In atomic contact sliding, it was discovered that friction is determined by four transition mechanisms, that is no wear, adhering, plowing and cutting, when the radius of asperity is kept constant and the depth of asperity indentation increases. On the other hand, a dislocation model suggested that the friction stress between two asperities is constant when the contact size is small but decreases when it reaches a critical value.
The present study aims to explore the mechanisms of the size effect of nano-friction using the molecular dynamics analysis. When the asperity radius varies, the indentation depth is kept unchanged so that any variance in the mechanisms of sliding will be caused by the different contact sizes solely. The molecular dynamics model used consists of a single spherical diamond or copper asperity, sliding across the (111) plane of a copper workpiece. The velocities of the atoms in the initial configuration of the model follow the Maxwell distribution and the interaction between atoms obeys the modified Morse potential.
It was found that a transition does exist when a diamond asperity is sliding over a copper workpiece. The friction stress is a constant independent of the asperity radius less than 12nm and the deformation of copper is purely elastic. The sliding experiences stick-slip and the atoms of the asperity slips concurrently over the workpiece atoms. However, when the radius of the asperity is more than 12nm, the friction stress decreases sharply. After this transition, the effect of stick-slip is not prominent and the asperity atoms do not slip concurrently over the workpiece atoms. This indicates that there exist slipped regions and unslipped regions. The contact length threshold at the transition varies with the indentation depth although all the shear stresses before transition fall into the regime of the theoretical shear stress of the material.
When sliding with a small copper asperity, the mechanism is similar to that with small diamond asperities of radii less than 12nm, i.e., the atoms of the asperity slip concurrently over the workpiece atoms and the friction stress remains constant regardless of asperity size. The stick-slip phenomenon is also prevalent. When the size of the asperity increases to 10nm, however, plastic deformation of the workpiece almost invariably occurs due to the strong adhesion between the asperity and workpiece atoms. This does not happen in diamond-copper sliding because the C-Cu interaction is weaker. With the intervention of plastic deformation, the transition from concurrent slip to single-dislocation assisted slip does not appear. Friction stress therefore increases rather than decreases when the asperity radius increases further.
In conclusion, the size effect on friction transition depends on sliding conditions, including material property, radius and indentation depth of an asperity. The understanding achieved is essential to an optimal design of nano-sliding systems.