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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. [ABSTRACT FROM AUTHOR]

With ultra-high mechanical strength and inherent lubrication property, graphene exhibits great potential as an atomically thin solid lubricant to address wear problems that affect mechanical function and longevity. Despite the excellent wear resistance of graphene at the nanoscale, wear of graphene is difficult to avoid at micro- to macroscale friction tests in ambient conditions and sensitive to the environmental atmosphere and humidity. As an atomically thin material, contaminants from the environment have a non-ignorable effect on graphene surface properties. To study the influence of airborne contaminants on graphene macroscale wear behavior, we performed sliding tests on new (as-prepared) and aged monolayer chemical vapor deposition grown graphene using a 5 mm-radius lens under load down to 0.1mN. The atomic force microscope characterization demonstrated the existence of airborne contaminants on aged graphene. The tribological experiments showed the aged graphene exhibited better wear resistance than the new one due to that the airborne contaminants separated the counterpart from the graphene wrinkles, from where graphene wear was initiated. Unlabelled Image • The aged graphene exhibits better wear resistance than the new one at macroscale. • The contaminants change the friction interface and affect the wear behavior. • The airborne contaminants on aged graphene surface can be mechanically cleaned. • Graphene wrinkles are too dense to avoid in macroscopic sliding tests. [ABSTRACT FROM AUTHOR]