# Molecular Science of Lubricant Additives +
## 1 Introduction
### Tribo-Chemical Reactions  
Mechano-chemical reactions have a long history from the alchemist ages [7; 8; 9; 10]. Since then, various chemical reactions have been empirically examined under mechanical stresses such as agitation in pestle-mortar or collision of solids. In addition to scientific approaches [11; 12], industrial applications of mechano-chemical reaction could be found in preparation of materials [13; 14] and waste treatments [15; 16]. The term, \"tribolysis\" was first published in 1928 [17]. It seems to indicate \"tribo-chemical reaction\" but this article is describing alterations of petroleum products by mechanical agitation (traditional tool for mechano-chemical reactions) compared with photolysis and thermolysis of these substances. Tribo-chemical reactions could be categorized as a part of mechano-chemical reactions. t should be defined precisely as \"conversion of molecules to others under the influence of rubbing\". Additionally, this review focuses on this phenomenon \"especially under lubricated conditions\". In this regard, chemical reactions of lubricants, both base fluids and additives, are of interest. It is generally agreed that tribo-chemical reactions are complicated phenomena. There are two major causes of the complexity. One is the problem with the analytes. Lubricants are mixture of various compounds, including base fluids and different additives. In most cases, their principle structure is hydrocarbons and hence clear identification of each substance is difficult. When we focus on the reaction mechanism of an additive of interest, the problem of the detection limit with instrumental analysis usually arises. Concentration of additive could be as low as in the range of mmol-kg-1. Specific analytical tools, such as surface sensitive analysis, combined with a model lubricant (e.g., a solution of single additive in a solvent) could be a solution for the analyte problem. Actually, many articles are being published on the mechanism of tribo-improvers (see Section 2.1) with this procedure. On the other hand, much progress in understanding of the energy for reaction is necessary. Chemical reaction is an arrangement of atoms within or between molecule(s), and is accompanied with breaking and making chemical bonds. This process needs energy to occur; tribo-chemical reaction is a conversion of mechanical energy into chemical energy in one sense. From this point, possible driving forces (sources of energy for chemical reactions) for tribo-chemical reactions are summarized in Figure 2. Six possible driving forces are considered for the initiation process of tribo-chemical reactions [18]. * Heat induces various chemical reactions. It was estimated that rubbing surface could reach as high as 250-450 \({}^{\circ}\)C [19]. Friction generates heat and this is considered as the major cause of tribo-chemical reactions. * Nascent surfaces could be exposed by wear of solid surfaces under mixed and boundary conditions. Similarly, a mechanical stress to crystalline materials can sometimes cause lattice defects. These produce a chemically active site on the rubbing surfaces [20]. The chemical activity of transition metals is induced by vacant d-atomic orbital. * Exo-electrons could be emitted from rubbing surfaces. Those electrons have low energy to promote chemical reactions but can produce radical intermediates for further chemical reactions [21]. * Elevated pressure up to Giga-Pascal (>10\({}^{4}\) bar) could be generated at tribo-contact [22]. Since chemical reactions are initiated by a collision of molecules, compression of reactant raises the probability of the collision. Chemical reactions at high pressure under static conditions are well known [23]. * Orientation of molecules may occur if they were flown through a narrow area [24]. This means the reacting functional groups are close to each other and increases the probability of reaction. This, together with elevated pressure, contributes to the entropy factor of the reaction. * Shearing can dissociate chemical bonds in a molecule directly.