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Friction
Published in Ahmed Abdelbary, Extreme Tribology, 2020
The contact region of solid surfaces subjected to loads will initially connect at only a few junctions in order to support the applied load. As the load is increased, the surfaces move closer together, a larger amount of higher asperities on both surfaces come into contact. The existing contacts grow and deform to support the increasing load which is referred to as junction growth. Thus, surface deformation occurs in the region of the contact asperities, establishing stresses that oppose the applied load. The mode of surface deformation may be elastic, which is characterized by linear relation between stress and strain. On the other hand, plastic deformation is characterized by a more complex stress-strain relation. The applied load induces a generally elastic deformation of the solid surfaces at the tips of the asperities. Local plastic deformation may take place. Thus, in most contact situations, we find a mixture of both elastic and plastic deformations.
Nanodrops on the Solid surface Contact Angle, Sticking Force
Published in Eli Ruckenstein, Gersh Berim, Wetting Theory, 2018
For a long time, a liquid drop on the surface of a solid was the object of intense experimental and theoretical investigations and numerous results were obtained using various methods. Particular attention was given to rough surfaces because of the large effect which roughness has on the wetting of a solid substrate. Two kinds of roughnesses were considered. One of them is due to the asperities present on the surface of a homogeneous solid substrate (physical roughness). The second, chemical roughness, occurs when the substrate has a smooth surface with nonuniform chemical composition. For both types of roughnesses, the contact angle of the drop on the rough surface is usually greater than on the smooth one, i.e. the roughness increases the hydrophobicity.1,2 Another important feature of a rough surface, which is absent for a smooth one, is the appearance of the sticking (pinning) of the drop-solid contact line to the solid surface due to the direct contact of this line with the asperities (see e.g. ref. 3).
The Tribosystem Analysis Form
Published in J. Blau Peter, Tribosystem Analysis: A Practical Approach to the Diagnosis of Wear Problems, 2017
Basically, there are three lubrication regimes discussed in the literature. The Martens-Stribeck curve forms the foundation for much of the interpretation of lubricated sliding and wear behavior, and for a more detailed discussion of lubrication regimes, the reader is referred to Jackson [5]. In summary, the three lubrication regimes are (i) boundary lubrication, (ii) hydrodynamic lubrication (HDL), and (iii) mixed film lubrication that falls between the first two regimes. These are based on the existence of a fluid film that either fully separates relatively moving surfaces or allows them to touch. If the fluid film allows opposing surfaces to touch all or most of the time, then we say there is boundary lubrication. A thin layer of fluid or chemical reaction products may coat the asperities on the wear surface during boundary lubrication and provide some level of friction reduction and wear protection. Even worse, if there is no lubricant present, we can call this a “starved” condition and expect to find the kind of wear and friction levels characteristic of bare, dry, sliding surfaces. In contrast, if the fluid flow into the contact creates sufficient pressure to force mating surfaces apart so that wear does not occur at all, then we call this hydrodynamic or “full-film” lubrication. Sliding bearings are commonly designed to produce full-film lubrication.
Tribological and rheological properties of the lubricant containing hybrid graphene nanosheets (GNs)/titanium dioxide (TiO2) nanoparticles as an additive on calcium grease
Published in Journal of Dispersion Science and Technology, 2022
Bahaa M. Kamel, Enas l. Arafa, Alaa Mohamed
Lubrication is a helpful technology because it works as a layer of separation between the surfaces, preventing asperities from coming into contact and as a sealant to minimize leakage and to keep out contaminants, which is used in many important applications like automotives, water pumps, agricultural machinery exposed to water and vibrating applications in transport. Friction and wear are a phenomenon of many mechanical devices with moving parts. Lubricants might fail to meet their lubricating characteristics in certain situations. Unwanted wear can result in significant material and energy waste.[1] Nanotechnology provides the possibility of increasing the performance of lubricant oil by utilizing nanoparticles to increase the properties of lubricant. In this regard, the advancement and modification of nano-lubricating technology, as well as a growing understanding of the distinctive mechanical and physical properties of functional nanoparticles employed as additives, demonstrate distinct mechanical and physical properties.
Roughness evolution of wheel surface in a simulated wheel–rail contact
Published in Tribology - Materials, Surfaces & Interfaces, 2022
J. A. Alberto Jaramillo, Juan C. Sánchez, Juan F. Santa, Mauricio Enrico Palacio, Alejandro Toro
The cross-sectional views for rail and wheel samples that reveal a sub-surface deformed microstructure can be observed in Figure 12. Figure 12(a,b) shows a thicker heavily deformed layer for wheel samples with higher initial roughness (W6.3), which also presented more significant delamination wear. The values of the deformed layer are 23 and 34 µm. The rail specimens present similar depths of deformed layer (21 µm) although the sample tested against W3.2 wheel specimen showed more material detachment. These observations are consistent with Figure 8 that reveals a better wear performance of rail samples tested against wheel samples with higher roughness. Figure 13 shows the cross-section of the specimen tested under lubricated conditions. The wheel samples denoted with W6.3 showed thicker wear lamellae and more cracks that emerge to the surface compared to the samples denoted W3.2. On the other hand, rails specimens tested against wheel samples denoted W6.3 showed a thicker deformed sub-surface layer compared to W3.2. Localized high contact stresses create a thicker deformed layer under the surface at the asperities.
Friction and Temperature Reduction in a Mechanical Face Seal by a Surface Texturing: Comparison between TEHD Simulations and Experiments
Published in Tribology Transactions, 2018
M. Adjemout, N. Brunetière, J. Bouyer
In many rotating machineries, such as pumps or compressors, mechanical face seals are used to avoid the leakage of pressurized fluids to the environment. These sealing components are composed of two flat annular rings, one of which is linked to the housing and the other one to the rotating shaft of the machine. The interface between these two rings constitutes the sealing interface that is lubricated with the sealed fluid. Research into the lubrication mechanism of these components first began several decades ago (Summers-Smith (1)). It was found that at low duty parameter values (i.e., low speed and high load), mixed lubrication and wear are experienced. To analyze the impact of surface roughness on mixed lubrication, Hamilton, et al. (2) created artificial roughness on one surface and demonstrated that these asperities work as microbearings, thus enhancing fluid lubrication. A few years later, Anno, et al. (3) showed that a similar mechanism of film formation and friction reduction is obtained with microcavities on the surface.