Force microscopy studies of the molecular origins of friction and lubrication

The atomic force microscope (AFM) has become a valuable instrument in recent years for studying the atomic and molecular origins of friction and lubrication. This paper reviews the effort in our laboratory using force microscopy to develop a molecular-scale understanding of friction and lubrication. Among the topics covered in this paper are 1) the nanomechanics and adhesive forces of lubricated and unlubricated surfaces, 2) the observation of atomic-scale features and their effect on the friction force for AFM tips sliding over graphite and mica surfaces, and 3) the effect of lubricating surfaces with bonded and unbonded perfluoropolyether polymers and with water molecules adsorbed from the ambient. Many of the descriptions developed using macroscopic continuum mechanics analysis are still applicable to these atomic-scale contact zones. However, a complete interpretation of the results requires incorporating descriptions of the atomic and molecular processes into the continuum mechanics analysis.