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Lubricants
Volume 6
Issue 4
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Lubricants, Volume 6, Issue 4 (December 2018) – 26 articles

Cover Story (view full-size image): Environmental concerns have led scientists to come up with alternative bio-degradable lubricants, such as water, to replace oil-based lubricants. However, this solution presents challenges regarding material selection. Furthermore, polymers might be subjected to harsh environmental conditions such as a hygrothermal conditions, which could possibly lead to tribological polymer composite failure, and hence must not be neglected in life estimation/design analyses. Therefore, understanding the absorption behaviour of polymers is crucial in environments where water is present. Consequently, further studies are required to investigate the influence of hygrothermal ageing on the tribological behaviour of polymeric materials. View this paper.
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24 pages, 11671 KiB  
Article
Experimental Studies of Lubricant Flow and Friction in Partially Filled Gaps
by Michael Müller, Lukas Stahl and Georg-Peter Ostermeyer
Lubricants 2018, 6(4), 110; https://doi.org/10.3390/lubricants6040110 - 18 Dec 2018
Cited by 13 | Viewed by 5061
Abstract
Starved lubrication is an important strategy for minimizing the amount of lubricant needed, and also inevitably occurs during idling and fail-safe lubrication. In this regime, however, the flow of the lubricant and the related friction coefficients are yet to be fully understood. This [...] Read more.
Starved lubrication is an important strategy for minimizing the amount of lubricant needed, and also inevitably occurs during idling and fail-safe lubrication. In this regime, however, the flow of the lubricant and the related friction coefficients are yet to be fully understood. This research aims to make fundamental contributions to the understanding of contact mechanics of partially lubricated contacts. Recent experiments with a pin-on-disk tribometer examined the microscopic behavior of partially filled gaps. Using a new experimental setup on a macroscale, new insights into partially filled gaps with rough surfaces were gained. This work presents the systematic analyses of the lubricant flow, friction coefficients, and other variables over a wide range of friction parameters. Distinct friction behaviors were observed, and similar effects occur on both the micro and macroscale. The experimental results show that a typical Stribeck characteristic is visible regarding not only the relative velocity, but also regarding the lubricant filling level in the gap. The fluid exhibits a variety of flow patterns for various velocities and viscosities. The patterns relate to different friction regimes, such as dry friction and mixed lubrication. It is concluded that the filling level is a valid parameter for regulating the transition from dry friction to hydrodynamic lubrication. These findings are quantified regarding the filling level and it is shown that for the identification of the friction regimes the filling level is an independent parameter in addition to the established parameters like speed, viscosity and pressure. Full article
(This article belongs to the Special Issue Dynamic Morphology of Tribological Interfaces in Theory, Simulation and Experiment)
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15 pages, 1212 KiB  
Article
Influence of Copper/Graphite Properties on the Tribological and Electrical Behavior of Copper-Graphite Third Body Layer
by Eric Chapteuil, Mathieu Renouf, Chaoqun Zeng and Yves Berthier
Lubricants 2018, 6(4), 109; https://doi.org/10.3390/lubricants6040109 - 14 Dec 2018
Cited by 7 | Viewed by 3305
Abstract
The understanding of rail braking is irrevocably dependent on the tribological analysis of contacts such as the wheel/rail contact or the wheel/brake disc contact. Because it is very complex to experimentally analyze the inside of a contact, a numerical approach based on discrete [...] Read more.
The understanding of rail braking is irrevocably dependent on the tribological analysis of contacts such as the wheel/rail contact or the wheel/brake disc contact. Because it is very complex to experimentally analyze the inside of a contact, a numerical approach based on discrete element modeling was used to model a third body composed of copper and graphite, the main elements present in sintered brake materials. Simulations were analyzed by measuring several global quantities as a function of the proportion of copper and the local properties of the material to determine the extent to which local parameters influence the electrical and tribological properties of the third body. Among the results noted was the fact that a certain proportion of mixture makes it possible to achieve a balance between electrical and tribological properties. Full article
(This article belongs to the Special Issue Dynamic Morphology of Tribological Interfaces in Theory, Simulation and Experiment)
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15 pages, 2921 KiB  
Article
Lubrication Regime Classification of Hydrodynamic Journal Bearings by Machine Learning Using Torque Data
by Jakob Moder, Philipp Bergmann and Florian Grün
Lubricants 2018, 6(4), 108; https://doi.org/10.3390/lubricants6040108 - 14 Dec 2018
Cited by 28 | Viewed by 5812
Abstract
Hydrodynamic journal bearings are used within a wide range of machines, such as combustion engines, gas turbines, or wind turbines. For a safe operation, awareness of the lubrication regime, in which the bearing is currently operating, is of great importance. In the current [...] Read more.
Hydrodynamic journal bearings are used within a wide range of machines, such as combustion engines, gas turbines, or wind turbines. For a safe operation, awareness of the lubrication regime, in which the bearing is currently operating, is of great importance. In the current study, highspeed data signals of a torque sensor, sampled with a frequency of 1000 hz in a time range of 2.5 s, obtained on a journal bearing test-rig under various operating conditions, are used to train machine learning models, such as neural networks and logistic regression. Results indicate that a fast Fourier transform (fft) of the highspeed torque signals enables accurate predictions of lubrication regimes. The trained models are analysed in order to identify distinctive frequencies for the respective lubrication regime. Full article
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17 pages, 6865 KiB  
Article
Experimental Evaluation of Brake Pad Material Propensity to Stick-Slip and Groan Noise Emission
by Alessandro Lazzari, Davide Tonazzi, Giovanni Conidi, Cristian Malmassari, Andrea Cerutti and Francesco Massi
Lubricants 2018, 6(4), 107; https://doi.org/10.3390/lubricants6040107 - 11 Dec 2018
Cited by 21 | Viewed by 5144
Abstract
Frictional and dynamic responses of brake pad materials, when sliding on brake disc counterfaces, are at the origin of noise, vibration and harshness (NVH) issues such as brake noise emissions. In more detail, groan is a low frequency noise emission often associated to [...] Read more.
Frictional and dynamic responses of brake pad materials, when sliding on brake disc counterfaces, are at the origin of noise, vibration and harshness (NVH) issues such as brake noise emissions. In more detail, groan is a low frequency noise emission often associated to the stick-slip frictional response of the brake system. The instability of such contact is the result of the coupling between the system dynamics and the frictional response of the materials in contact. In this work, an experimental approach is proposed for measuring the frictional response and the propensity to generate stick-slip of different lining materials, coming from commercial brake pads, when sliding on a worn surface of a brake disc, under the same controlled boundary conditions. The proposed methodology allowed for comparing the propensity of the tested pad materials to stick-slip vibrations, which is in agreement with feedback from automotive industry on groan emission. Full article
(This article belongs to the Special Issue Dynamic Morphology of Tribological Interfaces in Theory, Simulation and Experiment)
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12 pages, 2554 KiB  
Article
The Effect of Agglomeration Reduction on the Tribological Behavior of WS2 and MoS2 Nanoparticle Additives in the Boundary Lubrication Regime
by Yosef Jazaa, Tian Lan, Sonal Padalkar and Sriram Sundararajan
Lubricants 2018, 6(4), 106; https://doi.org/10.3390/lubricants6040106 - 10 Dec 2018
Cited by 16 | Viewed by 4289
Abstract
This study investigates the impact of different surfactants and dispersion techniques on the friction and wear behavior of WS2 and MoS2 nanoparticles additives in a Polyalphaolefin (PAO) base oil under boundary lubrication conditions. The nanoparticles were dispersed using Oleic acid (OA) [...] Read more.
This study investigates the impact of different surfactants and dispersion techniques on the friction and wear behavior of WS2 and MoS2 nanoparticles additives in a Polyalphaolefin (PAO) base oil under boundary lubrication conditions. The nanoparticles were dispersed using Oleic acid (OA) and Polyvinylpyrrolidone (PVP) to investigate their impact on particle agglomeration. The size distribution of the dispersed nanoparticles in PAO was measured by dynamic light scattering. The nanoparticles treated using PVP resulted in the most stable particle size. Friction studies showed that nanoparticle agglomeration reduction and the homogeneity of the suspension did not significantly impact the friction reduction behavior of the lubricant. Reciprocating wear experiments showed that, for our test conditions, both WS2 and MoS2 nano additives exhibited maximum wear depth reduction (45%) when using the PVP surface treatment compared to base oil. The wear results confirmed the significance of minimizing agglomeration and promoting high dispersion in promoting favorable wear resistance under boundary lubricant conditions. Analysis of the wear surfaces showed that a tribofilm formation was the primary wear reduction mechanism for WS2 particles treated by PVP while, in the case of MoS2 treated by PVP, the mechanism was load sharing via particles rolling and/or sliding at the interface. Full article
(This article belongs to the Special Issue Advanced Lubrication for Energy Efficiency)
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16 pages, 7965 KiB  
Article
Theoretical and Experimental Investigations on Transient Run-Up Procedures of Journal Bearings Including Mixed Friction Conditions
by Maximilian Prölß, Hubert Schwarze, Thomas Hagemann, Philipp Zemella and Philipp Winking
Lubricants 2018, 6(4), 105; https://doi.org/10.3390/lubricants6040105 - 1 Dec 2018
Cited by 13 | Viewed by 5899
Abstract
This paper focuses on the operating behavior of journal bearings for industrial machinery application during run-ups. For this purpose, a numerical simulation code that is based on a two-dimensional extended and generalized Reynolds equation and a full three-dimensional energy equation, was advanced by [...] Read more.
This paper focuses on the operating behavior of journal bearings for industrial machinery application during run-ups. For this purpose, a numerical simulation code that is based on a two-dimensional extended and generalized Reynolds equation and a full three-dimensional energy equation, was advanced by a theoretical model considering the effects of mixed friction and warming of journal components during start-up. The mixed friction routine contained the elastic half-spaces model proposed by Boussinesq, which considers the influence of rough surfaces by implementing flow factors and calculates additional stiffness and dissipation in areas with solid interactions. Furthermore, a transient term was added in the energy equation to consider the thermal inertia of journal, and bearing to ensure a realistic heating during run-ups. Results of the prediction were compared to experimental data taken from a special test rig built up for validation procedures. Besides the conventional sensors for temperature, oil flow, and relative motion between shaft and stator, a contact voltage measurement was installed to determine the intensity of mixed friction. The evaluation of experimental data by Stribeck curves, based on a shaft torsion measurement, indicated a significant influence of run-up time on frictional moment. The friction coefficient of the rotor bearing system was strongly influenced by the run-up time. A short run-up time reduced the frictional coefficient in the mixed lubrication regime while the opposite behavior was observed in the hydrodynamic lubrication regime. The numerical code predicted these tendencies in good agreement with experimental data, however, only if the transient energy model was applied. Full article
(This article belongs to the Special Issue Dynamic Morphology of Tribological Interfaces in Theory, Simulation and Experiment)
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12 pages, 2563 KiB  
Article
Mechanisms Involved in the Formation of Secondary Structures on the Friction Surface of Experimental Aluminum Alloys for Monometallic Journal Bearings
by Pavel Podrabinnik, Iosif Gershman, Alexander Mironov, Ekaterina Kuznetsova and Pavel Peretyagin
Lubricants 2018, 6(4), 104; https://doi.org/10.3390/lubricants6040104 - 28 Nov 2018
Cited by 18 | Viewed by 3600
Abstract
The processes taking place on the friction surface of high-alloyed aluminum alloys working with steel whilst replacing bronze journal bearings with aluminum are investigated. In this regard, eight experimental aluminum alloys with an Sn content from 5.4% to 11.0%, which also included Pb, [...] Read more.
The processes taking place on the friction surface of high-alloyed aluminum alloys working with steel whilst replacing bronze journal bearings with aluminum are investigated. In this regard, eight experimental aluminum alloys with an Sn content from 5.4% to 11.0%, which also included Pb, Zn, Si, Mg, and Cu, were cast. The surface and subsurface layer of experimental aluminum bearings were studied before and after tribological tests with a 38HN3MA steel counterbody by scanning electron microscopy including energy-dispersive analysis. The best aluminum alloy, which had an Sn content of 5.8% after the friction tests, showed 6.5-times better wear resistance and steel counterbody wear rate than the bronze reference. Both structural and compositional changes in the surface layer were observed. It was revealed that secondary structures formed on the surface during the friction process and included all of the chemical elements in the tribosystem, which is a consequence of its self-organization. Generally, the secondary structures are thin metal-polymer films generated as a result of the high carbon and oxygen content. The interaction behavior of some of the chemical elements in the tribosystem is shown and discussed. In addition, the influence that Sn, Pb, Cu, and C content in the secondary structures has on the tribological properties of low-tin and medium-tin alloys is shown. Full article
(This article belongs to the Special Issue Tribology for Mechanical Engineering--2018)
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20 pages, 18826 KiB  
Article
Effect of Hygrothermal Ageing on Tribological Behaviour of PTFE-Based Composites
by Mohammad-Reza Homayoun, Arash Golchin and Nazanin Emami
Lubricants 2018, 6(4), 103; https://doi.org/10.3390/lubricants6040103 - 26 Nov 2018
Cited by 8 | Viewed by 4530
Abstract
The present study investigates the influence of hygrothermal ageing on the tribological behaviour of polytetrafluoroethylene (PTFE) polymer composites. Three PTFE composites along with unfilled PTFE were tested in sliding contact against Inconel 625 (a Ni-based alloy) plates in both dry and water-lubricated conditions, [...] Read more.
The present study investigates the influence of hygrothermal ageing on the tribological behaviour of polytetrafluoroethylene (PTFE) polymer composites. Three PTFE composites along with unfilled PTFE were tested in sliding contact against Inconel 625 (a Ni-based alloy) plates in both dry and water-lubricated conditions, utilising a unidirectional pin-on-disc tribometer. The tribo-tests were performed at a constant sliding speed of 0.13 m/s with a normal load of 84N providing an apparent contact pressure of 5 MPa. Hygrothermal conditioning was carried out at two different temperatures, and the water absorption evolution and kinetic parameters were estimated. Various characterisation methods were used to identify the wear mechanisms and influence of hygrothermal ageing on the degradation of the filler/matrix. The different tribological behaviour for different PTFE composites was observed within the ageing timeframe. The wear resistance of the fibre-filled samples was reduced compared to the non-aged ones over the ageing timeframe. However, the friction and wear resistance of the bronze-filled PTFE were enhanced by hygrothermal ageing. Full article
(This article belongs to the Special Issue Tribology of High-Performance Polymer Composites in Extreme Conditions)
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13 pages, 3101 KiB  
Article
Reduction of CO2 Emissions and Cost Analysis of Ultra-Low Viscosity Engine Oil
by Keita Ishizaki and Masaru Nakano
Lubricants 2018, 6(4), 102; https://doi.org/10.3390/lubricants6040102 - 18 Nov 2018
Cited by 13 | Viewed by 9143
Abstract
This study is focused on the reduction of CO2 emissions and costs associated with ultra-low viscosity (ULV) engine oils for passenger vehicles. Specifically, the reduction in life cycle CO2 (LCCO2) emissions from lower-viscosity engine oil and the oil drain [...] Read more.
This study is focused on the reduction of CO2 emissions and costs associated with ultra-low viscosity (ULV) engine oils for passenger vehicles. Specifically, the reduction in life cycle CO2 (LCCO2) emissions from lower-viscosity engine oil and the oil drain interval (ODI) extension were estimated taking into account both mineral engine oil and synthetic engine oil. Furthermore, the cost-effectiveness of ULV engine oils were investigated by performing base-stock cost analysis. When the volatility limit of the Noack test (American Society for testing and materials (ASTM) D5800) was set to 15 wt %, the results indicated that the lower limit of kinematic viscosity at 100 °C (KV100) for mineral engine oil (with Group-III base-stock) and synthetic engine oil (with polyalphaolefin (PAO) base-stock) were approximately 5.3 and 4.5 mm2/s, respectively. Compared to conventional 0W-16 mineral engine oil (KV100 6.2 mm2/s), the effect of reducing LCCO2 emissions on ULV mineral engine oil (ULV-Mineral, KV100 5.3 mm2/s) was estimated at 0.6%, considering 1.5–1.8 L gasoline engines in New European Driving Cycles (NEDC) mode. ULV-Mineral, which continues to use a mineral base-stock, is considered highly cost-effective since its cost is similar to the conventional 0W-16 mineral engine oil. On the other hand, compared with ULV-Mineral, the vehicle fuel efficiency improvement from the use of ULV synthetic engine oil (ULV-PAO, KV100 4.5 mm2/s) was estimated to be 0.5%. However, considering CO2 emissions during engine oil production, the reduction of LCCO2 emission from ULV-PAO compared with ULV-Mineral was estimated to be only 0.1% or less using 2030 standards (assuming a vehicle fuel efficiency of 66.5 g-CO2/km) when ODI is set equivalent (7500 km) to mineral engine oil. As a result, ULV-PAO’s cost-effectiveness, considering the cost increase of PAO base-stock, was found to be nominal. Contrariwise, when the characteristics of PAO base-stock with higher oxidation stability are used comparatively with the mineral base-stock while extending the ODI to 15,000 km, the effect of reducing LCCO2 emissions of ULV-PAO was estimated to be 0.7% in 2030, making ULV-PAO a competitive and cost-effective alternative. In other words, the popularization of synthetic engine oil toward 2030 will require the consideration of both viscosity reduction and ODI extension. Full article
(This article belongs to the Special Issue Automotive Tribology)
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12 pages, 2337 KiB  
Article
On the Temperature and Lubricant Film Thickness Distribution in EHL Contacts with Arbitrary Entrainment
by Milan Omasta, Jakub Adam, Petr Sperka, Ivan Krupka and Martin Hartl
Lubricants 2018, 6(4), 101; https://doi.org/10.3390/lubricants6040101 - 15 Nov 2018
Cited by 10 | Viewed by 5488
Abstract
An understanding of mechanisms which are responsible for elastohydrodynamic lubricant film formation under high sliding conditions is necessary to increase durability of machine parts. This work combines thin-film colorimetric interferometry for lubricant film thickness measurement and infrared microscopy for in-depth temperature mapping through [...] Read more.
An understanding of mechanisms which are responsible for elastohydrodynamic lubricant film formation under high sliding conditions is necessary to increase durability of machine parts. This work combines thin-film colorimetric interferometry for lubricant film thickness measurement and infrared microscopy for in-depth temperature mapping through the contact. The results describe the effect of operating conditions such as speed, slide-to-roll ratio, ambient temperature, and sliding direction on lubricant film thickness and temperature distribution. Film thickness data shows how much the film shape is sensitive to operating conditions when thermal effects are significant, while the temperature profiles provides an explanation of this behavior. Full article
(This article belongs to the Special Issue Fluid-Film Lubrication)
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16 pages, 2676 KiB  
Article
Effect of Shaft Surface Roughness on the Performance of Radial Lip Seals
by Emilia Kozuch, Petros Nomikos, Ramin Rahmani, Nick Morris and Homer Rahnejat
Lubricants 2018, 6(4), 99; https://doi.org/10.3390/lubricants6040099 - 13 Nov 2018
Cited by 15 | Viewed by 5987
Abstract
Reduction of leakage from the shaft–radial lip seal conjunction is critical in ensuring enduring performance of entire lubrication system. This paper investigates leakage from three types of shaft surfaces, finished using different manufacturing processes. The measurement of surface topography is conducted in order [...] Read more.
Reduction of leakage from the shaft–radial lip seal conjunction is critical in ensuring enduring performance of entire lubrication system. This paper investigates leakage from three types of shaft surfaces, finished using different manufacturing processes. The measurement of surface topography is conducted in order to determine the pertinent roughness parameters which correspond to the observed sealing performance in real practical applications. It is found that the skewness of the surface topography correlates well with the anecdotal leakage failure rate. To quantify this association, a hydrodynamic model, accounting for shaft roughness in a deterministic manner is developed. The results from the numerical analyses confirm that the lubricant mass flow rate is reduced in the case of negatively skewed surface height distributions, when compared with the positively skewed profiles. Full article
(This article belongs to the Special Issue Multiphysics and Multiscale Models of Tribology)
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17 pages, 5928 KiB  
Article
Rheological and Wetting Properties of Environmentally Acceptable Lubricants (EALs) for Application in Stern Tube Seals
by F. Xavier Borras, Matthijn B. De Rooij and Dik J. Schipper
Lubricants 2018, 6(4), 100; https://doi.org/10.3390/lubricants6040100 - 13 Nov 2018
Cited by 11 | Viewed by 7352
Abstract
The use of Environmentally Acceptable Lubricants (EALs) for stern tube lubrication is increasing. Although the machine components of a sailing vessel are designed to operate together with mineral oil-based lubricants, these are being replaced by the less environmentally harmful EALs. Little is known [...] Read more.
The use of Environmentally Acceptable Lubricants (EALs) for stern tube lubrication is increasing. Although the machine components of a sailing vessel are designed to operate together with mineral oil-based lubricants, these are being replaced by the less environmentally harmful EALs. Little is known about the rheological performance of EALs in particular at the high shear rates that occur in stern tube seals. In this study, the viscosity and wetting properties of a set of different EALs is analysed and compared to traditional mineral oil-based lubricants using a set of experimental techniques. Some of the EALs present Newtonian behavior whereas other show shear thinning. No significant difference in surface tension was observed between the different lubricants. Full article
(This article belongs to the Special Issue Fluid-Film Lubrication)
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12 pages, 4390 KiB  
Article
Performance of Volcano-Like Laser Textured Cutting Tools: An Experimental and Simulative Investigation
by Zhengyang Kang, Yonghong Fu, Xingyu Fu and Martin Byung-Guk Jun
Lubricants 2018, 6(4), 98; https://doi.org/10.3390/lubricants6040098 - 7 Nov 2018
Cited by 7 | Viewed by 4172
Abstract
In recent years, surface texturing in micro-scale has been attempted on the surface of cutting tools for multiple purposes, e.g., cutting force reduction, prolonging life-span, anti-adhesion, etc. With respect to machinability and performance, micro-groove texture (MGT) has dominated in this field compared to [...] Read more.
In recent years, surface texturing in micro-scale has been attempted on the surface of cutting tools for multiple purposes, e.g., cutting force reduction, prolonging life-span, anti-adhesion, etc. With respect to machinability and performance, micro-groove texture (MGT) has dominated in this field compared to other textured patterns. In this study, a novel volcano-like texture (VLT) was fabricated on the rake face of cemented carbide inserts (WC-Co, YG6) by fiber laser. The following cutting experiment tested the flat, MGT and VLT tools in turning aluminum alloy 6061. The effects of coolant and cutting conditions were investigated. In addition, a validated FEM model was employed to explore the distribution of stress and temperature fields in the tool-chip interface. The initial forming process of adhesion layer on rake face was investigated as well. The results indicated that lower cutting force and less adhesion can be achieved by small scale VLT. This study not only introduced VLT on cutting tools but also revealed its comprehensive performance. Full article
(This article belongs to the Special Issue Tribological Performance of Textured Surfaces)
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15 pages, 1094 KiB  
Article
Thermal Turbulent Flow in Leading Edge Grooved and Conventional Tilting Pad Journal Bearing Segments—A Comparative Study
by Philip Croné, Andreas Almqvist and Roland Larsson
Lubricants 2018, 6(4), 97; https://doi.org/10.3390/lubricants6040097 - 2 Nov 2018
Cited by 9 | Viewed by 4194
Abstract
A comparative study between a conventional- and leading edge grooved (LEG) tilting pad journal bearing (TPJB) segment is performed. The developed model uses the Shear Stress Transport (SST) turbulence model, coupled with the energy equation and a partial differential equation for the fluid [...] Read more.
A comparative study between a conventional- and leading edge grooved (LEG) tilting pad journal bearing (TPJB) segment is performed. The developed model uses the Shear Stress Transport (SST) turbulence model, coupled with the energy equation and a partial differential equation for the fluid domain mesh displacement to predict the thermal flow characteristics. Instead of using an effective boundary condition to determine the inlet temperature of the LEG pad and excluding the additional LEG portion, as is common practice, the whole geometry of the LEG is modeled. Several sizes of the LEG portion is investigated and it is shown to have quite a small influence on pressure, temperature, film thickness and turbulence intensity. Moreover, the results also show that the conventional pad gives rise to higher levels of turbulence in the mid plane compared to its LEG counterpart, while the latter has a marginally higher value of turbulence when the volume average value is computed. The maximum value of turbulence is however present in the conventional model. Full article
(This article belongs to the Special Issue Multiphysics and Multiscale Models of Tribology)
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18 pages, 10206 KiB  
Article
On an Elastoplastic Sliding Model for a Coated Single Asperity
by Can Wang and Dik J. Schipper
Lubricants 2018, 6(4), 96; https://doi.org/10.3390/lubricants6040096 - 1 Nov 2018
Cited by 7 | Viewed by 3506
Abstract
In this study, a sliding friction model for coated single asperity contacts is proposed. A displacement-driven layered contact algorithm is firstly introduced and verified by the finite element method. Then, this algorithm is applied to simulate the contact between two semispherical asperities. The [...] Read more.
In this study, a sliding friction model for coated single asperity contacts is proposed. A displacement-driven layered contact algorithm is firstly introduced and verified by the finite element method. Then, this algorithm is applied to simulate the contact between two semispherical asperities. The full sliding contact process is discretized into a series of transient steps, and each of these steps are calculated by the displacement-driven contact algorithm. The effects of the interference depth and the properties of, respectively, the tribofilm (thickness, elastic modulus, and yield strength) and the nanocrystalline layer on the sliding coefficient of friction are investigated. The results suggest that when surface adhesion and asperity damage are ignored, the plastic deformation of the tribofilm is the main source of the sliding friction. Greater interference depth, tribofilm with greater thickness, higher elastic modulus or lower yield strength, and the presence of a nanocrystalline layer will lead to a higher coefficient of friction in single asperity sliding. Full article
(This article belongs to the Special Issue Multiphysics and Multiscale Models of Tribology)
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23 pages, 13581 KiB  
Article
Accuracy and Grid Convergence of the Numerical Solution of the Energy Equation in Fluid Film Lubrication: Application to the 1D Slider
by Silun Zhang, Mohamed-Amine Hassini and Mihai Arghir
Lubricants 2018, 6(4), 95; https://doi.org/10.3390/lubricants6040095 - 26 Oct 2018
Cited by 5 | Viewed by 3821
Abstract
The present work is focused on the numerical solution of the complete energy equation used in fluid film lubrication. The work was motivated by the fact that the complete energy equation has no analytical solution that can be used for validations. Its accuracy [...] Read more.
The present work is focused on the numerical solution of the complete energy equation used in fluid film lubrication. The work was motivated by the fact that the complete energy equation has no analytical solution that can be used for validations. Its accuracy and computation time are related to the employed numerical method and to the grid resolution. The natural discretization method (NDM) applied on different grids is systematically compared with the spectral method (the Lobatto Point Colocation Method or LPCM) with different polynomial degrees. A one dimensional inclined slider is used for the numerical tests, and the energy equation is artificially decoupled from the Reynolds equation. This approach enables us to focus all the attention on the numerical solution of the energy equation. The results show that the LPCM is one or two orders of magnitude more efficient than the NDM in terms of computation time. The energy equation is then coupled with the Reynolds equation in a thermo-hydrodynamic analysis of the same 1D slider; the numerical results confirm again the efficiency of the LPCM. A thermo-hydrodynamic analysis of a two-lobe journal bearing is then presented as a practical application. Full article
(This article belongs to the Special Issue Fluid-Film Lubrication)
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15 pages, 3707 KiB  
Article
In-Situ Synthesis and Characterization of Biodegradable Estolides via Epoxidation from Canola Biodiesel
by Venu Babu Borugadda and Ajay K. Dalai
Lubricants 2018, 6(4), 94; https://doi.org/10.3390/lubricants6040094 - 25 Oct 2018
Cited by 9 | Viewed by 4484
Abstract
Research on the formulation of estolides from plant seed oils has attracted substantial attention due to their favorable low-temperature properties and environmentally friendly nature. The present research investigates the formulation of canola biodiesel derived estolides for low-temperature applications. The dual-step research method includes [...] Read more.
Research on the formulation of estolides from plant seed oils has attracted substantial attention due to their favorable low-temperature properties and environmentally friendly nature. The present research investigates the formulation of canola biodiesel derived estolides for low-temperature applications. The dual-step research method includes ring opening of epoxidized canola biodiesel in the presence of oleic acid, followed by esterification with oleic acid to produce estolides using a mesoporous aluminosilicates possessing Modernite Framework Inverted (MFI) type pentasil structure as a heterogeneous acidic catalyst. Prepared catalyst was characterized to measure the properties essential for the effective catalysis. The catalyst demonstrated promising activity for the estolides formation, >95% conversion was achieved at 110 °C for 6 h using 15 wt % of catalyst loading. 1H NMR technique and oxirane oxygen titrimetric analysis were employed to determine product purity. Physicochemical properties of the reaction products were determined by standard methods and characterization results revealed that the formulated estolides had improved low-temperature, lubricity and rheological properties, and thermo-oxidative stability. Also, biodegradability of the estolides was found to be 92% within 28 days as per the bio-kinetic model. Wear scar diameter of 106 µm was noticed for 10% of alkoxide blend with standard diesel fuel. Overall, outcomes of the physicochemical characterization data indicated that the prepared estolides can act as possible alternative bio-lubricant basestock for various low-temperature applications. Full article
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12 pages, 13278 KiB  
Article
Combating the Tribo-Corrosion of LDX2404 Lean Duplex Stainless Steel by Low Temperature Plasma Nitriding
by Xiaoying Li, Wenbo Dou, Linhai Tian and Hanshan Dong
Lubricants 2018, 6(4), 93; https://doi.org/10.3390/lubricants6040093 - 19 Oct 2018
Cited by 14 | Viewed by 4031
Abstract
A lean duplex stainless steel, LDX2404, was DC plasma nitrided under a range of treatment conditions. The microstructure characterisation evaluation of the treated samples revealed that a dense, super-hard surface layer can be produced by low-temperature (<450 °C) plasma treatments. The original austenite [...] Read more.
A lean duplex stainless steel, LDX2404, was DC plasma nitrided under a range of treatment conditions. The microstructure characterisation evaluation of the treated samples revealed that a dense, super-hard surface layer can be produced by low-temperature (<450 °C) plasma treatments. The original austenite phase became S-phase and the ferrite phase was supersaturated with nitrogen and ε-Fe3N nitride precipitated from it. When plasma nitriding was carried out at above 450 °C, chromium nitrides precipitated in the surface nitrided layer. Compared to the untreated samples, the surface hardness of the lean duplex stainless steel (DSS) is increased up to four times. The dry wear resistance increased when increasing the treatment temperature. In contrast, the low-temperature treated samples showed the best performance in the electrochemical corrosion and corrosion-wear tests; the performance of the high temperature (>450 °C) plasma nitrided samples was found to be significantly worse than that of the untreated material. Full article
(This article belongs to the Special Issue Tribocorrosion of Surface Engineered Materials)
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14 pages, 4370 KiB  
Article
The Effect of Surface Texturing on Dry Gross Fretting
by Agnieszka Lenart, Pawel Pawlus, Slawomir Wos and Andrzej Dzierwa
Lubricants 2018, 6(4), 92; https://doi.org/10.3390/lubricants6040092 - 16 Oct 2018
Cited by 7 | Viewed by 3197
Abstract
The effect of steel disc surface texturing on dry gross fretting in a ball-on-disc configuration was studied. Dimples were created with abrasive jet machining. The tribological performance of sliding pairs, steel–steel and steel–ceramics, was experimentally studied. The character of surface texturing effect was [...] Read more.
The effect of steel disc surface texturing on dry gross fretting in a ball-on-disc configuration was studied. Dimples were created with abrasive jet machining. The tribological performance of sliding pairs, steel–steel and steel–ceramics, was experimentally studied. The character of surface texturing effect was related to the dominant wear type. During steel–steel contact, the presence of dimples on disc surfaces could lead to increases in wear and friction. However, the escape of wear debris into dimples could result in reductions of friction and wear in the steel–ceramics configuration. Full article
(This article belongs to the Special Issue Tribological Performance of Textured Surfaces)
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18 pages, 4966 KiB  
Article
The Influence of Surface Texturing on the Frictional Behaviour of Parallel Sliding Lubricated Surfaces under Conditions of Mixed Lubrication
by Dariush Bijani, Elena L. Deladi, Aydar Akchurin, Matthijn B. De Rooij and Dirk J. Schipper
Lubricants 2018, 6(4), 91; https://doi.org/10.3390/lubricants6040091 - 11 Oct 2018
Cited by 21 | Viewed by 3853
Abstract
In many industrial applications, a modification of the surface geometry can enhance the tribological behaviour of lubricated sliding contacts. In this paper, the effect of surface texturing on the coefficient of friction in parallel sliding lubricated surfaces is investigated. It is shown that [...] Read more.
In many industrial applications, a modification of the surface geometry can enhance the tribological behaviour of lubricated sliding contacts. In this paper, the effect of surface texturing on the coefficient of friction in parallel sliding lubricated surfaces is investigated. It is shown that surface texturing can improve film formation and, as a result, the load-carrying capacity as well as a reduction in the coefficient of friction. With the numerical model developed, and by considering cavitation, the effects of shape, depth, size, and the textured area fraction on the frictional behaviour of parallel sliding lubricated contacts under conditions of mixed lubrication is studied. In this article it is shown that the surface texturing can have a beneficial effect, in order to decrease friction. Full article
(This article belongs to the Special Issue Fluid-Film Lubrication)
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19 pages, 4027 KiB  
Article
Lubrication Performance of Engine Commercial Oils with Different Performance Levels: The Effect of Engine Synthetic Oil Aging on Piston Ring Tribology under Real Engine Conditions
by Pantelis G. Nikolakopoulos, Stamatis Mavroudis and Anastasios Zavos
Lubricants 2018, 6(4), 90; https://doi.org/10.3390/lubricants6040090 - 9 Oct 2018
Cited by 27 | Viewed by 10166
Abstract
To further improve efficiency in automotive engine systems, it is important to understand the generation of friction in its components. Accurate simulation and modeling of friction in machine components is, amongst other things, dependent on realistic lubricant rheology and lubricant properties, where especially [...] Read more.
To further improve efficiency in automotive engine systems, it is important to understand the generation of friction in its components. Accurate simulation and modeling of friction in machine components is, amongst other things, dependent on realistic lubricant rheology and lubricant properties, where especially the latter may change as the machine ages. Some results of research under laboratory conditions on the aging of engine commercial oils with different performance levels (mineral SAE 30, synthetic SAE10W-40, and bio-based) are presented in this paper. The key role of the action of pressure and temperature in engine oils’ aging is described. The paper includes the results of experiments over time in laboratory testing of a single cylinder motorbike. The aging of engine oil causes changes to its dynamic viscosity value. The aim of this work is to evaluate changes due to temperature and pressure in viscosity of engine oil over its lifetime and to perform uncertainty analysis of the measured values. The results are presented as the characteristics of viscosity and time in various temperatures and the shear rates/pressures. This paper also includes a Computational Fluid Dynamics (CFD) model, applying the experimental results in the piston ring tribology problem. Full article
(This article belongs to the Special Issue Tribology of Machine Elements--Smart Lubricants)
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7 pages, 1236 KiB  
Communication
Tribological Properties of Double-Network Gels Substituted by Ionic Liquids
by Hiroyuki Arafune, Fumiya Muto, Toshio Kamijo, Saika Honma, Takashi Morinaga and Takaya Sato
Lubricants 2018, 6(4), 89; https://doi.org/10.3390/lubricants6040089 - 8 Oct 2018
Cited by 2 | Viewed by 3783
Abstract
Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and [...] Read more.
Since human body joints have a gel-like structure with low friction that persists for several decades, hydrogels have attracted much interest for developing low-friction materials. However, such advantages can hardly be realized in industrial usage because water in the gel evaporates easily and the gel deswells. The substitution of water with an ionic liquid (IL) is one of the effective ways to overcome this problem. In this study, we substituted water in a double network (DN) hydrogel with 3-ethyl-1-methyl-imidazolium ethylsulfate (EMI-EtSulf), a hydrophilic IL, via a simple solvent exchange method to obtain a DN ion gel. A compressive test and thermogravimetric analysis showed that the DN ion gel has a high compression fracture stress and improved thermal properties, with the difference in 10% loss of temperature being ΔT10 = 234 °C. A friction test conducted using a reciprocating tribometer showed that the friction of a glass ball/DN ion gel was relatively higher than that of a glass ball/DN hydrogel. Because the minimum coefficient of friction (COF) value increased after substitution, the increase in polymer adhesion caused by the electrostatic shielding of the surface moieties of glass and poly 2-acrylamidomethylpropanesulfonic acid (PAMPS) was considered the main contributor to the high friction. As the COF value decreased with increasing temperature, the DN ion gel can achieve low friction via the restriction of polymer adhesion at high temperatures, which is difficult in the DN hydrogel owing to drying. Full article
(This article belongs to the Special Issue Ionic Liquids: Friction and Lubrication Mechanisms)
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15 pages, 2174 KiB  
Article
Combined Analytical and Experimental Evaluation of Frictional Performance of Lubricated Untextured and Partially Textured Sliders
by Wei J. Teo, Nader Dolatabadi, Ramin Rahmani, Nick Morris and Homer Rahnejat
Lubricants 2018, 6(4), 88; https://doi.org/10.3390/lubricants6040088 - 1 Oct 2018
Cited by 8 | Viewed by 3391
Abstract
The study of textured surface performance is one of the highly researched topics in recent times. This is mainly due to the advantages that such surfaces can potentially provide in practice, in mitigating adverse tribological conditions, such as friction and wear. However, considering [...] Read more.
The study of textured surface performance is one of the highly researched topics in recent times. This is mainly due to the advantages that such surfaces can potentially provide in practice, in mitigating adverse tribological conditions, such as friction and wear. However, considering the complexities found in practice, a methodological analysis and evaluation procedure is essential in order to gain an understanding of the benefits from utilising such features in a given contact. The current study provides a combined analytical and experimental approach towards an enhanced understanding of the behaviour of textured surfaces relative to their untextured counterparts. The developed analytical models are invaluable in providing an insight into the relationship between the many parameters involved in defining even simple surface texture feature geometry and the expected outcomes in practice, when corroborated with experimental results. The current study reports on such an endeavour. With the studied texture configuration, the results have shown the possibility of reducing friction by as much as 25%. Full article
(This article belongs to the Special Issue Tribological Performance of Textured Surfaces)
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18 pages, 1382 KiB  
Article
An Enhanced Stochastic Two-Scale Model for Metal-to-Metal Seals
by Francesc Pérez-Ràfols and Andreas Almqvist
Lubricants 2018, 6(4), 87; https://doi.org/10.3390/lubricants6040087 - 1 Oct 2018
Cited by 4 | Viewed by 3196
Abstract
Leakage in static metal-to-metal seals is predominantly determined by the topography of the contacting surfaces. The topography consists of features that span the entire range from its carefully engineered geometry down to micro-sized surface asperities. The mesh density necessary to fully resolve all [...] Read more.
Leakage in static metal-to-metal seals is predominantly determined by the topography of the contacting surfaces. The topography consists of features that span the entire range from its carefully engineered geometry down to micro-sized surface asperities. The mesh density necessary to fully resolve all the features, in this large span of length scales, generates too many degrees of freedom for a direct numerical approach to be applicable. Some kind of sophistication, either incorporated in the mathematical model or in the numerical solution procedure or even a combination of both is therefore required. For instance, in a two-scale model, the geometrical features can be addressed in the global-scale model, while the features belonging to length scales smaller than a given cut-off value are addressed in the local-scale model. However, the classical two-scale approaches do not explicitly address the stochastic nature of the surfaces, and this has turned out to be a requirement in order to obtain quantitative predictions of leakage in metal-to-metal seals. In this work, we present a continued development of an already existing two-scale model, which incorporates a stochastic element. The novelty lies in the way we characterise the permeability at the local scale and how this is used to build a more efficient and useful approach. Full article
(This article belongs to the Special Issue Multiphysics and Multiscale Models of Tribology)
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13 pages, 4325 KiB  
Article
Tribocorrosion Response of Surface-Modified Ti in a 0.9% NaCl Solution
by Richard Bailey
Lubricants 2018, 6(4), 86; https://doi.org/10.3390/lubricants6040086 - 25 Sep 2018
Cited by 7 | Viewed by 3399
Abstract
Titanium use is limited due to its poor tribological properties, and thermal oxidation (TO) and pack carburisation with limited oxygen diffusion (PCOD) are just two of the surface treatments that can be used to enhance the surface properties of Ti. In this study, [...] Read more.
Titanium use is limited due to its poor tribological properties, and thermal oxidation (TO) and pack carburisation with limited oxygen diffusion (PCOD) are just two of the surface treatments that can be used to enhance the surface properties of Ti. In this study, commercially pure titanium was surface modified using thermal oxidation (TO) and pack carburisation with limited oxygen diffusion (PCOD). Samples were tribological tested in a 0.9% NaCl solution under a contact load of 20 N to investigate the mechanical and electrochemical response of the surface treatments. The tests conducted show that a clear benefit can be obtained in terms of the overall material loss rate using both TO and PCOD. The TO and PCOD treatments generate very different surface structures: TO produces a rutile TiO2 surface film and the PCOD treatment produces a TiC network structure. Both treatments improve the load bearing capacity with the assistance of an oxygen diffusion zone (ODZ). When subjected to sliding contact in a 0.9% NaCl solution, the results show the PCOD-Ti produced the best overall results, with a material loss rate 7.5 times lower than untreated Ti and 2.4 times lower than TO-Ti. The improved wear rate of the PCOD-Ti is attributed to the TiC network structure. The TO-Ti suffers from rapid film failure and high friction. The reduced material loss rate (MLR) of the TO-Ti is attributed to the hard wearing ODZ. Full article
(This article belongs to the Special Issue Tribocorrosion of Surface Engineered Materials)
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17 pages, 3577 KiB  
Article
Contact-Patch-Size Distribution and Limits of Self-Affinity in Contacts between Randomly Rough Surfaces
by Martin H. Müser and Anle Wang
Lubricants 2018, 6(4), 85; https://doi.org/10.3390/lubricants6040085 - 20 Sep 2018
Cited by 20 | Viewed by 4997
Abstract
True contact between solids with randomly rough surfaces tends to occur at a large number of microscopic contact patches. Thus far, two scaling regimes have been identified for the number density n ( A ) of contact-patch sizes A in elastic, non-adhesive, self-affine [...] Read more.
True contact between solids with randomly rough surfaces tends to occur at a large number of microscopic contact patches. Thus far, two scaling regimes have been identified for the number density n ( A ) of contact-patch sizes A in elastic, non-adhesive, self-affine contacts. At small A, n ( A ) is approximately constant, while n ( A ) decreases as a power law at large A. Using Green’s function molecular dynamics, we identify a characteristic (maximum) contact area A c above which a superexponential decay of n ( A ) becomes apparent if the contact pressure is below the pressure p cp at which contact percolates. We also find that A c increases with load relatively slowly far away from contact percolation. Results for A c can be estimated from the stress autocorrelation function G σ σ ( r ) with the following argument: the radius of characteristic contact patches, r c , cannot be so large that G σ σ ( r c ) is much less than p cp 2 . Our findings provide a possible mechanism for the breakdown of the proportionality between friction and wear with load at large contact pressures and/or for surfaces with a large roll-off wavelength. Full article
(This article belongs to the Special Issue Multiphysics and Multiscale Models of Tribology)
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