Lubricating Action

the capability of certain materials to lower friction and to reduce wear and damage on sliding surfaces. Lubricating action is caused by a reduction in the adhesive forces acting on bodies in contact and in the friction forces acting on the surface layers of sliding bodies.

Under boundary conditions, the thickness of the lubricating layer usually does not exceed 0.1 micrometer and sometimes consists of one or several molecular layers. In this case, the lubricating action is determined by the physicochemical properties of the extremely thin film forced as a result of adsorption or chemisorption of substances from a liquid or a gaseous medium on the surface of the solid body. When organic surfactants are used, the absorption layer has the form of a molecular nap over which the surfaces can slide. The lubricating action on surfactants is also frequently caused by a lowering of the strength of the material in the surface layer through adsorption and by a plasticization of this material. Surfactants and substances that react chemically with the surface are used as additives in petroleum and cooling lubricants to improve the lubricating action. When polymeric materials are in friction, lubricating action can arise from low-molecular-weight substances that are formed as a result of the mechanical and chemical degradation of the polymers.

In the case of friction between liquids where the thickness of the lubricating layer exceeds tenths of a micrometer, the reduction in friction is brought about chiefly by a separation of the sliding bodies. This effect is explained by the hydrodynamic theories of lubricating action developed by such scientists as N. P. Petrovand O. Reynolds.

The lubricating action of solids, for example, graphite and molybdenum disulfide, derives from the layer structure and the weakness of the cohesive forces between layers. Soft metals, such as lead, tin, and cadmium, and certain other materials deposited on the surfaces of harder bodies form a layer with a low resistance to shearing. In this case, the lubricating action is caused by the localization of shearing deformation in the thin surface layer.