Effects Of The Defect And Substrate On Friction Properties Of Graphene By Molecular Dynamics Simulations

Numerous studies show that graphene is an ultra-smooth material. However, the friction properties might be influenced by the defects in graphene due to the limitation of preparation technology, which affects the performance of graphene-based devices. Moreover, the substrate in graphene devices might also decreases their performance. Therefore, it is important to study the effects of the defect and substrate on friction properties of graphene. It can deepen the understanding of graphene’s friction properties and provide a theoretical basis for the optimization of graphene devices’performance.Molecular dynamics simulations are performed to investigate the slipping of a silicon probe on graphene with vacancies and SW defects. The results show that the friction forces of graphene with both kinds of defects are larger than that of perfect graphene. The increased friction forces are caused by the enhanced energy dissipation resulting from the reinforced interfacial potential barrier of graphene by the vacancies. While for SW defects, wrinkles are easy to form, which hinders the probe’s movement, leading to an increased friction force. As the concentration of vacancies increases, the friction force of graphene with vacancies increases. Owing to the increased interfacial potential barrier and energy dissipation around vacancies. The friction force of graphene with SW defects of Type 2 is larger than that of Type 1, because it is easier for wrinkles to form in the former case with adjacent pentagon structures of carbon atoms in graphene than that in the latter. Larger wrinkles lead to larger friction forces in graphene with SW defects of Type 2.The slipping of silicon probe on graphene supported on substrates with different surface roughnesses and adhesive properties is also simulated by molecular dynamics method. The results show that the friction force of perfect graphene increases with the increasing substrate’s surface roughness. When the substrate’s surface roughness is low, the friction forces increases slightly with the number of graphene layers increasing. When the substrate’s surface roughness is high, the friction force of graphene decreases dramatically with the number of graphene layers increasing, which is on account of the coupling effect of the out-of-plane deformation and surface roughness of graphene with the probe applied. No obvious friction difference between perfect graphene and that with a single atom vacancy is found when supported on variously rough substrates. The friction force of graphene with SW defects is larger than that of perfect ones on any rough substrates, for the reason that wrinkles rooting in these SW defects impede the slipping of the probe. The friction force of graphene supported on smooth substrates decreases with the substrate’s adhesive property increasing. On the contrary, the friction force of graphene supported on rough substrates increases with the substrate’s adhesive property increasing. The reduction amplitude of the friction force of graphene with SW defects supported on substrates with the increasing substrate’s adhesive property is larger than that of perfect graphene and that with a single atom vacancy. When the well depth of LJ potential between graphene and the substrate is 3.73meV, the adhesive property is defined as a "normal value". When the substrate’s adhesive property is lower than the "normal value", the friction force of perfect graphene is larger than that of graphene with a single atom vacancy. While the substrate’s adhesive property is higher than the "normal value", the opposite result is found.