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Coefficients of Friction
Introduction All contacting surfaces are directly affected by surface friction. Surface friction allows
control of all forms of motion. The notes below provide some background into the
theory on the factors affecting the coefficient of surface friction. In practice
the coefficient of friction is generally derived from test rigs. The actual coefficient
of friction results as much from the surface conditions as from the materials involved The notes below relate to a classical basis for surface friction attributed to Coulomb When two metal surfaces are brought together the area of contact area is actually extremely small so the contact pressure is very high. Even at surface loads some plastic deformation occurs at the contact points, while the general surface metal is hardly affected. As the normal load is increased the contact points deform and fracture, thus increasing the real area of contact. The resulting real area of contact is much less than the apparent area which remains unchanged.
Assuming that the plastic yield stress is constant ( S_{y} ), the real area of
contact for each point taking a proportion of the load
A _{i} = F_{i} / S _{y}, so the total real contact area is:
A _{t} = F _{1} / S _{y} + F _{2} / S _{y}.....F _{i} / S _{y} = F _{t} /S _{y} The real area of contact is dependent on the load and is independent of
the area of the surfaces.. m = S _{s} / S _{y} Considering different failure regimes for a surface to fail in shear the associated direct stress is as follows
A variation on this theory is that the area of contact is F_{t} / H (hardness) which is
about 3 time the plastic yield value s _{y}. This reduces the
values for the coefficient of friction by a factor of 3. The resulting value for coefficient of friction based
on this variation of the theory is about 0,2 for all metals..
If the surface films can be eliminated then the coefficient of friction can be increased by an
order of magnitude. This has be done experimentally by heating test metals in a vacuum 
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Last Updated 17/01/2013