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Figure 16.1 dependence of the coefficient of friction of a steel-steel pair on the degree of rarefaction.
1 - low vacuum, 2 - medium, 3 - high vacuum
The components of liquid lubricants evaporate in vacuum at different rates. This can lead to a change in the composition of composite materials and deterioration of their lubricating characteristics. In a high vacuum, petroleum oils cease to perform their functions. The reason is that in a vacuum there is no oxygen and water vapor, which are necessary for the formation of soaps when rubbing oil-lubricated metal parts.
A proven method for reducing friction in a vacuum is the use of solid lubricants. An outstanding role in the creation of space technology was played by molybdenum disulfide. Teflon is widely used in the form of thin coatings and in rotaprint lubrication systems. Teflon coated rubber ("slippery" rubber) is used in vacuum seals, docking systems, etc.
A.A. Silin, E.A. Dukhovsky et al. discovered in 1969 the phenomenon of ultra-low friction. The coefficient of friction of polyethylene and some other materials in vapors with metals when exposed to a stream of helium atoms in a vacuum decreases by two orders of magnitude (up to 10 ^-3 Pa). This happens because a thin layer of material under the friction surface goes into an ordered state with low surface energy. The role of vacuum in this phenomenon is to clean the friction surfaces, without which the transition of the surface layer to a new energy state is impossible.
16.2 Friction at high temperatures
Many friction units of industrial equipment (furnaces, rolling mills, power plants, etc.) operate at temperatures of the order of 1273K. The tendency to increase temperatures in friction units up to the melting temperatures of materials is observed during the creation of aviation and rocket and space technology.
The phenomena that occur during friction at high temperatures depend on many factors that act simultaneously:
1) at high temperatures, the mechanical properties of materials change, the gaps in the friction units and the geometric shape of the parts is distorted;
2) the chemical activity of friction surfaces that interact with each other and the environment increases sharply;
3) the processes of diffusion and desorption in materials are accelerated, which leads to significant changes in the friction surfaces.
The limiting action of heating consists in melting the surface layer of one of the rubbing bodies. Then external friction spontaneously transforms into internal friction, which is described by the hydrodynamic theory of lubrication. Dependences of the coefficient of friction of structural materials on temperature have the form of curves with an electremum. This indicates the effect on friction of at least two competing processes:
1) an increase in adhesion due to an increase in the plasticity of materials and an acceleration of diffusion and sintering processes in the areas of contact;
2) reduction of deformation of surface layers due to self-orientation of structural elements on friction surfaces and changes in the properties of the surface layer when interacting with the environment.
On metals, the latter process usually proceeds in the form of oxidation of the friction surface. If the oxide layer is hard and brittle, and the underlying metal is soft and ductile (Al, Pb, Sn), then the oxide layer is easily destroyed, leading to seizing. If the deformation-strength parameters of the oxide and metal are close (Cu, Fe), the oxide layer is preserved even under heavy loads. A sharp change in the dependence of friction on temperature, as a rule, indicates a structural transformation in the material. Thus, in cobalt at 690 K, the hexagonal structure of the crystal lattice is transformed into a face-centered one. This is accompanied by an increase in the coefficient of friction.
The friction of most solid lubricants decreases with increasing temperature. After reaching a certain critical temperature, friction increases rapidly. This is due to the activation of surface phenomena or structural transformations. So, a sharp increase in the coefficient of friction of molybdenum disulfide in a pair with steel (Fig. 5.2) is caused by the transformation of a mixed-type crystal lattice into a hexagonal one in MoS _{2 . MoS 2} , graphite, and soft metal coatings (Au, Ag, Ni, Co, etc.) are used to lubricate high-temperature friction units . Silicate glasses are a specific lubricant. They are used in the temperature range 1273-1773K for pipe drawing, rolling, stamping, etc.