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Figure 16.2 Temperature dependence of the coefficient of friction of molybdenum disulfide in high vacuum
16.3 Friction at low temperatures
Friction at low temperatures takes place during the operation of cryogenic equipment , designed to obtain temperatures below 120 K ( cryogenic temperatures ), during the operation of machines in the conditions of Antarctica and the Arctic, rocket and space objects, equipment for nuclear, electronic and other industries.
The problem of friction at low temperatures, the development of which scientifically began in the 1950s, has not been sufficiently studied. The scientific discipline called low temperature physics singles out "cold" friction as a special area of research.
It is known that at low temperatures the hardness and brittleness of solids increase significantly. The effect of cold brittleness of metals is associated with the features of their crystal structure upon cooling. Cold brittleness is most characteristic of metals with a body-centered cubic lattice (Fe, Cr, Mo, Ta, W). Even small temperature differences, typical of friction at low temperatures, can cause the destruction of products made from these metals. Metals with face-centered cubic (Al, Ni, Pb, Cu) and hexagonal (Be, Mg, Zn, Ti) lattices are not so cold-breaking.
In "cold" friction, adhesion plays an important role. First, at low temperatures there are no conditions for the formation of oxide films on metals (with the exception of friction in liquid oxygen). Secondly, as the parts cool, the surface energy of the bodies increases. During friction in liquid nitrogen and liquid hydrogen, which is a strong reducing agent, the cleaned surfaces of metal parts become more prone to seizing. During the friction of metals in liquid oxygen, the formation of oxide films on the friction surfaces is so intense that it causes severe corrosion.
Liquefied gases cannot provide hydrodynamic lubrication due to their low viscosity. Frictional heating contributes to the evaporation of the liquefied gas in the contact zone. This can lead to cavitation - the formation of cavities in the liquid filled with gas. Cavitation causes a quick failure of friction units due to their damage.
The friction of organic polymers in ultrahigh vacuum is accompanied by a significant increase in the friction coefficient already at liquid nitrogen temperature (77K). At T = 13K, all polymers, including Teflon, seize with steel.
Cooling molybdenum disulfide to 203K has almost no effect on its coefficient of friction. Then, in a narrow temperature range, friction gradually increases by about 2 times. With further cooling, the change in the coefficient of friction MoS ₂ is insignificant.
Difficulties associated with the use of structural metals and traditional lubricants in low-temperature friction units have stimulated the development of special composite materials and coatings that ensure the operation of moving joints in space technology.