Effect of Polymer Grafting on the Tribological Performance of Graphene Oxide under Ambient Air and Vacuum.

Graphene is known to be an excellent lubricant in a dry air or vacuum environment; however, the modification of its surface chemistry and friction properties is a challenging task. In contrast, mono- and few-layered graphene oxide (GO) offers easier modulation of its surface properties, but its tribological properties under vacuum are currently unknown. In this work, the impact of GO modification by poly(methyl methacrylate) (PMMA) and poly(trifluoroethyl methacrylate) (PTFEMA) on the friction in ambient air and vacuum conditions has been investigated. The polymer chains were grafted from GO layers under surface-initiated atom transfer radical polymerization conditions followed by spin-coating on the sapphire substrates. The ambient air ball-on-disk investigations prove a good lubrication effect of all of the coatings as compared to the bare sapphire substrate. Modified GO shows a lower friction coefficient (COF) as compared to pristine GO (0.16-0.19 and 0.23, respectively) and up to five times longer steady-state friction stage, which is caused by the formation of the tribolayer, decreasing the surface hydrophilicity and probability of the debris agglomeration. In the vacuum, all samples show a COF ranging at 0.02-0.05. The observed difference is caused by morphological changes in the sliding contact. Generation of buckling ribbons on the sapphire surface in GO and PMMA-modified samples reduces the COF to 0.02-0.03, whereas an inhomogeneous tribolayer formed in PTFEMA-modified samples increases the COF to 0.05. Our results illustrate that grafting of the GO layers can improve the lifetime of the applications in ambient air, while it induces sliding mechanisms that can enhance the properties of the applications working in vacuum conditions.