Microscopic insights into the aggregation dynamics behavior and tribological properties of graphene

Abstract

The stable dispersion mechanism of graphene nanoparticles in solid-liquid two-phase lubrication systems still lacks in-depth insights into the dynamic aggregation process at the microscale. Herein, the effects of size difference and defect degree on the agglomeration dynamics behavior and tribological properties of graphene nanosheets were revealed by aggregation, pull-out, and friction molecular dynamics simulations. The results showed that for suspended graphene smaller than 100 nm, the increase in size and defect degree effectively improved the dispersion stability of nanosheets in base oil. Attributed to the rise in size improved solid-liquid chemical affinity and promoted out-of-plane deformation of nanosheets to enhance the steric hindrance effect. The increase in defect degree induced spontaneous wrinkling of nanosheets, leading to enhanced interlayer sliding resistance and thus inhibiting agglomeration behavior. Meanwhile, the rise in nanosheet size and degree of defects sufficiently bear the normal load, effectively avoiding the direct contact of asperities and thus improving the wear and temperature rise at the friction interface. However, friction-induced defective graphene curled to form a “carbon nanotube-like” structure, which reduced the oil film strength and led to deterioration of lubrication performance. The research results will provide certain microscale insights into the dispersion and friction of graphene in base oil.