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

7.4 Fretting


Fretting wear occurs as a result of small oscillatory, reciprocating movements with amplitudes from several units to hundreds of micrometers.
Damage to friction surfaces in this case resembles pitting and is concentrated in small areas.
Tribocouples exposed to fretting in a corrosive environment wear out most intensively as a result of fretting corrosion .
Wear as a result of fretting corrosion has a two-stage character.

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  At the first stage, friction strengthens the microprotrusions of the interacting surfaces in the places of their actual contact. At the same time, oxide films are destroyed, and the interatomic interaction of juvenile surfaces increases. Due to the destruction of the setting nodes and fatigue damage to the irregularities, wear products are formed, and a corrosive environment is formed.
  
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  At the second stage of wear, corrosion-fatigue damage predominates, leading to an increase in wear intensity.
  

Fretting corrosion is significantly affected by the relative slip amplitude, contact pressure, oscillation frequency, number of loading cycles, composition of the environment, and temperature. The intensity of destruction of surfaces also depends on the tendency to weaken the surface layers and their loosening due to electrochemical, corrosion-fatigue, alternating, cyclic deformation.
The corrosive activity of the environment also significantly affects the wear rate. For example, wear in air and oxygen is higher than in vacuum, nitrogen and other gases. The destruction of open surfaces is more intense in winter. As the temperature decreases, the wear rate increases.
Wear due to fretting corrosion, as a rule, occurs in rivet, bolted, splined, keyed, pin connections, with press fittings of parts, in steel ropes, hinges, coupling joints, springs, valves, electrical contact regulators, bearing elements of helicopters and aircraft, parts of gas turbine engines.
To reduce wear from fretting corrosion, lubricants are used in which oxygen has a low solubility, materials with a reduced diffusion coefficient, high antioxidant stability, and increased adhesive and adsorption properties to contact surfaces. Wear intensity is also reduced by applying non-metallic coatings, using gaskets of corrosion-resistant materials. Of essential importance is the correct choice of contacting materials with increased resistance to abrasive, fatigue, and corrosion failure.