Transfer wear

The scale models must be carefully designed. Failure to match the important dimensionless parameters will lead to erroneous simulation results. Modeling can be extended to particle convective heat transfer. Wear or erosion of in-bed surfaces can be qualitatively studied, although quantitative assessment requires the identification and simulation of additional wear-related parameters. 

Some of the tool materials incorporate different special metals providing improvements in heat transfer, wear resistance of mating mold halves, etc. These special metals include beryllium copper alloy, brass, aluminum, kirksite, and sintered metal. 

More detailed study indicates that transfer wear actually exists during CMP.52 The interactions between a copper wafer and urethane polishing pads were characterized to investigate the effects of friction on removal mechanisms of a polishing system of copper interconnected wafers in water. In situ... 

The wear characteristics of polytetrafluoroethylene (PTFE) have been widely studied it is an important commercial polymer. This special attention has sometimes created a thesis that this polymer has very unusual or special wear characteristics when compared with the response of other polymers. This review compares the wear behaviour of PTFE with that of a range of pol37mers and examines the basis of this belief. The experimental evidence indicates that it is only in the area of transfer wear that a major contrast in characteristics is seen. Even in this restricted wear mode the differences are arguably ones of extent and not kind. 

These things are well known and numerous specific papers and review articles emphasise these points from various viewpoints (1-9). In summary, the tribology of PTFE, althoi gh somewhat unusual in some respects, is not exceptionally different from that of other organic polymers, particularly low temperature (gross softening below ca. 350°C) thermoplastics. The review will focus on the way in which PTFE differs from other polymers bearing in mind that these differences are not really ones of kind but of extent. Three topics will be discussed abrasive wear, transfer wear and lubricated wear. 

If PTFE has anomolous wear characteristics> it is in the area of what is now described as transfer or adhesive wear. Much has been written on this topic and opinions do not really differ on the important features of this process (34 43). Before discussing PTFE in particular it is useful to briefly review two general aspects of transfer wear processes the transition between abrasive and transfer modes and the categories of transfer wear. 

PTFE is therefore amongst a class of polymers which exhibits cold, total transfer. It is the wear which arises from this type of transfer which is usually described as transfer wear. As we shall see the behaviour of PTFE is not unique but it is unusual in its extent. 

These general comments apply to some extent to all cold, "whole transfer wear processes. PTFE is unusual in that it transfer very readily in this type of process more so than any other known polymer. Further, the transfer films have particularly poor adhesion to most substrates and also have extremely high degrees of orientation. Some chain scission can be detected (38,42,50). They are also uncommonly thin sometimes no more than 10 nm in thickness (some workers believe they may be of the order of one molecular-chain cross section diameter in thickness, ca. 0.8 nm). 

In loose terms, the presence of ordered units or long range affine connections suppresses the necessary chain mobility required to generate oriented polymer surfaces and hence the formation of thin oriented transfer films by rupture at the oriented surface-bulk isotropic polymer interface. The solid particles included in PTFE to reduce transfer wear may act in the same way (41). 

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. 

To conclude this section we may state that, with respect to transfer wear, PTFE is not entirely unique. Even its special transfer behaviour is seen elsewhere. It is really a matter of the extent not the kind of its character which is remarkable. 

Figure 8. Wear rate of y-ii Tadiated PTFE as a function of y dose. Counterface mild steel, a =0.4 ym closed symbols gamma damage in vacuum open symbols gamma damage in air. In both cases the transfer wear reaches a minimum at about 20 M Rad. In the case of gamma treatment in air significant oxidation of the sample, particularly the surface, is found to occur.

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.

They are capable of providing insight into the presence or absences of transfer (wear), the adhesive strength of polymer to metal, amount of transfer, bond scission, mechanical effects such as loading of surfaces together, chemical effects on bonding and surface energetics. The field ion microscope coupled with the atom probe is the ultimate tool for the study of polymer wear because it allows... 

Transport velocities remain within defined limits and material flows are relatively constant, so characteristic wear data that have been determined under realistic conditions are quite readily transferable. Wear as a form of damage does not play a dominant part in chemical plants. Nevertheless, it does occur, and the following examples give an idea of the diversity. 

There does not seem to be any means of actually predicting the rate of interfacial wear processes, be they arising from transfer wear processes or through chemical degradation. This is not surprising in view of the uncertainty as to what is involved in the transfer process. However, what is not known is that if certain polymers are filled with hard or chemically active fillers then a securely bonded transfer film will attach to metal counterfaces and then the rates of wear will be reduced by several orders of... 

The overall view of the transfer wear process may be divided into its basic elements. These are... 

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... 

---