An analytical review of existing hypotheses about the physics of friction

Machine-building Engineering and Machine Science

Machine science, drive systems and machinery


Аuthors

Legaev V. P.*, Generalov L. K.**, Galkovskii O. A.***

State University named after Alexander and Nikolay Stoletovs, 87, Gorky str. Vladimir, 600000, Russia

*e-mail: legaev@vlsu.ru
**e-mail: generalov@vlsu.ru
***e-mail: ogvlsu@mail.ru

Abstract

Assigned the task to determine the laws of change in the coefficient of friction and determine the factors affecting it as part of the research work. The purpose of this work is to improve the performance parameters of precision machines.

Physics of the external friction process has the next form. When the contacting solids are shifted, the external friction force increases due to deformation of these solids, this phenomenon is called preliminary displacement. Static friction force Fs is the force of friction, corresponds the highest value of the preliminary displacement. One of the contacting solids moves irreversibly (slides) across the surface of another solid after a static friction force has been achieved, in


Relation of external friction force F to the movement x this case the external force is equal to the kinetic friction force Fk [2].

Friction interaction occurs in certain parts of the nominal contact only. Friction interaction is the third solid. The complete complex of frictional links forms a frictional interaction, which is discrete [1].

The preliminary displacement is caused by the redistribution of the contact irregularities in the support surface [1].

The total static friction force is the boundary point, under which preliminary displacement friction passes to kinetic friction force.

Kinetic friction is the friction of two solids that are in motion relative to each other [4].

Kinetic friction has a dual molecular-mechanical nature. It is caused by volumetric deformation of the material and overcoming intermolecular bonds


where Ffm - is the molecular component of the friction force; Ffd - is the mechanical (deformation) component of the friction force [2].

If the adhesion bond is less strong than the underlying layers, then there is a positive gradient of mechanical properties at depth:


Under normal friction process, the positive gradient rule is always come true.

The contact is always discrete and the area external friction depends on the applied load at external friction. Contact surface is continuous and independent on the applied load at internal friction.

The coefficient of friction depends on three factors almost equally: combination of materials; construction of friction pair; operating mode [1].

To execute the rule of positive gradient must be present lubrication film in the friction contact, or oxide film, soft components film [1].

The growth of the film slows down with increasing its thickness [1].

The growth of the film reduces the coefficient of friction to a known limit. Very thick films increase the coefficient of friction [1].

The relative sliding of two solids produces heat in a thin surface layer. The temperature rising can lead to local softening and melting of the material. The temperature field leads to a change in the mechanical properties of the material in a thin surface layer. The intensity of the heat flow depends on the friction work and the size of the area on which it is generated [1].

Important constructive characteristics of the friction units is the coefficient of mutual overlap, proposed by A.V. Chichinadze,


where Aa1 - the nominal friction area of the first element; Aa2 - the nominal friction area of the second element; Aa2 ≤Aa1 .

Wear products have a great impact on the strength and coefficient of friction [2].

Friction and wear characteristics and mechanical properties of friction pairs materials are in various nonlinear functional dependencies. At the same time, these dependencies can significantly change from the friction mode and from the thermal mode of friction pairs.

The construction of the friction unit significantly affects the force and coefficient of friction. In this regard, the nominal Aa, contour Ac and actual areas of friction Ar, the coefficient of mutual overlap Kov, the shape and size of the contact elements, their stiffness and elasticity is among the main parameters determining friction.

More rigid elements of surfaces intrusion into softer counterbody due to waviness, roughness, heterogeneity of mechanical properties and duality of molecular-mechanical nature of friction.

Accordingly, the speed ν of the indenter determines the friction force. At the same time, an increase in the load on the separately selected indenter leads to an increase in the friction force. However, the support reaction force N affects the area of the actual contact Ar in the actual operating conditions of the friction pair. The actual contact area depends on the load. Increasing the area of actual contact reduces specific pressure. Thus, the dependence of the friction force on the relative velocity of the friction pair and the load is not linear and differs for different materials.

Summary:

  1. In the research of the friction of polymeric and metallic materials should be used adhesion- deformation theory of friction, which includes the definition of the molecular and mechanical components of the friction forces.

  2. The thermal and mechanical properties of materials should be determined by the known friction force of the friction pair.

  3. A positive gradient rule should be observed and lubrication films, oxide films or films of a soft component in the friction contact should be provided.

  4. It is necessary to determine the area of the contacting surfaces at the given micronutrients and friction forces.

  5. It is necessary to take into account the shape and size of the friction unit and the coefficient of mutual overlap.


Keywords:

friction, coefficient of friction, friction force

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