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.