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

Lecture number 16.
Friction and wear under special conditions.
Friction in a vacuum, at low and high temperatures, under the influence of corpuscular and electromagnetic radiation has features that do not appear during friction under normal conditions. There are a lot of such friction units in modern technology and there are more and more. Their operation is associated with the specific influence of the environment on the friction mechanism.
16.1 Vacuum friction
Vacuum - the state of a gas or vapor with a pressure below atmospheric (10 ⁵ Pa). There are low (pressure above 10 ² Pa), medium (10 ² -10 ^-1 Pa), high (10 ^-1 -10 ^-5 Pa) and ultra-high (pressure below 10 ^-5 Pa) vacuum.
The problem of studying friction in a vacuum has become especially relevant with the advent of spacecraft. The first failures in their operation were caused by the failure of friction units. Ensuring the operability in space of bearings, devices for turning antennas, and devices for opening hatches required new technical solutions. These tasks have become more complicated in connection with the docking and undocking of ships, landing on the moon, the creation of self-propelled crews, etc. No less serious problems arose during the operation of friction units in vacuum equipment for melting, rolling, and vacuum arc welding of metals.
During evacuation, the dynamic equilibrium established in the atmosphere between the environment and the films adsorbed on the friction surfaces is disturbed. Adsorbed films are desorbed, and new ones are not formed due to the lack of adsorbed substances in vacuum. During friction in vacuum, the friction surfaces are gradually cleared of adsorbed films, as a result of which the friction coefficient, as a rule, increases with decreasing pressure (Fig. 5.1).
In the moving contact of clean surfaces, the release of heat increases and the probability of setting increases. The very first experiments, carried out in the 1940s in high vacuum, showed that friction leads to welding of metal parts. This is facilitated by a sharp decrease in heat transfer in vacuum compared to the atmosphere.
In high vacuum, the plasticity of metals often increases. This is caused, firstly, by facilitating the emergence of crystal lattice defects on clean surfaces and, secondly, by a decrease in gas impurities in the surface layers. Therefore, the adhesive component of the friction force in vacuum is higher than in air. Polymers lose mass in vacuum. The weight loss of polymeric materials in ultrahigh vacuum reaches 10% per year, which significantly worsens the properties of products made from them.