Lamellar solid lubricant

Table 14.1 Physical Properties of the 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. 

Graphite is probably the most widely used lamellar solid lubricant. Unlike M0S2, graphite has a lower friction and lower wear in the presence of moisture than in vacuum. Therefore, graphite is not recommended for vacuum or high-temperature applications. But another form of carbon, amorphous hydrogenated carbon films (also called diamond-like carbon), has the reverse behavior It works extremely well in vacuum, but its friction coefficient is increased by the presence of moisture [37]. 

FIGURE 9.20 Model of the functionally graded structure of the MoDTC/ZDDP tribofilm. A low-friction inner-skin layer formed hy MoSj nanostrips has low yield strength and acts as a lamellar solid lubricant in lowering the friction coefficient of the MoDTC/ZDDP tribofilm. High concentrations of Ca, O, P, and S oil additive elements in the middle layer decreased and disappeared from the base layer, while the Fe concentration increased from an ultralow level in the surface layer to the same level as the steel disk at the tribofihn-disk interface. 

Fig. 8.9 Schematic diagram of the mode of action of a lamellar solid lubricant when used in dispersion in a liquid such as oil. (a) Orientation of lubricant particles between sliding surfaces, (b) prevention of welding of asperities by the lubricant particle (From Hickman J, Middleton K (1970). Some surface chemical aspects of lubrication, advancement of science, June, p 442, with permission from the British Association for the Advancement of Science)...

While the interlayer shear mechanism is believed to be generally responsible for the low friction of most lamellar solid lubricants, extensive research by many scientists in previous years has also confirmed that a favorable layered crystal structure in itself is not sufficient... 

These coatings are also employed as solid lubricants in engine components. Refractory materials such as MoS2 and WSe2 are lamellar compounds and provide very effective solid-state lubrication in spacecraft bearings and components used in radiation environments where conventional organic liquid lubricants are not stable (see Lubrication and lubricants). 

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... 

Although a lamellar crystal structure is favourable for solid lubrication, the inter-lamellar spacing and the nature of the inter-lamellar bonding are of major importance in determining the resistance to inter-lamellar shear, and therefore the sliding friction, of lamellar compounds. 

Deacon, R.F. and Goodman, J.F., Orientation and Frictional Behaviour of Lamellar Solids on Metals, Proc. Instn. Mech. Engrs., 195C, 344, (1957). (Proc. Conf. on Lubrication and Wear). 

Giltrow, J.P., Factors Affecting the Endurance of Rubbed Lubricant Films of Lamellar Solids, Brit. J. Appl. Phys., 18, 831, (1967). 

Of the lamellar solids which do show lubricating action, the two most familiar to the lubrication engineer are molybdenum disulfide (M0S2) and graphite. The lubricant-grade M0S2 of commerce is made by processing... 

In view of its similarity to graphite in crystal structure and in the lamellar properties of the crystallites, it was expected that boron nitride would function effectively as a solid lubricant. However, it has been found that the friction of boron nitride is characteristically higher than that of graphite under the same circumstances. D. H. Buckley [82] observed a value of v = 1.0 for the friction of pyrolytic boron nitride in high vacuum (10 torr) at 296 K G. W. Rowe [54] obtained v... 

TABLE 19-13. SOME NON-LAMELLAR SOLIDS STUDIED AS LUBRICANTS... 

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 ... 

Among the above listed materials, M0S2 and graphite are the predominant solid lubricants. In dry powder form, these are effective lubricants due to their lamellar structures. The lamellae orient parallel to the surface in the direction of motion as... 

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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