Content area
Full Text
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, ??/W) in a lubricated contact. At high values of the Sommerfeld number, the behavior is well understood and describes well-known Theological 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 lowshear-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...