The influence of nanoscale roughness and substrate chemistry on the frictional properties of single and few layer graphene

Abstract

Nanoscale carbon lubricants such as graphene, have garnered increased interest as protective surface coatings for devices, but its tribological properties have been shown to depend on its interactions with the underlying substrate surface and its degree of surface conformity. This conformity is especially of interest as real interfaces exhibit roughness on the order of ∼10 nm that can dramatically impact the contact area between the graphene film and the substrate. To examine the combined effects of surface interaction strength and roughness on the frictional properties of graphene, a combination of Atomic Force Microscopy (AFM) and Raman microspectroscopy has been used to explore substrate interactions and the frictional properties of single and few-layer graphene as a coating on silica nanoparticle films, which yield surfaces that mimic the nanoscaled asperities found in realistic devices. The interactions between the graphene and the substrate have been controlled by comparing their binding to hydrophilic (silanol terminated) and hydrophobic (octadecyltrichlorosilane modified) silica surfaces. AFM measurements revealed that graphene only partially conforms to the rough surfaces, with decreasing conformity, as the number of layers increase. Under higher mechanical loading the graphene conformity could be reversibly increased, allowing for a local estimation of the out-of-plane bending modulus of the film. The frictional properties were also found to depend on the number of layers, with the largest friction observed on single layers, ultimately decreasing to that of bulk graphite. This trend however, was found to disappear, depending on the tip-sample contact area and interfacial shear strain of the graphene associated with its adhesion to the substrate.

Effect of roughness on the layer-dependent friction of few-layer graphene

“…In the transition regime represented by medium correlation lengths (between 1 and 3 nm), we observe a non-monotonic layer dependence trend for friction. Our results confirm and shed light on the experimental results of Spear et al 14 , where the typical layer dependence of graphene (decreasing friction with increasing number of layers) was only observed with large tip apices, on graphene samples deposited on a rough substrate consisting of silica nanoparticles. With such tips, the ratio of the tip size to the correlation length of the surface would be high, which, according to the results presented in Fig.…”

“…This behavior was discussed and correlated to interfacial adhesion between the tip and graphene layers 13 . Another set of recent experiments showed that the layer dependence of friction can be changed by scanning AFM tips with different radii against substrates of controlled nanoscale roughness covered with graphene 14 . In that study, a non-monotonic layer dependence of friction was observed using a sharp AFM tip and the behavior was explained and correlated to the interplay between surface roughness, tip radius and the relative adhesion between tip and substrate as well as between graphene and substrate 14 .…”

2D-nanomaterials for controlling friction and wear at interfaces

“…For example, studies of graphene on silica nanoparticle films with controlled nanoscopic roughness (ca. 10 nm rms) show that the frictional properties strongly depended upon the relative adhesion between the AFM probe tip and the substrate, with suppression of the ''puckering'' effect, mentioned above, occurring when there is large surface roughness and low adhesion to the tip [55].…”

Optimization of Process Parameters for a Chemi-Absorbed Graphene Coating and Its Nano Tribological Investigation

“…The nano-tribological properties of those coatings are usually researched under ultra-low applied loads [11]. When the applied load is raised to a certain extent, those physically adsorbed graphene coatings are apt to experience wear failure owing to poor adhesion with the substrate [16]. The tribological properties of graphene coatings have also been shown to depend on their interactions with the underlying substrate surface [17,18].…”