Micro and Nanotribology
3 Tribochemical Reactions in Boundary Lubrication: Nanoscale Events
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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 ; and (iii) the very active nascent surfaces created by the wear process, particularly metal surfaces and even on gold . 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 104 s−1. Under these conditions, friction-induced chemical reactions will take place in the so-called magma-state  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.