Wear evolution of monolayer graphene at the macroscale

The unprecedentedly-high mechanical strength and the intrinsically lubricating property make graphene an ideal candidate for atomically-thin solid lubricant. Despite its high potential, graphene was often found to get worn in micro- or macroscale friction tests in ambient conditions. To explore the detailed wear process and evaluate its impact on lubricating performance, we performed tribological experiments on monolayer graphene using a macroscale glass lens under normal loads down to 50 μN. Our ambient experiments show that there existed a critical normal load, below which wear of graphene was undetectable (with friction coefficient on the order of 10−2). Beyond the load threshold, graphene started to become worn and friction slowly increased. In contrast to the abrupt change in nanoscale tests, friction increase due to graphene wear was gradual at the macroscale and it only became apparent (with friction coefficient on the order of 10−1) when graphene damage was substantial. This suggests that a partially worn graphene can remain effectively lubricious at the macroscale. Our comparative experiments also demonstrate that humidity can noticeably minimize graphene wear and substantially extend lubrication life time. The study provides new insights on macroscale wear characteristics of graphene, which is crucial for mechanical applications of this atomically-thin lubricant.

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