Adsorbent solid lubricants

Special cases of solid lubrication arc boundary anil IIP (extreme pressure) lubrication In both cases ihe solid lubricant is formed by chemical reaction of special compounds, usually applied as oil solutions, with the metallic rubbing surfaces Typical boundary lubricants are the fatty acids which react with the metal surface to form metallic soaps which then carry the load. Strongly adsorbed hut nonreacting substances of linear structure, such as long chain tally alcohols, can also act as boundary lubricants but only under very mild conditions... 

The data can be used to obtain thermodynamic parameters characterising the nature of adsorbate-solid interactions which are important in gaining a fundamental understanding of selective adsorption mechanisms, the kinetics of adsorption, and associated processes such as catalysis, lubrication, dispersion technology, corrosion, adhesion, and the determination of surface areas of chemically different sites on solid surfaces. 

The definition of different lubrication regimes is a historic problem [41 ]. In boundary lubrication, molecules will be absorbed on a solid surface of a tribo-pair and form a monomo-lecular absorbed layer as described by Hardy [42] as shown in Fig. 1 (a). If the film thickness of lubricants in the contact region is from a few nanometres to tens of nanometres, different layers will be formed as shown in Fig. 1 (b) proposed by Luo et al. [3,4]. The layer close to the surfaces is the adsorbed film that is a monomolecular layer. The layer in the... 

However, the assumption of molecule orientation normal to the surface is not convincing enough for this author, and it does not consist well with the results of the molecular d5mamics simulations for the alkane confined between solid walls. An example in Fig. 3 shows that the chain molecules near the wall are found mostly lying parallel, instead of normal, to the wall [6]. This means that the attractions between lubricant molecules and solid wall may readily exceed the inter-molecule forces that are supposed to hold the molecules in the normal direction. Results in Fig. 3 were obtained from simulations for liquid alkane with nonpolar molecules, but similar phenomenon was observed in computer simulations for the functional lubricant PFPE (per-fluoropolyether) adsorbed on a solid substrate [7], confirming that molecules near a solid wall lie parallel to the surface. 

Thus, fundamentally the interest is in testing the limits and theory of polymer behavior in end-tethered systems, e.g., viscoelastic behavior, wetting and surface energies, adhesion, shear forces relevant to tribology, etc. It should be noted that relevant surfaces and interfaces can also refer to polymers adsorbed in liquid-liquid, liquid-gas, solid-gas, and solid-liquid interfaces, which makes these polymer systems also of prime importance in interfacial science and colloidal phenomena (Fig. 2). Correspondingly, a wide number of potential applications can be enumerated ranging from lubrication and microelectronics to bioimplant surfaces. 

An atom or molecule that approaches the surface of a solid always experiences a net attractive potential ). As a result there is a finite probability that it is trapped on the surface and the phenomenon that we call adsorption occurs. Under the usual environmental conditions (about one atmosphere and 300 K and in the presence of oxygen, nitrogen, water vapor and assorted hydrocarbons) all solid surfaces are covered with a monolayer of adsorbate and the build-up of multiple adsorbate layers is often detectable. The constant presence of the adsorbate layer influences all the chemical, mechanical and electronic surface properties. Adhesion, lubrication, the onset of chemical corrosion or photoconductivity are just a few of the many macroscopic surface processes that are controlled by the various properties of a monolayer of adsorbates. 

In this section we discuss dry friction, also called solid or Coulomb friction. Solid friction occurs when two solid surfaces are in direct contact without any other components like lubricants or adsorbed surface layers involved. The reader might object that in practice no such... 

Another method used to make lubricating greases, and also oil-based drilling fluids, involves the use of organic compounds to stabilize non-aqueous suspensions. For example, clays and other mineral solids can be dispersed in oils when polar organic compounds such as quaternary ammonium surfactants are adsorbed onto their surfaces, making the latter preferentially oil-wetting (see Sections 3.4 and 3.5.3). 

Zinc dialkyldithiophosphates (ZDDPs) function mainly as antioxidants and antiwear additives. Molecules of ZDDPs adsorb on metal surface to participate in surface tribofilm formation under conditions of boundary lubrication. The solid tribofilms are formed at the metal surface to protect even under conditions of coarse contact under load (Bom et al., 1992). 

The lubrication system is extremely complex. The mechanism of lubrication is partly dictated by the nature of interactions between the lubricant and the solid surface. Additives blended into lubricating oil formulations either adsorb onto the sliding surfaces, eg., fatty alcohols, fatty amines, amides, phosphoric acid esters (friction modifiers), or react with the surface, eg., ZDDP, MoDTC, MoDDP organic phosphates (extreme pressure). Some interactions affecting the surfaces of metals include adsorption, chemisorption, and tribochemical reactions-these form new compounds on the surface and lubrication by reaction products (Bhushan and Gupta, 1991 Briscoe et al., 1973 Briscoe and Evens, 1982 Heinicke, 1984 Hsu and Klaus, 1978 and 1979 Klaus and Tewksbury, 1987 Lansdown, 1990 Liston, 1993 McFadden et al., 1998 Studt, 1989). 

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