The shear strength of thin lubricant films

Abstract

This paper describes a study of the effects of pressure and temperature on the shear strength of very thin layers of a number of lubricants. The shear strength is deduced from measurements of the tangential (frictional) force required to slide glass spheres over glass plates coated with the lubricant. It is assumed that no glass-glass contact occurs through the lubricant film and that the area of lubricant sheared is the geometrical contact area determined by the elastic constants of the sliding surfaces. Consequently by varying the radius of the sphere and the normal load the contact pressure can be varied from 10⁷ to 5 x 10⁹ Pa (1 Pa = 1 Nm^-2). The results show that the shear strength of Langmuir-Blodgett films of calcium stearate is constant at low pressures, but at pressures greater than 5 x 10⁷ Pa it increases approximately in proportion to the pressure. At higher temperatures the behaviour is similar but the shear strength is reduced at all pressures. Similar experiments have been carried out on retracted stearic acid films, on vapour deposited sebacic acid films and on smeared films of copper stearate and anthracene. Thin films of high density polythene and PTFE have also been studied. For these materials both the absolute value of the shear strength and its variation with pressure resemble the behaviour of calcium stearate. Experiments on the effect of temperature on the shear strength show that calcium stearate, stearic acid and high density polythene exhibit similar ‘activation energies’ for the shear process. This suggests that these materials all shear by a similar process involving the sliding of linearly orientated methylene groups over one another. PTFE behaves in a similar way although its ‘activation energy’ is a little higher. By contrast anthracene and sebacic acid have much lower activation energies suggesting that a different shear process is involved.