The area of real contact and the shear strength of monomolecular layers of a boundary lubricant

Abstract

A method has been developed by which molecularly smooth surfaces may be placed together and the area of contact formed between them measured. Selected sheets of mica were cleaved to be free from cleavage steps on both sides of the sheets. These were bowed up and mounted as crossed cylinders in an apparatus in which normal and tangential loads could be applied. The area of contact formed between the surfaces has been examined using multiple-beam interference techniques. Examination of the hue and intensity of the central region of the interferograms enables a separation of the mica sheets to be detected even if this is of only a few ångströms. This means that the boundary of the region of contact may be determined with a greater precision than has hitherto been possible. Fringes of equal chromatic order provide a sensitive means for the detection of small particles of accidental contamination and have been used to show when the contact is intimate over the whole region. Normal and tangential forces have been applied to the mica specimens, both when in a clean condition and when covered with a monomolecular layer of calcium stearate. The area of contact was observed simultaneously. The area and the force required to shear it being known, the shear strength of the junctions has been calculated. The damage occurring during the process has been examined using reflexion electron microscopy. At certain values of the tangential force smooth sliding takes place on the lubricated surfaces and no surface damage can be detected. Further increase of the tangential force produces a rapid slip causing fragments to be torn out of the surface and some damage occurs. No smooth sliding was detected on unlubricated surfaces of mica and the damage produced during the slip was extensive. The force required to shear the junction formed between clean smooth mica surfaces is very high. A value of 10 Kg/mm 2 has been obtained. For the monomolecular layers of calcium stearate the value obtained is about forty times lower but is by no means negligible. This means that a considerable force is required to shear a film of boundary lubricant and accounts for the otherwise unexplained observation that when metals are lubricated with a single molecular layer of soap or fatty acid the frictional force is reduced by only a factor of 10 while the wear decreases by a factor of 10000 or more.