Fois, Giovanni (2008) Friction properties of homogeneous and micropatterned surfaces. [Tesi di dottorato]
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The tribological phenomena of adhesion, friction, and wear arise when solid objects make contact. As the size of devices shrinks to micro- and nanoscales, the surface-to-volume ratio increases and the effects of body forces (gravity and inertia) become insignificant compared with those of surface forces (van der Waals, capillary, electrostatic, and chemical bonding). In this situation, fluids lubrication and wear mitigation methods are ineffective. In microelectromechanical systems (MEMS), tribological and static interfacial forces are comparable with forces driving device motion. Then new strategies must be employed to reduce friction.
In this thesis I have studied the microscopic mechanisms involved in friction. Using a quartz crystal microbalance technique, I have measured the interfacial viscosity of pure gases monolayers deposited on very homogeneous Pb(111) surfaces. For nitrogen films at temperatures below 15 K, no dissipation is detected, suggesting that the N2 films are rigidly coupled to the oscillating electrode. By raising the temperature close to 20K, we find sliding of the nitrogen film for coverages above about 0,5 nominal layers. Similar behavior is shown by Ar films. For Ne films we have instead observed a pronounced depinning transition separating a low-coverage region, where the film is nearly locked to the oscillating electrode, from a high-coverage region characterized by slippage at the solid-fluid boundary. These data are suggestive of a structural depinning of the solid Ne film when it becomes incommensurate with the lead substrate. With this system I have also studied the electronic contribution to the total friction crossing the superconducting transition of lead. Ne monolayers do not show any anomalous behavior in the dissipation across Tc.
With the QCM technique I have also measured the friction of Ne films on gold surfaces. The calculated slip time was one order of magnitude smaller than for Pb. The effect of the electron coupling with the adsorbate should be high for this system. Then an insulating Kr overlayer was grown on the gold surface to reduce coupling. For Kr coverage lower than 2ML an increase in the slip time was measured while for increasing coverages a reentrant behavior was detected.
A preliminary study of the slippage of 15nm diameter gold nanoparticles deposited on gold has been carried out at room and cryogenic temperatures. Measurements reveal unexpected slippage at T=5,5K for a flat surface while for a rough substrate a strict dependance on the external applied load has been found.
Finally I have developed a formula to produce patterned silicon surfaces via micro contact printing of alkylsilane. Preliminary tribological studies have been done with a modified AFM tip having a square tip wih a contatct area of about ~2x2?m2. Measurements reveal a reduction of the friction coefficient for OTS patterned surface by a factor 2 with respect to bare silicon.
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