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Lubricants, Volume 7, Issue 11 (November 2019) – 9 articles

Cover Story (view full-size image): When predicting the piston skirt friction, it is important to consider the change of the roughness or waviness of the design patterns on the skirt. We analytically derived a new set of flow factors in the averaged Reynolds equation, as well as a new asperity contact model, for the trapezoid shape formed after wear of the original triangle shape. The calculation results showed quantitative agreement with an engine test in the trend of friction mean effective pressure during the break-in process. This work developed the necessary tools for optimizing the skirt design considering the break-in effect. View this paper.
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Article
Interactions of Ethanol with Friction Modifiers in Model Engine Lubricants
Lubricants 2019, 7(11), 101; https://doi.org/10.3390/lubricants7110101 - 15 Nov 2019
Cited by 4 | Viewed by 3378
Abstract
When employed as an engine fuel, ethanol can accumulate in the lubricant during use. Previous work has shown that ethanol contamination affects friction and elastohydrodynamic lubrication (EHL) film formation, and also the growth and stability of anti-wear tribofilms. The present work uses spacer-layer [...] Read more.
When employed as an engine fuel, ethanol can accumulate in the lubricant during use. Previous work has shown that ethanol contamination affects friction and elastohydrodynamic lubrication (EHL) film formation, and also the growth and stability of anti-wear tribofilms. The present work uses spacer-layer ultrathin interferometry and MTM tests to investigate how ethanol (both hydrated and anhydrous) interacts with friction modifiers in model lubricants. Small proportions (5 wt %) of ethanol were added to solutions of friction modifiers (one MoDTC and three organic friction modifiers) in a Group I base oil. For the three organic friction modifiers, the presence of ethanol promoted the formation of thick viscous boundary films so that very low friction coefficients were measured at low entrainment speeds. For the MoDTC additive, the presence of ethanol prevented the formation of a low friction film at low speeds at 70 °C, but this effect disappeared at 100 °C, probably due to ethanol evaporation. Full article
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Article
Investigation of the Tribofilm Formation of HiPIMS Sputtered MoSx Thin Films in Different Environments by Raman Scattering
Lubricants 2019, 7(11), 100; https://doi.org/10.3390/lubricants7110100 - 08 Nov 2019
Cited by 7 | Viewed by 2601
Abstract
Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoSx thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoSx films, the environment and [...] Read more.
Understanding the generation of third body particles and their contribution to the formation of tribofilms of MoSx thin films is still challenging due to a large number of influencing factors. Besides the structure of the as-deposited MoSx films, the environment and the conditions during the Ball-on-disk tests affect tribofilms and thus the friction. Therefore, the influence of the surface pressure and sliding velocity in air, argon and nitrogen environments on the generation of the third body particles and the tribofilm formation of randomly oriented MoSx films is investigated. A high surface pressure is one major factor to achieve low friction, especially under humid conditions, which is important considering the use in industrial applications, for example dry-running screw machines. However, the mechanisms leading to that frictional behavior are still affected by the surrounding environment. While low friction is caused by a more extensive tribofilm formation in air, in argon and nitrogen, large size third body particles dispensed all over the contact area contribute to a lower friction. Raman scattering reveal a different chemistry of these particles reflected in the absence of laser- or temperature-induced surface oxidation compared to the as-deposited film and the wear track. The Raman scattering results are discussed with respect to the wear particle size, its chemical reactivity and strain-induced bonding changes. Full article
(This article belongs to the Special Issue Selected Papers from the 60th German Tribology Conference 2019)
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Article
Effect of Normal Contact Vibration on Nano-Scale Friction
Lubricants 2019, 7(11), 99; https://doi.org/10.3390/lubricants7110099 - 07 Nov 2019
Cited by 2 | Viewed by 3031
Abstract
In this work, we investigate the effect of contact vibration on the friction of sliding single asperity contacts of different adhesion strength over a wide range of load and vibration amplitude. We convert the amplitude of vibration to its equivalent modulation force and [...] Read more.
In this work, we investigate the effect of contact vibration on the friction of sliding single asperity contacts of different adhesion strength over a wide range of load and vibration amplitude. We convert the amplitude of vibration to its equivalent modulation force and tip-oscillation velocity. We observe a logarithmic relationship between friction and the ratio of the modulation force to the normal force and between friction and the ratio of sliding velocity to the tip-oscillation velocity. We discuss these logarithmic dependencies based on an induced corrugation of the tip-sample interaction potential. Full article
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Article
Study of Break-In Process and its Effects on Piston Skirt Lubrication in Internal Combustion Engines
Lubricants 2019, 7(11), 98; https://doi.org/10.3390/lubricants7110098 - 02 Nov 2019
Cited by 4 | Viewed by 3041
Abstract
The piston skirt is one of the main contributors to the total mechanical loss in internal combustion engines. Usually, the skirt friction experiences a rapid change during the break-in period largely due to the wear of the machine marks or roughness against soft [...] Read more.
The piston skirt is one of the main contributors to the total mechanical loss in internal combustion engines. Usually, the skirt friction experiences a rapid change during the break-in period largely due to the wear of the machine marks or roughness against soft coatings. It is thus important to consider the effect of the change of the roughness for a realistic prediction of the piston skirt friction and system optimization. In this work, an existing model of piston skirt lubrication was improved with the consideration of a breaking in process for the most commonly used triangle machine marks. A new set of flow factors in the averaged Reynolds equation were analytically derived for the trapezoid shape formed after wear of the original triangle shape. A new asperity contact model was developed for the trapezoid shape. The calculation results reflect the trend of friction mean effective pressure (FMEP) during break-in in an engine test and showed quantitative agreement under the same amount of wear. Full article
(This article belongs to the Special Issue Tribology of Powertrain Systems)
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Article
Multiphase Computational Fluid Dynamics Analysis of Hydrodynamic Journal Bearing Under the Combined Influence of Texture and Slip
Lubricants 2019, 7(11), 97; https://doi.org/10.3390/lubricants7110097 - 30 Oct 2019
Cited by 6 | Viewed by 2947
Abstract
The drive to maintain the environmental sustainability and save the global energy consumption is urgent, making every powertrain system component a candidate to enhance efficiency. In this work, the combined effects of the slip boundary and textured surface in hydrodynamic journal bearing as [...] Read more.
The drive to maintain the environmental sustainability and save the global energy consumption is urgent, making every powertrain system component a candidate to enhance efficiency. In this work, the combined effects of the slip boundary and textured surface in hydrodynamic journal bearing as one of the critical components in industrial powertrain and engine systems are assessed using a multiphase computational fluid dynamic analysis that allows for phase change in a cavitation process and arbitrary textured geometry. The texture studied consists of regularly spaced rectangular dimples through two-dimensional (infinitely long) journal bearing. The modified Navier–slip model is employed to describe the slip boundary condition. A systematic comparison is made for various textured configurations varying the texture depth and the length of the texturing zone with respect to the performance of a smooth (untextured) bearing for several eccentricity ratios. The effectiveness of the texture with or without slip at enhancing the load support over a corresponding smooth bearing is investigated with the parameters. The detrimental or beneficial effect of surface texturing as well as the slip promotion is explained in terms of the mechanisms of pressure generation for several eccentricity ratios. The results of the present work indicate that journal bearing textured by a proper texturing zone and dimple depth are characterized by substantial load support levels. However, in the range of high eccentricity ratios, the promotion of texturing and slip can significantly degrade the performance of the load support. Full article
(This article belongs to the Special Issue Tribology of Powertrain Systems)
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Perspective
Rapid Thermal Characterization of Graphene Oxide—Nanocalorimetry as a Pathway for Novel Insights in Tribology
Lubricants 2019, 7(11), 96; https://doi.org/10.3390/lubricants7110096 - 29 Oct 2019
Cited by 3 | Viewed by 2329
Abstract
The use of solid lubricants such as graphene, graphene oxide, and other nanoparticles have gained notable attention in the tribological community to reduce friction and wear thus aiming at improved energy efficiency and sustainability. Tribological experiments unify rather extreme conditions such as high [...] Read more.
The use of solid lubricants such as graphene, graphene oxide, and other nanoparticles have gained notable attention in the tribological community to reduce friction and wear thus aiming at improved energy efficiency and sustainability. Tribological experiments unify rather extreme conditions such as high contact pressures, small contact areas, relative sliding motion, and rapid heating. This combination leads to mechanically- and/or thermally induced chemical, structural and microstructural modifications of the lubricating nanoparticles during rubbing thus altering their material’s properties. Due to the high sensitivity, we propose nanocalorimetry as the method of choice to shed more light on the thermally-induced processes and changes. As a model material for solid lubricants, we explore the transitions of graphene oxide under heating with 1000 °C/s up to 600 °C using quasi-adiabatic nanocalorimetry. We identify a strong exothermic runaway reaction at 317 °C. This runaway is preceded by exothermic reactions between 75–125 °C, which are correlated with the release of intercalated species and the formation of CO and CO2. Full article
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Review
Multi-Scale Surface Texturing in Tribology—Current Knowledge and Future Perspectives
Lubricants 2019, 7(11), 95; https://doi.org/10.3390/lubricants7110095 - 28 Oct 2019
Cited by 111 | Viewed by 7563
Abstract
Surface texturing has been frequently used for tribological purposes in the last three decades due to its great potential to reduce friction and wear. Although biological systems advocate the use of hierarchical, multi-scale surface textures, most of the published experimental and numerical works [...] Read more.
Surface texturing has been frequently used for tribological purposes in the last three decades due to its great potential to reduce friction and wear. Although biological systems advocate the use of hierarchical, multi-scale surface textures, most of the published experimental and numerical works have mainly addressed effects induced by single-scale surface textures. Therefore, it can be assumed that the potential of multi-scale surface texturing to further optimize friction and wear is underexplored. The aim of this review article is to shed some light on the current knowledge in the field of multi-scale surface textures applied to tribological systems from an experimental and numerical point of view. Initially, fabrication techniques with their respective advantages and disadvantages regarding the ability to create multi-scale surface textures are summarized. Afterwards, the existing state-of-the-art regarding experimental work performed to explore the potential, as well as the underlying effects of multi-scale textures under dry and lubricated conditions, is presented. Subsequently, numerical approaches to predict the behavior of multi-scale surface texturing under lubricated conditions are elucidated. Finally, the existing knowledge and hypotheses about the underlying driven mechanisms responsible for the improved tribological performance of multi-scale textures are summarized, and future trends in this research direction are emphasized. Full article
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Article
Investigation of the Influence of Aging on the Lubricity of Metalworking Fluids by Means of Design of Experiment
Lubricants 2019, 7(11), 94; https://doi.org/10.3390/lubricants7110094 - 23 Oct 2019
Cited by 2 | Viewed by 2473
Abstract
The influence of complex aging processes in water-miscible metalworking fluids on process performance is of high relevance for the metalworking industry. Because of the highly dynamic interactions in the complex “metalworking fluid” ecosystem, a distinct correlation between the aging process and the performance [...] Read more.
The influence of complex aging processes in water-miscible metalworking fluids on process performance is of high relevance for the metalworking industry. Because of the highly dynamic interactions in the complex “metalworking fluid” ecosystem, a distinct correlation between the aging process and the performance of the fluid in metalworking processes is hardly possible. Consequences of the aging process on physical, chemical, and biological properties of the fluid include aspects such as the decrease of the pH value, the increase of the droplet size in emulsions, the presence of bacterial cells, or the modification of the metalworking fluid composition. In the presented work, the influences of these aging aspects on the lubricity of metalworking fluids were investigated individually. A test series has been carried out, which was planned with a design of experiments method, to investigate interactions between the aging aspects regarding lubricity. In addition, the results enabled the development of an empirical regression model, which allowed an integrated description of the influence of the relevant aging aspects. Full article
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Technical Note
New Approach to Interpreting Seizure Tests on the Translatory Oscillation Tribometer (SRV)
Lubricants 2019, 7(11), 93; https://doi.org/10.3390/lubricants7110093 - 23 Oct 2019
Cited by 2 | Viewed by 1990
Abstract
When looking in detail at analyses of the tribological load-carrying capacity of lubricants, it becomes apparent that an exclusive evaluation of the development of the coefficient of friction cannot provide any sufficient criteria for determining the occurrence of adhesive failure. This is due [...] Read more.
When looking in detail at analyses of the tribological load-carrying capacity of lubricants, it becomes apparent that an exclusive evaluation of the development of the coefficient of friction cannot provide any sufficient criteria for determining the occurrence of adhesive failure. This is due on the one hand to the increasing complexity of lubricant formulae, and on the other hand to the increasing power capacity of modern drive and control concepts in the construction of tribometers. For this reason, it is urgently needed to examine the adhesive processes and their detection in more detail with the help of appropriate tribological values and criteria. The evolution of the friction can be coupled with the stroke, contact resistance, and other parameters. Besides, from new criteria for adhesive failure, which were compiled by the workgroup for the relevant ISO, DIN, and ASTM standards, this contribution discusses the inclusion of additional parameters. The SRV® test system—where SRV stands for the German acronym for oscillation, friction, and wear—is one that was developed to analyze adhesive failure from many measurements alongside the coefficient of friction. Testing with the SRV® system is done via step tests with standardized procedures and parameters, which are in accordance with ASTM test methods. The system continuously monitors electrical resistance, zero stroke position signals, temperature, and other measurements, and can derive further parameters that also help to identify adhesive failure and other adhesive events. These dimensionally reduced parameters can provide new insight on the mechanism of the adhesive behavior. This paper aims to discuss how the interpretation of these step tests beyond the development of coefficient of friction can lead to new knowledge and insight in tribological research, and explores the applicability of Stribeck’s theory to the oscillatory and reciprocating motion utilized in the SRV® tribometer. Full article
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