Friction mechanisms

The demonstration that superlubricity is possible, at least from a theoretical standpoint, indicates that solids are not required to exhibit friction under all conditions. However, virtually all surfaces exhibit friction as a result of various mechanisms that lead to deviations from the conditions necessary 

If k is very large, i.e., k is greater than the maximum curvature of the potential, V ax = (2n/a)2Vo, there will always be a unique equilibrium position for the atom of xeq t o/ and the atom will always be close to xeq. Consequently, the friction will be linear in t o for small values of t o. 

Many other mechanisms lead to energy dissipation, although they may be less universal than those related to boundary lubricant-induced geometric frustration. Chemical changes in lubricant molecules, reversible or irreversible, produce heat. Examples are configurational changes in hydrogen-terminated 

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Numerous AFM imaging techniques have been developed and commercialized to monitor topography, friction, mechanical response, capacitance, magnetic properties, etc. However, adhesion measurements require the tip to come into, and out of, contact to measure attractive and adhesion forces. Therefore, other than to select an analysis region, most imaging techniques are not useful for adhesion studies. Instead, measurements are necessarily based on force-displacement curve approaches. 

Although this treatment does not explicitly involve interactions at a solid-liquid interface, the application of Green s function to find the stochastic friction force may be an excellent opportunity for modeling interfacial friction and coupling, in the presence of liquid. An interesting note made by the authors is that the stochastic friction mechanism is proportional to the square of the frequency. This will likely be the case for interfacial friction as well. 

Halochromism) Tribochromism Kezochromism (pH change) Mechanical friction Mechanical pressure (ditto)... 

A fuze is a complicated mechanical device, metallic or plastic, housing combustible and/or explosive components in a row known as "train". It is designed to initiate an item of ammunition on being subjected to one of the following actions stab, percussion, friction, mechanical time, chemical, electrical or hydrostatic pressure... 

The internal viscosity force is defined phenomenologically by equations (2.26) formulated above. Various internal-friction mechanisms, discussed in a number of studies (Adelman and Freed 1977 Dasbach et al. 1992 Gennes 1977 Kuhn and Kuhn 1945 Maclnnes 1977a, 1977b Peterlin 1972 Rabin and Ottinger 1990) are possible. Investigation of various models should lead to the determination of matrices Ca/3 and Ga and the dependence of the internal friction coefficients on the chain length and on the parameters of the macromolecule. 

Uemura, M., Okada, K., Mogami, A. and Okitsu, A., Effect of Friction Mechanisms on Friction Coefficient of MoSj in an Ultrahigh Vacuum, Lubric. Eng., 43, 937, (1987). 

Silin A.A. and Aparin, V.I., The Role of Adhesion in the Orientation of Molybdenite During Friction, Mechanical Sciences, 1, 92, (1979). 

Figure 1. Schematic representation of various possible friction mechanisms (a) Geometric interlocking of asperities with typical angle 0, (b) elastic deformation (stretched dashed bonds) to interlock atoms and/or macroscopic peaks, resulting in multiple metastable states, (c) defect pinning (circles), (d) pinning by an intervening layer of weakly bound material, (e) plastic deformation or plowing, and (f) material mixing or cold welding.

The power law found in their simulations for the interaction of an amorphous tip with a crystalline substrate is similar to that observed in AFM experiments in argon atmosphere [75,76]. Also AFM studies of friction on layered materials [77] showed striking similarity with Fig. 6 in the wearless regime, although the experiments were conducted in atmosphere. In the latter case, there is even an order of magnitude agreement between the experimentally measured friction and simulated friction. The prefactors observed in another set of experiments [75,76] are, however, distinctly larger than those in the simulations. Moreover, the experimental results by Schwarz et al. [76] obtained with tips of different radii of curvature seem consistent with a picture in which the friction force is proportional to the area of contact and independent of the normal pressure. It thus remains an open question what friction mechanism is predominant in these experiments. 

It might yet be possible to predict trends from Eq. (52). Persson and Tosatti [27] argue that decreases quickly with increasing normal pressure. This has important consequences for tectonic motion for instance, small earthquakes typically do not occur close to the earth s surface. Sokoloff [122] concluded recently that even for his small value of the contribution to kinetic friction due to elastic instabilities that result from a competition between surface roughness and elastic interactions would lead to rather small friction coefficients on the order of 10 provided that no contamination layer or other local friction mechanisms were present in the contacts. 

Proposed Gel Friction Mechanism A Repulsion-Adsorption Model... 

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