Other Lamellar Solid Lubricants

The lubricating performance of the lamellar crystal structure is strongly dependent on the inter-lamellar separation and bonding, and these can be modified by the intercalation of other substances between the lamellae. The general subject of intercalation will therefore be discussed before proceeding to descriptions of the individual lamellar solid lubricants. 

Just like other lamellar solids, boric acid is also self-lubricating. Its lubrication mechanism is similar to that of the other solid lubricants. It can be manufactured as very fine nanopowders. Because of their light weight, these nano-powders can easily be dispersed in oils and used as colloidal anti-friction and -wear additives. Tribological studies have confirmed that they have very high capacity to reduce friction and wear. TOF-SIMS and XPS studies... 

As a result, the number of lamellar solids which are of interest as lubricants is reduced to eight including molybdenum disulphide, and the density and resistivity of the other seven are listed in Table 14.1. These materials, together with PTFE, are the main alternatives to molybdenum disulphide for solid lubrication. Some of the more important aspects of PTFE have been discussed in Chapter 12, and the properties and... 

Solid lubricants need low shear strength in at least one dimension. Solid lubricants fall into three main classes - inorganic solids with a lamellar (layer-Uke) crystal structure, solids that suffer plastic deformation easily, and polymers in which the constituent chains can slip past each other in an unrestricted way. Although soft metals such as tin and lead have been used for many years as bearings, and polymers, especially Teflon (PTFE, polytetrafluoro-ethylene) are used as a coating to create nonstick ... 

Such lamellar structures are even able to prevent contact between highly loaded stationary surfaces. In the direction of motion, the lamellae easily shear over each other resulting in low friction. While larger particles perform best on relatively rough surfaces at lower speeds, finer particles perform best on relatively smoother surfaces and at higher speeds. A comparison of various solid lubricants with respect to their coefficients of friction is shown in Figure 19.2 [4]. 

Most of the solid lubricants mentioned above owe their low-Mction characteristic primarily to a lamellar or layered crystal structure (see two of them in Figure 6.1 as typical examples). When present at a sliding contact interface, these solids shear easily along their atomic shear planes and thus provide low friction. Some of the solid lubricants do not have such layered crystal structures, but in applications, they too provide very low friction and wear. For example, certain soft metals (In, Pb, Ag, Sn, etc.), PTFE, a number of solid oxides and rare earth fluorides, diamond and diamondlike carbons, etc., can also provide fairly good lubrication despite the lack of a layered crystal structure like the ones shown in Figure 6.1 [1]. In fact, diamondlike carbon films are structurally amorphous but provide some of the lowest friction and wear coefficients among all other solid materials available today [8]. 

The lubrication mechanism of the bulk and thin film forms of boric acid is similar to that of the other known lamellar solids. Specifically, under shear stresses, plate-like crystallites of H3BO3 can align themselves parallel to the direction of relative motion and then slide over one another with relative ease to provide low friction, as shown in Figures 6.9 and 6.10 [24]. 

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