Left of the Stribeck curve

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge December 2012

Adsorbed molecules influence the way lubricated contacts change their friction with speed.
 

WE ARE ALL FAMILIAR WITH THE STRIBECK CURVE, which describes the way in which friction varies with viscosity, speed and load (captured by the Sommerfeld number, ηU/W) in a lubricated contact. At high values of the Sommerfeld number, the behavior is well understood and describes well-known rheological effects, but the behavior at low values generally remains shrouded in mystery.

Many textbooks simply show this boundary- lubrication region as a horizontal straight line, with the friction locally unaffected by the value of the speed, load or viscosity. Reality differs from this picture.

In the boundary region, where the contacts are close and chemistry rules supreme, effective lubrication generally relies on the use of additives. These compounds adsorb on the sliding surfaces and provide a lows-hear- strength interface, where sliding takes place with less friction and wear than it would be on the hard, asperity-ridden surfaces immediately beneath.

It turns out that the molecular structure of the additives plays a significant role in determining the shape of this boundary region of the Stribeck curve. In a recent paper published in Tribology Letters, Sophie Campen and Hugh Spikes of Imperial College, London, collaborating with Jonathan Green, Gordon Lamb and David Atkinson of Castrol, Ltd., have revisited the issue, both reviewing existing reports and generating interesting new data (1).

Many previous studies had shown that friction increases with sliding speed in lubricants containing organic friction modifiers such as stearic acid. Such behavior has important consequences, since a positive slope in this region of the Stribeck curve guarantees that stick-slip will not occur (thus avoiding clutch shudder in automatic transmissions, for example). The new experiments explored the boundary region, using a variety of additives dissolved in hexadecane. While all chains were 18 carbons in length, some contained double bonds. While straight-chained, saturated stearic acid showed the expected positive slope in the boundary region of the curve, the unsaturated oleic acid showed a slight negative slope, as shown in the figure.

The issue of whether it was the double bond itself or the bent shape of oleic acid that led to this difference was resolved by examining elaidic acid. This molecule, like oleic acid, contains a single double bond, but in the so-called trans configuration, which leads to a straight-chained molecule. In the Stribeck curve, elaidic acid behaved like stearic acid, showing that the straightness of the chain, rather than the double bond itself and presumably the ability of the molecules to pack into ordered monolayers, were influencing the sign of the slope.

Another approach to modifying the low Sommerfeld number region of the curve was recently reported by Robert Bielecki, Maura Crobu and Nic Spencer at the ETH Zurich, Switzerland (2). In this work, closely spaced, oil-compatible polymer chains (collectively referred to as a “polymer brush”), several hundred nanometers in length, were grown out of silicon and steel surfaces. The behavior under a variety of oils differed markedly from that of the bare surfaces (and the textbook Stribeck curve), in that the hydrodynamic region seemed to extend much further to the left and provided lower friction coefficients, finally leveling off at a roughly constant value.

In contrast to hard-hard interactions, even when modified with small-molecule additives, it appears that soft contact between oil-swollen brushes provides an intrinsically low value of friction, presumably provided by a readily sheared, oil-rich layer at the outer edges of the contacting polymer brushes.

FOR FURTHER READING:
1. Campen, S., Green, J., Lamb, G., Atkinson, D. and Spikes, H. (2012), “On the Increase in Boundary Friction with Sliding Speed, Tribology Letters, 48 (2), pp. 237-248.
2. Bielecki, R., Crobu, M. and Spencer, N.D., “Polymer-Brush Lubrication in Oil: Sliding Beyond the Stribeck Curve,” Tribology Letters, Currently in press.
 

Eddy Tysoe is a Distinguished Professor of Physical Chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.


Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland. Both serve as editors-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.