Engineering Materials Program, Mechanical, Automotive and Materials Engineering Department, University of Windsor, Windsor, Ontario, N9B 3P4, Canada.
Center for Nanoscale Materials, Argonne National Laboratory, Cass Ave, Lemont, IL, 60439, USA.
Sci Rep. 2018 Jan 9;8(1):121. doi: 10.1038/s41598-017-17971-1.
Sliding contact experiments and first-principles calculations were performed to elucidate the roles of structural defects and water dissociative adsorption process on the tribo-chemical mechanisms responsible for low friction of graphene. Sliding friction tests conducted in ambient air and under a dry N atmosphere showed that in both cases a high running-in coefficient of friction (COF) occurred initially but a low steady-state COF was reached only when the sliding was continued in air with moisture. Density functional theory (DFT) calculations indicated that the energy barrier (E ) for dissociative adsorption of HO was significantly lower in case of reconstructed graphene with a monovacancy compared to pristine graphene. Cross-sectional transmission electron microscopy of graphene transferred to the counterface revealed a partly amorphous structure incorporating damaged graphene layers with d-spacings larger than that of the original layers. DFT calculations on the reconstructed bilayer AB graphene systems revealed an increase of d-spacing due to the chemisorption of H, O, and OH at the vacancy sites and a reduction in the interlayer binding energy (E ) between the bilayer graphene interfaces compared to pristine graphene. Thus, sliding induced defects facilitated dissociative adsorption of water molecules and reduced COF of graphene for sliding tests under ambient and humid environments but not under an inert atmosphere.
采用滑动接触实验和第一性原理计算，阐明了结构缺陷和水离解吸附过程在解释石墨烯低摩擦的摩擦化学机制中的作用。在环境空气中和干燥氮气气氛下进行的滑动摩擦测试表明，在这两种情况下，初始时都出现了较高的运行摩擦系数（COF），但只有在空气中存在水分时继续滑动，才能达到较低的稳定状态 COF。密度泛函理论（DFT）计算表明，与原始石墨烯相比，具有单空位的重构石墨烯上水的离解吸附的能垒（E）显著降低。转移到对摩面的石墨烯的横截面透射电子显微镜揭示了部分非晶结构，其中包含具有大于原始层的 d 间距的受损石墨烯层。对重构双层 AB 石墨烯体系的 DFT 计算表明，由于空位处 H、O 和 OH 的化学吸附，以及与原始石墨烯相比，双层石墨烯界面之间的层间结合能（E）降低，导致 d 间距增加。因此，滑动诱导的缺陷促进了水分子的离解吸附，并降低了在环境和潮湿环境下进行滑动测试时石墨烯的 COF，但在惰性气氛下则没有。
