There are mainly three sites concerned with tribochemical reactions in a tribological system. Figure 3.1 schematically shows the tribochemical reactions: (i) the contact area under the combined effect of pressure, shear and friction-induced temperature-rise, (ii) the outside of the contact zone, where microplasma can be generated mainly in the case of non-conducting materials in friction processes [1]; and (iii) the very active nascent surfaces created by the wear process, particularly metal surfaces and even on gold [2]. In this section, we will deal with triboinduced chemical reactions occurring in the stressed zone of boundary-lubricated contacts that are directly concerned with friction and wear processes. A rough estimation of the quantity of material concerned by the reaction indicates very small masses, typically between 10 and 100 picograms, depending on the tribofilm thickness and real area of contact. On the other hand, the solicitation time is often very low, typically a few milliseconds, depending on the sliding speed and the contact diameter. The maximum contact pressure is usually calculated to lie between 500 MPa and 1 GPa and the shear rate is generally higher than 10 4 s −1 . Under these conditions, friction-induced chemical reactions will take place in the so-called magma-state [3] and metastable amorphous structures will more probably be generated. The process is mainly governed by nanometer-scale events, and it is correct to note that macrotribology is certainly dependent on nanotribological events. However, it is generally difficult and hazardous to extrapolate directly from micro to macroscale and vice versa.

Friction, wear and their related phenomena in the tribological systems take place essentially due to complex dynamic interactions between the frictional surfaces. The friction occurs mainly because of the shearing of direct contact parts and deformation of asperities of frictional surfaces. The wear occurs mainly by adhesion, abrasion, erosion, surface fatigue and tribological reactions at the solid/(lubricant)/solid interface. Compared with the other lubrication system in the Stribeck curve, tribology of the boundary lubrication system is most complex due to various interactions at the interface and parameters involved. In boundary lubrication, especially, the bulk rheological properties and hydrodynamic effects of lubricating oil are less important to the friction and wear phenomena, and the load applied is carried almost entirely through the contact and deformation of the asperities of frictional surfaces and the lubricating surface films formed on them. The formations of lubricating films by physicochemical interactions, such as adsorption and wetting, and by chemical reactions at the lubricant/solid interface are very important for reduction of the direct contact resulting in low friction and wear. The formation of these lubricating films is generally obtained for frictional metal surfaces and for ceramics with a polar surface that enables adsorption of lubricant molecules. The surface active substances, such as fatty acids and aliphatic alcohols, are in general added lubricating oils as friction-reducing agents to improve their tribological properties. These agents are adsorbed by or react with frictional solid surfaces and form lubricating films to minimize the direct solid contact, contributing to friction and wear reduction. On the other hand, the chemically reactive boundary lubricants (extreme pressure agent) containing sulfur, chlorine and phosphorus atoms in the molecule react with frictional solid (metal) surfaces to form lubricating films (sulfide, chloride and phosphide) of low shear strength and high melting point. These films are more stable and durable than adsorbed films, and can be applied under higher loads, higher sliding speeds and higher temperatures. However, it is generally hard to obtain such chemical reaction films with extreme pressure agents for chemically stable ceramics. The aim of this chapter is to discuss the tribological properties of boundary lubricants such as friction-reducing agents in boundary lubrication.