power Losses)
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Article
Study on Tribological Characteristics of Textured Surface under Convergent Oil Film Gap
Lubricants 2022, 10(8), 183; https://doi.org/10.3390/lubricants10080183 - 11 Aug 2022
Viewed by 322
Abstract
Under the condition of convergence, a textured non-parallel 2D slider bearing model was established, and the tribological properties of textured surface under the convergence gap were numerically studied with the load-carrying capacity as an indicator. Firstly, whether the convergence ratio parameter can accurately [...] Read more.
Under the condition of convergence, a textured non-parallel 2D slider bearing model was established, and the tribological properties of textured surface under the convergence gap were numerically studied with the load-carrying capacity as an indicator. Firstly, whether the convergence ratio parameter can accurately characterize the joint effects of film thickness difference and oil film thickness on the surface tribological properties was verified, and the effects of film thickness difference and oil film thickness on the load-carrying capacity of textured and non-textured surface were studied, respectively. The results show that the efficiency of improving the load-carrying capacity of the surface structure first increases and then decreases with the increase of the oil film thickness. In the case of large film thickness difference, the surface texture will reduce the efficiency of improving the load-carrying capacity. In addition, the effects of texture depth, texture width, and sliding velocity on the load-carrying capacity under the convergence gap are also studied. In particular, an optimal texture width to maximize the load-carrying capacity exists. Full article
(This article belongs to the Special Issue Tribology and Reliability of Mechanical Transmission)
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Article
Flow Mechanisms and Lubrication Performance of Water-Lubricated Thrust Bearings with Herringbone Grooves
Lubricants 2022, 10(8), 182; https://doi.org/10.3390/lubricants10080182 - 10 Aug 2022
Viewed by 358
Abstract
Due to their excellent stability and zero leakage capability, thrust bearings with herringbone spiral grooves are frequently used in transmission mechanisms. However, the lubrication mechanism of thrust bearings has not been clearly understood and explained, preventing the optimization of the bearing performance. Thus, [...] Read more.
Due to their excellent stability and zero leakage capability, thrust bearings with herringbone spiral grooves are frequently used in transmission mechanisms. However, the lubrication mechanism of thrust bearings has not been clearly understood and explained, preventing the optimization of the bearing performance. Thus, this paper is devoted to solving this problem by building a three-dimensional finite element flow model. In this model, the change in viscosity temperature is considered using Roelands equation, and the turbulence and cavitation are taken into consideration. Using the established model, the influence of parameters such as spiral angle, groove width ratio, and rotational speed on the cavitation area of the thrust bearing are analyzed. The pressure contour and speed distribution are obtained inside the clearance, as well as the volume fraction of the gas phase at the end face. Finally, according to the analysis results, the optimum structural parameter for the herringbone spiral groove structure is proposed, which enables higher bearing stability and provides a reference for engineering practice. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermal Sciences)
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Article
Wear Properties of C-MoS2-PTFE Composite Coating Prepared on 4032 Aluminum Alloy
Lubricants 2022, 10(8), 181; https://doi.org/10.3390/lubricants10080181 - 10 Aug 2022
Cited by 1 | Viewed by 373
Abstract
A large number of joint friction pairs work during the work of scroll compressors, resulting in high energy consumption and short service life of scroll compressors. To improve the tribological performance of friction pairs of the scroll compressors, the C-MoS2-PTFE (Polytetrafluoroethylene) [...] Read more.
A large number of joint friction pairs work during the work of scroll compressors, resulting in high energy consumption and short service life of scroll compressors. To improve the tribological performance of friction pairs of the scroll compressors, the C-MoS2-PTFE (Polytetrafluoroethylene) lubrication coating is prepared through spraying technology on the surface of 4032 aluminum alloy, a common material for scroll compressors. The microstructure of the C-MoS2-PTFE coating was analyzed by X-ray diffractometer (XRD) and scanning electron microscope (SEM), and the wear behavior of the coating under different loads was studied by reciprocating friction and wear testing machine and three-dimensional profiler. The surface of the grinding marks was analyzed by SEM and energy density spectrum (EDS). The performance was compared with the anodized film of 4032 aluminum alloy and the Ni-coated coating of 4032 aluminum alloy. The experimental results show that the C-MoS2-PTFE coating has a dense structure, and the hardness is 35 HV0.1. Under dry friction conditions, the C-MoS2-PTFE coating has excellent wear reduction and wear resistance, and the coefficient of friction and wears rate under different loads are less than those of the anodized film of 4032 aluminum alloy and the Ni-plated coating. The wear mechanisms of C-MoS2-PTFE coating are fatigue wear, adhesive wear, abrasive wear, and oxidation wear under different loads. Full article
(This article belongs to the Special Issue Friction and Wear of Coatings/Films)
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Article
Experimental Investigation of Triboelectrification Behaviour in the Friction Process
Lubricants 2022, 10(8), 180; https://doi.org/10.3390/lubricants10080180 - 09 Aug 2022
Viewed by 370
Abstract
The triboelectrification phenomenon can occur during the friction process of metal contact pairs. An in-depth understanding of triboelectrification behaviour is incredibly beneficial to controlling friction and wear. However, due to the complexity of the driving mechanism, it is still challenging to gain a [...] Read more.
The triboelectrification phenomenon can occur during the friction process of metal contact pairs. An in-depth understanding of triboelectrification behaviour is incredibly beneficial to controlling friction and wear. However, due to the complexity of the driving mechanism, it is still challenging to gain a thorough understanding of the triboelectrification behaviour of metal–metal contact pairs. To further reveal the triboelectrification behaviour during the friction process of metal pairs, wear experiments of GCr15 steel–cast iron were carried out on a CFT-I tribometer under oil-free and oil lubrication conditions. The triboelectric current signal was collected during the investigation, and its variation was discussed. The result shows that the varying trend of the triboelectric current was consistent with that of the friction coefficient in the friction process. The triboelectrification of similar metal contact pairs primarily driven by material transfer was closely related to friction and wear conditions. Full article
(This article belongs to the Special Issue Frictional Behavior and Wear Performance of Cast Irons)
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Article
Design and Performance Analysis of Dual Membrane Restrictor for Hydrostatic Bearing
Lubricants 2022, 10(8), 179; https://doi.org/10.3390/lubricants10080179 - 08 Aug 2022
Viewed by 301
Abstract
In this paper, a dual membrane restrictor design was proposed to improve the stiffness performance of the compensated hydrostatic bearing. Theoretical models for the proposed dual membrane restrictors were derived. Analysis of these models showed that a high stiffness region could be achieved [...] Read more.
In this paper, a dual membrane restrictor design was proposed to improve the stiffness performance of the compensated hydrostatic bearing. Theoretical models for the proposed dual membrane restrictors were derived. Analysis of these models showed that a high stiffness region could be achieved at the desired loading region through the proper selection of the design parameters. A series of simulations were conducted to study the variations in the design parameters on the stiffness and clearance variations. It was found that the dimensionless membrane stiffness in the inlet restrictor, Kmi*, was the most dominant parameter for the performance of the compensated bearing system. The main advantages of the proposed dual membrane restrictor are the increase in flexibility by providing high stiffness at the desired loading region; and improving the stiffness performance of the bearing system especially at the desired loading region. Full article
(This article belongs to the Special Issue Friction and Lubrication of Sliding Bearings, Volume II)
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Article
Improvement of Mechanical Properties and Adhesion of Ti-Al-Si-N Coatings by Alloying with Ta
Lubricants 2022, 10(8), 178; https://doi.org/10.3390/lubricants10080178 - 07 Aug 2022
Viewed by 367
Abstract
The effect of the Ta content on the structure, mechanical properties and adhesion of magnetron-sputtered Ti1-x-y-zAlxTaySizN coatings was studied. According to the energy-dispersive X-ray spectroscopy analysis, the coatings studied had the following chemical compositions: Ti [...] Read more.
The effect of the Ta content on the structure, mechanical properties and adhesion of magnetron-sputtered Ti1-x-y-zAlxTaySizN coatings was studied. According to the energy-dispersive X-ray spectroscopy analysis, the coatings studied had the following chemical compositions: Ti0.41Al0.49Si0.10N, Ti0.38Al0.47Ta0.05Si0.10N, Ti0.36Al0.44Ta0.10Si0.10N and Ti0.35Al0.40Ta0.15Si0.10N. The X-ray diffraction experiments revealed the B1-type fcc crystal structure of the coatings. The increasing Ta content was found to induce the texture evolution from (200) to (111), which was attributed to a significant increase in the residual compressive stress in the coatings. The hardness monotonically increased from 32.7 to 42.2 GPa with increasing the Ta content, while the reduced Young’s modulus decreased from 369 to 353 GPa. The adhesion of the coatings to the Ti substrate was evaluated by scratch testing. It was found that the Ti0.36Al0.44Ta0.10Si0.10N coating was characterized by maximum adhesion strength, while incorporation of a larger amount of Ta resulted in earlier coating spallation due to the high residual compressive stress. Full article
(This article belongs to the Special Issue Friction and Wear of Coatings/Films)
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Review
High-Temperature Solid Lubricants and Self-Lubricating Composites: A Critical Review
Lubricants 2022, 10(8), 177; https://doi.org/10.3390/lubricants10080177 - 07 Aug 2022
Viewed by 667
Abstract
Solid lubricants are described as solid materials of intentionally introduced or in situ formed on contact surfaces in relative motion for the purpose of lowering friction and wear and providing protection from damage. Solid lubricants and advanced self-lubricating materials are widely used in [...] Read more.
Solid lubricants are described as solid materials of intentionally introduced or in situ formed on contact surfaces in relative motion for the purpose of lowering friction and wear and providing protection from damage. Solid lubricants and advanced self-lubricating materials are widely used in modern industries, especially in aerospace, aviation, automotive, metallurgy, materials forming, and machining industries, and have attracted great interest in lubrication applications under very severe circumstances such as elevated temperatures, heavy loads, ultrahigh vacuum, extreme radiation, strong oxidation, and chemical reactivity environments. Many efforts have been made to develop self-lubricating composites by a variety of material preparation techniques, which include powder metallurgy, physical/chemical vapor depositions, thermal spraying, electrodeposition, laser cladding, and additive manufacturing. Although several reviews on the development of high-temperature solid lubricants have been published, most of them only focus on a type of material, a specific process, or application. In this paper, a comprehensive review is provided to present the state-of-the-art progress in solid lubricants, self-lubricating composites/coatings, and their effective functions that can be used over a wide variety of environmental conditions, especially at elevated temperatures. The solid lubricants considered include representative soft metals, layered structure materials (e.g., graphite, hexagonal boron nitride, transition metallic dichalcogenides, MAX phase), chemically stable fluorides, binary or ternary metallic oxides, especially alkaline earth chromates, and sulfates, and synergistic effects from these solid lubricants. This paper also provides new insights into design considerations of environmental adaptive solid lubrication, and the challenges and potential breakthroughs are further highlighted for high-temperature solid lubrication applications. Full article
(This article belongs to the Special Issue Multiscale Tribology of Solid Lubricants)
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Review
Toxicity Analysis of Nano-Minimum Quantity Lubrication Machining—A Review
Lubricants 2022, 10(8), 176; https://doi.org/10.3390/lubricants10080176 - 06 Aug 2022
Viewed by 464
Abstract
The lubrication properties of nanoparticles are of great interest to the manufacturing industry and led to the development of the nano-minimum quantity lubrication (NMQL) cooling strategy. To evaluate the sustainability characteristics of nano-minimum quantity lubrication, apart from analyzing the benefits of increasing machining [...] Read more.
The lubrication properties of nanoparticles are of great interest to the manufacturing industry and led to the development of the nano-minimum quantity lubrication (NMQL) cooling strategy. To evaluate the sustainability characteristics of nano-minimum quantity lubrication, apart from analyzing the benefits of increasing machining efficiency, it is also essential to evaluate the potential detrimental effects of nanoparticles on human health and the environment. Existing literature provides substantial data on the benefits of nano-minimum quantity lubrication machining. However, the current literature does not provide researchers in the machining sector a comprehensive analysis of the toxicity of the nanoparticles used in nano-minimum quantity lubrication. This study aims to provide a comprehensive review that addresses the toxicity levels of the most frequently used nanoparticles in NMQL machining. To understand the impacts of nanoparticles on the human body and the environment, in vitro studies that evaluate the nanoparticles’ toxicity on human cells and in vitro/in vivo studies on other living organisms are considered. The results from toxicity studies on each of the chosen nanoparticles are summarized and presented in chronological order. The reviewed studies indicate transition metal dichalcogenides (MoS2 and WS2) exhibit very low toxicity when compared to other nanoparticles. The toxicity of hBN and AL2O3 nanoparticles varies depending on their lengths and crystalline structures, respectively. In conclusion, a chart that maps the toxicity levels of nanoparticles on seven different human cell lines (human lung epithelial cells (A549), human bronchial epithelial cells (Nl-20), AGS human gastric cells, human epidermal cells (HEK), human liver-derived cells (HepG2), human endothelial cells and human peripheral cells), representing exposures by inhalation, ingestion and dermal contact, was developed for easy and quick insights. This is the first attempt in open literature to combine the results of the experimental investigations of nano-minimum quantity lubrication cooling and the toxicity studies of nanoparticles, allowing researchers to make informed decisions in the selection of the most sustainable nanoparticles in the nano-minimum quantity lubrication machining process. Full article
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Article
Synergistic Lubricating Performance of Graphene Oxide and Modified Biodiesel Soot as Water Additives
Lubricants 2022, 10(8), 175; https://doi.org/10.3390/lubricants10080175 - 05 Aug 2022
Viewed by 334
Abstract
The tribological performance of graphene oxide (GO) nanosheets, modified biodiesel soot (MBS) nanoparticles, and their mixture (MBS–GO) nanoparticles as lubricant additives in water was evaluated using a reciprocating ball-on-plate tribometer. The effects of different mass ratios of GO to MBS, additive concentrations, and [...] Read more.
The tribological performance of graphene oxide (GO) nanosheets, modified biodiesel soot (MBS) nanoparticles, and their mixture (MBS–GO) nanoparticles as lubricant additives in water was evaluated using a reciprocating ball-on-plate tribometer. The effects of different mass ratios of GO to MBS, additive concentrations, and loads, as well as corresponding lubrication mechanisms, were studied. The tribological measurements showed that the water-containing 0.5 wt% additives at a mass ratio of 60:40 (GO to MBS) resulted in larger reductions in friction coefficient (69.7%) and wear volume (60.5%) than water. Owing to the synergistic effect of GO nanosheets and MBS nanoparticles, the MBS–GO aqueous sample showed superior lubricating properties compared to water as well as GO and MBS aqueous samples. The good tribological properties of MBS–GO nanoparticles in water are attributed to the formation of a tribofilm of hybrid nanoparticles that effectively protects the friction interface. Moreover, the MBS nanoparticles can provide lubrication by acting as ball bearings. Full article
(This article belongs to the Special Issue Advances in Novel Lubricant Additives)
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Article
Study on Distribution of Lubricating Oil Film in Contact Micro-Zone of Full Ceramic Ball Bearings and the Influence Mechanism on Service Performance
Lubricants 2022, 10(8), 174; https://doi.org/10.3390/lubricants10080174 - 01 Aug 2022
Viewed by 337
Abstract
Compared with metal ball bearings, full ceramic ball bearings have more outstanding service performance under extreme working conditions. In order to reveal the lubrication mechanism and improve the operation performance and service life of full ceramic ball bearings, in this paper, the friction, [...] Read more.
Compared with metal ball bearings, full ceramic ball bearings have more outstanding service performance under extreme working conditions. In order to reveal the lubrication mechanism and improve the operation performance and service life of full ceramic ball bearings, in this paper, the friction, vibration, and temperature rise characteristics of 6208 silicon nitride full ceramic deep groove ball bearing, under the condition of oil lubrication, are studied experimentally. Based on the test results, and through theoretical calculation and simulation analysis, the distribution of the lubricating oil film in bearing contact micro-zone under different working conditions was simulated. After that, the surface of contact micro-zone of full ceramic ball bearing was analyzed. It was found that there is an optimal oil supply for full ceramic ball bearing oil lubrication in service. Under the optimal oil supply lubrication, full film lubrication can be achieved, and the bearing exhibits the best characteristics of friction, vibration, and temperature rise. Compared with the load, the rotational speed of the bearing has a decisive influence on the optimal oil supply. When the rotational speed and load are constant, the minimum oil film thickness and oil film pressure in the contact area of the rolling body decrease with the increase of angle ψ from the minimum stress point of the rolling body. Under the action of high contact stress, thin oil film will be formed in the bearing outer ring raceway. In the field of full ceramic ball bearings, the research content of this paper is innovative. The research results of this paper have an important guiding significance for revealing the oil lubrication mechanism of full ceramic ball bearing and enriching its lubrication theory and methods. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermal Sciences)
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Article
Enhancement of Deep Drilling for Stainless Steels by Nano-Lubricant through Twist Drill Bits
Lubricants 2022, 10(8), 173; https://doi.org/10.3390/lubricants10080173 - 29 Jul 2022
Viewed by 462
Abstract
This paper represents a new lubricant method which is able to one-stroke drill deep holes with a length-to-diameter of 8, on the AISI SUS 304 stainless steel. By adding graphene nanosheet into typical soluble emulsion and then mixing with water, a nano fluid [...] Read more.
This paper represents a new lubricant method which is able to one-stroke drill deep holes with a length-to-diameter of 8, on the AISI SUS 304 stainless steel. By adding graphene nanosheet into typical soluble emulsion and then mixing with water, a nano fluid can be made. The results revealed that using nanofluid can provide a reduction of 4.4-fold of the drilling torque, and thus expand the tool life as many as 20 times, compared with using typical emulsion lubricant. The proper set of cutting parameters was found by using Taguchi L9 experiments as 550 rpm spindle speed and 0.05 mm/rev. The results can be expanded to apply in other deep drilling of hard-to-cut material, using inexpensive devices and avoiding peck-drilling. The proposed lubricant can also be promissing for other machining operations. Full article
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Article
CPU Performance Improvement Using Novel Thermally Conductive Carbon Nano Grease
Lubricants 2022, 10(8), 172; https://doi.org/10.3390/lubricants10080172 - 27 Jul 2022
Viewed by 347
Abstract
Electronics depend on their ability to shed operational heat to maintain operating temperature. Inferior grease can create preventable problems in electronics. This is typically achieved through a thermal grease or paste. If this grease fails to dissipate heat or maintain thermal contact, then [...] Read more.
Electronics depend on their ability to shed operational heat to maintain operating temperature. Inferior grease can create preventable problems in electronics. This is typically achieved through a thermal grease or paste. If this grease fails to dissipate heat or maintain thermal contact, then the equipment will have faults. Greases with less-than-optimal performance create excessive wear, heat, and reduced life expectancy. This can cause equipment failures and malfunctions at the most inopportune moments. Thermal greases are applied to Central Processors (CPU) and Graphics Processors (GPU) in avionics LRUs, computers, Solar panels, HVAC systems, and other electronics. A high-performance novel nano grease will shed excess heat and increase device life expectancy. The fabricated nano greases show improvements of up to 80% in thermal conductivity measurements. CPU testing resulted in a 100% decrease in the standard deviation of temperature variation from commercial greases. Full article
(This article belongs to the Special Issue Thermally and Electrically Conductive Nanomaterials Lubricants)
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Review
Turbulence Effects in Tilting Pad Journal Bearings: A Review
Lubricants 2022, 10(8), 171; https://doi.org/10.3390/lubricants10080171 - 26 Jul 2022
Viewed by 473
Abstract
This paper reviews the current knowledge on turbulence effects in tilting pad journal bearings. Turbulence is becoming increasingly important in the design of hydrodynamic bearings due to the trend to increase power density in turbomachines and consequently the operating speeds of the hydrodynamic [...] Read more.
This paper reviews the current knowledge on turbulence effects in tilting pad journal bearings. Turbulence is becoming increasingly important in the design of hydrodynamic bearings due to the trend to increase power density in turbomachines and consequently the operating speeds of the hydrodynamic bearings. Turbulence has a series of effects on the bearing performance which may be beneficial or detrimental, depending on the operating conditions. The main turbulence models are recalled and a historical overview on the evolution of numerical simulations of turbulent flow in tilting pad journal bearings is presented. The two broad simulation strategies used are the generalized Reynolds equation and computational fluid dynamics. The main experimental works are then reviewed, and a unified comparison of these works is provided. Novel results on the critical Reynolds number in a tilting pad journal bearing are reported. Much emphasis is given on the experimental evidence for laminar-to-turbulent transition. The evidence used in the literature is reviewed and its reliability is discussed. Lastly, some future research directions are suggested. Full article
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Article
Prediction of Remaining Service Life of Rolling Bearings Based on Convolutional and Bidirectional Long- and Short-Term Memory Neural Networks
Lubricants 2022, 10(8), 170; https://doi.org/10.3390/lubricants10080170 - 26 Jul 2022
Cited by 1 | Viewed by 428
Abstract
Predicting the remaining useful life (RUL) of a bearing can prevent sudden downtime of rotating machinery, thereby improving economic efficiency and protecting human safety. Two important steps in RUL prediction are the construction of a health indicator (HI) and the prediction of life. [...] Read more.
Predicting the remaining useful life (RUL) of a bearing can prevent sudden downtime of rotating machinery, thereby improving economic efficiency and protecting human safety. Two important steps in RUL prediction are the construction of a health indicator (HI) and the prediction of life. Traditional methods simply use the time-series characteristics of the vibration signal, for example, using root mean square (RMS) as HI, but this HI does not reflect the true degradation of the bearing. Meanwhile, existing prediction models often cannot consider both the time and space characteristics of the signal, thus limiting prediction accuracy. To address the above problems, in this study, wavelet packet transform (DWPT) and kernel principal component analysis (KPCA) were combined to extract HI from the original vibration signal. Then, a CNN-BiLSTM (convolutional and bidirectional long- and short-term memory) prediction network with root mean square as input and HI as output was constructed by combining convolutional neural network (CNN) and bi-directional long- and short-term memory neural network (BiLSTM). The network improved prediction accuracy by considering the temporal and spatial characteristics of the input signal. Experimental results on the PHM2012 dataset showed that the method proposed in this paper outperformed existing methods. Full article
(This article belongs to the Special Issue Advances in Bearing Lubrication and Thermal Sciences)
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Article
Validation of Experimental and Finite Element Biomechanical Evaluation of Human Cadaveric Mandibles
Lubricants 2022, 10(8), 169; https://doi.org/10.3390/lubricants10080169 - 25 Jul 2022
Viewed by 518
Abstract
Background: Biomechanical analysis of human mandible is important not only to understand mechanical behavior and structural properties, but also to diagnose and develop treatment options for mandibular disorders. Therefore, the objective of this research was to generate analytical and experimental data on mandibles, [...] Read more.
Background: Biomechanical analysis of human mandible is important not only to understand mechanical behavior and structural properties, but also to diagnose and develop treatment options for mandibular disorders. Therefore, the objective of this research was to generate analytical and experimental data on mandibles, construct custom 3D models, and compare the analytically derived maximum strains with strain gage data in five areas of interest for each mandible. Methods: We investigated the surface strains in the cadaveric human mandibles under different configurations of cyclic compressive loads in an experimental setting and compared these experimental strain data with results derived from computational finite element analysis (FEA), accurately replicating the experiments. Strains on the surface of each mandible were measured with strain gauges, and subsequently a subject-specific finite element (FE) volume mesh was generated from computed tomography (CT) scans of each mandible. Strain patterns of each mandible were derived from the FEA simulating the experimental setup and matched with the experimental data. Findings: Analysis of experimental data showed that strain as measured at the condylar locations was significantly different from those at other locations on the mandible, and that the sex and age of the subject did not have a significant correlation with the strain. Comparing the FE numerical predictions with the experimental data, we found a good statistical correlation and statistical agreement between in-vitro measurements and FE results. Interpretation: The study demonstrates that our methodology of generating subject-specific FE models is a valid and accurate, non-invasive method to evaluate the complex biomechanical behavior of human mandibles. Full article
(This article belongs to the Special Issue Synovial Lubricated Joints—Devices and Mechanical Behavior)
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