The Atomic Force Microscope (AFM) is an instrument that can offer an ability to investigate friction arising from a single ÔperfectÕ contact between surfaces. We have studied the variation of frictional force with externally applied load for a silicon AFM tip contacting a quartz surface at pressures likely to be experienced by quartz particles in a soil. At low loads, the frictional force varies with pressure in proportion to a Hertzian contact area. However, at loads equivalent to pressures of ~2.5GPa, the data becomes scattered in relation to the Hertzian model, indicating the onset of surface wear. At pressures exceeding ~3GPa, a reduced friction and Hertzian contact area are obtained with increasing applied load. Scanning electron microscope images of the AFM tip used reveal extensive plastic deformation, wear and possible melting of the tip contact region. It is thought that these processes produced a flow in the material that acted as a Ôself lubricantÕ for the tip during sliding, thus contributing to a significantly reduced friction at high contact stresses.
The last 20 years have seen the advent of several new devices for measuring surface and intermolecular forces. During operation interatomic forces between the tip and surface induce displacement of the cantilever, and this movement produces a proportional change in direction of a laser beam that is reflected from the back of a cantilever. The relative change in position of the laser onto the photodetector is converted to an image of cantilever deflection as it interacts with a surface. The AFM is today used for imaging most surfaces, from surfactant layers (Bhushan et al., 1995) to electronic materials (Teuschler et al., 1996) through to biological molecules such as DNA (Bustamante and Keller, 1995).
