Lubricants molecular layer

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. 

If the speed is smaller than that of the failure point, the film thickness will suddenly decrease to a few molecular layers, or it is in boundary lubrication [34]. 

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. 

Three theories were proposed to explain wall-slip (a) adhesive failure at the wall, (b) cohesive failure within the material as a result of disentanglement of chains in the bulk and chains absorbed on the wall, and (c) the creation of a lubricating surface layer at the wall either by a stress-induced transition, or by a lubricating additive. If the polymer contains low molecular weight components or slip-additives, their diffusion to the wall will create a thin lubricating layer at the wall, generating apparent slip. 

These results are confirmed by Soviet work which studied the effect of velocity and temperature upon the eutectic mixture MBBA, EBBA with a of 52 C and compared their results with the lubrication of "MS 20 aviation oil". Lubrication by LC reduced the friction coefficient to one-fifth of that of the oil (Fig. 22a). In ref 9 LC is compared to vaseline oil with the same result. (Fig. 22b). In all these experiments,vi is lower than 0.05 which indicates the presence of a poly molecular layer of lubricant. 

Conventional oil-based lubricants are not suitable for application in MEMS, as the viscous forces can be quite large when compared with the forces involved in operating these components because the size of the oil molecules is of the same order as those of MEMS components [5], Therefore, many researchers have proposed ultrathin organic molecular layers as the lubricants for Si-based MEMS systems [3, 5, 8]. These ultrathin molecular lubricant layers are generally formed by two methods (a) the Langmuir-Blodgett (LB) method and (b) the self-assembly method [9]. The LB monolayers have two main difficulties for their use as the lubricant layers for MEMS components (a) LB films are less wear resistant, as the interaction forces between LB films and substrate (van der Waals) are weaker [10], and (b) application of the LB method is restricted to flat surfeces only, and it is not practical to coat three-dimensional surfaces of a structure [11-14],... 

In general, the reduction of the liquid film thickness to fewer than 4-6 molecular layers can induce lateral ordering and lead to freezing. It has been demonstrated that water confined to nanospaces exhibits anomalous phase behaviors that are typically illustrated experimentally or via MD simula-tion. " Moreover, there is evidence that a possible liquid-solid phase transition occurs for ionic liquids in confined systems. We also reported the first simulation results of a liquid-solid freezing transition of an 1,3-dimethylimidazolium chloride ([Dmim][Cl]) ionic liquid between two parallel graphite walls. " This result is important to understand the microstructure and freezing processes of ILs in confined systems, such as lubrication, adhesion, and IL/nanomaterial composites. 

Solid lubricants are layered materials such as graphite and M0S2. Intercalation of atomic or molecular species can substantially improve the lubricating properties of these sohds. The reason is that the intercalated species may widen the van der Waals gap. ... 

The lubricant is required to be very thin on the order of a mono-molecular layer. Therefore, the frictional properties depend not only on the molecular structure, but also on the microscopic structure of the lubricant film.(Seki Kondo, 1991 Kondo Seki, 1993a Kondo et al, 1996 Kondo, 2008) Microscopic coverage of this alkylammonium-based protic ion liquid film on the medium surface is also examined using FTIR and X-ray photoelectron spectroscopy (XPS) and related to the spectra to the frictional properties. 

Another type of load-carrying additives is EP additives, which can react with metal surfaces to produce inorganic sur ce films. These films protect sub-surface metal and thereby reduce metal-metal contact. In this study, tri n-butylphosphate (n-BuP) was used as a representative of this type of additive. n-BuP forms a metal phosphate layer on the sur ce tribochemically. It is also believed that the contribution of the organic portion as an adsorbed molecular layer cannot be expected from the n-butyl group, because the chain length is too short. Therefore, the major contribution of n-BuP to lubrication is the formation of an inorganic surhice layer. 

Friction on the macroscopic scale can take place either between dry contacts or between lubricated surfaces. An intermediate case called boundary lubrication is friction in which the surfaces are not separated by a thick layer of lubricant but just by surface layers such as oxide layers on metals or by a few molecular layers of adsorbed lubricants. 

Film stability is a primary concern for applications. LB films of photopoly-merizable polymeric amphiphiles can be made to crosslink under UV radiation to greatly enhance their thermal stability while retaining the ordered layered structure [178]. Low-molecular-weight perfluoropolyethers are important industrial lubricants for computer disk heads. These small polymers attached to a polar head form continuous films of uniform thickness on LB deposi-... 

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). 

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