Lubrication at Extreme Conditions: A Discussion About the Limiting Shear Stress Concept

Martinie, Laëtitia; Vergne, Philippe

doi:10.1007/s11249-016-0709-4

Cited by 28 publications

(39 citation statements)

References 55 publications

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“…For example, when the applied conditions yield shear localisation, the friction coefficient of L−J fluids can decrease with increasing pressure and shear rate. Such behaviour is not commonly observed experimentally for lubricants, but is more similar to how traction fluids behave [204] above their critical shear stress [185]. The nonequilibrium phase diagrams and friction maps for L−J fluids were also found to be sensitive to the degree of the wall roughness on the atomic scale [188].…”

Section: Shear Localisationmentioning

confidence: 52%

“…Slip has also been inferred from optical interferometry experiments on glass and even steel surfaces under EHL conditions for viscous polybutadiene and polyphenyl ether (5P4E) lubricants [184]. However, slip has not yet been observed experimentally for realistic lubricants confined between conventional surfaces and thus its significance in tribology remains uncertain [185].…”

Section: Boundary Slipmentioning

confidence: 99%

“…However, it has also been inferred from film thickness measurements in realistic mineral oils [202,203] and there is currently a growing consensus that this phenomenon may also be important for more realistic lubricants under extreme EHL conditions [185]. NEMD simulations provide a useful complement to these experiments since they can more readily study the challenging conditions of interest.…”

Section: Shear Localisationmentioning

confidence: 99%

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Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

2018

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Nonequilibrium molecular dynamics (NEMD) simulations have provided unique insights into the nanoscale behaviour of lubricants under shear. This review discusses the early history of NEMD and its progression from a tool to corroborate theories of the liquid state, to an instrument that can directly evaluate important fluid properties, towards a potential design tool in tribology. The key methodological advances which have allowed this evolution are also highlighted. This is followed by a summary of bulk and confined NEMD simulations of liquid lubricants and lubricant additives, as they have progressed from simple atomic fluids to ever more complex, realistic molecules. The future outlook of NEMD in tribology, including the inclusion of chemical reactivity for additives, and coupling to continuum methods for large systems, is also briefly discussed.

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Section: Shear Localisationmentioning

confidence: 52%

Section: Boundary Slipmentioning

confidence: 99%

Section: Shear Localisationmentioning

confidence: 99%

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Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

2018

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“…The focus is not so much on the boundary lubrication and AFM type experiments, but more on the fully lubricated thick film limit which is a protype model closer to elastohydroynamic lubrication in real experiments. 37,73 The emphasis is on the nonequilibrium states-friction map relationships. The advantage of NEMD is it provides both the tribology and the associated confined film nonequilibrium phase diagram as a function of the wall sliding speed and applied normal pressure, the two being linked manifestations of the same state.…”

Section: Discussionmentioning

confidence: 99%

“…For example, Plug-Slip, 34,35 and Central Localization, 36 steady states have been detected using a novel methodology based on phosphorescence imaging. Martinie and Vergne, 37 also gave evidence that the LJ shear localised states are present in real lubricants under traction experiment conditions despite the simplicity of the computer model. These non-equilibrium phase transformations affect the lubrication performance by in general decreasing the traction coefficient below values expected for Couette flow acting across the system.…”

Section: Introductionmentioning

confidence: 99%

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

Maćkowiak

Heyes

Dini

et al. 2016

The Journal of Chemical Physics

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The phase behavior of a confined liquid at high pressure and shear rate, such as is found in elastohydrodynamic lubrication, can influence the traction characteristics in machine operation.Generic aspects of this behavior are investigated here using Non-equilibrium Molecular Dynamics (NEMD) simulations of confined Lennard-Jones (LJ) films under load with a recently proposed wall-driven shearing method without wall atom tethering [Gattinoni, Maćkowiak, Heyes, Brańka and Dini, Phys. Rev. E 90, 043302 (2014)]. The focus is on thick films in which the nonequilibrium phases formed in the confined region impact on the traction properties. The nonequilibrium phase and tribological diagrams are mapped out in detail as a function of load, wall sliding speed and atomic scale surface roughness, which is shown can have a significant effect. The transition between these phases is typically not sharp as the external conditions are varied. The magnitude of the friction coefficient depends strongly on the nonequilibrium phase adopted by the confined region of molecules, and in general does not follow the classical friction relations between macroscopic bodies, e.g., the frictional force can decrease with increasing load in the Plug-Slip (PS) region of the phase diagram owing to structural changes induced in the confined film. The friction coefficient can be extremely low (∼ 0.01) in the PS region as a result of incommensurate alignment between a (100) face-centered cubic wall plane and reconstructed (111) layers of the confined region near the wall. It is possible to exploit hysteresis to retain low friction PS states well into the Central Localization high wall speed region of the phase diagram.Stick-Slip behavior due to periodic in-plane melting of layers in the confined region and subsequent annealing is observed at low wall speeds and moderate external loads. At intermediate wall speeds and pressure values (at least) the friction coefficient decreases with increasing well depth of the LJ potential between the wall atoms, but increases when the attractive part of the potential between wall atoms and confined molecules is made larger.2

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Molecular simulation to discover rheological properties and soil‐binding ability of PHPA polymer on montmorillonite surface

Yang

Adhikari

2020

Surface & Interface Analysis

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Synthetic polymer fluids are increasingly being applied to support excavations in deep foundations. As these fluids are molecularly engineered, their underlying microstructure interaction with in situ soils significantly affect excavation stability and soil dispersion. However, little molecular‐scale research has been done on the rheological behavior of partially hydrolyzed polyacrylamides (PHPA) polymer fluids on the clay surface. Molecular models of the clay–polymer systems are constructed using PHPA on montmorillonite (MMT) clay surface. Initial rheological properties and soil‐binding ability at different shear rates, temperatures, and polymer concentrations are first studied using molecular dynamics (MD) simulations. It is found that the functional groups of PHPA can interact with the MMT surface and form a viscous film under the atomic interaction of hydrogen bonds, water bridges, and electrostatic attraction. The shear stress, σ increases with the shear rate and follows the power‐law model. And the viscosity, η decreases as the shear rate increases, which is consistent with the experimental trend. However, the σ and η decrease with the increase of temperature. And the action mode of PHPA concentration has been identified from the MD perspective. This work provides insight into the molecular mechanism for PHPA's rheology on the clay surface and their interaction.

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Lubrication at Extreme Conditions: A Discussion About the Limiting Shear Stress Concept

Cited by 28 publications

References 55 publications

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

Advances in nonequilibrium molecular dynamics simulations of lubricants and additives

Non-equilibrium phase behavior and friction of confined molecular films under shear: A non-equilibrium molecular dynamics study

Molecular simulation to discover rheological properties and soil‐binding ability of PHPA polymer on montmorillonite surface

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