Strain-Induced Nonlinear Frictional Behavior of Graphene Nanowall Films.

Graphene nanowall (GNW) films, a representation of three-dimensional (3D) carbon nanomaterial films, are emerging as promising candidates for applications in electric devices and composites, on account of their 3D structures and exceptional properties of graphene sheets. However, the frictional responses of GNW films, which exhibit significant influence on their performances, have seldom been reported. Herein, we reported a growth process of a GNW film by the chemical vapor deposition method and studied the frictional behavior of the GNW film for the first time. The results demonstrated the nonlinearity between the frictional force of the GNW film and normal load. Based on the structural evolution of the GNW film with normal load and frictional tests on precompressed GNW films, the influence of the strain property of the GNW film, namely, the strengthening effect, could be confirmed. The results of molecular dynamics simulations show that the bending force of GNWs in front of the tip plays a determinate role in the frictional force of the GNW film. Furthermore, the bending force is proportional to the bending contact area, which increases nonlinearly with the normal load due to the strengthening effect of the GNW film. The result suggests that the nonlinear increase of the bending contact area induced by the strengthening effect of the GNW film is the key factor that leads to its nonlinear frictional force. This study provides a novel insight into the frictional responses of GNW films, which would be beneficial for the design and application of electric devices and composites made of GNW and other 3D carbon nanomaterial films.