Molecular origin of nanofriction contrast between graphite and MoS2 homojunctions in oil

Graphite and molybdenum disulfide (MoS2) have been widely utilized as oil lubricating additives exhibiting superior friction-reducing capability, but there is debate about which one possesses better lubrication performance in the oil environment. Here, we report the nanotribological properties of graphite and MoS2 homojunctions in the oil environment of hexadecane examined by atomic force microscope (AFM) and molecular dynamic (MD) simulations. The friction coefficient of the MoS2 homojunction is 0.003, while the graphite homojunction processes a much better lubrication performance with a lower friction coefficient of 0.001 (67% reduction). There is a confined molecular layer of hexadecane in the contact zone, and the shear occurs at the graphite/hexadecane and MoS2/hexadecane interfaces for graphite and MoS2 homojunctions, respectively. MD simulation demonstrates that the hexadecane molecules are directionally aligned along the zigzag direction of graphite for the corrugated and anisotropic potential energy surface, and this orderly arrangement contributes to the lower energy barrier.