Lubricant flow in thin-film elastohydrodynamic contact under extreme conditions

Šperka, Petr; Křupka, Ivan; Hartl, Martin

doi:10.1007/s40544-016-0134-6

Cited by 10 publications

(8 citation statements)

References 26 publications

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“…9 PS has been observed experimentally for viscous polybutadiene 30,31 and has also been inferred from film thickness measurements in realistic mineral oils. 4,5 Fig . 8 shows the change in the velocity profile with sliding velocity for a representative fluid, DM2H.…”

Section: Flow Behaviourmentioning

confidence: 99%

“…1 There is also growing evidence to suggest that very high shear stresses can lead to nonequilibrium phase transitions which result in deviations from Couette flow. [4][5][6] However, such phase transitions have proved difficult to capture experimentally and neither the physical driving force behind them, nor their influence on the friction behaviour, is yet well understood. 2,6 Confined nonequilibrium molecular dynamics (NEMD) simulations have proved a valuable tool with which to investigate the friction behaviour of thin films of a range of fluids.…”

Section: Introductionmentioning

confidence: 99%

“…[27][28][29][30][31][32][33] However, there is currently a growing consensus that the phenomenon may also be important for more realistic lubricants under extreme conditions, for example those experienced in the EHL regime. [4][5][6] In NEMD simulations of binary mixtures of Lennard-Jones particles, crystallisation was suppressed and only liquid or glassy phases were formed. 2 In these binary systems, the friction coefficient increased linearly with log(sliding velocity), 2 as is frequently observed for realistic lubricants, such as polyalphaolefin (PAO), under EHL conditions.…”

Section: Introductionmentioning

confidence: 99%

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On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

Ewen

Gattinoni

Zhang

et al. 2017

Phys. Chem. Chem. Phys.

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A detailed understanding of the behaviour of confined fluids is critical to a range of industrial applications, for example to control friction in engineering components. In this study, a combination of tribological experiments and confined nonequilibrium molecular dynamics simulations has been used to investigate the effect of base fluid molecular structure on nonequilibrium phase behaviour and friction. An extensive parameter study, including several lubricant and traction fluid molecules subjected to pressures (0.5-2.0 GPa) and strain rates (10-10 s) typical of the elastohydrodynamic lubrication regime, reveals clear relationships between the friction and flow behaviour. Lubricants, which are flexible, broadly linear molecules, give low friction coefficients that increase with strain rate and pressure in both the experiments and the simulations. Conversely, traction fluids, which are based on inflexible cycloaliphatic groups, give high friction coefficients that only weakly depend on strain rate and pressure. The observed differences in friction behaviour can be rationalised through the stronger shear localisation which is observed for the traction fluids in the simulations. Higher pressures lead to more pronounced shear localisation, whilst increased strain rates lead to a widening of the sheared region. The methods utilised in this study have clarified the physical mechanisms of important confined fluid behaviour and show significant potential in both improving the prediction of elastohydrodynamic friction and developing new molecules to control it.

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Section: Flow Behaviourmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the effect of confined fluid molecular structure on nonequilibrium phase behaviour and friction

Ewen

Gattinoni

Zhang

et al. 2017

Phys. Chem. Chem. Phys.

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“…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%

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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“…[4][5][6] An example is plug ow where the lubricant ows as a solidied plug at the mean entrainment speed, with the shear zones close to the walls. [7][8][9][10] As a consequence of the uncertainty surrounding actual ow conditions, there is still disagreement as to which constitutive equation most accurately describes the rheology of lubricants in rolling-sliding contacts under EHD conditions. 1 There is, therefore, a clear need for in situ experimental techniques which measure important physical quantities such as ow, pressure, temperature and viscosity in EHD contacts with good spatial and temporal resolution.…”

Section: Introductionmentioning

confidence: 99%