Various effects can develop that are dependent on
the materials being used. For instance, when polymers
are in contact with each other there can be new
polymers formed between the two original surfaces.
Surface links may catch on to each other, and chemical
interactions may occur between the two surfaces.2
Another model makes use of the discovery of flash
temperatures, which helps describe how heat created
by friction is distributed at surfaces. Rather than heat
being spread uniformly along the surface, there are
“hot spots” that arise since boundaries are jagged at
small size scales.2,3 The regions that heat up may cause
a local uplifting of the surface, which in turn leads
to more hills and valleys and increased shear forces,
and ultimately increased friction forces. On the other
hand, for certain materials flash temperatures have
been shown to cause softening, which actually reduces
friction.4,5 The problem may be further complicated
when surface lubricants are used. In either situation,
there still is no single theoretical model that sufficiently
explains this complicated phenomenon we
call friction, because the fine structure and properties
of surfaces for different materials vary greatly. It is
worthwhile for students to gain some experience with
this way of thinking (i.e. macroscopic properties are
often determined by microscopic behavior) that goes
beyond what their textbooks say and tend to reduce to
something much simpler.