Molecular dynamics simulation of rolling friction using nanosize spheres

The rolling resistance of a sphere on an atomically flat surface was studied by molecular dynamics simulation carried out by rolling a rigid Ni sphere on a copper (100) surface. Spheres of 6 and 12 nm diameters were used for rolling simulations after indentation up to 10 Å in depth and the computations carried out using embedded atom potentials of Ni and Cu, assuming either active molecular interaction at the contacts (normal potentials) or the presence of a passivation layer on the sphere. Results showed that the sphere size, penetration depth, and adhesion at the rolling interface strongly affected the rolling friction. When molecular interactions were allowed at the rolling contacts, the average rolling friction coefficient was higher and severe oscillations in the friction force was observed. On the other hand, a sphere with a passivation layer produced more dislocations in the copper substrate during rolling and the motion of the dislocations affected the coefficient of rolling friction and the size of the friction force oscillations. This work also suggested that rolling friction at the nanoscale level was similar to the macroscopic rolling condition during strain hardening of metals due to severe dislocation multiplication underneath the nanosphere in nanorolling.