Friction sliding

Coefficient of friction for acrylonitrile-butadiene rubber sliding on wavy glass, plotted against logarithm of velocity times shift factor ar reduced to 20 C. Temperature dependence of ar corresponds to equation 40 of Chapter 11 with 7, 30 C. Points identify 13 different temperatures from —15 (at right) to 85 C (at left). (Grosch. ) 

For a polymer in the glassy or crystalline state, sliding on either another surface of the same polymer or a metal surface, the coefficient of friction passes through another maximum as a function of either velocity or temperature whose magnitude is lower (of the order of 0.4) than those of rubbery polymers. This has been attributed to energy losses associated with a secondary viscoelastic mechanism. 

---

Figure Bl.19.24. Friction loop and topography on a heterogeneous stepped surface. Terraces (2) and (3) are composed of different materials. In regions (1) and (4), the cantilever sticks to the sample surface because of static friction The sliding friction is tj on part (2) and on part 3. In a torsional force image, the contrast difference is caused by the relative sliding friction, Morphological effects may be...

Persson B N J 1997 Sliding Friction—Physical Principles and Applications (New York Springer)... 

Persson B N J and Tosatti E (eds) Physics of Sliding Friction (NATO ASI Series E311) (Dordreeht Kiuwer)... 

The PTFE micropowders are commonly used in plastics, inks, lubricants, and finishes such as lacquer. Lubricants containing micropowders are used for bearings, valve components, and other moving parts where sliding friction must be minimized or eliminated. Nonstick finishes that require good release properties, for example, in the food and packaging industry, commonly use PTFE micropowders. 

Wall Friction Angles. Wall friction values, important when characterizing the dow properties of a bulk soHd, are expressed as the wall friction angle or coefficient of sliding friction. The lower the friction, the less steep the hopper walls need to be to achieve mass dow. 

The following variables can affect wall friction values of a bulk soHd. (/) Pressure as the pressure acting normal to the wall increases, the coefficient of sliding friction often decreases. (2) Moisture content as moisture increases, many bulk soHds become more frictional. (3) Particle size and shape typically, fine materials are somewhat more frictional than coarse materials. Angular particles tend to dig into a wall surface, thereby creating more friction. (4) Temperature for many materials, higher temperatures cause particles to become more frictional. (5) Time of storage at rest if allowed to remain in contact with a wall surface, many soHds experience an increase in friction between the particles and the wall surface. (6) Wall surface smoother wall surfaces are typically less frictional. Corrosion of the surface obviously can affect the abiUty of the material to sHde on it. 

A slide surface is a surface where the tangential velocity can be discontinuous as shown in Fig. 9.9. Separate velocities are calculated for each side. Slide lines are useful for modeling phenomena such as sliding friction or flow through pipes. 

The sliding friction coefficient is another area of evaluation in gear couplings. It produces a resistance to the necessary axial movement as rotors heat... 

The cylinder is generally constructed of cast iron and includes the water jacket. The bore is machined and brought to a good finish to reduce the vane sliding friction. The inlet and outlet connections are flanged. The heads, which also house the bearings and stuffing box, are also made of cast iron. 

Boundary lubrication is perhaps best defined as the lubrication of surfaces by fluid films so thin that the friction coefficient is affected by both the type of lubricant and the nature of the surface, and is largely independent of viscosity. A fluid lubricant introduced between two surfaces may spread to a microscopically thin film that reduces the sliding friction between the surfaces. The peaks of the high spots may touch, but interlocking occurs only to a limited extent and frictional resistance will be relatively low. 

All sliding friction forces are dramatically affected by surface contamination. If the surface is covered with a material that prevents the adhesive forces from acting, the coefficient is reduced. If the material is a liquid which has low shear viscosity the condition exists of lubricated sliding where the characteristics of the liquid control the friction rather than the surface friction characteristics of the materials. It is possible by the addition of surface materials that have high adhesion to increase the coefficient of friction. 

Friction Sensitivity. By friction pendulum, it snaps when tested with a steel shoe (Ref 11). With an ABL-designed sliding friction machine, AP required a sliding dist of 16cm for 50% explns (NO 15cm) (Ref 24, p 20)... 

It has been found from MD simulations that friction of SAMs on diamond decreases with the increasing chain length of hydrocarbon molecules, but it remains relatively constant when the number of carbon atoms in the molecule chain exceeds a certain threshold [44], which confirmed the experimental observations. In simulations of sliding friction of L-B films, Glosli and McClelland [45] identified two different mechanisms of energy dissipation, namely, the viscous mechanism, similar to that in viscous liquid under shear, and the plucking mechanism related to the system instability that transfers the mechanical energy into heat, similar to that proposed in the Tomlinson model (see Chapter 9). On the basis of a series work of simulations performed in the similar... 

Glosli, J. N. and McClelland, G., "Molecular Dynamics Study of Sliding Friction of Ordered Organic Monolayers, Phys. Rev. Lett., Vol. 70,1993, pp. 1960-1963. 

FIGURE 26.1 Experimental friction data (left) as function log speed at different temperatures and master curve (right) of an acrylate-butadiene rubber (ABR) gum compound on a clean dry silicon carbide 180 track surface referred to room temperature. (From Grosch, K.A., Sliding Friction and Abbrasion of Rubbers, PhD thesis. University of London, London, 1963.)... 

K.A. Grosch, PhD thesis. Sliding Friction and Abrasion of Rubbers, University of London, 1963. 

Kagata, G. and Gong, J. P. (2007) Surface sliding friction of negatively charged... 

Sliding Friction between Concentric Carbon Nanotubes. 

Bright, J.D., Danchaivijit, S. and Shetty, D.K. (1991). Interfacial sliding friction in silicon carbidc-borosilicate glass composites a comparison of pullout and pushout tests. J. Am. Ceram. Soc. 74, 115-122. 

Jero, P.D. and Keran, R.J. (1990). The contribution of interfacial roughness to sliding friction of ceramic fibers in a glass matrix. Scripta Metall. Mater. 24, 2315-2318. 

---