ABSTRACT
In the preceding chapters we reviewed the foundations of mechanics and thermodynamics that are of relevance to friction. We discussed the basics of the theory of mechanical vibrations and stability, non-linear effects and selforganization, thermodynamic kinetic relationships, and stability of the stationary states. In this chapter we will discuss friction in depth: the empirical data about friction, its laws and mechanisms, thermodynamics of friction, and various models of friction. We will identify properties of friction that are invariant for different friction mechanisms and thus constitute the essence of friction as a physical phenomenon. The main characteristic of friction is that it is an interface phenomenon. The interactions across the interface are usually weaker than those in the bulk of the material. As a result, there is always a small parameter present in systems with friction. When the friction force is presented as a series by the small parameter, the dominating term is linear. This is why various mechanisms of friction lead to the same linear dependency of the friction force on the normal load. We will discuss also how linear phenomenological coefficients of non-equilibrium thermodynamics are related to viscous friction and how three-dimensional (3D) viscous friction can be reduced to dry Coulomb friction as a limiting case of the asymptotic transition to the two-dimensional (2D) interface. All of these considerations are intended to answer the question of why friction is so universal and what the general features of friction are.
