Surface growth processes induced by AFM debris production. A new observable for nanowear.

Loss of material due to abrasion, adhesion, erosion or other types of wear mechanisms is a fundamental phenomenon occurring between two surfaces in relative motion on each other. Generally, in a wide range of length scales, from macroscale down to nanoscale, wear is quantified by measuring the volume loss after a wear test, and the quantification of the wear volume is the main observable to be measured in a wear test. In this chapter, we present some recent results showing that in precise experimental conditions, as ultrahigh vacuum (UHV) environments, surface growth processes induced by atomic force microscopy (AFM) tip sample abrasion can be estimated to have an accurate knowledge of atomic and molecular onset mechanisms involving the occurrence of wear mechanisms, mainly abrasion. In fact, recent UHV scratching AFM experiments made on ionic crystals showed the formation of small clusters, larger aggregates or regular patterns on the surface being scanned, and a theory capable of capturing the basic mechanisms producing the formation of such structures has been proposed. Such cluster structures, generally self-organised in regular structures, are mainly produced by the flux of adatoms generated by the AFM tip stripping off adatoms during the continuous passage of the probe tip on the surface being analysed. In UHV environments, surface diffusion is the dominant mass transport mechanism, and a non-equilibrium thermodynamic framework for the self-organised growth process has been developed demonstrating that the surface growth processes maintain a sort of coherence with respect to the flux rates of the adatomic debris induced by the AFM tip during the wear test making the wearing and the surface growth specular. As a consequence, the physical nature of the growth processes induced by AFM debris could represent a new observable to be measured for a new and accurate comprehension of wear mechanisms on nanoscale.