Light-Scattering Study of the Glass Transition in Lubricants

Al-Saad, Mohammed; Winer, W. O.; Medina, F. D.; O’Shea, Donald C.

doi:10.1115/1.3453201

Cited by 12 publications

(4 citation statements)

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“…As for spherical molecules, confinement produces large shifts (∼ 0.8GP a) in the bulk transition pressure. These shifts are comparable to the glass transition pressures (∼0.5GPa) of bulk lubricants at room temperature [50], and one may expect that many lubricants vitrify in thin films.…”

Section: Equilibrium Structurementioning

confidence: 61%

Structure and Shear Response in Nanometer‐Thick Films

Thompson

Robbins

Grest

1995

Israel Journal of Chemistry

169

162

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Simulations of the structure and dynamics of fluid films confined to a thickness of a few molecular diameters are described. Confining walls introduce layering and in-plane order in the adjacent fluid. The latter is essential to transfer of shear stress. As the film thickness is decreased, by increasing pressure or decreasing the number of molecular layers, the entire film may undergo a phase transition. Spherical molecules tend to crystallize, while short chain molecules enter a glassy state with strong local orientational and translational order. These phase transitions lead to dramatic changes in the response of the film to imposed shear velocities v. Spherical molecules show an abrupt transition from Newtonian response to a yield stress as they crystallize. Chain molecules exhibit a continuously growing regime of non-Newtonian behavior where the shear viscosity drops as v −2/3 at constant normal load. The same power law is found for a wide range of parameters, and extends to lower and lower velocities as a glass transition is approached. Once in the glassy state, 1 chain molecules exhibit a finite yield stress. Shear may occur either within the film or at the film/wall interface. Interfacial shear dominates when films become glassy and when the film viscosity is increased by increasing the chain length.

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Section: Equilibrium Structurementioning

confidence: 61%

Structure and Shear Response in Nanometer‐Thick Films

Thompson

Robbins

Grest

1995

Israel Journal of Chemistry

169

162

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“…3, which does not match very well with the phase diagram, would fit if dimensionless pressure was chosen. Indeed, MCS1218 has a low glass transition pressure (P g of order of 117 MPa at 293.15 K [58], leading to P/P g & 1.2). However, this condition does not seem to be sufficient.…”

Section: Validation Of the Full Picture Of The Lubricant Behaviormentioning

confidence: 98%

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

Martinie

Vergne

2016

Tribol Lett

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Although limiting shear stress is a common behavior that is observed in many highly loaded lubricated contacts, its physical origins are still a matter of debate. Several scenarios are proposed in the literature to explain the macroscopic manifestations of the shearing response of a thin film of highly pressurized lubricant. Experimental measurements have been taken to capture these scenarios. However, limiting shear stress has never been simultaneously measured to validate any of them. Over the last decade, more and more molecular dynamics simulations have been performed to go further into details that are experimentally unattainable. This paper aims at comparing both approaches to studying the limiting shear stress, and more specifically at experimentally validating a lubricant phase diagram derived from molecular dynamics simulations. It suggests that the physical mechanism that initiates limiting shear stress is shear localization.

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“…e breakdown phenomenon could also be treated similar to electric breakdown of insulating oils [25][26][27][28][29][30][31]. is assumption should be taken with precaution since a higher load of operation (in order of 100's of MPa to a few GPa) on the liquid lubricating lm in the bearing results in a glassy state of the lubricant, where the lubricant does not have liquid-like properties [32][33][34].…”

Section: Electrical Breakdowns Mechanisms Inside Bearingmentioning

confidence: 99%

Investigations of Particle-Initiated Insulation Breakdowns in Bearings

Joshi

Blennow

2019

Journal of Renewable Energy

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Early failures in bearings of wind turbine drivetrains have increased after introduction of power electronic switches, which leads to shaft voltages and bearing currents. In presence of voltage, a rupture of bearing insulation could occur due to several plausible electro-physical mechanisms viz., asperities, electric breakdowns, particles, etc. The flow of high amperage current through the bearing during a breakdown mechanism could lead to early failures. Our aim is to understand the electrical behaviour of a bearing and elaborate by an equivalent electric circuit model, emphasizing on particle-initiated breakdowns. In presence of a shaft voltage, the particles form a path of low resistance through the bearing and results in flow of shaft or bearing currents, which could cause pre-mature failure of the bearing. Particles such as Arizona Test Dust (ATD), carbon black, aluminium powder and fine iron powder were mixed in lubricant at particle concentrations ranging between 7.5 mg/L and 150 mg/L. The breakdown characteristics of electrical insulation of the bearing during a given test is quantified as time of conduction, which is expressed as a percentage of the time the bearing is in the conducting state during a test. An investigation of time of conduction for different lubricant samples was conducted along with studying the effects of start and stops of the rotating shaft. The electrical conductive nature of the particle played no role in breakdown of bearing voltage. At a fixed concentration of 150 mg/L, the insulation breakdown events were highest in lubricant with ATD, followed by iron powder, aluminium power and carbon black particles. The time of conduction increases up to 24 times for the same test lubricant, as the particle concentration was increased from 7.5 mg/L to 150 mg/L. The current activity reduced to almost half in the test after stopping the shaft rotation. The resistance of bearing during insulation breakdown events is highest for aluminium powder, followed by fine iron powder, carbon black and ATD.

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Light-Scattering Study of the Glass Transition in Lubricants

Cited by 12 publications

References 0 publications

Structure and Shear Response in Nanometer‐Thick Films

Structure and Shear Response in Nanometer‐Thick Films

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

Investigations of Particle-Initiated Insulation Breakdowns in Bearings

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