The study of contact, adhesion and friction at the atomic scale by atomic force microscopy

The physical behavior of materials in contact with one another is generally not understood at the atomic level. In an attempt to quantitatively elucidate the fundamental mechanisms involved in contact, friction, and adhesion, atomic force microscopy (AFM) studies in ultrahigh vacuum (UHV) were performed with various single crystal samples. With low applied loads, the sharp tip on the end of the AFM cantilever forms a nanometer-sized single asperity contact with a sample. Adhesion, loading, and friction forces acting between the tip and each sample were measured for these ideal contacts.;To perform the experiments, a novel UHV AFM was designed, built and characterized. The instrument is the first variable temperature UHV AFM, and allows flexibility for sample exchange, AFM measurement positioning, and surface science investigations of the sample.;In order to calibrate AFM measurements accurately, a novel technique was developed for the calibration of lateral forces and was applied whenever possible. The relative lateral to normal force sensitivity is determined by measuring these forces on surfaces which are tilted with respect to the scanning plane. The predicted geometrical coupling of forces is compared with the output signals to determine the relative sensitivity of the instrument.;The occurrence of atomic-scale stick-slip friction forces was investigated with a number of samples. Consideration of instrumental effects reveals that the apparent topography displayed in these measurements is in fact due to two-dimensional frictional forces.;Friction between the mica(0001) surface and various tips was measured as a function of applied load in UHV. At low applied loads, friction is observed to deviate from the macroscopic law of Amonton. Instead of being proportional to the applied load, friction is proportional to the area of contact predicted by the theory of elastic contact mechanics. The variation of friction with applied load was observed to depend upon the tip shape in accordance with the theory of contact mechanics. This result demonstrates that quantitative knowledge of the tip shape is crucial for extracting meaningful and reproducible results from AFM measurements.;A new instrumental technique was developed which allowed the measurement of the lateral contact stiffness. This measurement provides further insight into the properties of the tip-sample contact, as it is proportional to the contact radius and the elastic shear modulus. A comparison between measurements in ambient and vacuum on mica samples with silicon nitride tips produces dramatically contrasting friction and contact area behavior, due to the presence of a water meniscus in ambient conditions.;A series of alkali halides were studied to compare their frictional properties. KF(001), KCl(001) and KBr(001) samples were cleaved and probed in UHV with a silicon nitride tip. The surfaces exhibit atomically flat terraces with predominantly monatomic steps. With these materials, tip-sample contact creates higher friction domains on the terraces. The structure, topography and degree of friction force contrast of these domains is material dependent. We propose that the observed domains result from surface structural changes created by low load tip-sample contact on these relatively soft materials and that this behavior is an example of the initial stages of wear at the atomic scale. (Abstract shortened by UMI.)...