Speaker
Description
Molybdenum disulfide (MoS$_2$) is a layered material that has been used as a solid lubricant for decades[1]. The experimental verification of superlubricity in the early nineties for crystalline MoS$_2$ coatings in ultrahigh vacuum conditions[2] sparked again the interest for such a material. Since then, a lot of effort has been devoted in order to understand (and possibly control) the complex phenomena taking place during sliding in such a system, e.g. the formation of crystalline layers from molecular precursors[3] or from amorphous material[4]. On the contrary, the understanding of abrasive wear processes on such a material is still in an early stage.
In this study[5], we performed molecular dynamics (MD) simulations in order to shed light on the elementary steps in the wear process of MoS$_2$ with atomistic detail. A rigid diamond tip has been used to indent a six-layer thick system, and then the tip has been dragged in order to scratch the material. The position and the force acting on the tip has been followed for more than 20 ns, together with the damage done on the substrate. Different MoS$_2$ orientations and normal loads have been considered. The trajectory analysis revealed that the tip moved in a stick-slip fashion, somehow recalling the dynamics that takes place during sliding, but with the slip events occurring on a much longer length scale. Remarkably, it turned out that about only one fifth of the input energy is irreversibly spent in surface damage. The system has been studied also with atomic force microscopy (AFM) experiments, where two different regimes were identified in terms of distributions of the force drop during the slip phase: a generalized extreme value distribution (at high loads, also observed in MD simulations), and a Gaussian distribution (at low loads). These findings provide new insight into the fundamental mechanisms of friction and wear in layered materials and establish a framework for precision nanomachining of van der Waals solids, relevant for next-generation devices at sub-micrometer scales.
References
[1] T. Spalvins, J. S. Przybyszewski, NASA Technical Note D-4269 (1967).
[2] J. M. Martin et al., Phys. Rev. B, 48, 10583 (1993).
[3] S. Peeters et al., Appl. Surf. Sci., 606, 154880 (2022).
[4] P. Nicolini et al., ACS Appl. Mater. Interfaces, 10, 8937 (2018).
[5] P. Koczanowski et al., currently under review in Small (2025).
