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Lubricants, Volume 7, Issue 6 (June 2019) – 8 articles

Cover Story (view full-size image): A quartz crystal microbalance was employed to measure the nanotribological properties of statistically diverse materials combinations of aqueous suspensions of nanoparticles and substrates deposited on an electrode. Five nanoparticles (SiO2, SiO2-P (with surfactant polyethylene glycol), TiO2, Al2O3, and γ-Fe2O3) and ten substrates (Au, Al, Cr, Cu, Mo, Ni, Pt, SiO2, Al2O3, and 304 stainless steel) were studied. The changes in QCM frequency and motional resistance led to identification of statistical performance factors that could be related to the macroscale friction coefficients and identify material genomes that underlie the performance of nanoparticle systems as lubricants. View this paper.
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Article
Avoiding Starvation in Tribocontact Through Active Lubricant Transport in Laser Textured Surfaces
Lubricants 2019, 7(6), 54; https://doi.org/10.3390/lubricants7060054 - 25 Jun 2019
Cited by 7 | Viewed by 3071
Abstract
Laser texturing is a viable tool to enhance the tribological performance of surfaces. Especially textures created with Direct Laser Interference Patterning (DLIP) show outstanding improvement in terms of reduction of coefficient of friction (COF) as well as the extension of oil film lifetime. [...] Read more.
Laser texturing is a viable tool to enhance the tribological performance of surfaces. Especially textures created with Direct Laser Interference Patterning (DLIP) show outstanding improvement in terms of reduction of coefficient of friction (COF) as well as the extension of oil film lifetime. However, since DLIP textures have a limited depth, they can be quickly damaged, especially within the tribocontact area, where wear occurs. This study aims at elucidating the fluid dynamical behavior of the lubricant in the surroundings of the tribocontact where channel-like surface textures are left after the abrasion wear inside the tribocontact area. In a first step, numerical investigations of lubricant wetting phenomena are performed applying OpenFOAM®. The results show that narrow channels (width of 10 μ m ) allow higher spreading than wide channels (width of 30 μ m ). In a second step, fluid transport inside DLIP textures is investigated experimentally. The results show an anisotropic spreading with the spreading velocity dependent on the period and depth of the laser textures. A mechanism is introduced for how lubricant can be transported out of the channels into the tribocontact. The main conclusion of this study is that active lubricant transport in laser textured surfaces can avoid starvation in the tribocontact. Full article
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Article
Influence of the Nonequilibrium Material State on Wear Resistance
Lubricants 2019, 7(6), 53; https://doi.org/10.3390/lubricants7060053 - 23 Jun 2019
Cited by 2 | Viewed by 1879
Abstract
The influence of the nonequilibrium state of a material on its wear resistance is investigated in this study. Using methods of non-equilibrium thermodynamics and the theory of self-organization, a non-equilibrium material is shown to possess an overall lower wear rate than a material [...] Read more.
The influence of the nonequilibrium state of a material on its wear resistance is investigated in this study. Using methods of non-equilibrium thermodynamics and the theory of self-organization, a non-equilibrium material is shown to possess an overall lower wear rate than a material in an equilibrium state. This was experimentally demonstrated to be the case in different materials and tribosystems, such as Babbit operating in complete lubrication conditions, copper in the current collector and a cutting tool with a coating applied on it. Full article
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Review
Tribology and Dentistry: A Commentary
Lubricants 2019, 7(6), 52; https://doi.org/10.3390/lubricants7060052 - 20 Jun 2019
Cited by 15 | Viewed by 2838
Abstract
Since 1966 the term “tribology” has integrated different topics like friction, lubrication and wear. After a few years, interest in this type of phenomena rapidly spread out around the world of biology and medicine, determining a new research area defined as biotribology. This [...] Read more.
Since 1966 the term “tribology” has integrated different topics like friction, lubrication and wear. After a few years, interest in this type of phenomena rapidly spread out around the world of biology and medicine, determining a new research area defined as biotribology. This commentary is conceived within this framework with the aim of underlining the close link between tribology and dentistry regarding both physiological and restorative issues. The contact between teeth requires investigation into their tribological behavior focusing on the enamel wear process against natural teeth and/or artificial teeth, allowing us to obtain useful information on the tribological behavior of restorative materials. Thus, tested materials may be natural teeth, restorative materials (metal alloys, ceramics, composites) or both. This work aims to make a contribution to underlining the need for greater standardization of tribological experimental procedures as well as to obtaining more homogeneous and indicative results on the tested tribo systems. Full article
(This article belongs to the Special Issue Biotribology in Human Body)
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Editorial
Carbon Nanomaterials—Promising Solid Lubricants to Tailor Friction and Wear
Lubricants 2019, 7(6), 51; https://doi.org/10.3390/lubricants7060051 - 18 Jun 2019
Cited by 1 | Viewed by 2020
Abstract
It is our pleasure to launch this Special Issue related to the application of carbon nanomaterials as solid lubricants to tailor friction and wear [...] Full article
Article
Principal Stress Ratio Effect at Residual Stress Determination Utilizing the Variation of Indentation Hardness
Lubricants 2019, 7(6), 50; https://doi.org/10.3390/lubricants7060050 - 11 Jun 2019
Cited by 1 | Viewed by 1898
Abstract
The determination of residual stresses is an important issue when it comes to material failure analysis. The variation of global indentation properties, due to the presence of residual stresses, can serve as a guideline for the size and direction of such stresses. One [...] Read more.
The determination of residual stresses is an important issue when it comes to material failure analysis. The variation of global indentation properties, due to the presence of residual stresses, can serve as a guideline for the size and direction of such stresses. One of these global indentation properties, the material hardness, is unfortunately invariant of residual stresses when metals and alloys are at issue. In this situation, one has to rely on the size of the indentation contact area for residual stress determination. For other materials such as ceramics and polymers, where elastic deformations are of greater importance at indentation, such invariance is no longer present. Here, this variation is investigated based on finite element simulations. The aim is then to determine how the indentation hardness is influenced by the principal residual stress ratio and also discuss if such an influence is sufficient in order to determine the size and direction of such stresses in an experimental situation. It should be emphasized that this work does not suggest a new approach to residual stress determination (by indentation testing) but investigates the applicability of previously derived methods to a situation where the surface stress field is not simplified as equi-biaxial or uniaxial. For simplicity, but not out of necessity, only cone indentation of elastic-perfectly plastic materials is considered. Full article
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Article
Nanotribological Performance Factors for Aqueous Suspensions of Oxide Nanoparticles and Their Relation to Macroscale Lubricity
Lubricants 2019, 7(6), 49; https://doi.org/10.3390/lubricants7060049 - 07 Jun 2019
Cited by 6 | Viewed by 2421
Abstract
Quartz crystal microbalance (QCM) measurements of nanotribological properties of statistically diverse materials combinations of nanoparticles and substrate electrodes in aqueous suspensions are reported and compared to macroscale measurements of the same materials combinations for a subset of the nanoparticle combinations. Four ceramic nanoparticles, [...] Read more.
Quartz crystal microbalance (QCM) measurements of nanotribological properties of statistically diverse materials combinations of nanoparticles and substrate electrodes in aqueous suspensions are reported and compared to macroscale measurements of the same materials combinations for a subset of the nanoparticle combinations. Four ceramic nanoparticles, TiO2, SiO2, Al2O3, and maghemite (γ-Fe2O3) and ten substrate materials (Au, Al, Cr, Cu, Mo, Ni, Pt, SiO2, Al2O3, and SS304) were studied. The QCM technique was employed to measure frequency and motional resistance changes upon introduction of nanoparticles into the water surrounding its liquid-facing electrode. This series of experiments expanded prior studies that were often limited to a single nanoparticle - solid liquid combination. The variations in QCM response from one nanoparticle to another are observed to be far greater than the variation from one substrate to another, indicating that the nanoparticles play a larger role than the substrates in determining the frictional drag force levels. The results were categorized according to the direction of the frequency and motional resistance changes and candidate statistical performance factors for the datasets were generated. The performance factors were employed to identify associations between the QCM atomic scale results and the macroscale friction coefficient measurements. Macroscale measurements of friction coefficients for selected systems document that reductions (increases) in motional resistance to shear, as measured by the QCM, are linked to decreases (increases) in macroscale friction coefficients. The performance factors identified in the initial study therefore appear applicable to a broader set of statistically diverse samples. The results facilitate full statistical analyses of the data for identification of candidate materials properties or materials genomes that underlie the performance of nanoparticle systems as lubricants. Full article
(This article belongs to the Special Issue Recent Research in Nanolubricants)
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Article
The Impact of Lubricant Film Thickness and Ball Bearings Failures
Lubricants 2019, 7(6), 48; https://doi.org/10.3390/lubricants7060048 - 02 Jun 2019
Cited by 4 | Viewed by 3069
Abstract
An effort was made to find a relationship between the lubricant thickness at the point of contact of rolling element ball bearings, and empirical equations to predict the life for bearings under constant motion. Two independent failure mechanisms were considered, fatigue failure and [...] Read more.
An effort was made to find a relationship between the lubricant thickness at the point of contact of rolling element ball bearings, and empirical equations to predict the life for bearings under constant motion. Two independent failure mechanisms were considered, fatigue failure and lubricant failure resulting in seizing of the roller bearing. A theoretical formula for both methods was established for the combined probability of failure using both failure mechanisms. Fatigue failure was modeled with the empirical equations of Lundberg and Palmgren and standardized in DIN/ISO281. The seizure failure, which this effort sought to investigate, was predicted using Greenwood and Williamson’s theories on surface roughness and asperities during lubricated contact. These two mechanisms were combined, and compared to predicted cycle lives of commercial roller bearing, and a clear correlation was demonstrated. This effort demonstrated that the Greenwood–Williams theories on the relative height of asperities versus lubricant film thickness can be used to predict the probability of a lubricant failure resulting in a roller bearing seizing during use. Full article
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Article
Numerical Study of Power Loss and Lubrication of Connecting Rod Big-End
Lubricants 2019, 7(6), 47; https://doi.org/10.3390/lubricants7060047 - 01 Jun 2019
Cited by 4 | Viewed by 2704
Abstract
A hydrodynamic lubrication analysis for connecting rod big-end bearing is conducted. The effects of engine speed, operating condition, lubricant viscosity and oil temperature on tribological performance of big-end bearing have been examined. Force equilibrium is solved to define instantaneous eccentricity between journal and [...] Read more.
A hydrodynamic lubrication analysis for connecting rod big-end bearing is conducted. The effects of engine speed, operating condition, lubricant viscosity and oil temperature on tribological performance of big-end bearing have been examined. Force equilibrium is solved to define instantaneous eccentricity between journal and bearing to have accurate estimation of oil film thickness at interface of connecting rod big-end bearing and crankpin. Connecting rod big-end is treated as π film hydrodynamic journal bearing and finite difference scheme is applied to calculate generated hydrodynamic pressure and frictional power loss at each crank angle. Beside the development of analytical formulation, well-known Mobility model introduced by Booker has been employed to be compared with the analytical model. The presented analytical model reduces the complexity and the numerical effort with respect to Mobility method, thus shortening the computation time. The simulation results show good agreement between analytical model, Mobility approach and experimental data. Full article
(This article belongs to the Special Issue Tribology of Powertrain Systems)
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