Wear Volume of Self-Mated Steel at the Submicron-Scale: An Atomic Force Microscopy Study

Wear phenomena at the nanoscale are essential for applications involving miniaturized specimens. Furthermore, stochastic nano-events affect in general tribological processes, eventually also at the macroscale. Hence, it is of fundamental importance to perform nanotests with materials—such as steel—which are widely used also at the macroscale. In this paper, we present the analysis of tribotests performed with self-mated 100Cr6 steel (AISI 52100) at the submicron scale by means of an atomic force microscope. To this aim, steel particles with micrometer size were glued to the cantilever as “colloidal particles”. The microscope was employed for wear generation, for the imaging of scars and colloidal particles, and for the determination of wear volumes of both specimens. The analysis is focused on wear volume and its dependence on normal force and total sliding distance. Nanotests are compared with previously presented macrotests, also performed with self-mated steel. Nanotests exhibit, compared with macrotests, a significantly larger scattering and poor repeatability. Especially the analysis of these features reveals that, with small forces (≤10 µN) and surfaces (≤2 µm²), the random number of asperities inside the contact surface plays a crucial role, by far more decisive than the normal force or the sliding distance. Moreover, in several cases, only few asperities (<10) are involved in the wear process. Such low numbers lead to a breakdown in the applicability of tribological laws (e.g., Archard's law) based on statistical methods and on average variables.