Fundamental Studies of Friction, Lubrication, and Wear by Atomic Force Microscopy

摘要

To probe friction, lubrication, and wear on an atomic scale, a capacitance-basedultrahigh vacuum force microscope was developed. It measures both parallel and perpendicular forces between a tip and a surface. Friction between a diamond tip and chemical-vapor-deposited (CVD) diamond films at loads below one micronewton showed a sublinear dependence of friction on load, with the effective friction coefficient varying between 0.2 and 0.8 depending on load and location. Stick-slip behavior resulting from both surface heterogeneity and static/dynamic friction was observed. A CVD process was developed for growing single crystal diamond tips with radii as small as 30 nm. The normal force between these tips and diamond (100) and (111) surfaces agrees with calculated dispersion forces. The frictional force variation on the (100) surface are tentatively associated with a reconstructed geometry convoluted over an asymmetric tip shape, while the (111) surface exhibits features which cannot be simply associated with the surface structure. Friction is approximately 3 nanonewtons independent of loads up to 100 nanonewtons. A field emission technique was developed for continuously observing the motion of individual adsorbed atoms and molecules. The hopping of individual Cs atoms between sites on a tungsten tip was observed with 2 ps and 5 Angstrom resolution.