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........ published in NEWSLETTER # 58

PHYSICS OF SLIDING FRICTION
by Dr. B.N.J. Persson, Forschungszentrum, Juelich (Germany)

The study of sliding friction is one of the oldest in physics and certainly one of the most important from a practical point of view. Low-friction surfaces are increasingly in demand in high-tech components such as computer storage sytems, miniature motors, and aerospace devices. It has been estimated that about 6% of gross national product in developed countries is "wasted" on friction and related wear.

The interfacial topic has experienced a major burst of activity since 1987, much of which has developed quite independently and spontaneously. This volume (NATO ASI SERIES E311) contains contributions from leading scientists on fundamental aspects of sliding friction. Two articles cover dry friction dynamics at low sliding velocities. A phenomenological theory is presented based on the non-linear interplay between slow plastic ageing, leading to reinforcement of the microcontracts, and creep motion of the interface under shear.

Four contributions address the origin and nature of stick-slip dynamics of lubricated surfaces. Experimental results show that the kinetic friction force in some cases exhibits giant fluctuations, indicating that the "liquid" formed structures at a constant rate and snapped suddenly, so that observed stress reflected competition between equilibration tendencies and imposed shear drive. Molecular dynamic simulations of the sliding dynamics of lubrication layers exposed to an external shear stress are presented. Sliding velocity as a function of the external shear stress exhibits hysteresis, giving a microscopic explanation for stick-slip motion.

Six contributions are presented concerning energy transfer at surfaces addressing the fundamental question of whether the electronic or phononic friction gives the main contribution to the friction force.

Two contributions are presented on the physics of snow friction and the sliding of glaciers. It is now fairly well established that snow friction is controlled by generation of melt water by frictional heating rather than pressure melting. The question of how thick the water layer needs to be in order to give the observed low sliding friction is addressed.

The use of atomic force and friction force microscopies to study adhesion and sliding friction is covered in seven articles. Information about the mechanical properties and deformation path of ultra-small metallic contacts is presented. High resolution friction force maps show that the friction force on a microscopic scale does not obey Amonton's law. Finally, the question of the role of self-organized criticality in sliding friction is addressed in two papers.

Because of the universal nature of friction and its economic impact, the present book should interest a broad range of scientists and mechanical and chemical engineers. It may also form the basis for a university course on tribology.
Reference books: E220, E311

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