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TECHNICAL PAPERS

Formation of Boundary Lubricating Layers With Water-Based Lubricant in a Concentrated Elastohydrodynamic Contact

[+] Author and Article Information
Ph. Bouré, D. Mazuyer, J.-M. Georges

École Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Systèmes UMR CNRS 5513, B.P. 163, F-69131 Écully Cedex, France

A. A. Lubrecht

INSA Lyon, Laboratoire de Mécanique des Contacts, UMR CNRS 5514, 20, Boulevard Albert Einstein, F-69621 Villeurbanne Cedex, France

G. Lorentz

Rhodia recherche, C. R. A., 52, Rue de la Haie Coq, F-93308 Aubervilliers Cedex, France

J. Tribol 124(1), 91-102 (Mar 07, 2001) (12 pages) doi:10.1115/1.1398549 History: Received October 10, 2000; Revised March 07, 2001
Copyright © 2002 by ASME
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References

Figures

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Principle of the EHL machine used for the experiments. The ball and the disc are accurately and independently entrained by 2 stepping motors in the range 0.5 mm/s to 3 m/s. The applied load is between 0 and 25 N (contact pressure <0.5 GPa).
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Validation of the film thickness measurements based on image analysis of the interferometric image of the contact. The evolution of the film thickness is plotted versus entrainment speed, for a mineral oil with known viscosity (41 mPa.s) and pressure viscosity coefficient (22 GPa−1 ). The agreement of the experimental points with the predicted values from Hamrock-Dowson law show the efficiency of the film thickness measurement.
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Evolution of the lubricant film thickness with time for the lamellar dispersion at constant entrainment speed of 0.4 m/s. The curve can be divided in 2 zones: first, the thickness increases according to a growth rate given the origin slope of the curve h(t); second, the film thickness reaches a steady-state value hl. Then a critical time tc above which the stationary regime is obtained, can be defined as: tc=hl/(dh/dt)t=0.
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Pictures of the boundary at different steps of its formation in the EHL contact (entrainment speed=0.5 m/s): (a) in the beginning of the film build-up, the surface is partially covered by thin film patches; (b) when the steady-state regime is reached, the patchy films gather to homogeneously cover the whole track and the boundary layers become thicker. Thick pile-ups appear on both sides of the hertzian area.
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Evolution of the central film thickness versus time at two entrainment speeds Ue for the lamellar dispersion. The critical time tc is longer at 0.4 m/s than at 0.5 m/s. For entrainment speed higher than 0.5 m/s, this time is independent of speed. It can be noted that the limiting thickness hl increases with entrainment speed. When the entrainment speed is increased in the steady-state regime, from 0.4 m/s to 0.5 m/s, the film thickness increases and reaches the thickness of the boundary films formed at 0.5 m/s. The continuous curves derive from a starved lubrication model explained in part 4 and is in good agreement with the experimental points.
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Effect of sliding on the evolution of the central film thickness. When sliding is introduced (sliding speed=0.1 m/s and rolling speed=0.5 m/s) at constant entrainment speed during a pure rolling EHD experiment, the film thickness quickly drops to the value of the steady-state thickness of the film directly formed under sliding at the same slide/roll ratio. The decrease in the limiting film thickness is related to the shear stress induced by the introduction of sliding.
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Pictures of the boundary layers formed with the lamellar dispersion on a chromium disc in contact with a steel ball under: (1) pure rolling (Ue=0.5 m/s): the tracks on both the ball and the disc are covered by homogeneous layers whose thickness on each surface is half the limiting thickness hl; (2) partial sliding (Ue=0.5 m/s,Us=0.1 m/s, and Ud<Ub): Only the track on the disc is covered by a thin film whose thickness is the half of the thickness of the film formed under pure rolling at the same entrainment speed; and (3) Partial sliding (Ue=0.5 m/s,Us=−0.1 m/s, and Ud>Ub): as in the previous case, whatever the sign of the sliding speed, only the surface of the chromium disc is covered by a homogeneous film. It can be deduced that the appearance of the film on one of the surfaces is not related to an adsorption process and that the sliding induces a shearing at the interface between the film and the steel ball. This shows that the boundary layers are more adherent on chromium than on steel surfaces.
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Pictures of the boundary layers formed with the lamellar dispersion on a chromium disc in contact with a brass ball under: (1) Pure rolling (Ue=0.5 m/s): The tracks on both the ball and the disc are covered by homogeneous layers whose thickness on each surface is half the limiting thickness hl; (2) Partial sliding (Ue=0.5 m/s,Us=0.1 m/s, and Ud<Ub): only the track on the brass ball is covered by a thin film whose thickness is half of the thickness of the film formed under pure rolling at the same entrainment speed; and (3) partial sliding (Ue=0.5 m/s,Us=−0.1 m/s, and Ud>Ub): as in the previous case, whatever the sign of the sliding speed, only the surface of the ball is covered by a homogeneous film. The sliding induces a shearing at the interface between the film and the chromium. This shows that location of the shear plane depends on the nature of the surface.
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Summary of the different film formation according to the kinematics conditions and the nature of the contacting surfaces. Under pure rolling, viscous boundary films are formed on each surface and both ensure a starved lubrication of the contact. The film thickness is half the steady-state value hl. When sliding is introduced by keeping the entrainment speed constant, shearing occurs at the interface between the film and one of the surface. Only one of the surface is covered by a homogeneous film. Experiments performed with a brass/chromium contact and with steel/chromium contact show that the boundary layers are more adherent on the brass than on chromium surface and they are also more adherent on a chromium surface than on a steel surface.

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