Lubricant film thicknesses

The molecular dipstick microscope is related to the AFM. It measures lubricant film thickness. The probe is lowered into the oil film on a surface (like the automobile engine crankcase dipstick). The tip is attracted to the surface by the surface tension of the film but repelled by van der Waal s forces from the hard substrate. By noting the height of the probe from the two surfaces as it makes contact, the film thickness can be measured with a precision of about 0.5 nm. 

In the past decade, effects of an EEF on the properties of lubrication and wear have attracted significant attention. Many experimental results indicate that the friction coefficient changes with the intensity of the EEF on tribo-pairs. These phenomena are thought to be that the EEF can enhance the electrochemical reaction between lubricants and the surfaces of tribo-pairs, change the tropism of polar lubricant molecules, or help the formation of ordered lubricant molecular layers [51,73-77]. An instrument for measuring lubricant film thickness with a technique of the relative optical interference intensity (ROII) has been developed by Luo et al. [4,48,51,78] to capture such real-time interference fringes and to study the phenomenon when an EEF is applied, which is helpful to the understanding of the mechanism of thin film lubrication under the action of the EEF. 

For a point contact problem, the local lubricant film thickness can be expressed as ... 

Choo, J. W., Glovnea, R. R, Olver, A. V., and Spikes, H. A., The Effects of Three-Dimensional Model Surface Roughness Fea- [58] tures on Lubricant Film Thickness in EFIL Contacts," ASME J. IHbol,Voi. 125,2003,pp. 533-542. 

Lubricant Film Thickness in Rough Surface, Mixed Elastohydrodynamic Contact, ASME J. Tribol., Vol. 122, 2000, pp. 

Bhushan et al. [9] independently confirmed the lack of polishing activity due to hydrodynamic lubrication. The depth of the wafer carrier in CMP was adjusted so that samples either projected above the surface of the carrier (the normal case), were essentially coplanar with the carrier, or were recessed below the plane of the carrier. This produced wafer-pad lubrication film thicknesses of controlled dimension. For the case of a wafer recess of 75 um ( 3 X the lubrication film thickness reported in Ref. [7]), removal rate was negligibly small. 

Coppeta et al. [10] made slurry film measurements during using laser-induced fluorescence. By addition of a fluorescent dye to the polishing slurry film thickness was experimentally from the fluorescence intensity of the lubrication film as measured through a transparent substrate. Film thickness measurements were in good agreement with those of Levert et al. [7,8]. This technique can also be used to study slurry transport across the wafer surface, diameter variation in lubrication film thickness, and slurry mixing effects [11]. 

For a fixed geometry, the friction force depends solely on the viscosity of the lubricant. We could try to decrease the viscosity of the lubricant to reduce friction. There is, however, a limit to this The lubrication film thickness must always be kept higher than the surfaces asperities. Otherwise the surfaces will come into direct contact, resulting in much higher friction. Therefore, it is common to use an oil with a viscosity that is just high enough to maintain a continuous lubrication layer. 

Fig. 8.2 Comparison of lubrication film thickness with the average size of lubricant molecules, redrawn and adapted from [6]...

Fig. 2 Relative lubricant film thicknesses (after Dowson [12])...

Surface-oriented tribological analysis bonding state Thin film structure, internal pressure defects, etc. (including neutron diffraction) Surface-oriented tribological analysis, bonding state, lubricant film thickness (+ ion gun)... 

Figure 4.9 Two-dimensional asperity orientation with a significant effect on lubrication film thickness.

In principle the theory of electrical capacitance can be applied to the determination of lubricant film thickness between the bounding surfaces of two solids, but in practice the geometrical difficulties described in Section 5.1.1 for electrical resistance methods are even more troublesome for capacitance. If the solids are bounded by two flat surfaces parallel to each other, the electrical capacitance problem becomes that of a parallel-plate condenser, for which the formula is... 

Another approach to the detection of fluid film failure is the study of wear in relation to lubricant film thickness. Figure 6-18 shows observations by E. M. Landen [25] of the wear of two disks rubbing with velocities in the ratio 1 1.25. The film thicknesses were calculated from the operating parameters of the apparatus and elastohydrodynamic theory. As shown by curve D, for an oil film 65 nm thick wear ceases after a break-in period of one hour. When the oil film is only 20 nm... 

With the assumption that a is a slowly varying function of x, we can again use the dip coating momentum equation (Eq. 10.3.2) to calculate the lubrication film thickness 8f, the term 8" daldx being neglected as small. The boundary condition at the bubble surface (Eq. 10.3.3b) changes to... 

Carrying out the integration of Eq. (10.3.2) and applying the new conditions given above, we obtain the same equation as Eq. (10.3.7), except that the factor 3 is replaced by 6. The resulting equation can thus be transformed to the universal form of Eq. (10.3.8) with the 3 in the definition of replaced by 6. The solution procedure follows exactly as in the dip coating and clean bubble problem, giving for the lubrication film thickness, in place of Eq. (10.3.17),... 

The following regimes or types of lubrication maybe considered in the order of increasing severity or decreasing lubricant film thickness (Figure 50.1) ... 

Several examples of experimental approaches to thin-film lubrication have been reported [3]. It is important in examining these techniques to make the distinction between methods that are used to determine lubricant film thickness under hydrodynamic or elastohydrodynamic conditions (e.g., optical interference, electrical capacitance, or x-ray transmission), and methods that are used to determine the occurrence or frequency of contact. As we will see later, most experimental studies of synovial joint lubrication have focused on friction measurements, using the information to determine the lubrication regime involved this approach can be misleading. 

It is well known that this type of wearing in presence of a liquid film is of the so called typ>e "corrosion mechanical wearing". The main reason for its occurrence is the washing out of the lubricating film and disturbance of the laws of hydrodynamic friction, as well as decrease of the lubricating film thickness. Nevertheless, this effect of wearing in the presence of the batch P-1 was significantly weaker. 

Figure 11.21 Friction coefficient plotted as a function of fluid viscosity and shear velocity divided by load (Stribeck curve) with corresponding lubrication film thickness.

Figure 2. A direct comparison of the coefficient of friction (y-axis on the left-hand-side) and the lubricant film thickness (y-axis on the right-hand-side) for both polymer-free and PLL(20)-g[3.4]-PEG(2)-containing HEPES buffer solution, measured by MTM and ultra-thin-fihn interferometry. Squares are for the coefficient of friction and circles for the lubricant film thickness. The lines between data points serve as a guide for the eye. Ball = stainless steel (19 mm in diameter), substrate = silica, buffer solution = 10 mM HEPES (pH 7.4), polymer concentration...

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. 

Bupara [35] have proposed an energy equation in which fluid velocities are calculated from Reynolds equation assuming a constant viscosity across the film the method is justified because of its simplicity. Tipei and Degueurce [36] have studied THD problems for exponential lubricating film thicknesses. With this method, Reynolds and energy equations can be solved independently. Motosh [37] treated the problem from a... 

An attempt was made In the present work to Include the effect of elastic and thermal distortions on the lubricant film thickness. But because of the low speeds and low loads chosen In the present experimental work It Is considered that the effect of elastic and thermal distortions on the prediction of operating temperature Is reasonably small and hence these effects are neglected. 

The ideal bearing system should be self-feeding, maintenance-free and have infinite life. Additionally, when water (or sea water with possible particulate contamination) is to be used as the lubricating medium, the bearing, as well as exhibiting corrosion resistance and material stability, must, due to the low viscosity of the medium, operate with very small lubricant film thickness unless very conservative and often impractical designs are adopted. 

Experimental measurement of elastohydro-dynamic film thicknesses is difficult because Che film is generally less chan a micrometer Chick. The use of the optical interferometry developped by Cameron and Gohar (9) and Foord et al (10) is recognised to be a powerful technique for determining film thicknesses. The Hamrock and Dowson formulae are seen to predict lubricant film thicknesses which are in reasonable agreement with measured values for both centre and minimum film thicknesses. However, for lubricant L2, as found previously by Koye and Winer (11), the measured film thickness data show Chat the experimental data are roughly 30 per cent greater than Che calculated values with the Hamrock and Dowson minimum film thickness formula. This result... 

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