Solid lubricants shear planes

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]. 

Solid lubricants, low shear compounds A solid lubricant used in low torque applications where the lubrication is provided by shear between crystallographic planes. Example M0S2. 

Lubrication theory suppose that lubrication is due to the high pressure which sets up in very thin films of liquids sheared between two plane surfaces slightly inclined to one another. In a smectic structure, the layers fow easily past one another while they have a solid strength in the direction perpendicular to the layer, thus the pressure effect may be expected to be stronger... 

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