O’zbekistоn respublikasi оliy va o’rta maxsus ta`lim vazirligi

4.3 Friction with boundary lubrication
Friction without lubricant is quite rare in technology: in brakes and friction gears, in equipment of the textile, food, and pharmaceutical industries, where lubricant is unacceptable in order to avoid damage to products, as well as in high-temperature friction units, when any lubricant is unsuitable. In most other cases, the friction surfaces are separated by a layer of lubricant. Friction during boundary lubrication occurs when the thickness of the lubricating layer is commensurate with the size of several molecules and amounts to hundredths of a micrometer. The physical properties of such layers differ from the bulk properties of the lubricant. Boundary layers are able to withstand high normal pressures without collapsing.
Boundary layers are formed by the adsorption mechanism. A feature of the molecular structure of fatty acids, esters and other components of oils is the presence of a long chain. During adsorption, long molecules with an active group are arranged along the normal to the friction surface parallel to each other. This gives the boundary layer sufficient strength due to the forces of molecular attraction. The large length of the chains allows them to tilt and bend under the influence of sliding, which helps to reduce friction. For boundary friction, it is sufficient to form a saturated monolayer of molecules; however, in practice, boundary layers have a multimolecular structure, Fig. 3.2.
The disorientation (desorption) of the molecules of the boundary layer essentially depends on the temperature. Desorption temperature T _d T _p , where T _p is the melting point of the lubricant. The boundary layer formed by the mechanism of physical adsorption evaporates at T=T _p . The chemically adsorbed layer is more heat resistant. The main position of the "chemical" theory of boundary lubrication developed by F. Bowden is the presence of chemical bonds between the lubricating layer and the friction surface. MM. Khrushchev and R.M. Matveevsky developed on this basis a temperature method for assessing the limiting lubricity of oils.