ABSTRACT
Abstract .................................................................................................................. 437 17.1Introduction .................................................................................................. 438 17.2Experimental Section ...................................................................................440 17.3Results and Discussion ................................................................................. 441
17.3.1Surface Chemistry of Model Chlorinated Hydrocarbon Additives on Iron Surfaces ............................................................... 441
17.3.2Surface Chemistry of Tribological Additives ................................... 441 17.3.3Relationship between Surface Chemistry and
Chemistry at Tribological Interface..................................................444 17.3.4Tribological Properties of Thin, Lubricous Films ............................446
17.4 Conclusions ................................................................................................... 453 References .............................................................................................................. 454
In general, lubricants are rather complex mixtures of components, each of which is added to provide a particular function, for example, to improve the viscosity, prevent the growth of bacteria, or inhibit the formation of foams. This chapter focuses on a class of lubricant additives that react at the tribological interface to form some type of surface ”lm. Depending on the particular application of the lubricant, this ”lm may be required to reduce friction or minimize wear, or both. However, these additives react with the surface to form a ”lm of average thickness X where the resulting ”lm thickness depends on the nature of the additive, the surface, and the reaction conditions. In fact, one of the most widely used antiwear additives, zincdialkyldithiophosphate,wasoriginallyusedasanantioxidant,butitwasalso found to reduce wear by reacting with the surface to form a ”lm. In the following, we refer to these as “tribo”lms,” and they are formed by a chemical reaction with the contacting surfaces that are moving relative to each other. The resulting thickness of the ”lm arises from a balance between the rate that it is formed by reaction with the surface and the rate at which it is removed (the wear rate) and is therefore inherently a kinetic process. Accurately modeling the wear rate is a complex issue, but is most simply discussed in terms of a simple Archard wear model [1] where the wear (the amount of material removed) per unit sliding length is proportional to the applied load and inversely proportional to the shear strength or hardness of the materials that constitute the contacting interface. In principle, modeling the ”lm-formationkineticsshouldberelativelystraightforwardsincechemicalkinetic processes are rather well understood [2].
