Slide conditions

Sliding wear is perhaps the most complex in the way different materials respond to sliding conditions. The metallic materials that perform best... 

Fig. 24—Measured film thickness perturbation for square features passing through the contact area under different sliding condition, taken from Felix [54]. (a) Slip=0 (b) Slip=1.0 and (c) Slip=-1.0.

As a final point, we note that typical surfaces are usually not crystalline but instead are covered by amorphous layers. These layers are much rougher at the atomic scale than the model crystalline surfaces that one would typically use for computational convenience or for fundamental research. The additional roughness at the microscopic level from disorder increases the friction between surfaces considerably, even when they are separated by a boundary lubricant.15 Flowever, no systematic studies have been performed to explore the effect of roughness on boundary-lubricated systems, and only a few attempts have been made to investigate dissipation mechanisms in the amorphous layers under sliding conditions from an atomistic point of view. 

Plug-like flow and uniform volume heating (or cooling) leading to a flat chemical reaction front (uniform conversion at each section of the channel) may be regarded as an ideal flow systems. Maintenance of complete (or close to it) sliding conditions at the walls is one of the ways to achieve this goal. 

Fig. 11 Calculated surface profiles of the octahedral shear stress at yield assuming a modified Von Mises criterion (a), and of the octahedral shear stress for a glass/epoxy contact under gross sliding condition (b). The grey area delimits the region at the leading edge of the contact where the octahedral shear stress is exceeding the limit octahedral shear stress at yield (a is the radius of the contact area) (from [97])...

Such approaches have been applied to the contact cracking problems under sliding conditions in order to assess the values of the stress intensity factors K and Ku under cyclic tangential loading. As an example, Fig. 18 shows the numerical simulations corresponding to a cracked epoxy substrate. 

Tsuya, Y. and Kitamura, M., Effect of Sliding Conditions on the Wear Rate of Organic and Inorganic Bonded Solid Lubricant Films, Proc.2nd. ASLE Inti. Conf. on Solid Lubrication, Denver, Colorado (15-17 Aug. 1978) ASLE SP-6 p.85. 

Bermudez et al. [67] studied the tribological properties of PC+0.5%ZnO+1.5% IL composites and showed an 80% friction reduction and wear reduction of nearly two orders of magnitude with respect to PC+0.5% ZnO. They then investigated ILs as an effective lubricant additive of epoxy resin under the pin-on-disk configuration and showed a friction reduction higher than 50% and a wear rate reduction of two orders of magnitude for epoxy resin (RE)+1.5% IL with respect to the neat Re and Re+0.5% ZnO [68], The ability of ILs to act as lubricant additives of polymers under sliding conditions can thus be confirmed by the above report. 

Under sliding conditions, films formed by ZDDPs behave in a plastic manner and boundary friction coefficients of ZDDP films are 0.13-0.15 [39]. In addition, they increase friction in the mixed lubrication region, secondary ZDDPs more so than primaries, which has an obvious impact upon energy efficiency. It may be that there is a thick viscous layer of organophosphate material on top of the solid reaction films but evidence for [40] is slightly weaker than the evidence against [41] and further studies are required to resolve this issue. 

Friction is defined as the resistance encountered when one body moves tangentially over another and they are in contact. Friction often embraces two classes of relative motion sliding and rolling. In industrial processes, frictional energy is usually dissipated as waste heat. The friction force is represented by F and the friction coefficient by p. Under many sliding conditions, the p for a given pair of materials and fixed conditions of lubrication is mostly constant. The three laws of friction are ... 

Ethyl chloride shows the same kind of linear response to static exposure but at a much lower level. Under static conditions vinyl chloride is adsorbed more readily on the metal than is ethyl chloride. The two-stage interaction seen with vinyl chloride under sliding conditions may indicate its polymerization on the surface when the exposure factor is high enough. 

Belyi et al. W have discussed various methods for decreasing wear and/or friction in polymer-metal sliding. These involve modification of the supramolecular structure of the polymer, structural transformations by gamma-irradiation, thermo-activation effects from the decomposition of filled materials under sliding conditions, and selective transfer by incorporating specific fillers in the polymeric material. The use of cuprous oxide as a filler in polytetra-fluoroethylene (PTFE) has been shown to reduce the wear rate by a factor of 3. It is believed to be due to the selective transfer of copper to the steel surface because cuprous oxide is reduced to pure copper at the temperatures produced in sliding 9). 

Figure 1. Coefficient of friction of high density polyethylene sliding against metal surfaces. Sliding conditions 49 N load, 2.1 m/s speed.

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