The tribological behavior of graphene and its role as a protective coating
The scope of this thesis is to explore the fundamental tribological behavior of graphene as a two-dimensional (2-D) nanomaterial and evaluate its performance as a protective coating. Graphene is the strongest material ever measured, gas-impermeable, chemically and thermally stable, and atomically-thin, making it an excellent candidate as a protective coating. The fundamental tribological behavior of graphene and other 2-D materials under sliding conditions has only just begun to be explored. In particular, the wear of graphene has hardly been explored. The objective of this work is to investigate the tribological behavior of graphene through atomistic simulation as well as experimental testing under various sliding regimes and length scales. Wear in a graphene monolayer, after scratch tests with a nanoindenter, was characterized for the first time using Raman spectroscopy, revealing new insights into the failure of graphene after sliding. These sliding tests revealed a new frictional phenomenon where friction increased linearly with sliding length over large distances. This was caused by delamination likely due to the coalescence of small bubbles of gas trapped between the graphene monolayer and substrate during sliding, confirmed with atomic force microscopy. Furthermore, atomistic simulations of an asperity sliding over a graphene bubble mimicked experimental results, further supporting this bubble coalescence hypothesis. Graphene's potential as an anti-corrosive coating was demonstrated for macro-scale, commercially-available electrical connectors. It was demonstrated that even a monolayer of graphene can prevent oxide and reduce electrical contact resistance by orders of magnitude.
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