Failure Films

The mechanism of the film failure is due to the limiting shear stress of the fluid film in the nano-scale [34,53]. According to Newtonian fluid theory ... 

Figure 9.2 shows some results of friction testing at 5 x 10 Torr on coatings produced by Ermakov by heating molybdenum in contact with elemental sulphur at 500° - 600°C. It is interesting that he found an increase in friction as the test temperature was raised, but that there was no film failure even at 700°C. 

Lead Not usable in air because oxidation leads to film failure Lubrication of rolling bearings in vacuum... 

The vortex action in Benard cells can cause the defects knovm as flooding and floating because pigments of different sizes and weights will move at different velocities, will separate, and show nonuniformity of colors in films. The pigments also can be concentrated at certain local spots and thus leave pure binder at other spots that are vulnerable to film failure upon exposure. 

Because moduli are temperature dependent, the ambient conditions for drying determine whether a catastrophe will be encountered, signaling the onset of film failure. One of the practical consequences of this dependence is that high-temperature cures can be effected with materials whose rigidity at service temperatures is high enough to be serviceable. 

MEASUREMENT OF FLUID FILM THICKNESS AND DETECTION OF FILM FAILURE... 

It is obvious from scrutiny of the nature and mechanism of fluid film lubrication that in order for the to lubricate the rubbing of one solid surface against another it must separate them. Hydrodynamic calculations will show whether or not a fluid film of the requisite thickness theoretically can exist under the given pressure distribution over the opposing surfaces. We may therefore take as an idealized criterion of fluid film failure a calculated film thickness of zero. The physical consequence implied by this criterion is that the solid surfaces can then come into direct contact. 

If the idealized concept of fluid film failure proposed above is to have any significance in the world of experimental mechanics and engineering, we must find a basis for its validity and utility. The study of fluid film failure in a practical sense then becomes the study of the behavior of the boundary surfaces of the solids and of the intervening fluid lubricant as the thickness of the lubricant film approaches zero. An important aspect is the reliability of the measurement technique for very thin films. We must be careful not to think of fluid film failure as rupture or breakdown by exceeding the intrinsic strength of the lubricant material. Bulk liquid films do not behave in that way. We know by hydrodynamic theory that the pressure a film of fluid is able to... 

Let us examine the process of film failure conceptually by considering the progressive thinning of a fluid film bounded by two perfectly smooth solid surfaces. We might say that as long as there is a film of lubricant one molecule thick between these surfaces they are not in contact. But we cannot be sure the surfaces are truly isolated from each other in the sense that the atomic force fields of the surfaces do not interact through the molecular film of lubricant. However, the theoretical possibility of such interaction is not necessarily a criterion for film failure, for as we shall learn in subsequent chapters, there are types of adsorbed films only one or two molecules thick that function recognizably and satisfactorily as lubricants. 

If the fluid film separating two metallic bounding surfaces is an electrical insulator, then loss of insulating behavior and appearance of conductance can be interpreted as contact between these surfaces. This is the premise behind the electrical conductance method of detecting lubricant film failure. A practical limit on the applicability of this premise is electrical breakdown of the fluid as thinning of the film concentrates the field intensity. Another complication arises from the fact that for the structured surfaces of every day experience first contact is at the highest asperities rough surfaces may therefore come into initial contact sooner than indicated by the overall trend of the resistance measurements. 

Figure 6-13. Circuitry for detecting film failure in a plain bearing.

Figure 6-14. Oscilloscope traces of film condition in a plain journal bearing. (a) Film intact. (b) Total film failure. (c) Intermittent failure. Data by C. M. Allen [21].

DETECTION OF FLUID FILM FAILURE BY FRICTION OR BY EXAMINATION OF SURFACE CONDITION... 

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... 

Experimental confirmation of the elastohydrodynamic lubrication theory has been obtained in certain selected systems using electrical capacitance, x-ray transmission, and optical interference techniques to determine film thickness and shape under dynamic conditions. Research is continuing in this area, including studies on micro-EHL or asperity lubrication mechanisms, since surfaces are never perfectly smooth. These studies may lead to a better understanding of not only lubricant film formation in high-contact-stress systems but lubricant film failure as well. 

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