Investigation Of Nanofriction Lubrication Mechanisms

The fast growth of micro- and nano-electromechanical systems (M/NEMS) technology has engendered a wealth of information on the atomistic processes of friction. The tribological issues, of which the M/NEMS are subject to, have motivated further understanding of the friction mechanism and new attempts to control the frictional characteristics of these devices have emerged. This thesis aims at investigating the underlying friction behavior at the atomic scale through theoretical and experimental methods under opened environmental conditions and through molecular dynamics (MD) simulation under ultra-high vacuum (UHV) conditions.First, the liquid lubrication, thermolubricity and dynamic lubricity due to mechanical oscillations are investigated with an atomic force microscope (AFM) in ambient environmental conditions with different relative humidity (RH) levels. Experimental results demonstrate that high humidity at low temperature regime enhances the liquid lubricity while at high temperature regime it hinders the effect of the thermolubricity due to the formation of liquid bridges. Friction response to the dynamic lubricity in both high and low temperature regimes keeps the same trends, namely the friction force decreases with increasing the amplitude of the applied vibration on the tip regardless of the RH levels. An interesting finding is that for the dynamic lubricity at high temperature, high humidity condition leads to the friction forces higher than that at low humidity condition while at low temperature the opposite trend is observed. An extended two-dimensional dynamic model accounting for the RH is proposed to interpret the frictional mechanism in ambient conditions.Second, frictional ageing is investigated through simulating a silicon tip sliding over a diamond substrate with a MD model. It is demonstrated that contact strengthening in UHV is mainly caused by surface dimerization. With the increase of temperature, tip-substrate contact evolves from incommensurate to commensurate due to the lattice transition from (1×1)to (2×1) lattice ordering. The combination effects of contact strengthening and thermal lubrication lead to a nonmonotonic variation of the mean friction force with temperature rise. However, the friction increase trend induced by the surface dimerization can be suppressed efficiently with the structural lubrication.Third, MD simulation of dynamic lubricity AFM achieved by sinusoidal modulation of the normal interaction forces at the tip-sample interface is thoroughly investigated in UHV condition. In order to find a frequency at which the friction unequivocally decays with increase of imposed amplitude oscillations, the phonon density of state (DOS) calculations are invoked. It is first found that the best candidate frequency lies on the narrow overlapping profiles of the tip and sample DOS curves. However, this high frequency on the order of THz is shown to slightly reduce friction as it provokes the tip to hastily vibrate in the tip-sample interaction repulsive regime, irrespective to the range of vibration amplitude applied. Therefore, it is tentatively concluded that this phenomenon is the reason behind the friction enhancement previously reported at higher frequency. Besides, by lowering the frequency to GHz order of magnitude which is close to the wash-board frequency, the friction precipitously decreases as the tip is enabled to jump off contact even at low amplitude modulation. At larger amplitude the tip oscillates in both the repulsive and attractive regime. Finally, with the chosen frequency of GHz level it is shown that for part of the oscillation cycle at which the tip tends to retract from the sample, the energy barrier to sliding increases and the higher the amplitude modulation the larger is the barrier that can be surpassed. On the other hand, for part of the oscillation cycle at which the tip contacts the sample, the energy barrier decreases with increase amplitude modulation to enable the tip transition.The contribution of this thesis which attempts to provide answers to some of the pivotal questions of nano-tribology is deemed relevant for the understanding of some aspects of atomic-scale friction.