The nature of the frictional force at the macro-, micro-, and nano-scales

Nowadays it is accepted that the friction force is a combined effect arising from various phenomena: adhesive forces, capillary forces, contact elasticity, topography, surface chemistry, and generation of a third body, etc. Any of them can dominate depending on the experimental force and length scales of the study. Typical forces in macro-tribology are in the Newtons, while are reduced to milli-/micro-Newtons, and nano-Newtons in micro- and nano-tribology, respectively. In this paper, experimental friction results from fullerene like CNx films and single-crystal Si at the three scales will be discussed. The complex and broad variety of processes and phenomena connected with the dry friction coefficient at the macro-, micro-, and nano-scale point of view will be highlighted.