Control of Nanoscale Ripple Formation on Ionic Crystals by Atomic Force Microscopy

On the most fundamental level, nanoscale wear can be considered as a process of atom-by-atom removal during mechanical contact between surfaces. But at the same time, nanoscale wear processes are often accompanied by the formation of quasi-periodic surface structures, i.e., ripples, in a self-enhancing process driven by lateral force variations. Understanding and potentially controlling the complex mechanisms of ripple formation are interesting from a general tribological point of view, since our experiments bridge the gap between the early stages of atomic scale wear to the ensuing phenomena of abrasive wear on larger length scales. In this work, we have now analyzed this phenomenon by reciprocating single asperity scratching of an atomic force microscopy (AFM) tip across a flat surface of an ionic crystal under ultrahigh vacuum (UHV) conditions. In particular, the influence of dynamic scan parameters like sliding velocity \(v_{x}\) and the vertical adjustment velocity for topography changes \(v_{z}\) has been explored. Our experiments show that the sliding velocity \(v_{x}\) does not influence friction, wear, and the resulting surface structure, with the latter confirming numerical simulations for ripple formation. However, the vertical velocity \(v_{z}\) can be used as a direct control parameter for ripple formation, where low values of \(v_{z}\) seem to enhance the elastic instabilities that drive the surface patterning.