On the Application of Transition State Theory to Atomic-Scale Wear

The atomic force microscope (AFM) tip is often used as a model of a single sliding asperity in order to study nanotribological phenomena including friction, adhesion, and wear. In particular, recent work has demonstrated a wear regime in which surface modification appears to occur in an atom-by-atom fashion. Several authors have modeled this atomic-scale wear behavior as a thermally activated bond breaking process. The present article reviews this body of work in light of concepts from formal transition state theory (also called reaction rate theory). It is found that this framework is viable as one possible description of atomic-scale wear, with impressive agreements to experimental trends found. However, further experimental work is required to fully validate this approach. It is also found that, while the Arrhenius-type equations have been widely used, there is insufficient discussion of or agreement on the specific atomic-scale reaction that is thermally activated, or its dependence on stresses and sliding velocity. Further, lacking a clear picture of the underlying mechanism, a consensus on how to measure or interpret the activation volume and activation energy is yet to emerge. This article makes suggestions for measuring and interpreting such parameters, and provides a picture of one possible thermally activated transition (in its initial, activated, and final states). Finally, directions for further experimental and simulation work are proposed for validating and extending this model and rationally interrogating the behavior of this type of wear.