Transfer wear rates, PTFE composites

A discussion of the wear of PTFE would not be complete without some reference to PTFE composites. This has been a popular field of study simply because without fillers the wear of PTFE is normally unacceptable. A good filler will reduce transfer wear rates by up to three orders of magnitude. Various mechanisms have been proposed and the subject has been reviewed by the present author (8,9) and others (2,52). The simplest idea is that fillers wear less than the polymer when exposed at the interface. They may also suppress transfer and improve transfer film adhesion, A good deal of effort of high quality has been put into the search for chemically induced adhesion promotion at the transferred film-substrate interface but the evidence is equivocal (53,54). Chemical changes are detected but their precise contribution to the adhesion is uncertain in commercial applications. PTFE is a remarkably stable polymer to chemical attack even at sliding interfaces. 

Figure 9. Transfer wear rates in "lubricants" of two PTFE composites (, PTFE-polyimide and 0, PTFE-25% w.w. I carbon fibre) as a function of the solubility parameter, 6, of the lubricating media. The wear of the dry contacts is shown at 6 = 0 and the calculated value of 6 for PTFE is ca. 6.0. The solubility parameter is defined as the square root of the cohesive energy density and is therefore nearly proportional to the square root of the surface tension, of the fluid. The trend for the wear to increase with y and 6 is apparent. In the dry contact secure transfer films are formed but they are not evident in lubricated contacts. It is reasonable to suppose that as y increases the wetting of the steel counterface improves and hence the transfer films are more readily displaced. Data adapted from Lancaster and Evans.

Fig. 12. Transfer film formed on abraded steel (AISI 02 tool steel) surface (i a = 0.11 fim) as a result of dry sliding (a) unfilled and (b) filled PEEK after 70,000 cycles of sliding. The filled specimen had 25 vol% of CuS + 10 vol% of PTFE. The unfilled PEEK specimen does not show uniform film formation and, in contrast, the filled specimen forms uniformly covered film on the steel counterface. The tests were conducted on a pin-on-disk apparatus with 63 mm track diameter, sliding velocity 1 m/s and a normal load of 19.6 N (pressure 0.654 MPa). There was about 90% reduction in the wear rate as a result of transfer film formation, while the coefficient of friction for the composite was higher ( 0.43) in comparison to that for the virgin PEEK ( 0.4) (75). It was concluded in this work that first Cu atoms are formed as a result of reduction of CuS during the sliding interaction. Then fluorine atoms in the PTFE molecules react with Fe atoms of the counterface in the presence of Cu and thus forming FeF2. This chemical reaction helped in the formation of strong transfer layer on the counterface which was not possible for the case of PEEK with CuS (40 vol%) alone without the presence of PTFE molecules. Reprinted from Ref. 75, copyright 1995, with kind permission om Elsevier Science.

First, the filler may retard reorientation at the composite interface and thereby suppress the rate of transfer film deposition. Second, the filler may produce local stress intensifications within the transferred layer and hence, by some u e gain means, produce a more strongly attached transferred film. These two effects would be potentially capable of retarding the overall rate of transfer wear and there is a body of indirect evidence which indicates that they are likely to play a significant role. For example, there are certain synergistic effects when mixed oxide fillers are incorporated into PTFE a density polythene and the composites... 

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