Physical nature and properties of dynamic surface layers in friction

Simulations of the dynamic processes in micro contacts with the Method of Movable Cellular Automata (MCA) show that their common feature is formation of a boundary layer where intensive plastic deformation and mixing processes occur. The boundary layer is well localized and does not spread to deeper layers. We call this layer a ‘quasi-fluid layer’. The thickness of the boundary layer is roughly proportional to the viscosity of solid. This parameter thus should play an important role in determining the wear rate of materials in friction.To better understand the physical nature of the dynamic surface layers, we consider a simplified model of a solid consisting of many thin sheets, interacting with each other according to a ‘friction law’ of Coulomb type. A quasi-fluid layer is always developing if the ‘friction law’ does allow a bi-stability in some range of stresses with one static and one dynamic state at the same stress.The existence of the boundary layer motivates us to change the existing approach to calculating wear in frictional contacts. The wear should be understood not as ‘fracture’ but as ‘mass transport out of friction zone’. The process of stochastic transport of wear particles in the closed friction zone is at the same time the main mechanism of development of surface topography.A very important fact is that the conditions for appearance of a quasi-fluid layer depend on the minimal size of structural elements of the medium, which means that this effect cannot be principally described in the frame of a continuum model.