ABSTRACT
Commercial lubricants that are required to operate under severe conditions are composed of several components. The most abundant of these is the base fluid, which may be a mineral oil or, in some cases, water. The lubricants are “formulated” by adding various components. Some of these function, e.g., to stabilize the oil against oxidation or biological decay. Others improve the tribological performance. This chapter addresses the role of additives that thermally decompose at the lubricated surfaces to deposit reactively formed films so that the surface film arises from a chemical reaction between the additive and the surface. The key parameter in understanding the properties of these reactive additives is the temperature of the interface, because an increase in temperature of the rubbing surfaces correspondingly increases the rate at which the tribological film is formed. Additives that thermally decompose to form tribological films are therefore, in some sense, “smart” additives because they form films just in those regions of the interface that most require them [1]. This chapter therefore focuses on understanding the way in which these lubricant additives react at the tribological surface, the nature of the film that is formed, and the role of this film in improving tribological behavior. This improvement may be in the reduction of friction, in allowing an increase in the loads that can be sustained, or in generally providing a protective coating on the surface. Fully understanding these phenomena requires the synthesis of mechanical approaches with an understanding of the chemical and physical properties of the interface, and therefore
requires a truly interdisciplinary strategy. The chapter will thus focus on an area where it has been well established that surface chemical reactivity plays a key role in forming tribological films, that of extreme-pressure (EP) lubricants that are used for processes such as machining, wire drawing, fineblanking, etc. A wide range of compounds have been claimed to be effective EP additives but the ones that are currently most commonly used generally contain chlorine, sulfur, or phosphorus [2-12]. These are most often added as organic compounds, which render them soluble in the base lubricating fluid. Because many of the compounds that are currently used for this purpose are either environmental pollutants or health hazards, or both, these will ultimately have to be replaced by more benign alternatives [13]. Because these additives are understood to thermally decompose at the lubricated interface, the nature of this interface is of crucial relevance to understanding the behavior of the additive. This means that the additive+ surface combination must be considered as a whole because the lubricant film is formed by a chemical reaction between them. This clearly means that what may be a good lubricant additive for one metal may not be such a good additive for another. In addition, the presence of surface contaminants, e.g., oxide films or carbonaceous layers, can profoundly affect the reactivity at the surface. This problem is to some extent obviated in the EP regime because wear rates are usually sufficiently high that these surface contaminants are worn away.
