Abstract:
Graphene is a single graphite plane consisting in sp
2-carbon atoms, and the excellent properties, such as high carrier mobility, strong mechanical strength and excellent transparency and potential applications make it a research hot in the last decades. Nowadays, lots of studies investigated the tribological properties of graphene in forms of lubricating additives or thin films deposited on flat substrate surface. In this paper, ball milling was employed to make steel balls rolling and colliding adequately with each other in graphene powder to prepare graphene wrapped steel ball. With the help of the interlayer slipping, graphene sheets were overlapped, and large area continuous graphene film was prepared on the surface of the highly curved steel ball. The coating degree, morphology change, structure evolution, binding property and tribological properties were studied under different milling time (5, 25 and 50 h). The results showed that the amorphous structure was introduced by collision between the steel balls during the ball milling making the graphene film present a three-dimensional carbon network with structured graphene structure. Graphene film presented excellent adhesion testing by adhesive tape adhesion for 100 times and remained attached to the steel ball surface after ultrasonic cleaning for 30 minutes. At the same time, along with the increase of ball milling time, graphene adhered to the surface of steel ball as flakes and steel ball surface became more and more flat. When the ball milling time further increased to 50 h, large area graphene film distributed uniformly and continuous on the steel ball surface. The graphene film was still adhered to the surface of steel ball when it was rubbed with diamond-like film for 6 000 cycles under a normal load of 5 N, and was not wear out of the friction contact area. It depended on the weak van der Waals force between graphene layers, which can reduce the average friction coefficient from 0.043 of the pristine steel ball to 0.022 of the 50 hours ball milling sample, and the depth and width of the wear tracks were obviously reduced. The friction interface between steel ball and diamond-like carbon film after rubbing was investigated through micro Raman spectrometer and high resolution transmission electron microscope (HRTEM). Raman spectra showed that there was still a significant D peak in the friction contact area of ball milling steel balls after 6 000 cycles and the
ID/
IG value was basically staying the same as that of the samples before friction indicating the graphene film was still adhered to the steel ball surface. What’s more, the D peak strength of the wear scars after friction was much higher than that of pristine steel ball, the difference of the results further eliminated the possibility of the structure evolution of diamond-like carbon film induced D-peak enhancement. HRTEM results showed that the wear debris of the pristine steel ball were amorphous carbon structure, however, the sample after 50 h ball milling presented an ordered graphene structure confirming the wrapping of graphene layers on the steel ball. It can be concluded that the sustainable low friction for 6 000 cycles was not caused by diamond-like carbon film, but can be attributed to graphene films which were distributed unevenly and adhered firmly on the surface of ball milling steel balls. Therefore, the results not only enriched the preparation of graphene film on spherical metal surfaces on the basis of deposition, but also improved the friction-reducing and anti-wear properties of diamond-like film.