Four great challenges confronting our understanding and modeling of sliding friction

This paper addresses four challenges whose solutions may together enable significant progress in the predication and control of friction for energy conservation. Posed as questions, these are: (1) How can materials with greatly different properties and compositions produce similar kinetic friction coefficients when tested under comparable conditions? (2) How is it possible that the kinetic friction coefficient for the same materials pair can differ greatly when it is slid in different tribosystems? (3) How can frictional phenomena at different size scales be reconciled? and (4) How can the effects of the machine, the materials (including lubricants), and the environment be successfully incorporated into quantitative and predictive friction models? Examples related to these four challenges are provided, as are possible approaches fur attacking than in future research efforts.