Page 187 - Handbook of Surface Improvement and Modification

182 The Coefficient of Friction

the macroscale was observed using a combination of graphene flakes and nanodiamond on
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a SiO surface under variable load, velocity of movement, and temperature. The gra-
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phene flakes wrap around nanodiamond particles forming nanoscale “scrolls” (Figure
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11.23). During sliding, the nanodiamond particles facilitate scroll formation for two rea-
sons: dangling bonds on the particles adhere to the edges of the graphene flakes and the
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three-dimensional nature of the particles acts as a physical barrier to the sheets. The
nanoscrolls decrease friction by reducing the contact area between the graphene and dia-
mond-like carbon-coated surfaces, while van der Waals forces stabilize the structure
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itself. The one-atom thick graphene is flexible and easily forms scrolls around nanodia-
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monds. It is also inert, providing very low adhesion energy. The humid or damp condi-
tions affect the lubrication process because graphene remains strongly attached to the
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surface and friction increases. Water on the surface also prevents the scrolling of gra-
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phene flakes during sliding.
Friction-induced, nano-structural evolution of graphene produced lubricating perfor-
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mance. A different number of layers and interlayer spacing by exfoliation were stud-
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ied. The additives providing a higher degree of exfoliation gave better lubricating
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properties. The ordered tribofilm on the frictional interfaces was parallel to the sliding
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direction and the exfoliated graphene caused slippage between its layers. The friction
mechanism of the structure evolution of the graphene additives during friction is presented
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as a schematic view in Figure 11.24. The graphene is uniformly dispersed in the lubrica-
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tion oil and it becomes physically absorbed on the friction interface. Under pressure and
shear, the graphene with higher exfoliation overlaps and then restacks to a lamellar tribo-
film parallel to the sliding direction (Figure 11.24a), improving lubrication properties. 27
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The opposite trend is displayed in Figure 11.24b. During friction, the integrated and ori-
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ented graphene layers are prone to damage and scratch the friction interfaces.
The effects of fluorine additives on ice friction of ultra-high molecular weight poly-
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ethylene has been studied. Perfluoropolyalkylether improved the surface and sliding

Figure 11.24. Schematic demonstration of the lubrication mechanism of structural evolution of graphene addi-
tives. (a) and (b) respectively show the friction-induced structure changing of the exfoliated and oriented gra-
phene additive. [Adapted, by permission, from Zhao, J; Mao, J; Li, Y; He, Y; Luo, J, Appl. Surf. Sci., 434, 21-7,
2018.]