Mixed-film lubrication regime

While high A values seem optimal to minimize wear, many commercial implants, such as those of Firkins et al [6], seem to operate within a mixed film lubrication regime. If lubrication protection improves as wear proceeds, it may not be essential to manufacture implants to produce high initial A. 

Full film lubrication, usually assumed in EHD contact modelling, is not compatible with this type of problem where local metal to metal contact between the contacting surfaces may occur, as considered in the mixed film lubrication regime. 

Figure 5 shows the Ai Cheng model. The pressure field obtained may be regarded as an upper limiting case of the mixed film lubrication regime -the full film EHD rough contact. 

The strong influence of surface roughness on the friction coefficient in mixed film lubrication regime is justified. 

The mixed film lubrication model, being more efficient in modelling the effect of surface roughness on the lubricant film shear stresses, gives closer results to the experimental values, as explained above. These results also demonstrate how inqmrtant surface roughness is for the evaluation of the friction coefficient in mixed lubrication regime. 

Fig. 28—Different stages in transition of lubrication regimes, (a) Full-film lubrication with film thickness much larger than roughness h/cr> ). (b) Surfaces are separated but roughness effect becomes significant (5>/i/cr>3). (c) Asperities interfere with each other but hydrodynamic films carry the most load (h/cr 3). (d) Typical mixed lubrication with load shared by lubrication and asperity (h/cr<3). (e) Boundary lubrication when asperities carry the most part of load (h/a-<0.S).

At higher normal force, or at lower lubricant viscosity and velocity, the mixed lubrication regime applies. The thickness of the liquid film, in this case, is comparable to the average surface roughness some asperities can therefore touch one another. 

By virtue of our modem lifestyle, more than one-third of the readers of this article will likely experience the failure of their native hip joint(s)—sooner or later [1], Fortunately, hip-joint arthroplasty is on hand to deal with this condition. A major unsolved problem, however, is the mechanical wear of the artificial joint, which hmits the fifetime to about 10-15 years [2]. While most lubrication of the healthy natural joint relies on a film of synovial fluid (SF), the artificial joint consists of synthetic materials and is mainly lubricated in the mixed and boundary regimes [3]. In a widely used arthroplastic design [4], the acetabular cup consists of a linear of ultra-high-molecular-weight poly(ethylene)... 

The fact that vegetable oils are generally liquid or fluid at room temperature allows them to be readily applicable in lubrication processes where formation of thick lubricant films is necessary. As described above, formation of high lubricant film thickness is important in processes that occur in hydrodynamic and mixed-film regimes. Thus, vegetable oils are suitable for formulating lubricants that will be applied in hydrodynamic and mixed-film regimes. 

Greenberg et al. compared the effects of IF addition in oil under three lubrication regimes hydrodynamics, mixed and limit [44]. They based the lubrication mechanism of IF on a film formation on surfaces and showed that the IF are most effective at mixed Inbrication because all conditions for a good efficiency of IF are combined, hi the hydrodynamic regime, fullerenes do not have interactions with surfaces. In a boundary Inbrication regime, film formed on surfaces is quickly removed, due to contact severity. 

As the oil film thickness (OFT) and hence lubrication at the cam / follower interface is variable between EHD, mixed and boundary regimes there are clearly a range of conditions present throughout a complete cycle. To all intents... 

The same boundary and mixed lubrication regimes are found in small diameter hard-on-hard metallic implants [10,11,12]. However, beyond a certain diameter, or below a certain clearance, the severity of mixed lubrication diminishes, as hydrodynamic action becomes increasingly significant [1], The severity of surface interaction and wear can thus be controlled through careful design and manufacture to optimise fluid film lubrication effects. 

Some relatively small diameter joints, which generated only modest film thicknesses, less than about 7 nm, operated in the boundary lubrication regime, whilst some of relatively large diameter and small clearance enjoyed the benefits of fluid-film action. However, the majority of the implants tested operated in the mixed... 

Bearing friction is measured as a function of speed at a range of temperatures (up to 150 °C). Stribeck curves generated using this rig show the variation of friction with speed as the lubrication regime changes from full film hydrodynamic to mixed hydrodynamic. 

Many investigators have studied the effects of the solid-surface texture on friction, mainly in full-film EHL or mixed lubrication where the hydrodynamic action of the fluid plays a role [1-3]. The consensus on the effect of surface rouglmess orientation on friction is that transversely oriented roughness results in a much thicker hydro namic film than does longitudinally oriented roi hness. However, the effect of si ce texture in the performance of lubricants in a boundary lubrication regime is much less understood, and the difference between mixed and boundary lubrication is basically unclear. The reason for friis insufficient understanding is hydrodynamic action, including micro-EHL, is not well understood and we cannot yet predict where it should appear. More research on mixed lubrication is necessary [4]. 

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