Sliding friction behavior

Fig. 22 Effect of clay content of NC gels on the sliding frictional behavior. The force profiles were measured in air and wet for N-NC gels with different clay contents (NC1-NC5). Insef. The principle of sliding frictional force measurement on the gel surface and a picture of sliding friction measurement on NC gel. Reprinted from Haraguchi and Takada [98], Copyright 2005, with permission of Wiley...

Phenomenal studies were made to observe the frictional behavior of L-B films and SAMs and its dependence on applied load and sliding velocity, which has been summarized in a review article by Zhang [33]. It has been confirmed that in comparison to the bare surface of the substrates, the friction on molecular films is significantly reduced, with friction coefficients in a range of 0.05-0.1. Friction forces are found... 

When the water film is squeezed out, the thick water layer is removed and the surfaces are separated by lubricant film of only molecular dimensions. Under these conditions, which are referred to as BL conditions, the very thin film of water is bonded to the substrate by very strong molecular adhesion forces and it has obviously lost its bulk fluid properties. The bulk viscosity of the water plays little or no part in the frictional behavior, which is influenced by the nature of the underlying surface. By comparing with the friction force of an elastomer sliding on a rigid surface in a dry state, Moore was able to conclude that for an elastomer sliding on a rigid surface under BL conditions, one can expect ... 

The frictional behavior of 316 stainless steel is shown in Fig. 8 for sliding in air or in different atmospheres. No clear trend in the dependence of friction coefficient on environment was observed. 

Fig. 40 Comparison of the friction behavior of PTFE-EPDM composites as a function of sliding distance...

This was corroborated by experimental results obtained in single particle experiments carried out for polypropylenes. It was possible to show that the relative attrition behavior under pure sliding friction stresses was identical to that observed in a pipe bend whereas deviations to that under pure normal impact conditions were found. For polymethylmethacrylate and polystyrene on the other hand, the relative attrition behavior in the pipe bend corresponds to that of the normal impact experiments. 

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... 

Erdemir, A., Kovalchenko, A., McNallan, M.J., Weltz, S., Lee, A., and Gogotsi, Y. Effects of high-temperature hydrogenation treatment on sliding friction and wear behavior of carbide-derived carbon films. Surf. Coat. Technol. 188-189, 2004 588-593. 

Figure 7-1 is a simple, generalized representation of the friction of two sliding bodies but it suffers from the drawback that conceptual probes must be introduced into the model in order to describe what occurs at the rubbing interface. Figure 7-3 depicts a direct experimental demonstration of frictional behavior and at the same time illustrates the basis of a method for the detection and measurement of the forces involved. The technique of elastic restraint shown there is not the only way to detect and measure friction, but it is the one used most frequently. 

The treatment o friction in the preceding chapter was primarily phenomenological, with emphasis on the observation and measurement of frictional behavior. In this chapter our inquiry will be directed toward the basic mechanisms of friction, particularly how the properties of solid surfaces and the conditions of sliding govern observable friction phenomena. 

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