  It has been recently reported that graphene is able to significantly reduce
the friction coefficient of steel-on-steel sliding contacts. The microscopic
origin of this behavior has been attributed to the mechanical action of load
carrying capacity. However, a recent work highlighted the importance of the
chemical action of graphene. According to this work graphene reduces the
adhesion of iron interfaces by reducing the surface energy thanks to a
passivation effect. The aim of the present work is to clarify the still debated
lubricating behavior of graphene flakes. We perform pin-on-disc experiments
using liquid dispersed graphene solution as a lubricant. Two different
materials, pure iron and bronze are tested against 100Cr6 steel. Raman
spectroscopy is used to analyze the surfaces after the friction tests. The
results of these tests prove that graphene flakes have a beneficial effect on
the friction coefficient. At the same time they show a tendency of graphene to
passivate the native iron surfaces that are exposed during sliding as a
consequence of wear.

Materials Science