Friction dissipating contact mechanical work

The so-called unit processes would then be the way in which friction is able to provided a means to dissipate contact mechanical work. Then how that work might produce irreversible chemical and mechanical damage. Finally, how that the damaged material could be displaced as debris from the contact zone and from without the contact to produce the wear debris and the mechanical material wear within the system. The remainder of this particular review will show what progress has been made in these areas of lnit research. 

Figure L The scheme showing the various units of a tribological system as developed by Salomon and Czichos using the so-called systems approach . Friction arises from contact mechanical work which causes dissipation and deformation which leads to damage which produces debris and when the debris is displaced in the contact wear accrues. In the process, significant interfacial modifications, third body formation , occurs. The latter can be in the form mechanical, morphological chemical or topographical modifications.

The energy used to overcome the frictional force is spent either in the form of mechanical work or as heat. As a result of the heat, two bodies in relative motion become warmed. As the contact areas of two bodies in relative motion are not smooth on a microscopic scale, the dissipation of mechanical energy in the form of heat will mainly be localised. The points of local heating, called hot spots, are initiators of explosive chemical reactions. The temperature of the hot spots depends upon... 

Two types of friction are commonly measured and calculated. The static friction Fj is defined as the minimum lateral force needed to initiate sliding of one object over a second, while the kinetic friction Fk v) is the force needed to maintain sliding at a steady velocity v. Observation of static friction implies that the contacting solids have locked into a local free-energy minimum, and Fj represents the force needed to lift them out of this minimum. It is a threshold rather than an actual force acting on the system, and it limits lateral motion in any direction. No work is done by the static friction, since no motion occurs. The kinetic friction is intrinsically related to dissipation mechanisms, and it equals the work done on the system by external forces divided by the distance moved. 

Fig. 1 Schematic description of cohesive and interfacial wear processes from the two terms non interacting model of friction (from [96]). Bulk ploughing involves the dissipation of the frictional work within a volume of the order of the cube of the contact radius. Interfacial shear corresponds to the dissipation of the frictional energy in much thinner regions and at greater energy densities. Cohesive wear processes (cracking, tearing, microcutting...) are governed by the cohesive strength of the polymer. Mechanisms such as transfer film formation correspond to interfacial wear and do not readily correlate with accessible bulk failure properties...

Work and Heat. In classical mechanics, when a force F displaces a body by an amount Js, the work done dW = ds. Work is measured in joules. Dissipative forces, such as friction between solids in contact, or viscous forces in liquids, can generate heat from work. Joule s experiments demonstrated that a certain amount of work, regardless of the manner in which it is performed, always produces the same amount of heat. Thus, the following equivalence between work and heat is established ... 

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