Unexpected speed-dependent friction with graphene

Graphene is being examined with a view to potential use as a lubricating layer. If it is applied to a platinum surface, it has a significant impact on the measurable friction forces. An international team of researchers (Israel, Switzerland) has reported that, in this instance, the friction depends on the speed at which the tip of an atomic force microscope (AFM) is moved across the surface. In conjunction with the platinum substrate, graphene no longer forms only the hexagonal honeycomb pattern of carbon atoms and instead forms Moiré superlattices. The surface is then no longer completely flat and exhibits a certain degree of roughness. When theye moved the AFM tip across the slightly corrugated surface at low speed, they measured a weak and almost constant frictional force. Above a certain threshold, however, the friction increased with the speed of the AFM tip. The larger the Moiré superstructure, the lower the threshold at which the friction becomes speed dependent. They found greater resistance at the ridges of the Moiré superstructures during the movement of the tip. The ridges underwent elastic deformation due to the pushing tip before relaxing again when the pressure is sufficiently high. This effect resulted in greater frictional forces that increase with the speed of the tip…read more. TECHNICAL ARTICLE