Quantitative understanding of single-asperity contact via molecular simulations

Abstract

Single-asperity contact is a critical topic in nanotechnology, not only its technical relevance in terms of reliability in probe-based characterization instruments and memory storage devices, but also its fundamental importance as a pathway to multi-asperity contact. Recent experimental studies using AFM have discovered a new wear mechanism named atomic wear. However, the atomic level understanding of the nanoscale wear mechanism is still lacking due to experimental challenges such as precise measurements of wear rates. Aside from debris generation, single- asperity contact can also cause plasticity and/or cracking. For instance, the critical crack initiation in metallic glass under nanoindentation is yet not understood at an atomic level.

In this thesis, I investigated single-asperity sliding and nanoindentation using molecular simulations. By tuning the interfacial adhesion and contact stress, it was found that there are two distinct wear modes (atomic wear and plastic wear) in terms of mathematical formulation, debris spatial orientation, and debris cluster size distribution. The transition from plastic wear to atomic wear can be also enhanced by adding lubricant. By controlling the sliding temperature, velocity and contact area, we found that atomic wear is shear-assisted, athermally-activated, and re- depositable. In a cluster analysis of atomic wear, we demonstrate a scaling behavior of the cluster distribution, which can be analogically understood by the critical phenomenon of phase transition. Through nanoindentation simulations, the crack initiation mechanism within a glassy material was revealed. The atomic insights obtained from our simulations will improve the quantitative reliability assessment for probe-based devices, help develop new crack resistant materials, and pave the way to quantitatively understand macroscopic wear and cracking.