Static and Sliding Friction

So you might have already thought to ask, If the world of friction is split into two simple cases of static and sliding friction, then why is the table shown in Figure 14.11 curvy instead of just having two horizontal lines Well, it turns... 

Orders of magnitude of both static and sliding friction coefficients of selected sohds are presented in Table 1.5. 

It is useful to differentiate between static and dynamic friction. Dynamic friction, also called kinetic friction, is the mechanical force between sliding or rolling surfaces that resists the movement. Static friction must be overcome to start the movement between two bodies which are initially at rest. 

In view of these complexities, it is remarkable that Eq. 4.1-4 represents numerous metal-metal, dry frictional data rather well, for both the static and sliding cases. Polymers, on the other hand, exhibit an even more complex frictional behavior on metal. This is, perhaps, not surprising, since the physical situation involves a relatively soft, viscoelastic, and temperature-dependent material in contact with a hard, elastic, and much less temperature- and rate-dependent material. Empirical evidence of these complexities is the nonlinear relationship between the frictional force and the normal load... 

Figure 7-5. Sliding behavior in static and kinetic friction. 1 Plate moving. 2 Block moves with plate. 3 Block slides on plate. 4 Plate and block stop. 5 Plate stops block slides back.

This table gives characteristic values of both the static and sliding coefficients of friction for a number of material combinations. In each case Material 1 is moving over the surface of Material 2. 

Figure 5o Relation between the coefficient of static and kinetic friction, Us and in the sliding of steel sphere and crystal-Unity Q > Us . Uk ...

Surface properties The molecular configuration of PTEE imparts a high degree of antiadhesiveness to its surfaces, and for the same reason these surfaces are hardly wettable. PTFE possesses the lowest friction coefficients of all solid materials, between 0.05 and 0.09. The static and dynamic friction coefficients are almost equal, so that there is no seizure or stick-slip action. Wear depends upon the condition and type of the other sliding surface and obviously depends upon the speed and loads. 

The friction force, which must be overcome between two surfaces at rest, is greater lhan the friction force, which develops between the siufaces sliding on each another. One speaks of static friction and sliding friction. The respective coefficients of proportionality // and //q are called coefifi-... 

The field of nanotribology evolves around the attempts to understand the relationship between macroscopic frictional forces and the microscopic properties of the embedded system. Recent revival of interest in friction [/, 2, 3, 4, 5, 6, 7, 8] has unraveled a broad range of phenomena and new behaviors which help shed light on some fundamental concepts which are already considered textbook material. These include the static and kinetic friction forces, transition to sliding, thinning, and memory effects, which have been widely discussed but whose microscopic origins are still lacking. 

This table gives characteristic values of both the static and sliding coefficients of friction for a number of material combinations. In each case Material 1 is moving over the surface of Material 2. The type of lubrication or any other special condition is indicated in the third column. All values refer to room temperature unless otherwise indicated. It should be emphasized that the coefficient of friction is very sensitive to the condition of the surface, so that these values represent only a rough guide. 

The only smooth surfaces we know are those of liquids at rest. No sliding friction is observed on them. Boundary lubricants fill the depressions on the rough surfaces and thus lower the effective angle of slope usually, they have also other functions which are of no interest to the problem at hand. Frictional vibrations (or "stick-slip" friction) has been explained long ago by the difference between static and dynamic friction. Perhaps you will be interested to consult my paper in Rev. Modern Physics (19 ). 

Figure 2. Coefficients of static and kinetic friction, Pg, and Pi, respectively plotted against concentration of HtP (mol percent) in a range of TFE - HFP copolymers, Load Ikgf sliding speed Imm s temperature 20 C.

A number of friction studies have been carried out on organic polymers in recent years. Coefficients of friction are for the most part in the normal range, with values about as expected from Eq. XII-5. The detailed results show some serious complications, however. First, n is very dependent on load, as illustrated in Fig. XlI-5, for a copolymer of hexafluoroethylene and hexafluoropropylene [31], and evidently the area of contact is determined more by elastic than by plastic deformation. The difference between static and kinetic coefficients of friction was attributed to transfer of an oriented film of polymer to the steel rider during sliding and to low adhesion between this film and the polymer surface. Tetrafluoroethylene (Telfon) has a low coefficient of friction, around 0.1, and in a detailed study, this lower coefficient and other differences were attributed to the rather smooth molecular profile of the Teflon molecule [32]. 

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...

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