Lubrication physical adsorption

Further examples reported in Table 40 arc given by the alkyl ketone moieties deriving from oxidation of polyenes, which arc subjected to Mannich aminomethylation in order to produce compounds 559 having dispersant properties for lubricating oils, by benzotriazoles 557, capable of forming, due to physical adsorption, thin layers over the surfaces subjected to friction, and by S-Mannich bases 558, combining antifriction and antioxidant properties. 

The mechanisms of film formation previously described involve both physical and chemical processes. It follows that factors favourable to film formation can influence friction and wear. Coefficients of friction were shown to vary from 0.04 for a reactive metal such as zinc, lubricated with 1% lauric acid, to 0.55 for an inert metal, silver, with the same lubricant [10], These factors include strong dipole interactions or strong hydrogen bonding which aid physical adsorption and the ease of chemical reaction from this adsorbed layer. Both interactions favour the formation of low-shear-strength films and similar influences have been reported by many workers including wear for mixtures of dilinoleic and linoleic acids [11], and for ZDDPs [6]. 

Thus, the interfacial friction can be evaluated from measurement of AT and A/. This procedure has been applied to a number of systems in which weak physical adsorption occurs, such as the adsorption of Xe, Kr, N2 on Au, and of H2O and CeH on Ag [34-38]. In all the above cases slippage was observed, and the ratio of the coefficient of sliding friction to the mass density was of the order x/Amf = (10 - 10 )s As an example, the frictional stress acting on the monolayer Xe film sliding on a Ag(lll) surface at a velocity v = 1 cms F = xv, equals about 10Nm [40]. It is much smaller than typical shear stresses involved in sliding of a steel block on a steel surface under boundary lubrication condition (Eq. 6), which is of order 10 Nm 2 [39]. 

A lubricant is any material that can be placed between surfaces to lessen friction. The purpose of a lubricant is to reduce the frictional resistance between two contacting surfaces forced to slide over one another, to minimize wear, and prevent corrosion. Antiwear agents produce a surface fihn either by a chemical or by a physical adsorption mechanism to minimize friction and wear under boundary lubrication conditions. 

FIGURE 18.13. Adsorption for boundary layer lubrication can occur through either physical adsorption (a) or chemisorption (b). While physical adsorption is rapid and essentially universal, it is relatively weak and may produce thin, easily disrupted films. Chemisorption, on the other hand, is usually much stronger and may produce more... 

In practice, metal oxides are covered with organic molecules and water adsorbed from the atmosphere.Other common sources of surface contamination are residual processing oils and lubricants. Whether the interacting species are considered physically adsorbed or chemisorbed depends on the strength of interaction. Although somewhat arbitrary, it has been stated that interactions up to 10 kcal/mole are considered physical adsorption, while those greater than 10 kcal/mole are regarded as chemisorption. 

As noted before, thin film lubrication (TFL) is a transition lubrication state between the elastohydrodynamic lubrication (EHL) and the boundary lubrication (BL). It is widely accepted that in addition to piezo-viscous effect and solid elastic deformation, EHL is featured with viscous fluid films and it is based upon a continuum mechanism. Boundary lubrication, however, featured with adsorption films, is either due to physisorption or chemisorption, and it is based on surface physical/chemical properties [14]. It will be of great importance to bridge the gap between EHL and BL regarding the work mechanism and study methods, by considering TFL as a specihc lubrication state. In TFL modeling, the microstructure of the fluids and the surface effects are two major factors to be taken into consideration. 

In summary, for metal surfaces in boundary lubrication, complex tribochemical reactions occur along with the physical/chemical adsorptions, which lead to the formation of surface hlms, consisting of reaction products, oxide layer, the mixture of particles and organometallic polymer, and perhaps a viscous layer. The surface hlms operate as a sacri-... 

The forces acting between two surfaces in contact or near - contact determine the behavior of a wide spectrum of physical properties. These can include friction, lubrication, the flow properties of particulate dispersions, and, in particular, the adsorption and adhesion phenomena, the stability of colloidal system [1,2] and the ability to form Langmuir monolayer at the air - water interface. 

Nascent surface Explain the difference in the concept of liquid lubrication mechanism in (a) hydrodynamic, (b) elastohydrodynamic and (c) boundary lubrication. Which of the following characterize (a), (b), and (c) lubrication regime continuous fluid film, negligible deformation, complete separation of the surfaces, elastic and plastic deformation, no wear takes place, no contact between the sliding surfaces, involving surface topography, physical and chemical adsorption, catalysis and reaction kinetics, and tribochemical film formation ... 

The critical temperature of lubricated friction has been related to the physical chemistry of adsorption by interpreting the transition from smooth sliding with a low coefficient of friction to high values of friction with scuffing as the critical depletion of the adsorbed lubricant film. The critical transition temperature is identified with the critical temperature of desorption. Frewing [31] developed the following relation for the stable existence of a film of adsorbed additive in equilibrium with its oil solution ... 

Adsorption of polymers on a solid flat substrate is an important physical chemistry issue in many applications of polymers, such as composites, coatings, adhesion, lubricates, gel chromatography, wetting, colloidal stability, piping transportation. 

The process of surfactant adsorption from a solution onto a solid surface is perhaps the most relevant in the field of tribology. It is the process by which all surfactant-mediated lubrication occurs. The physical model for this can be described by a series of steps. As with other systems, there will be diffusion from the bulk to a subsurface layer, adsorption of (initially) a monolayer of surfactant molecules, and the possibilities of subsequent rearrangement of molecules on the surface, deposition of a bilayer and subsequent further layers, and also desorption. The interactions that govern the adsorption and desorption processes can be hydrophilic, hydrophobic, and/or electrostatic, depending on the nature of the surfactant and surface concerned. Now the situation is very complex, and because of this, it is sensible to tackle the kinetics of nonionic and ionic surfactants separately. 

Lubricating greases are essential for the performance of friction units in various machines and mechanisms. In these cases, surfactants play a dual role at early stages of their use, they enhance wear and thus facilitate a breaking-in of the friction surfaces, while at later stages, they form protective adsorption layers that help to retard wear in the parts. The role of surface-active media in the processes of friction and wear is the subject of tribology, which constitutes a separate area of physical-chemical mechanics [95-106]. 

Systematic surface analytical studies on model or ideal systems. These are performed in order to understand the mechanisms and underlying physics and chemistry of various phenomena, such as the reactions promoted by additives, the effects of adsorption on friction and adhesion, and the degradation of a thin lubricant coating in a severe environment. 

Superhydrophobic materials have surfaces that are extremely difficult to wet, with water contact angles in excess of 150° or even greater, see Fig. 20.6 shows that surfaces with ultrahydrophobicity have aroused much interest with their potential applications in self-cleaning coatings, microfluidics, and biocompatible materials and so on. Many physical-chemical processes, such as adsorption, lubrication, adhesion, dispersion, friction, etc., are closely related to the wettability of materials surfaces [52, 53]. Examples of hydrophobic molecules include alkanes, oils, fats, wax, and greasy and organic substances with C, N, O, or F as the key constituent element. 

Reply bv the Authors Most probably, the transitions in tribological behaviour can be attributed to changes in the chemical and physical properties of the lubricant itself in the contact region and of boundary layers formed by adsorption as well as chemical reactions. As to the lubricant, there is certainly an influence of temperature on viscosity which may lead to a breakdown of partial micro-EHD-films when temperature rises. As to adsorbed layers, they will detach from the surface at a critical temperature. For reaction layers formed by additive-surface-interaction, it is known that threshold temperatures do exist. 

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