Solid lubricants transfer layers

It is known that even condensed films must have surface diffusional mobility Rideal and Tadayon [64] found that stearic acid films transferred from one surface to another by a process that seemed to involve surface diffusion to the occasional points of contact between the solids. Such transfer, of course, is observed in actual friction experiments in that an uncoated rider quickly acquires a layer of boundary lubricant from the surface over which it is passed [46]. However, there is little quantitative information available about actual surface diffusion coefficients. One value that may be relevant is that of Ross and Good [65] for butane on Spheron 6, which, for a monolayer, was about 5 x 10 cm /sec. If the average junction is about 10 cm in size, this would also be about the average distance that a film molecule would have to migrate, and the time required would be about 10 sec. This rate of Junctions passing each other corresponds to a sliding speed of 100 cm/sec so that the usual speeds of 0.01 cm/sec should not be too fast for pressurized film formation. See Ref. 62 for a study of another mechanism for surface mobility, that of evaporative hopping. 

This paper studies the local evolution of the surface layer. The technique used is based on spatially resolved measurements in a reciprocating contact. This approach called triboscopy was described elsewhere [12]. It has been previously used to study phenomena occurring in a sliding interface, especially the build-up of a transfer layer and local wear of a solid lubricant in thin films [13]. 

One of the major themes of boundary lubrication is to transfer the shear stress at the interface of direct solid contact to somewhere inside the lubricating layer, to achieve low friction and high wear resistance. In this sense, materials with low shear strength, such as liquid films, soft metals, and lamella solids, can be employed as candidate lubricants. 

Transference of monolayers from water to solid surfaces. Multilayers. In 1920 Langmuir1 dipped a clean glass plate into water covered by a monolayer of oleic acid, and found that an area of the monolayer equal to the area of the plate dipped was deposited on withdrawing the plate this layer lubricated the surface and rendered it more or less hydrophobic. In 1934 Miss K. B. Blodgett, in Langmuir s laboratory,2 found that any number of layers could be deposited successively by repeated dippings and these multilayers , consisting of monolayers deposited one by one, have received considerable attention since that time.3... 

Furthermore, insofar as additive action is concerned, the films of interest are oriented films. Our particular interest is in such films formed on the adsorbing surface by the additive which is dissolved in the liquid carrier fluid of the lubricant. These are the films which are found in everyday technology, in contradistinction to Langmuir-Blodgett films which are deposited on the solid surface by transfer of a preformed layer from the water surface of a film balance and which, despite their interesting properties, may be regarded as artifacts, as was pointed out by Zisman [5] in a review of the relation between wettability and the nature of adsorbed films. 

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