Lubrication regimes

Thin film lubrication (TFL), as the lubrication regime between elastohydrodynamic lubrication (EHL) and boundary lubrication, has been proposed from 1996 [3,4], The lubrication phenomena in such a regime are different from those in elastohydrodynamic lubrication (EHL) in which the film thickness is strongly related to the speed, viscosity of lubricant, etc., and also are different from that in boundary lubrication in which the film thickness is mainly determined by molecular dimension and characteristics of the lubricant molecules. 

The definition of different lubrication regimes is a historic problem [41 ]. In boundary lubrication, molecules will be absorbed on a solid surface of a tribo-pair and form a monomo-lecular absorbed layer as described by Hardy [42] as shown in Fig. 1 (a). If the film thickness of lubricants in the contact region is from a few nanometres to tens of nanometres, different layers will be formed as shown in Fig. 1 (b) proposed by Luo et al. [3,4]. The layer close to the surfaces is the adsorbed film that is a monomolecular layer. The layer in the... 

The failure of TFL only means a loss of mobility here, but monolayers can stay on solid surfaces to separate the solid surfaces in relative motion, and subsequently sustain a feasible boundary lubrication state [10]. Because the film thickness of TFL is of the nano scale or molecular order, from a mechanical point of view, TFL is the last one of the lubrication regimes where the Reynolds equation can be applied. 

Thin him lubrication is essentially a transition lubrication regime between elastohydrodynamic lubrication and boundary lubrication regimes. Papers devoted to the investigations of this lubrication regime are not enough for engineering needs. In this section, a function to describe the viscosity distribution is proposed in order to attain predictive results and to describe the characteristics of TFL in the viewpoint of engineering. 

Figure 22 shows variation of the him thickness with velocities. The three curves in the hgure are results from the EHL solution, experimental data, and TFL solution, respectively. The maximum Hertzian contact pressure is 0.125 GPa and the atmosphere viscosity of oil is 0.062 Pa s. While the velocity is higher than 100 mm s, i.e., the him is thicker than 50 nm, all the results from EHL, TFL, and experimental data are very close to each other, which indicate that when in the EHL lubrication regime, bulk viscosity plays the main role and the results of three types are close to each other. When... 

TFL is essentially a transition lubrication regime between EHL and boundary lubrication. A new postulation based on the ordered model and ensemble average (rather than bulk average) was put forward to describe viscosity in the nanoscale gap. In TFL, EHL theories cannot be applied because of the large discrepancies between theoretical outcomes and experimental data. The effective viscosity model can be applied efficiently to such a condition. In thin him lubrication, the relation between Him thickness and velocity or viscosity accords no longer with an exponential one. The studies presented in this chapter show that it is feasible to use a modi-Hed continuous scheme for describing lubrication characteristics in TFL. 

Due to lack of deterministic solutions of mixed lubrication in the literatures, it is difficult to conduct a direct result comparison in the mixed lubrication regime. Considering the fact that at the ultra-low speed mixed lubrication would transit to the boundary lubrication or lubricated contact... 

Transition of Lubrication Regime—A Study Based on the DML Modei... 

The Stribeck curve gives a general description for the transition of lubrication regime, but the quantitative information, such as the variations of real contact areas, the percentage of the load carried by contact, and changes in friction behavior, are not available due to lack of numerical tools for prediction. The deterministic ML model provides an opportunity to explore the entire process of transition from full-film EHL to boundary lubrication, as demonstrated by the examples presented in this section. 

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).

The model has been applied successfully to examine the performances of mixed lubrication, such as the effects of the roughness height distribution, the behavior of asperity deformation under different conditions of contact and sliding, and the transition of lubrication regime from hydrod5mamic to boundary lubrication. 

See also Beer s Law Boundary film lubrication regimes, 15 212 Boundary layer capacitors, carbon black application, 4 800... 

Elastic recovery, 19 744 in olefin fibers, 11 227—228 Elastic scattering, 24 88-89 Elastic springs, in virtual two-way SMA devices, 22 346-347 Elastic waves, 17 422 Elastohydrodynamic (EHD) lubrication regime, 15 211-212 Elastomer-coated dies, in bar soap manufacture, 22 752 Elastomer designations, ASTM, 9 552t Elastomeric fibers, dyeing, 9 204 Elastomeric polycarboranylsiloxanes,... 

Extrapolating properties, defined, 16 729 Extra spring copper alloys, 7 723t Extreme ambient conditions, lubrication and, 15 252-256 Extreme-case analysis, 9 547 Extreme environments, solid and liquid lubricants for, 15 256 Extremely low toxic substances, 23 113 Extreme pressure (EP) lubrication regime, 15 214. See also EP entries Extreme purity gases, analyses of, 13 468 Extreme ultraviolet lithography, 15 189-191... 

Hydrodynamic injection, capillary electrophoresis, 4 633-634 Hydrodynamic lubrication regime, 15 210-211... 

Any and all of these particles can become attached to the pad. As the surface of the pad accumulates more and more particles, the surface glazes and becomes smoother and less abrasive. Consequently, the removal rate declines. Also, as the pad becomes glazed it becomes smoother, which causes a decline in the ability of the polish pad to distribute slurry under the wafer. At present there is considerable debate as to whether CMP takes place in a contact regime or in a lubrication regime. It is likely that resolution of this issue will be necessary to establish what fundamental limits there are on slurry flow, as well as how these limits affect pad conditioning. 

Depending on the thickness of the lubricating layer, we distinguish between two different lubrication regimes. In hydrodynamic lubrication the lubrication layer is thicker than the maximum height of the surface asperities resulting in a complete separation of the friction partners. In boundary lubrication the lubrication layer is typically only a few molecular layers thick and therefore thinner than the surface roughness. In many practical applications we are between the two extremes, which is referred to as mixed lubrication. 

Poly(a-olefins) or PAOs, polyol esters and diesters are now used in automotive and marine engine oils. To understand how an ester lubricates, it is important to consider its behavior in the different lubrication regimes, especially boundary lubrication when the properties of the bulk lubricant (e.g. viscosity) are of minor importance. The chemical properties of the lubricant responses under extreme conditions will become increasingly important. The polar ester will preferentially stick to the surface of metal when a small amount of ester is added to a low viscosity nonpolar fluid (PAO), (Randles, 1999 Spikes, 1999). When the two metal surfaces come closer together, the polar ester molecules stay in the contact zone. The use of fully synthetic engine oil formulations has produced some improvement in viscosity control and engine cleanliness in the piston and valve train areas over petroleum-based oils (Boehringer, 1975 Frame et ah, 1989 Kennedy, 1995 Lohuis and Harlow, 1985). 

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

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