Chemical wear, polymer mechanics

Several mechanisms of polymer wear have been discussed in the literature (5-7) adhesive wear, abrasive wear, fatigue wear, tribo-chemical wear, corrosive wear and impact wear. We shall limit this discussion to the four basic mechanisms shown in Figure 1. Neither corrosive(5) nor impact wear(8,9) are common, and we do not plan to discuss these in this paper. 

Tribo-chemical Wear. Besides the above three wear mechanisms, we should discuss tribo-chemical wear. Tribo-chemical wear(7) takes many forms. Some of these wears result from the interactions of the polymer with its environment, e.g., oxygen, ozone, heat (e.g., friction heat), surface contaminants, etc.- The application of mechanical energy at the interface can also cause mechanochemical degradation(35) to generate free radicals which can further lead to cross-linking or other interactions. In the composites, polymer-filler interactions can also take place through mechanochemical mechanisms. 

The predominantly carbon material created during implantation is very hard. Not surprisingly, the density also increases and results in a microstructure that is remarkably resistant to chemical solvents. The mechanical and chemical behavior of implanted polymers is currently being studied for applications such as nonperme-able chemical barriers and wear resistance [9]. 

Chemical Wear. Chemical wear, which can be operative in both abrasive and adhesive wear types depending upon the deformational and thermal conditions, is the mechanism of wear debris formation generated as a result of some chemical reaction between the interacting materials and their environment. Such kinds of chemical reactions are also known as tribochemical reaction as they are promoted only in a tribological interaction. In the case of polymers sliding against metal surfaces, there are four important mechanisms by which a tribochemical reaction can be promoted (2). The first one is due to the interfacial heating... 

There are many other examples of chemical reactions being induced by shearing stresses. A mechanism involving metallization seems plausible. Areas of application include photochemistry, degradation of polymers, friction and wear, mechanical alloying and cutting processes. 

There is some discussion that the primary degradation of MRR is due less to pad asperity deformation than to byproduct accumulation. The optimum approach to maintaining consistent MRR could be quite different depending on the true nature of the mechanism (physical or chemical). For instance, handling a byproduct accumulation may be addressable chemically, which would reduce the wear on the pad (also increasing its lifetime). More work should be be done to separate the MRR performance based on the extent of polymer roughness or byproduct presence. 

Polymer wear can take place in various modes, e.g., adhesive, abrasive, transfer, fatigue, and tribo-chemical. In reality, several mechanisms can also operate simultaneously. If impaction is involved, an impact wear can be the chief mechanism. The predominance of any one type of wear can be influenced by the form of polymers, e.g., thermoplastics, elastomers or 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. 

Thus, the new method for studying the molecular characteristics of the thermoplastic polymer transfer and wear products makes it possible to establish the orientation of mechano-chemical processes in the rubbing zone and to find ways of dispersion and wear control. The method also enables the wear mechanisms of antifriction polymers to be studied in terms of the transfer phenomena. 

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... 

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