Tribology is concerned with the influence of
mechanically applied forces on interfacial phenomena that
accompany and control sliding. A wide range of models
has been developed to describe these phenomena, which
include frictional dissipation, wear and tribochemical reactions.
This paper shows that these apparently disparate
models are based on the same fundamental concept that an
externally applied force accelerates the rate of thermal
transition of atoms or molecules across energy barriers
present in solid and liquid materials, thereby promoting
flow, slip or bond cleavage. Such ‘‘stress-assisted’’ effects
and the associated thermal activation concepts were developed
independently and in different forms by Prandtl (Z
Angew Math Mech 8:85, 1928) and Eyring (J Chem Phys
4(4):283–291, 1936). These two works have underpinned
subsequent theories of dry friction, boundary lubrication,
EHD rheology, tribochemistry and nanoscale wear modelling.
This paper first reviews the historical development
of the concepts, focussing in particular on the models of
Prandtl and Eyring and how they have subsequently been
used and adapted by others. The two approaches are then
compared and contrasted, noting that although superficially
similar, they contain quite different assumptions and constraints.
First, the Prandtl model assumes that the force is
exerted through a compliant spring, while constant force
sliding is assumed by Eyring. Second, different approximations
are made in the two models to describe the change in energy barrier with external force. Prandtl explores
the asymptotic behaviour of the energy barrier as the
applied force become sufficiently high to reduce it to zero,
while Eyring assumes that the energy barrier is reduced by
an amount equal to the external work carried out on the
system. The theoretical underpinnings of these differences
are discussed along with the implications of compliant
coupling and constant force sliding on the velocity and
temperature dependence of the friction forces for the two
models.