At very high normal loads the contact is no longer discrete, slip is localized in the solid surface layers, and only the plasticizing action of adsorption-active liquids may be considered in the analysis of the lubricating action (12, 14).

Physico-chemical aspects of the lubricating action are analyzed, and it is shown this action entails the formation of a boundary film and adsorption-induced plasticizing of metal surfaces. Data relating to the influence exerted on these phenomena by various (including new) iubricating materials is reported. It is shown that friction under boundary lubrication conditions is related to the thickness and stability of the lubricant film, and factors affecting these properties of Iubricant films are analyzed in the light of experimental data. For a long time the lubricating action of mineral oils and certain simple surface-active substances (SAS) was associated with the deviation from the hydronamic theory of lubrication [I-4]. The appearance of numerous new lubricating materials** and the progress in the theory of surface phenomena (including friction) necessitated a new approach to the old problem. In general terms the lubricating action may be defined as a reduction produced in the friction bond strength by lubricating materials or products of their transformation present in the friction zone. As is known [11], the friction bond depends on the adhesion force, the real surface area over which this force is acting, the resistance to shear in the contact zone being deformed or dispersed and the depth of this zone, i.e., the volume of deformed material. Accordingly, the friction force consists of adhesive and mechanical components. A lubricant may affect one or both of these components. And so, SAS not only screen intermolecular forces [2] but also plasticize metal surface layers [12] These substances, as well as lubricating liquids, powdered solid lubricants and antifriction coatings, concentrate the "weak" points of the friction bond and reduce the thickness of the region of shear. The localization of slip in a single plane is an important factor in boundary lubrication of real (rough) surfaces [18]. Effects that determine the lubricating action are of mechanical, chemical and physico-chemical character. The most intensely studied is the hydrodynamic (mechanical) aspect of theproblem. In considering the physical chemistry of the lubricating action it is pertinent to point out that under liquid lubrication conditions the slip is fully localized in the liquid flm whose thickness h and stability depend on the viscosity ~ of the liquid and on the friction parameters (contact pressure P, slip rate V, geometry of the parts in contact), while the friction coefficient is proportional to hV/P; in the first approximation h is an inverse function of P. In the case of physico-chemical effects of the lubricating.action (especially under boundary lubrication conditions), not only no simple qualitative relations between these effects and properties of lubricating flms have been established, but even the qualitative aspects of the problem have not been fully elucidated. We shall therefore analyze physico-chemical factors determining the stability and thickness of lubricating films and the influence of these factors on friction.