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Chapter 11
On the Evaluation of Thermal and Mechanical Factors in Low-Speed Sliding

Excerpt

Tribochemistry is defined as the branch of chemistry dealing with chemical and physiochemical changes of matter due to the influence of mechanical energy [1]. It was suggested in [2] that the high pressures and high temperatures at the interface between two surfaces can cause chemical reactions, which under normal conditions would proceed very slowly or not at all. Tribochemical reactions always occur in dynamic and complex situations, which make them difficult to describe with a mathematical model. Several physical, chemical and mechanical factors can affect tribochemical reactions and thus influence the production of different reaction products in various situations. Further, most of these factors influence one another via a number of feedback loops. For example, mechanical contact stresses depend on the topography of surfaces, which are determined by the deformation of asperities and by wear processes, which in turn depend on contact stresses. Chemical reactions are also activated by a number of mechanisms [1, 2] and influence mechanical processes by modifying the surface topography and the materials' properties. In accordance with these findings, it was recognized that each tribochemical reaction must be considered a simultaneous function of mechanical and chemical phenomena. Therefore, it is clear that it is extremely difficult to come to a conclusion about the most influential factors in tribochemical reactions that can result in tribochemical changes at the interface, since mechanical and thermal effects act simultaneously and both influence the result.

An indicative and interesting example is that of sliding at very low sliding speeds, for example, in fretting, since at low sliding speeds contact temperatures should be low and, therefore, tribochemical reactions should be a consequence of the broadly prevailing mechanical factors. We know that tribochemical reactions determine the sliding behavior of silicon nitride ceramics, and this also applies to low-speed sliding and fretting [3, 4, 5, 6, 7, 8, 9]. In the literature, low temperatures and negligible thermal effects on tribochemical reactions are typically assumed for low-speed sliding. Even in some of the rare attempts to measure the contact temperature at very low sliding velocities, for example, in fretting [10], rather low temperature increases were determined, which also agreed well with the theoretical calculations. On the other hand, evidence for phase transformations in materials that are typical of high temperatures was also obtained several times in low-speed sliding experiments [5, 9, 11, 12, 13, 14, 15]. However, since these processes occur within a “closed” contact where any exact measurements are impossible, we have to rely on the indirect evidence of a subsequent surface analysis, or theoretical calculations, and this is bound to introduce an element of uncertainty. Therefore, different, and sometimes contradictory, assessments of the importance of thermal and mechanical effects on tribochemical reactions are reported in the literature.

  • 1. Introduction
  • 2. Calculation of the Contact Temperature
  • 3. Surface Analyses and Metallographic Studies
  • 4. Measurement of the Contact Temperature
  • 5. Discussion
  • Conclusions
  • References

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