Interfacial wear processes

Fig. 1 Schematic description of cohesive and interfacial wear processes from the two terms non interacting model of friction (from [96]). Bulk ploughing involves the dissipation of the frictional work within a volume of the order of the cube of the contact radius. Interfacial shear corresponds to the dissipation of the frictional energy in much thinner regions and at greater energy densities. Cohesive wear processes (cracking, tearing, microcutting...) are governed by the cohesive strength of the polymer. Mechanisms such as transfer film formation correspond to interfacial wear and do not readily correlate with accessible bulk failure properties...

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 important features of interfacial wear processes continue to evolve. Transfer wear processes are common but it is still not yet clear which factors control its occurrence and its general characteristics when it is observed. The extent of chemical degradation and its contribution to wear is also only vaguely understood. 

Erosive wear can be considered to be an interfacial wear where the energy evolved in the wear is dissipated. The third component in the wear process removes some of the frictional heat generated when the wear particle strikes the surface of the sample. 

If one considers that wear damage also results in energy dissipation, this approach can tentatively be applied to wear. Accordingly, wear processes can be classified as cohesive or interfacial depending on the length scales associated with particles detachment mechanisms (Fig. 1). 

Although it has not been established by systematic study, the operating parameter that determines whether the wear process is adhesive transfer and oxidation or oxidation and denudation is most likely rubbing speed, which in the ultimate analysis means interfacial temperature. If the temperature is high enough, both the rider and the track will acquire a coherent film of oxide which will effectively block adhesive transfer of metal from the rider to the track. Below some critical temperature only the more activated sites will be oxidized, which affords an opportunity for transfer of metal from unoxidized sites on the rider to the track oxidation of the transferred metal on the track is probably a consequence of its activated condition there. There is no clear-cut behavioristic demarcation between metallic transfer and the oxidation/ denudation process in the loss of material from the rider. Observers have frequently reported that wear experiments whose steady state proceeds by oxidation/denudation at a moderate rate may have as the initial stage severe wear with metallic debris (e.g. [39, 41]). 

It is interesting to note that very thick transfer films were established on smooth counterfaces (Figures 8 and 9). Such films are typically 10 ym thick and clearly submerge the details of the initial counterface profile, rather like snow drifts on the initial land topography. In this case the bulk polymer will slide on relatively thick transfer films and it will then be the Interfacial conditions between the two polymeric features which dictate the wear process, rather than the details of the burled metallic counterface. 

Glass fibres are beneficial in reducing wear rate provided that the fibre fraction is less than 30 v/o. The interfacial bond between matrix and fibres plays an important role in the wear process. To ensure that cracks do not initiate and propagate in the matrix, leading to failure in these... 

Figure 4. Wear process of short carbon fiber-reinforced epoxy composite (5 vol.% graphite, 5 vol.% PTFE, 15 vol.% SCF) (a) fiber thinning, (b) fiber breakage, and (c) fiber pulverization and Interfacial removal. Wear conditions normal pressure = 1 MPa, sliding velocity = 1 m/s, duration = 20 h, pin-on-disk apparatus.

Particulate reinforced polymer matrix composites are widely used in dentistry as esthetic restorative materials. Although the normally measured mechanical and physical properties of this class of materials approach the properties of dental amalgam (1) composite restorative materials exhibit limited durability in clinical service. Compared to amalgam restorations, composites undergo loss of material through wear processes and exhibit breakdown at the interfacial region between the restoration and tooth structure. 

The recovery of petroleum from sandstone and the release of kerogen from oil shale and tar sands both depend strongly on the microstmcture and surface properties of these porous media. The interfacial properties of complex liquid agents—mixtures of polymers and surfactants—are critical to viscosity control in tertiary oil recovery and to the comminution of minerals and coal. The corrosion and wear of mechanical parts are influenced by the composition and stmcture of metal surfaces, as well as by the interaction of lubricants with these surfaces. Microstmcture and surface properties are vitally important to both the performance of electrodes in electrochemical processes and the effectiveness of catalysts. Advances in synthetic chemistry are opening the door to the design of zeolites and layered compounds with tightly specified properties to provide the desired catalytic activity and separation selectivity. 

Organic surfaces are encountered in a wide range of situations where interfacial properties impact a material s performance characteristics. For example, a polymer s interfacial characteristics determine chemical and physical properties such as permeability, wettability, adhesion, friction, wear, and biocompatibility. " However, polymers frequently lack the optimum surface properties for these applications. Consequently, surface modification techniques have become increasingly desirable in technological applications of polymers. - These processes are capable of tuning the properties of... 

Wear is the process of physical loss of material. In sliding contacts this can arise from a number of processes in order of relative importance they are adhesion, abrasion, corrosion and contact fatigue. Wear occurs because of local mechanical failure of highly stressed interfacial zones and the mode of failure is influenced by environmental factors. 

Application to Adhesional Wear. For adhesional wear, as opposed to adhesion itself, the property or process that is of direct interest is the rupture of interfacial crazes and the transfer of polymeric material, rather than the force or energy requirement. It is clear that the inequalities, (lb), (3b) and (4b) are the criteria that predict the occurrence of polymer wear. In terms of molecular properties, small values of the yield strength, and a rapid decrease in Oy with temperature, should lead to the occurrence of polymer wear, and to the transfer of polymer to the solid. Large values of Oy and small values of dOy/dT will be conducive to the absence of wear. 

PEEK and polytetrafluoroethylene (PTFE) are highly incompatible. However, fine PTFE powder is commonly added to PAEK to act as an internal lubricant in tribiological applications. The PTFE smears across the wear surface and reduces interfacial friction. This reduces interfacial forces and the heat build-up that can lead to failure by melting. PTFE is particularly suitable in applications where there is no external lubricant and the compounds are often reinforced with carbon fibre. PEEK can also be added to PTFE to improve the wear properties of PTFE - although other less expensive polymers can have similar effects. More recently PAEK and PTFE have been blended so as to produce melt-processable PTFE which has a number of interesting properties [24]. This is perhaps the most luilikely example of the use of PAEK to improve the melt-processability of an otherwise hard-to-process material. 

The function of a lubricant is to (1) prevent the moving interacting surfaces from coming into direct contact, (2) to provide an easily sheared interfacial film, (3) to remove the heat evolved in the process, and (4) to reduce wear of the surfaces. Solid lubricants can only satisfy (1), (2), and (4), but only hquids and gas can also satisfy (3). Lubricants can thus conveniently be classified into gaseous, hquids, and solids. 

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