Study of nanoscale friction under ambient conditions

Nanehkarani, Sajjad Salimi (2020) Study of nanoscale friction under ambient conditions. Masters thesis, Memorial University of Newfoundland.

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Abstract

Macroscopic friction involves interaction between many asperities of two surfaces in contact. After the invention of an atomic force microscope (AFM), the physics of friction can be studied at a single-asperity level. While the majority of such studies are performed in ultrahigh vacuum (UHV), friction in ambient conditions is more relevant to our everyday life. In this thesis, the results of AFM friction measurements on amorphous glass and crystalline mica surfaces in ambient conditions are presented. AFM friction in ambient conditions is found to differ significantly from vacuum conditions. While in UHV, the motion of the AFM cantilever is of the stickslip type, in ambient conditions, only steady sliding is observed. Furthermore, in UHV, the average friction force is known to increase logarithmically with the pulling velocity. In ambient, on the other hand, it may either increase or decrease with the puling velocity. These experimental findings strongly suggest that AFM friction in ambient conditions is produced by water bridges between the AFM tip and the surface. A version of the mechano-kinetic model (MKM) is developed, in which the water bridges can spontaneously be created and broken. The main difference between the MKM proposed in this thesis and the one existing in the literature is that we assume that a water bridge may slide along the surface, whereas in the standard MKM, it is rigidly coupled to the surface. The main motivation for this modification is that it suppresses the onset of the stick-slip motion, making the steady sliding the generic regime of motion at slow pulling. A simple analytical formula is obtained for the average friction force that reproduces the experimental results both qualitatively and quantitatively.

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