Control of friction on the atomic scale

"The modern world depends upon the smooth and satisfactory operation of countless tribological systems.” B.N.J. Persson Measuring and controlling friction on the atomic scale is the main goal of this thesis. Nowadays, fundamental studies of friction on nanometer-scale are mandatory, since frictional forces become more and more relevant as the scale of nanoelectromechanical devices is reduced. Despite the increased ratio between surface and volume forces in these devices, we will show how an appropriate design and manipulation of the sliding components can result in smooth motion with minimum energy consumption. All our frictional studies are performed by means of a home-built Atomic Force Microscope (AFM) under ultra high vacuum (UHV) and room temperature conditions. The preparation of samples under UHV conditions allowed the study of clean surfaces, free of water or adsorbates. Friction experiments were thence conducted on dry and clean surfaces, without lubricants. Typical friction signals obtained in AFM measurements present stickslip characteristics, when the tip moves over the atomic corrugated surface. The jump of the tip from one energy minimum to another is accompanied by instabilities essential for dissipation. Chaptershows that by decreasing the normal force a transition from atomic stick-slip to continuous sliding is observed and a new regime of ultra-low friction is encountered. The transition is described in the framework of the classical Tomlinson model introducing a parameter η, which compares the strength of the lateral atomic surface potential to the stiffness of the contact under study. For η ≫a dissipative regime of sliding is encountered, whereas for η ≤sliding occurs with negligible dissipation. This parameter can be tuned experimentally by varying the normal load on the contact. Chapterpresents an alternative method based on induced perturbations under resonance condition, which lead to a reduction of friction to negligible values in a controlled way. The regime of zero friction is achieved by applying a periodic excitation between tip and sample at frequencies corresponding to the normal modes of the combined tip-surface system. This method was verified on different surfaces, ionic crystals and mica. An opposite effect on the nanoscale is wear. Under wear the surfaces involved in the contact experience irreversible changes. In order to understand the conditions under which this process is initiated, and how it develops, we studied the wear process between the AFM tip and insulating and metallic surfaces. Chapterpresents a detailed study of the formation of regular topographic structures on a KBr surface under repeated scanning. After the removal of single atomic layers has started, the debris is moved and reorganized due to the interplay between friction-induced strain and erosion, transport of material by the action of the tip, and possibly diffusion. 1D and 2D ripple structures are thus developed. All these results contribute to a better understanding and control of fundamental friction problems which may help to improve the functioning of nanoscale
devices.