Sliding speed and

The effective temperatures in the sliding contact will of course, be significantly affected by frictional heating and therefore by sliding speed and load, but the important conclusion is that the increase in friction with rising temperature occurs at a temperature at least 100°C higher with tungsten disulphide than with molybdenum disulphide. 

In this section the interaction of sliding speed and friction, in particular the interpretation of the instrumental displays in the measurement of motion, will be examined more closely than it was in Sections 8.3 and 8.4. We are interested specifically in the reliable determination of with an apparatus that employs elastic coupling of the... 

Figure 3.90 Coefficient of friction of FEP vs sliding speed and pressure. ...

For low-carbon steels, typical sliding speeds, pressures, and friction welding process times are on the order of 1-5 m/sec, 50-140 MPa, and few seconds, respectively [30, 31]. In these conditions the contact interface can reach 1500 C, higher than the melting point of the bulk material. The sliding speeds and pressures needed to friction weld materials varies, but it appears that temperatures close to the melting point of the lower-melting-point component are necessary for the process. 

Polytetrafluoroethylene (PTFE, or Teflon) is reported to resist friction welding to other polymers even at high speeds and pressures [24]. PTFE has a much lower coefficient of friction and relatively higher softening point among the polymers. To reach the necessary conditions to weld it, sliding speed and pressure should be increased by a factor of 50 and 20 in comparison with high-density polyethylene [19]. 

I think so. Unless experiments are carried out with varying sliding speed and load conditions, it is not possible to say when this change will occur. 

With the X-ray technique described stresses in polymer wear films can be measured. One could incorporate a pin-on-disk friction device within the opening of the X-ray camera and conduct in situ measurements. The stresses developed in the film with repeated passes, increasing load, varied sliding speeds and polymer compositions can be measured. This could lead to a better understanding of the role of stresses in polymers on their wear behavior. 

The selection of an appropriate material, and the estimation of its likely performance in a particular application, can become complicated because of the combined effects of bearing pressure, temperature, counterface roughness, sliding speed, and other parameters, on the wear rate of the material. Guidance on the selection of polymer materials for bearings is available, for example... 

UHMWPE tends to form transfer layers on metal coimterfaces in water through interlameUar shear of the polymer in the same way as other low-friction polymers, such as polytetrafluorethylene (PTFE). The low friction coefficients of these materials are attributed to the ease with which the polymer molecules shear against each other. Most other polymers show poor friction properties due to lumpy transfer of material to the metal surface. However, lumpy transfer, in which debris adheres to the metal surface, can also occur for PTFE or UHMWPE under certain conditions. For example, lumpy transfer of PTFE occurs at low sliding speeds and was shown to give a friction coefficient that was approximately twice that of the thin transfer fQm. ... 

Coefficient of friction - 0.15-0.7 (against steel depending on sliding speed, and applied pressure) Mimaroglu, A Unal, H Arda, T, Wear, 262, 1407-13, 2007. 

These results show that at low sliding speeds and moderate temperatures P.T.F.E. and high density P.E. give a low coefficient of friction (of order ii-0.06) and produce a thin adherent film of polymer on the counter surface. Its thickness is of order 30-lOOA and the molecular chains are highly oriented in the direction of sliding. The behavior does not appear to depend on the crystallinity or band structure of the polymer or, in the case of P.E., on spher-ulite size. On the other hand if bulky side groups are incorporated into the molecular backbone the friction and transfer increase and resemble those of "normal polymers". These results suggest that the low-friction low-transfer behavior is connected with a smooth molecular profile. 

In order to minimize the effects of frictional heating all the measurements described earlier were made at sliding speeds below 1 cm s . Some measurements have been made at higher sliding speeds and at a contact pressure of 2 kg cm s . This combination covers the practical range over which tyres operate although the samples used are unfilled compounds and not therefore tyre compounds. The results, shown in Figure 12, indicate that the friction coefficient... 

Figure 9.21 Dynamic coefficient of friction versus sliding speed and pressure of Celanese GUR UHMWPE [9].

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