Amorphous shear sliding

In recent experiments, the application of stress orthogonally to a shearing surface caused a ductile failure of brittle polymer (5, 6). In the first series (5), a variety of plastomers and elastomers were made to slide one on the surface of another, at a constant velocity of 215 cm/sec, under increasing normal loads. The wear characteristics of polymers, including several brittle ones such as PMMA and PS, depend on the applied normal stress. At relatively low pressure, almost no wear was observable, even under magnification the little observed was apparently brittle, ill-defined, microparticulate debris. At intermediate normal loads, 3 to 20 kg/cm2, roll formation was the dominant mode of wear. Such rolls appear on the surfaces of all uncrosslinked polymers whose Tg is below the test temperature and on amorphous and semicrystalline polymers whose Tg is above... 

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]. 

---