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Lubricants, Volume 8, Issue 6 (June 2020) – 6 articles

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Cover Story (view full-size image) The strive for improved efficiency of machine components implies increasingly severe operating [...] Read more.
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Open AccessArticle
Friction and Temperature Behavior of Lubricated Thermoplastic Polymer Contacts
Lubricants 2020, 8(6), 67; https://doi.org/10.3390/lubricants8060067 - 24 Jun 2020
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Abstract
This work focuses on the friction and temperature behavior of thermo-elastohydrodynamically lubricated (TEHL) contacts under rolling-sliding conditions. For this purpose, a twin-disk test rig is used with a hybrid setup of plain and fiber-reinforced polyamide (PA) 66 and polyetheretherketone (PEEK) disks paired with [...] Read more.
This work focuses on the friction and temperature behavior of thermo-elastohydrodynamically lubricated (TEHL) contacts under rolling-sliding conditions. For this purpose, a twin-disk test rig is used with a hybrid setup of plain and fiber-reinforced polyamide (PA) 66 and polyetheretherketone (PEEK) disks paired with case-hardened steel disks and three different lubricants. Experimental investigations include various lubrication regimes by varying sum velocity and oil temperature as well as load and slip ratio. The measured friction in thermoplastic TEHL contacts is particularly very low in the area of high fluid load portion, which refers to the large deformation of the compliant polymer surface. Newtonian flow behavior mainly determines fluid friction. The low thermal effusivity of polymers insulates the contact and can further reduce the effective lubricant viscosity, and thus the fluid friction. For low sum velocities, solid friction influences the tribological behavior depending on the solid load portion. Although the interfacial contact friction is comparably small, material damping strongly contributes to power losses and increases bulk temperature, which in turn affects the TEHL contact. Thus, loading frequency and the resulting bulk temperature are identified as one of the main drivers of power losses and tribological behavior of lubricated thermoplastic polymer contacts. Full article
(This article belongs to the Special Issue Friction Reduction at Interfaces)
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Open AccessArticle
Study on the Quantitative Evaluation of the Surface Force Using a Scanning Probe Microscope
Lubricants 2020, 8(6), 66; https://doi.org/10.3390/lubricants8060066 - 17 Jun 2020
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Abstract
There are two types of friction modifiers (FMs) used as lubricant additives: Reaction film FMs (RF-FMs) and adsorption film FMs (AF-FMs). While RF-FMs provide good performance in severe conditions, AF-FMs excel in mild conditions. This empirical evidence leads us to combine these two [...] Read more.
There are two types of friction modifiers (FMs) used as lubricant additives: Reaction film FMs (RF-FMs) and adsorption film FMs (AF-FMs). While RF-FMs provide good performance in severe conditions, AF-FMs excel in mild conditions. This empirical evidence leads us to combine these two FMs to cover broader conditions. However, the effects of their combination are highly complicated due to the interaction between these FMs. If the interaction force of AF-FMs with various materials can be evaluated, it would help us to improve tribological performances of lubricants. Although atomic force microscopy seems suitable for this application, we found some obstacles, such as fluid resistance, electrostatic force, and laser positioning of the cantilever, to achieve proper measurements of the adsorption force. In this study, the adsorption force between the polar group and the surface was directly measured in oil with a 1 µm silica probe modified with CH3 or COOH. This paper proposed how to eliminate errors included in the adsorption force measurement using AFM and a calibration method for obtaining an accurate adsorption force of the polar group, and a test of normality of the measured data was conducted by 400 measurements. As a result, it was shown that approximately 100 tests were needed to obtain an accurate adsorption force in this study. Full article
(This article belongs to the Special Issue Friction Reduction at Interfaces)
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Lifting Lightweight Metals to a New Level—Tribological Improvement by Hybrid Surface Solutions on Aluminium and Magnesium
Lubricants 2020, 8(6), 65; https://doi.org/10.3390/lubricants8060065 - 13 Jun 2020
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Abstract
Nanocrystalline PEO (plasma electrolytical oxidation) coatings were performed on Al 6082 and Mg AZ31 alloys, resulting in hard, dense and wear-resistant surfaces to increase the wear resistance of those alloys. To soften the wear influence on the counter body side and to reduce [...] Read more.
Nanocrystalline PEO (plasma electrolytical oxidation) coatings were performed on Al 6082 and Mg AZ31 alloys, resulting in hard, dense and wear-resistant surfaces to increase the wear resistance of those alloys. To soften the wear influence on the counter body side and to reduce friction in a tribological application, the high-performance polymer PEEK (poly-ether-ether-ketone) was added to the load-supporting PEO surface by a laser melting technique to avoid heat influence on the lightweight substrate. The usage of additives in the PEEK dispersion led to stable conditions in the tribological system by decreasing the wear on the coated substrate as well as the counter body to a minimum accompanied by a low coefficient of friction during the whole life-time. The adopted hybrid coating systems were characterized using laser scanning microscopy (LSM), secondary and back scattered electron microscopy (SEM) and energy dispersive spectroscopy (EDS). A pin-on-disc test was employed to analyze the wear behavior of the different PEO and hybrid coatings and the influences of these surfaces on the coefficient of friction. Full article
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Open AccessArticle
On Waviness and Two-Sided Surface Features in Thermal Elastohydrodynamically Lubricated Line Contacts
Lubricants 2020, 8(6), 64; https://doi.org/10.3390/lubricants8060064 - 13 Jun 2020
Viewed by 532
Abstract
Machine components are designed to endure increasingly severe operating conditions due to the strive for improved energy efficiency of mechanical systems. Consequently, lubricated non-conformal contacts must rely on thin lubricant films where the influence of surface topography on the lubricating conditions becomes significant. [...] Read more.
Machine components are designed to endure increasingly severe operating conditions due to the strive for improved energy efficiency of mechanical systems. Consequently, lubricated non-conformal contacts must rely on thin lubricant films where the influence of surface topography on the lubricating conditions becomes significant. Due to the complexity of the multiphysical problem, approximate assumptions are often employed to facilitate numerical studies of elastohydrodynamically lubricated (EHL) contacts. In this work, the rough, time dependent, thermal EHL problem is solved with focus on two main analyses. The first analysis focuses on the influence of sinusoidal roughness and the difference between a thermal non-Newtonian approach and an isothermal Newtonian approach. The second analysis is focused on the lubricating mechanisms taking place when two-sided surface features overtake within the thermal EHL contact. The results indicate that the film thickness in the outlet of the contact may be significantly overestimated by an isothermal Newtonian approach and that differences in the high-pressure region may also occur due to viscosity variations in the inlet of the contact. Moreover, for the studied two-sided surface features, it became evident that not only the surface feature combination but also the overtaking position influence the film thickness and pressure variations significantly. Full article
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Open AccessEditorial
Tribology and Dowson
Lubricants 2020, 8(6), 63; https://doi.org/10.3390/lubricants8060063 - 09 Jun 2020
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Abstract
It is with great sadness that we note the passing of Professor Duncan Dowson on 6th January 2020. Duncan was an esteemed member of the Editorial Board of this journal. He will be remembered as one of the founding fathers of tribology and [...] Read more.
It is with great sadness that we note the passing of Professor Duncan Dowson on 6th January 2020. Duncan was an esteemed member of the Editorial Board of this journal. He will be remembered as one of the founding fathers of tribology and as a true gentleman. He was the last living member of the Jost Committee, set up by the UK Government (1964–1966) to investigate the state of lubrication education and research, and to establish the requirements of industry in this regard [1]. This committee coined the term “tribology”.Duncan contributed to many areas of tribological research and established many of them, including elastohydrodynamic theory and biotribology.[...] Full article
Open AccessArticle
Improvement in the Hard Milling of AISI D2 Steel under the MQCL Condition Using Emulsion-Dispersed MoS2 Nanosheets
Lubricants 2020, 8(6), 62; https://doi.org/10.3390/lubricants8060062 - 05 Jun 2020
Viewed by 290
Abstract
The present work shows the process for MoS2 nanosheet production by liquid N2-queched bulk, a novel method having highly efficient, green, and facile operation. The produced MoS2 nanoparticles are suspended in minimum quantity cooling lubrication (MQCL)-based fluid to form [...] Read more.
The present work shows the process for MoS2 nanosheet production by liquid N2-queched bulk, a novel method having highly efficient, green, and facile operation. The produced MoS2 nanoparticles are suspended in minimum quantity cooling lubrication (MQCL)-based fluid to form nanofluid used for the hard milling of AISI D2 steel. The study aims to improve the hard-milling performance assisted by the MQCL technique using MoS2 nanofluid. ANOVA analysis is used to evaluate the effects of three input machining variables, including nanoparticle concentration, cutting speed, and material hardness on cutting forces. The results indicate that the better cooling effect from the principle of the Ranque–Hilsch vortex tube of the MQCL device combined with the better lubricating performance from MoS2 nanofluid brings out the sustainable alternative solution for machining difficult-to-cut material. Moreover, the experimental results provide the technical guides for the selection of proper values of nanoparticle concentration and cutting speed while ensuring the technological, economic, and environmental characteristics. Full article
(This article belongs to the Special Issue Tribology of Smart Materials)
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