Lubrication monolayer

Salmeron, M., Generation of Defects in Model Lubricant Monolayers and Their Contribution to Energy Dissipation in Friction," THbology Letters, Vol. 10, No. 1-2, 2001, pp. 69-... 

FMM was successfully applied in order to image local stiffness variations occurring on surfaces of polymer samples. For instance, Overney et al. [46] and Bar et al. [47] demonstrated by means of FMM that the frictional properties of lubricating monolayers not only depend on their adhesion properties but also on their compression stiffness. Consequently, stiffness should... 

It is evident that boundary lubrication is considerably dependent on the state of the monolayer. Frewing [48] found that, on heating, the value of fi rose sharply near the melting point sometimes accompanied by a change from smooth to stick-slip sliding. Very likely these points of change correspond to the transition between an expanded film and a condensed film in analogy with... 

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. 

The lubricant properties of alkanethiols and fluorinated alkanes have been studied extensively by scanning probe techniques [163]. In agreement with experiments on LB monolayers it was found that the fluorocarbon monolayers show considerably higher friction than the corresponding hydrocarbon monolayers [164, 165 and 166] even though the fluorocarbons are known to have the lowest surface free energy of all organic materials. 

Ultra-high vacuum (UHV) surface science methods allow preparation and characterization of perfectly clean, well ordered surfaces of single crystalline materials. By preparing pairs of such surfaces it is possible to fonn interfaces under highly controlled conditions. Furthennore, thin films of adsorbed species can be produced and characterized using a wide variety of methods. Surface science methods have been coupled with UHV measurements of macroscopic friction forces. Such measurements have demonstrated that adsorbate film thicknesses of a few monolayers are sufficient to lubricate metal surfaces [12, 181. 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

In addition, efforts have been made to explore the possibility of applying a monolayer as a lubricant to the surfaces of the magnetic head and disk, or both [32-35]. The results indicate ... 

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. 

A correlation between the packing in the monolayer and its lubricating properties is detected, which suggests that monolayers in higher degree order may exhibit better boundary lubricating properties. 

The expectation of the structural dependence of lubrication motivated great numbers of investigations that intended to prepare and use highly ordered organic films, such as the Langmuir-Blodgett (L-B) hlms and Self-Assembled Monolayers (SAMs), as solid lubricant, which will be discussed more specihcally in Section 4. 

When the surface is subjected to wear, the weakly bonded monolayers would be worn off in a few passes of sliding, but active molecules in the lubricant would adsorb successively on the surface to repair the worn part of the him. The wear resistance depends on the balance of the rate of him removal and generation. 

A key limitation to the application of the monolayer as a boundary lubricant is the sensitivity to temperature. Molecules are desorbed from the surfaces as the temperature rises, and the coverage ratio would drop to zero above 200 ° C at which the monolayer becomes completely ineffective. Boundary lubrication functions at high temperature regions rely mainly on chemical reactive hlms. 

In summary, impressive progress has been achieved to understand the role of adsorbed layers in boundary lubrication, but the effect of molecular orientation on tribology performance and the shear strength of adsorbed monolayers are the issues that remain to be clarihed in a long future study of boundary lubrication. 

The most commonly used amphiphiles to build L-B hlms for tribological applications are the straight chain hydrocarbon compounds with simple functional groups such as the fatty acids, including stearic acids, arachidic acids, and behenic acids [32], but other amphiphilic molecules, e.g., 2,4-heneicosanedione and 2-docosylamina-5-nitropyridine, are also applied in some cases. There are two major systems of self-assembled monolayers, namely the alkylsilance derivatives (e.g., OTS, octadecyltrichlorosilane) on hydroxylated surfaces and the alkanethiols on metal substrates, which have been investigated extensively to examine their properties as solid lubricants and protective surface films [31 ]. 

It has been proposed recently [28] that static friction may result from the molecules of a third medium, such as adsorbed monolayers or liquid lubricant confined between the surfaces. The confined molecules can easily adjust or rearrange themselves to form localized structures that are conformal to both adjacent surfaces, so that they stay at the energy minimum. A finite lateral force is required to initiate motion because the energy barrier created by the substrate-medium system has to be overcome, which gives rise to a static friction depending on the interfacial substances. The model is consistent with the results of computer simulations [29], meanwhile it successfully explains the sensitivity of friction to surface film or contamination. 

We performed SPEM studies on hard disc coupons covered with lubricant films of different types and thicknesses [70]. No evidence of dewetting was observed with films of submonolayer average thickness (17 A for Zdol and 14 A for Zdol-TX). Dewetting was observed, however, when the film thickness was above one monolayer. The stracture of these films during dewetting differs depending on the type of lubricant. 

Moreover, stable liquid systems made up of nanoparticles coated with a surfactant monolayer and dispersed in an apolar medium could be employed to catalyze reactions involving both apolar substrates (solubilized in the bulk solvent) and polar and amphiphilic substrates (preferentially encapsulated within the reversed micelles or located at the surfactant palisade layer) or could be used as antiwear additives for lubricants. For example, monodisperse nickel boride catalysts were prepared in water/CTAB/hexanol microemulsions and used directly as the catalysts of styrene hydrogenation [215]. 

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