Friction is a result of complex interaction of physical, chemical and mechanical forces at the sliding interface. Due to mentioned complexity, truly predictive models of friction are yet to be developed. As a result of the complexity, various phenomena rise, as for example friction hysteresis due to the change of the normal load. The friction value measured at loading is not the same as the friction curve at unloading. The fact that this behavior is reproduced from cycle to cycle, indicates that it is an important feature of the friction process. This hysteresis was observed for several materials, including graphene, silica, mice and others.
An international group of researchers from USA and Canada recently explored this peculiar behavior of friction using Atomic Force Microscopy (AFM) and Molecular Dynamic (MD) simulations. They considered the hysteresis in friction coefficient between silicon AFM tip and graphene by loading from to and unloading till the pull off. By repeating the measurements several times, the same results were obtained, indicating absence of visco-elastic forces or wear.
To find out the mechanism responsible for the observed hysteresis, the MD simulations were performed. At first, the out-of-plane deformation of the graphene was suspected and analyzed, however, it was found to be negligible in contribution to the process. It must be noted, that the hysteresis was observed only in the presence of a water molecules, indicating a major role of humidity. Working from this point, the researchers performed several simulations and found that the contact area exhibited a clear load-dependent hysteresis due to the water meniscus. The contact angle evolution of the water meniscus during loading and unloading revealed a hysteresis behavior. From MD simulations it was further concluded, that the contact angle hysteresis is an intrinsic aspect of water-graphene interaction and is responsible for the observe behavior.
The details of the discovered hysteresis mechanism can be found in the original article: “Load-Dependent Friction Hysteresis on Graphene” by Zhijiang Ye, Philip Egberts, Gang Hee Han, A. T. Charlie Johnson, Robert W. Carpick and Ashlie Martini.
Credit for image: adapted with permission from “Load-Dependent Friction Hysteresis on Graphene”. Copyright 2016 American Chemical Society.