Adhesive wear, polymer mechanics

The wear rate of plastics is governed by several mechanisms. The primary one is adhesive wear which is characterised by fine particles of polymer being removed from the surface. This is a small-scale effect and is a common occurrence in bearings which are performing satisfactorily. However, the other mechanism is more serious and occurs when the plastic becomes overheated to the extent where large troughs of melted plastic are removed. Table 1.7... 

Czichos, H. "Influence of Adhesive and Abrasive Mechanisms on the Tribological Behaviour of Thermoplastic Polymers", Wear 88 (1983) 27. 

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. 

For polymer-polymer sliding pairs,(ll) adhesion appears to be the dominant mechanism of polymer friction. Adhesion can also take place at the polymer-metal interface provided that surface roughness is below a certain value.(12) For a smooth surface, it is difficult to differentiate fatigue from adhesive wear.(5)... 

When polymers slide on machined metal surfaces, it is quite possible that steady-state wear Involves a combination of abrasive, fatigue, and adhesive wear mechanisms. To study fatigue wear, it would be desirable to minimize the contributions of the abrasive and adhesive wear modes. In this paper, the following polymers polycarbonate, polyvinyl chloride, ultra-high molecular weight polyethylene, siloxane modified epoxies, and polylmldes are tested in experiments in which the fatigue wear mode is predominant. 

In comparison with metals, most conventional polymers are low in wear resistance. For wear control, we need to understand various wear mechanisms for each polymer system (V). As discussed in a previous paper, for adhesive wear, surface energetics can determine the extent of surface wear. Thus, a low surface energy is preferred to minimize the surface attrition. In addition, a harder polymer is desired to lower the wear rate. For abrasive wear, fracture energetics become important a harder and tougher material should be more wear resistant. 

For systems consisting of common materials (e.g., metals, polymers, ceramics), there are at least four main mechanisms by which wear and surface damage can occur between solids in relative motion (1) abrasive wear, (2) adhesive wear, (3) fatigue wear, and (4) chemical or corrosive wear. A fifth, fretting wear and fretting corrosion, combines elements of more than one mechanism. For complex biological materials such as articular cartilage, most likely other mechanisms are involved. 

The frictional properties of thermoplastics, specifically the reinforced and filled composites, vary in a way that is unique from metals. In contrast to metals, even the highly reinforced resins have low modulus values and thus do not behave according to the classic laws of friction, as developed by theories from the ICI-LNP. Metal-to-thermoplastic friction is characterized by adhesion and deformation resulting in frictional forces that are not proportional to load, because friction decreases as load increases, but are proportional to speed (see Tables 3-21 through 3-24) [323]. The wear rate is generally defined as the volumetric loss of material over a given unit of time. Several mechanisms operate simultaneously to remove material from the wear interface. However, the primary mechanism is adhesive wear, which is characterized by having fine particles of polymer removed from the surface. 

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... 

Blends of elastomers are routinely used to improve processability of unvulcanized rubbers and mechanical properties of vulcanizates like automobile tires. Thus, cis-1,4-polybutdiene improves the wear resistance of natural rubber or SBR tire treads. Such blends consist of micron-sized domains. Blending is facilitated if the elastomers have similar solubility parameters and viscosities. If the vulcanizing formulation cures all components at about the same rate the cross-linked networks will be interpenetrated. Many phenolic-based adhesives are blends with other polymers. The phenolic resins grow in molecular weight and cross-link, and may react with the other polymers if these have the appropriate functionalities. As a result, the cured adhesive is likely to contain interpenetrating networks. 

Styrene-butadiene rubber (SBR) is a random polymer made from butadiene and styrene monomers. It possesses good mechanical property, processing behavior, and can be used like natural rubber. Moreover, some properties such as wear and heat resistance, aging, and curing property are even better than in natural rubber. Styrene-butadiene rubber was the first major synthetic rubber to be produced commercially. Now it has become the most common rubber with the largest production and consumption in the synthetic rubber industry. It can be widely used in tire, adhesive tape, cables, medical instruments, and all kinds of rubberware. 

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. 

The theory that we have developed does not demand that all fibrils and their bases be the same size, or that their mechanical properties be identical. Chain entanglement is a statistical property, and (as already noted) the number of chains per fibril is small enough that serious fluctuations in local properties are to be expected. Even if, in a particular case, the vast majority of fibrils detach cleanly and without leaving any polymer on the solid, a small fraction of the fibrils may rupture, in an experiment of "adhesive" separation. Likewise in a frictional experiment even when, at the majority of points of adhesional attachment, detachment occurs without appreciable fibril drawing, there may be fibrildrawing and rupture at a few sites and consequently, wear will occur. 

The wear of polymers Is general believed to be caused by one or more of the following mechanisms adhesive transfer, abrasive cutting,... 

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. 

Wear in a strict sense occurs whenever material is lost from a solid. The mechanism of loss can be abrasion, adhesion, erosion, cavitation, corrosion or fatigue. This loss can occur at the atomic level. At this level analytical tools such as the field ion microscope and the atom probe can be used to study wear loss of polymers. These tools have been used in the authors laboratory for many years to study polymer adhesion and transfer to metal surfaces ... 

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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