Hydrodynamic, boundary lubrication

Due to the absence of a hydrodynamic effect, boundary film thickness is expected to be independent of speed of surface movement, as can be observed in the left part of the Stribeck curve. This is a significant criterion that distinguishes boundary lubrication from EHL and mixed lubrica-... 

Boundary Lubrication as a Limiting State of Hydrodynamic Lubrication... 

The present author has performed computer simulations to examine the transition of pressure distributions and shear response from a hydrodynamic to boundary lubrication. Figure 4(a) shows an example of a smooth elastic sphere in contact with a rigid plane, the EHL pressure calculated at a very low rolling speed coincides perfectly with the... 

Fig. 4—Illustration of the transition from hydrodynamic to boundary lubrication (a) a comparison of pressure of thin EHL film with Hertzian distribution (b) a schematic stress-velocity map showing the dependence of shear stress of lubricating films on sliding velocity.

Unlike traditional textbooks of tribology, in this book we regard boundary lubrication as a limit state of hydrodynamic lubrication when film thickness is down to molecular dimension and independent of the velocity of relative motion. The discussions are based on the existing results, some from literatures but mostly from the authors own work. The topics are mainly focused on the mechanical properties of boundary films, including rheology transitions, molecular ordering, and shear responses. Ordered molecule films, such as L-B films and SAM, are discussed, with emphasis on the frictional performance, energy dissipation and the effects from structural features. Boundary films can be modeled either as a confined substance, or an adsorbed/reacted layer on the... 

The process of transition from hydrodynamic to boundary lubrication can be described qualitatively by plotting the measured friction coefficients against film thickness, which depends on the operational conditions, such as load, sliding velocity and lubricant viscosity. A typical diagram known as the "Stribeck Curve is schematically shown in Fig. 27, in which the friction coefficients are given as a function of, ... 

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).

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. 

At low sliding velocities and high loads, the lubricating film is squeezed out of the gap. This leads to so-called boundary lubrication. Friction coefficients under these conditions are typically 100 times higher than under hydrodynamic lubrication conditions, but still substantially smaller than for dry friction under UHV conditions. This is due to the fact that the surfaces are still wetted by molecular layers of the lubricant, even under conditions where the local stress is high enough to deform the surface asperities. Under these conditions friction depends more on the chemical constitution of the lubrication layer than on its viscosity. 

In practical applications we often encounter a combination of boundary and hydrodynamic lubrication which is called mixed lubrication. For example, bearings that are usually lubricated hydrodynamically, experience mixed lubrication when starting and stopping. This is shown in the Stribeck diagram (Fig. 11.12) At low speeds, boundary lubrication with high friction... 

Figure 11.11 Different lubrication situations in a journal bearing. Left At low velocities and high loads, boundary lubrication with a high friction coefficient dominates. The shaft climbs the journal on the right side. Right At high speeds and low loads hydrodynamic lubrication leads to much lower friction. The build up of the hydrodynamic wedge moves the shaft to the upper left.

Figure 11.12 Stribeck diagram Plot of friction coefficient /u versus ijkv/P,where rjk is the kinematic viscosity, v the velocity, and P the contact pressure. From left to right there are three different friction regimes Boundary lubrication with high friction and wear, mixed lubrication with medium friction and wear, and hydrodynamic lubrication with low friction and wear.

Liquid lubrication mechanism. There are four defined regimes of liquid lubrication hydrodynamic (thickness of lubricant film (h), h > 0.25 pm), elastohydrodynamic (h 0.025 to 2.5 pm), boundary (h 0.0025 pm), and mixed. These regimes are dependent on oil viscosity (Z) and relative velocity (V) and are inversely proportional to the load (L), (ZV/L). Fig. 5.1, known as the Stribeck-Hersey curve, depicts these regimes in terms of friction coefficient versus viscosity, velocity, and load (ZV/L) (Fusaro, 1995). 

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 ... 

This application illustrates the fact that friction will often be reduced by the addition of molybdenum disulphide to an oil where the geometry is such that the particles can enter the oil film and be loaded against the bearing surfaces. This is the case when boundary lubrication is occurring, and frequently when elastohydrodynamic lubrication is taking place. It can also arise where hydrodynamic lubrication is marginal, and it is often in such cases that frictional heating problems and premature failures occur. 

A qualitative model for lubrication of the condensed alcohol layer is not fully developed yet. We present here the best qualitative model based on the literature. As the applied pressure is very large and the scanning speed is very low in AFM, one can rule out the hydrodynamic lubrication even in the presence of condensed alcohol layer on the substrate. The AFM tip and substrate interface must be in the boundary lubrication regime. Figs. 7-10 show that the adhesion force reduction upon alcohol adsorption is accompanied by reduction of the friction force. Part of the adhesion force reduction is because of the surface tension decrease of the water layer on the substrate. When alcohol is dissolved in water, the surface tension decreases significantly because of segregation of alcohol molecules to the liquid-air interface. ... 

Previous studies have indicated that no hydrodynamic lubrication occurs during CMP.28 3la There is always a physical contact between the wafer and the polishing pad asperities. In the following section, we will see that there is enough evidence to prove interactions between a wafer and a pad. The boundary lubrication associated with tribochemical interactions plays a dominant role. In order to understand the mechanisms of boundary lubrication in CMP, the physical, electrochemical, and mechanical processes of interfaces must be considered. The mechanisms can be classified into the following categories based on the surface physical chemistry of materials involved during CMP. 

In looking at the basic mechanisms of lubricated sliding friction, the major emphasis falls on the adhesive process because a p/ilonj. it is the one most likely to be influenced by the presence of the lubricant at the rubbing interface. The mechanisms to be considered here in particular are those that make their effect felt in thin film or boundary lubrication. The action of macroscopic liquid films, generated hydrodynamically or otherwise, are not included in this treatment because the surfaces are completely separated from each other the meaning of friction in such cases is discussed in Chapter 2. 

ZoandaAy lubAd.catd.on is a familiar term in the vocabulary of the tribologist. In the general concept of the boundary lubricated condition, the lubricant film between the two surfaces is no longer a liquid layer instead the surfaces are separated by films of only molecular dimensions. Friction is influenced by the nature of the underlying surface and by the chemical constitution of the lubricant films. This view of lubricating action at the solid surface was introduced by Sir W. B. Hardy [1] as an extension of Osborne Reynolds concept that hydrodynamic action within the liquid film is a process treated by continuum methods which are not applicable at the discontinuity or "boundary" between liquid and solid. 

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