In order to gain a fundamental understanding of friction, and the
closely related phenomenon of lubrication, one must understand, at the
molecular level, how the energy associated with the work to overcome
friction is converted to heat.[1] Such knowledge is key to understanding
the rate at which an interface will heat, and in addition how chemical
reactions and other physical processes triggered by heat will be
affected by friction. One of the simplest possible geometries in which
friction can occur, and thus be studied, is that of a fluid or
crystalline monolayer adsorbed on an atomically flat surface. This
geometry is experimentally accessible to experiments with a Quartz
Crystal Microbalance (QCM), to numerical simulation techniques, and to
analytic theory.
Measurements of the tribological properties of "model system" and "real
world" lubricants have been performed for rare gases, octane and TCP
adsorbed on lead, iron and/or copper surfaces in extreme temperature
operating environments ranging from 4-700K. The measurements have been
performed in both open (with QCM) and confined geometries (by bringing a
STM tip into tunneling contact with the QCM electrode). Lead substrates
are of particular interest on account the recent observation of
superconductivity-dependent sliding friction. Iron and copper substrates
are of interest for a variety of practical applications. Interaction
potentials for adsorbed rare gases are known to a high degree of
accuracy, allowing highly reliable comparisons of theory to experiment.
TCP is meanwhile a "real-world" lubricant known for its demonstrated
anti-wear properties for macroscopic systems. Although this lubricant
has been the subject of much research for over 40 years, the
atomic-scale details of its lubrication mechanisms are far from being
satisfactorily understood.
[1] "Surface science and the atomic-scale origins of friction: what once
was old is new again.", J. Krim, Surf. Sci. 500, 741 (2002)