Tribology of Mechanical Systems: A Guide to Present and Future Technologies
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Polymers are used in a wide variety of tribological applications such as dry bearings, seals, piston rings, slides, prosthetic joints, gears, tires and brake shoes. In view of their commercial significance, the friction and wear behavior of these materials has been widely studied. The phenomena involved in sliding contacts are complex because of interactions between sliding members and the environment. As compared to metals and ceramics, polymers have much lower melting points, so they tend to soften or melt due to frictional heating in sliding. The low thermal conductivity of these materials confines the heat generated in the sliding contact zone. The viscoelastic behavior of organic materials also makes them more sensitive to deformation rate and temperature. Because of the combination of elastic and viscous behaviors of these materials, there possess relaxation mechanisms that are not observed in other materials. The long chain molecular structure and morphologies that result from the arrangement of these chains also account for the tribological behavior of these materials. Most cross-linked polymers are usually hard and brittle. Amorphous and crystalline polymers have a wide range of properties. The moduli of elasticity of all polymers are about two orders of magnitude lower than those of metals. The physical and mechanical properties of polymers can be varied over a wide range with a suitable choice of fillers and reinforcements.
The discussion of tribological behavior in this chapter is limited to sliding friction and wear under dry conditions only. The reason for this is that most tribological studies on polymers have been made with unlubricated surfaces because polymers are considered self-lubricating materials. The topics of erosion and fretting are not considered because of their limited practical significance. There are many reviews on this subject [1, 2, 3, 4, 5], plus a number of books [6, 7, 8, 9] and conference/symposia proceedings [10, 11] that address specific aspects of polymer tribology. It is to be expected that many topics discussed in this chapter are covered in greater details in the references cited. The objective of this chapter is to cover basic concepts and emphasize recent work.