Dynamics of sliding mechanisms in nanoscale friction

Abstract

Nanotribology is the study of friction and wear at the nanoscale, with relevance to such applications as micromechanical systems (MEMS) and thin, hard coatings. For these systems, classical laws of friction are inappropriate due to the small dimensions of the sliding elements and the lack of excessive plastic deformation. This thesis presents a theoretical investigation of friction at the sliding interface by Molecular Dynamics (MD) simulations of ideal Lennard-Jones solids. The effect of the interfacial structure on the frictional behavior is investigated by simulating a variety of interface configurations: commensurate, noncommensurate (or grain boundary), and amorphous. The effect of adhesion on the frictional behavior is also explored through a parametric study. For the commensurate interface, the degree of adhesion determines whether sliding occurs in the frictional or "frictionless" regime; the former is distinguishable by the presence of jump phenomena, the principal mechanism of friction in the MD model. The Sigma-5 [100](310) symmetric tilt grain boundary exhibits three distinct sliding regimes which are, in the order of increasing adhesion, frictionless sliding, frictional sliding, and sliding coupled with grain boundary migration. Twist grain boundaries of the (111) plane exhibit frictionless sliding for all degrees of adhesion. Among the structures simulated, the grain boundary systems have the lowest friction due to the intrinsic misorientation at the sliding interface. In the amorphous system, sliding occurs by a series of random local slips due to the individual atomic motion associated with the disordered structure.

(cont.) Increasing the adhesion leads to the initiation of a shear-induced crystallization process followed by an extremely rapid growth of the crystalline cluster. Friction in the amorphous system increases with adhesion only up to a certain limit due to the onset of bulk deformation. Similar trends have been observed in AFM measurements of the friction of thin, hard coatings.