Friction sliding conditions

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. 

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 ... 

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.

At low ambient temperatures, oxidation occurs at asperity contacts because of frictional heating. At higher ambient temperatures, general oxidation of the entire surface occurs and affects wear. In the case of steel, the predominant oxide present in the debris depends on the sliding conditions. The predominant oxide at low speeds and ambient temperatures is a-Fc203, at intermediate conditions it is Fc304, and at high speeds and temperatures the oxide is FeO. Oxidation of iron and many metals follows a parabolic law with the oxide... 

The rope and the friction wheel will not occur for sliding conditions ... 

Qiao HB, Guo Q, Tian AG, Pan GL, Xu LB. A study on friction and wear characteristics of nanometer AI2O3/PEEK composites under the dry sliding condition. Tribol Int 2007 40(1) 105-10. 

The breakthrough of ionic liquid tribology took place in 2011 with the publicatiOTi [48] of the friction coefficients of a number of material contacts in the presence of imidazolium ILs with fluorine-containing anions. The first ILs described as neat lubricants were imidazolium [BFJ and [PFs] derivatives. They undergo a complex surface chemistiy which depends on the chemical composition of the surfaces, the environment and the sliding conditions. 

Pin-on-disk tribometers (CSEM, Neuchatel, Switzerland) were used to measure macroscopic frictional forces under pure sliding conditions. Two tribometers operating in different speed ranges were employed to enable the sliding speed to be varied over a wide range. The slower tribom-eter measures frictional forces in the speed range of 0.1-20 mm/s and the faster tribometer from 25 to 400 mm/s. A fixed pin that holds the steel ball (diameter = 6 mm, DIN 5401-20 G20, Hydrel AG, Romanshom, Switzerland) was... 

Investigation of Wear and Friction Properties Under Sliding Conditions of Some Materials, Suitable for Cages of Rolling-Contact Bearings, RX. Johnson, MA. Swikert, and E.E. Bisson, NACA Report 1062,1952. 

Compounded Oils. These are usually petroleum-based oik containing 3 to 5 percent fatty or synthetic fatty oik (usually animal fat or acidless tallow). They are usually used for worm-gear lubrication, where the fatty content helps reduce the friction generated nnder high-sliding conditions. 

In return, a corrosion process can modify the surface states of materials and in consequence the contact conditions. In that way corrosion can affect the sliding conditions (coefficient of friction, wear regime,...). The interaction between friction and corrosion therefore induces a complex phenomenon of synergy. 

Can the observed behaviour on the macroscopic and microstructural scales be reconciled with what we know about frictional sliding under these conditions To answer this question, we turn to other reported work with mica in which the real and apparent areas of contact coincide. Johnson reports that experiments with an atomic friction microscope (AFM), on mica in which the contact dimension is 2 to 10 nm, indicate a frictional shear stress of 1 GPa. Other measurements performed with a surface force apparatus (SFA), in which the contact dimension is in the order... 

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