Entry behavior into Friction Interface and Forming Unstable Lubricating Accumulation of Graphene Oxide between SUJ2 Ball and Glass Disk under Boundary Lubrication

Carbon nanomaterials have been studied for their promising tribological properties under boundary lubrication. However, the behavior of carbon nanomaterials upon entering narrow friction interfaces under boundary lubrication is poorly understood. In this study, to clarify how graphene oxide (GO), which is one of the carbon nanomaterials, enters and works on a friction interface under boundary lubrication, in situ observations of a friction interface between an SUJ2 ball and soda glass-lime disk in a GO water dispersion, with a load of 1 N, were conducted using optical microscopy. Isolated dispersed GO accumulated only in front of the contact areas. GO accumulations formed, grew, and peeled off due to the friction. Although the GO accumulations were significantly unstable, they functioned as friction reduction and anti-wear. We firstly found that the unstable GO lubricating accumulations that formed in front of the contact areas were able to widen between the ball and the disk similar to a wedge; this wedge effect would cause a decrease in friction and wear. Unlike the wedge effect under hydrodynamic lubrication, the GO accumulations that functioned as the wedge stayed on the ball surface.