Use of the Atomic Force Microscope to Study Mechanical Properties of Lubricant Layers

Abstract

Advances in our understanding of the phenomena of adhesion, friction, and lubrication are facilitated by the recent development of new tools that allow the study of contacts in close-to-ideal conditions. These new tools are the surface force spparatus (SFA) and the atomic force microscope (AFM). The first was developed by Israelachvili in the 1970s. In this device, contact between two atomically flat surfaces of mica occurs over an area of several micrometers in diameter after the mica sheets, glued onto two perpendicular cylindrical lenses, are compressed. Force, area of contact, and separation distance can be controlled at the atomic scale. The second device, the AFM, was developed by Binnig et al. in 1986. The sharp tip of the AFM is a convenient idealization of a single asperity. In addition, the AFM can be used to image the surface in the weak repulsive or in the attractive modes so that minimum perturbation is introduced by the imaging process itself. These two devices have the necessary sensitivity to allow the application of forces weak enough not to dislodge atoms from their sites during contact. The order of magnitude of the force that can lead to the rupture of chemical bonds is a convenient figure to keep in mind in this context. A simple estimate of this force is obtained by considering a bond-length increase of 1 Å as leading to dissociation. For a bond energy of ≈1 eV, Fb ≈ 1 eV/1 Å ≈ 1 × 10−9 N.