Effect of materials on tribochemical reactions between hydrocarbons and surfaces

Tribochemistry can be defined as the chemical reactions between the surface and the lubricant molecules inside a sliding contact under boundary lubrication conditions. These reactions are important because they control the lubrication processes, and therefore they determine the reliability and durability of moving parts in machineries. In spite of its practical significance, our understanding of the nature of tribochemistry is limited, and to a large extent, empirical in nature, and mainly restricted to the steel-hydrocarbon system. As new materials emerge and are being applied to various applications, users are often hampered by the lack of knowledge of how these new materials can be effectively lubricated via tribochemical reactions, resulting in occasional premature failure and product recall. There are many tribochemical reactions which occur in a sliding system, such as tribocorrosion, electro-chemistry, hydrolysis of ceramics and anti-wear additive film formation. However, the fundamental basis of lubrication in most of the practical systems starts with the reactions between materials surfaces and hydrocarbon base oils. Without this reaction, for example, an anti-wear additive such as zinc dithiophosphate would not be effective. Therefore, in this paper, we will focus on the effect of materials on the basic tribochemical reactions between hydrocarbons and surfaces. This paper integrates existing information in the literature on the lubrication chemistry of various materials and provides a consistent framework in the context of tribochemistry. For discussion purposes, we will examine metals, semiconductors, and insulators. The preponderance of data in the metals-hydrocarbon systems suggests that thermochemistry and organometallic chemistry dominate the interfacial chemical reactions. For semiconductors and insulators, there is evidence that electrostatic charge induced electron emission plays an important role in the tribochemistry of these materials. Various measurement methods used to characterize tribochemical behaviours are introduced and discussed in terms of reactivity analysis and tribochemical mechanisms.