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Lubricants, Volume 8, Issue 9 (September 2020) – 9 articles

Cover Story (view full-size image): Adhesion has a tremendous impact on cutting-edge applications, such as the development of nano- and micromechanical systems and bioinspired adhesives. Usually, roughness destroys adhesion; however, with some special types of roughness, adhesion enhancement may occur. In this study, the contact problem of a sphere that is in contact with a wavy axisymmetric single-scale substrate is studied. It is shown that adhesion enhancement either stops for "rigid" materials or is stopped by large waviness amplitudes due to the appearance of internal cracks. The enhancement effect is well captured by an equation in terms of the Johnson parameter. View this paper.
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Open AccessArticle
Simulation Analysis of Erosion–Corrosion Behaviors of Elbow under Gas-Solid Two-Phase Flow Conditions
Lubricants 2020, 8(9), 92; https://doi.org/10.3390/lubricants8090092 - 22 Sep 2020
Viewed by 585
Abstract
In the production and gathering process of coal gas, the complex composition of the coal gas, harsh environments, the complex medium, and high content of solid particles in slurry cause the equipment malfunctions and even failure because of erosion and corrosion. In the [...] Read more.
In the production and gathering process of coal gas, the complex composition of the coal gas, harsh environments, the complex medium, and high content of solid particles in slurry cause the equipment malfunctions and even failure because of erosion and corrosion. In the present study, COMSOL multi-physics finite element simulation software is used to simulate the erosion–corrosion behaviors of elbow in key chemical equipments. The electrochemical corrosion, solid particle erosion, chemical reaction, and turbulent flow are coupled together. The particle count method is proposed to clarify the erosion phenomenon. The simulation results show that particles with high turbulent intensity hit the wall of elbow directly, which forms a slanted elliptical erosion zone on the extrados surface at 40°–50°. The chemical reaction in turbulence has a difference in the concentration distribution of substances, and this phenomenon leads to different magnitudes of the corrosion current densities in the tube. Moreover, 1/6 released particles hit the extrados surface of the elbow. These findings are beneficial to understand the erosion–corrosion phenomena and design the elbow in key chemical equipment. Full article
(This article belongs to the Special Issue Interfacial Dissipative Phenomena in Tribomechanical Systems)
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Open AccessReview
Review on Friction and Wear Test Rigs: An Overview on the State of the Art in Tyre Tread Friction Evaluation
Lubricants 2020, 8(9), 91; https://doi.org/10.3390/lubricants8090091 - 16 Sep 2020
Cited by 1 | Viewed by 1136
Abstract
The future evolution of autonomous mobility and road transportation will require substantial improvements in tyre adherence optimization. As new technologies being deployed in tyre manufacturing reduce total vehicle energy consumption, the contribution of tyre friction for safety and performance enhancement continues to increase. [...] Read more.
The future evolution of autonomous mobility and road transportation will require substantial improvements in tyre adherence optimization. As new technologies being deployed in tyre manufacturing reduce total vehicle energy consumption, the contribution of tyre friction for safety and performance enhancement continues to increase. For this reason, the tyre’s grip is starting to drive the focus of many tyre developments nowadays. This is because the tread compound attitude to maximize the interaction forces with the ground is the result of a mix of effects, involving polymer viscoelastic characteristics, road roughness profiles and the conditions under which each tyre works during its lifespan. In such a context, mainly concerning the automotive market, the testing, analysis and objectivation of the friction arising at the tread interface is performed by means of specific test benches called friction testers. This paper reviews the state of the art in such devices’ development and use, with a global overview of the measurement methodologies and with a classification based on the working and specimen motion principle. Most tyre friction testers allow one to manage the relative sliding speed and the contact pressure between the specimen and the counter-surface, while just some of them are able to let the user vary the testing temperature. Few devices can really take into account the road real roughness, carrying out outdoor measurements, useful because they involve actual contact phenomena, but very complex to control outside the laboratory environment. Full article
(This article belongs to the Special Issue Interfacial Dissipative Phenomena in Tribomechanical Systems)
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Open AccessFeature PaperArticle
A Numerical Study on Roughness-Induced Adhesion Enhancement in a Sphere with an Axisymmetric Sinusoidal Waviness Using Lennard–Jones Interaction Law
Lubricants 2020, 8(9), 90; https://doi.org/10.3390/lubricants8090090 - 04 Sep 2020
Cited by 3 | Viewed by 831
Abstract
Usually, roughness destroys adhesion and this is one of the reasons why the “adhesion paradox”, i.e., a “sticky Universe”, is not real. However, at least with some special type of roughness, there is even the case of adhesion enhancement, as it was shown [...] Read more.
Usually, roughness destroys adhesion and this is one of the reasons why the “adhesion paradox”, i.e., a “sticky Universe”, is not real. However, at least with some special type of roughness, there is even the case of adhesion enhancement, as it was shown clearly by Guduru, who considered the contact between a sphere and a wavy axisymmetric single scale roughness, in the limit of short-range adhesion (JKR limit). Here, the Guduru’s problem is numerically solved by using the Boundary Element Method (BEM) with Lennard–Jones interaction law, which allowed us to explore the contact solution from the rigid to the JKR limit. It is shown that adhesion enhancement stops either for low Tabor parameter, or by large waviness amplitudes, due to the appearance of internal cracks within the contact patch. We do not seem to find a clear threshold for “stickiness” (complete elimination of adhesion), contrary to other recent theories on random roughness. The enhancement effect is well captured by an equation in terms of the Johnson parameter derived by Ciavarella–Kesari–Lew, and is much larger than the Persson–Tosatti enhancement in terms of increase of real contact area due to roughness. The Persson–Tosatti energetic argument for adhesion reduction seems to give a lower bound to the effective work of adhesion. Full article
(This article belongs to the Special Issue Interfacial Dissipative Phenomena in Tribomechanical Systems)
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Open AccessArticle
Analytical Elastostatic Contact Mechanics of Highly-Loaded Contacts of Varying Conformity
Lubricants 2020, 8(9), 89; https://doi.org/10.3390/lubricants8090089 - 02 Sep 2020
Cited by 1 | Viewed by 756
Abstract
In applications requiring high load carrying capacity, conforming contacting pairs with a relatively large contact footprint are used. These include circular arc, Novikov, and Wildhaber gears found, for example, in helicopter rotors. Closely conforming contacts also occur in many natural endo-articular joints, such [...] Read more.
In applications requiring high load carrying capacity, conforming contacting pairs with a relatively large contact footprint are used. These include circular arc, Novikov, and Wildhaber gears found, for example, in helicopter rotors. Closely conforming contacts also occur in many natural endo-articular joints, such as hips, or their replacement arthroplasty. The main determining factors in contact fatigue are the sub-surface shear stresses. For highly loaded contacts, classical Hertzian contact mechanics is used for many gears, bearings, and joints. However, the theory is essentially for concentrated counterforming contacts, where the problem is reduced to a rigid ellipsoidal solid penetrating an equivalent semi-infinite elastic half-space. Applicability is limited though, and the theory is often used inappropriately for contacts of varying degrees of conformity. This paper presents a generic contact mechanics approach for the determination of sub-surface stresses, which is applicable to both highly conforming as well as concentrated counterforming contacts. It is shown that sub-surface shear stresses alter in magnitude and disposition according to contact conformity, and lead to the different modes of fatigue failure noted in practice. Full article
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Open AccessArticle
The Identification of an Adequate Stressing Level to Find the Proper Running-In Conditions of a Lubricated DLC-Metal-System
Lubricants 2020, 8(9), 88; https://doi.org/10.3390/lubricants8090088 - 31 Aug 2020
Viewed by 627
Abstract
Using a tribometer equipped with a high-resolution wear measurement unit (RNT), the running-in of a diamondlike carbon (DLC) iron spray coating contact was analyzed and optimized. The optimization comprised an initial parameter field with different load and speed levels to find key operation [...] Read more.
Using a tribometer equipped with a high-resolution wear measurement unit (RNT), the running-in of a diamondlike carbon (DLC) iron spray coating contact was analyzed and optimized. The optimization comprised an initial parameter field with different load and speed levels to find key operation points. These points were used to compose a dedicated running-in parameter field. The analysis underlined the importance of identifying the adequate stressing conditions. With respect to our concept of the running-in corridor, a high-power running-in has to be preferred to obtain a tribological system with low friction, small total wear and wear rate, high system stability, and low sensitivity to external changes. Full article
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Open AccessReview
Tribochemistry as an Alternative Synthesis Pathway
Lubricants 2020, 8(9), 87; https://doi.org/10.3390/lubricants8090087 - 29 Aug 2020
Cited by 1 | Viewed by 870
Abstract
While reactions driven by mechanical force or stress can be labeled mechanochemical, those specifically occurring at a sliding interface inherit the name tribochemical, which stems from the study of friction and wear: tribology. Increased perception of tribochemical reactions has been gained through technological [...] Read more.
While reactions driven by mechanical force or stress can be labeled mechanochemical, those specifically occurring at a sliding interface inherit the name tribochemical, which stems from the study of friction and wear: tribology. Increased perception of tribochemical reactions has been gained through technological advancement, and the development of new applications remains on-going. This surprising physico-kinetic process offers great potential in novel reaction pathways for synthesis techniques and nanoparticle interactions, and it could prove to be a powerful cross-disciplinary research area among chemists, engineers, and physicists. In this review article, a survey of the history and recent usage of tribochemical reaction pathways is presented, with a focus on forging new compounds and materials with this sustainable synthesis methodology. In addition, an overview of tribochemistry’s current utility as a synthesis pathway is given and compared to that of traditional mechanochemistry. Full article
(This article belongs to the Special Issue Friction Reduction at Interfaces)
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Open AccessReview
An Overview of Grease Water Resistance
Lubricants 2020, 8(9), 86; https://doi.org/10.3390/lubricants8090086 - 21 Aug 2020
Cited by 1 | Viewed by 675
Abstract
Many grease-lubricated components operate in wet environments, making them susceptible to water contamination which degrades their performance, functionality, and useful life. Hence, selecting a grease with appropriate water-resistant properties can have a significant influence on the life of the machine. While industry standards [...] Read more.
Many grease-lubricated components operate in wet environments, making them susceptible to water contamination which degrades their performance, functionality, and useful life. Hence, selecting a grease with appropriate water-resistant properties can have a significant influence on the life of the machine. While industry standards attempt to evaluate a grease’s water resistance, research indicates that a more thorough understanding of water resistance is needed to properly match a grease to an application. This paper provides an overview of the interaction of grease and water, covers existing water-resistance standards, discusses the results of available experiments aiming to describe the effects of water on grease, demonstrates the need for more meaningful standards, and suggests additional measures for characterizing a grease’s water resistance. Full article
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Open AccessCommunication
Comparative Tribological Properties of Pd-, Pt-, and Zr-Based Bulk Metallic Glasses
Lubricants 2020, 8(9), 85; https://doi.org/10.3390/lubricants8090085 - 21 Aug 2020
Viewed by 916
Abstract
We present a comparative study of the tribological properties of Pd-, Pt-, and Zr-based bulk metallic glasses (BMG-Pd, BMG-Pt, and BMG-Zr, respectively) under unlubricated conditions. In particular, micro-tribometry is utilized with a 52,100 steel ball, showing that BMG-Pt exhibits a significantly higher coefficient [...] Read more.
We present a comparative study of the tribological properties of Pd-, Pt-, and Zr-based bulk metallic glasses (BMG-Pd, BMG-Pt, and BMG-Zr, respectively) under unlubricated conditions. In particular, micro-tribometry is utilized with a 52,100 steel ball, showing that BMG-Pt exhibits a significantly higher coefficient of friction (COF) (0.58 ± 0.08) when compared with BMG-Pd (0.30 ± 0.02) and BMG-Zr (0.20 ± 0.03). Topographical roughness on and off wear scars is characterized via atomic force microscopy (AFM), with results that do not correlate with the observed frictional behavior. On the other hand, scanning electron microscopy (SEM) is utilized to reveal contrasting wear mechanisms for the three samples: while BMG-Pd and BMG-Zr exhibit predominantly abrasive wear, there is evidence of adhesive wear on BMG-Pt. Consequently, the occurrence of adhesive wear emerges as a potential mechanism behind the observation of relatively high coefficients of friction on BMG-Pt, suggesting stronger interactions with steel when compared with the other BMG samples. Full article
(This article belongs to the Special Issue Applied Tribology in Mechanical Engineering)
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Open AccessReview
A Practical Approach to Gear Design and Lubrication: A Review
Lubricants 2020, 8(9), 84; https://doi.org/10.3390/lubricants8090084 - 20 Aug 2020
Viewed by 720
Abstract
The modern design of mechanical parts, such as gears, goes through the continuous demand for a high level of efficiency and reliability, as well as an increased load carrying capacity and endurance life. The aim of the present paper was to perform a [...] Read more.
The modern design of mechanical parts, such as gears, goes through the continuous demand for a high level of efficiency and reliability, as well as an increased load carrying capacity and endurance life. The aim of the present paper was to perform a review and to collect practical examples in order to provide interesting tips and guidelines for gear design, including both its dimensioning and its lubrication. From this point of view, this paper is particularly novel, as it is a full-comprehensive collection of all the tools supporting gear design. Several practical aspects have been taken into account, including the definition of the right profile shifting, the selection of a proper lubricant, and the definition of the quality grade and of the tolerances needed to obtain the correct backlash. Finally, a numerical example is provided, addressing the research of the best solution to fit a given space, while maximizing the transmittable torque over weight ratio for two mating spur gears. Full article
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