Engineering laboratories have measured
the coefficient of friction for a number of materials
and have tabulated the results. These coefficient of
friction values apply only to hard, clean surfaces
sliding against each other. Since various experimental
parameters such as surface conditions are not listed,
considerations should be made in using these tabulated
values because they may not directly relate to your
application.
Questions you may have include:
-
What considerations must be made in
using such tables?
-
Why are surface conditions
important?
-
What are the coefficient values?
Considerations
The information available on the various
coefficients of friction provide a starting point on the
subject, but unfortunately the values lack a good
scientific basis. The values should be used just as a
guide.
Effective scientific measurements
typically state the exact condition of materials and
that of the surrounding environment. In the
determination of the coefficient of friction of various
materials, it is important to at least given an
indication of the surface roughness of the tested
materials, as well as their physical condition.
Describing the test setup is important
in order to allow others to duplicate and verify the
measurements. With friction, the normal force can be a
factor in the case of hard materials that may deform
slightly under high pressure. That value should be part
of the test description.
It is also important to state the
testing environment. Although it is unlikely
measurements were made under extreme temperature and
humidity conditions that affect the coefficient, at the
very least those items should be noted.
Surface conditions
Other effects like oxidation of a metal
surface, dirt, water or grease can dramatically change
the coefficient of friction for the given materials.
For example, clean dry steel sliding on
steel has a coefficient of friction of μ = 0.78, but if
the surface has oxidized, the coefficient changes to μ =
0.27.
Likewise, clean dry copper sliding on
copper has a coefficient of μ = 1.21, while oxidized
copper has a value of μ = 0.76.
The biggest problem in using values
established by others in such tables is that you do not
know the actual surface condition of the materials used
or how the values were determined.
Table
The following chart lists the static and
kinetic coefficient of frictions for a variety of common
material combinations. In most cases, the materials are
assumed to be clean and dry. A few are listed as being
wetted by water.
These values are the average of those
from a number of sources. In some cases there are no
values listed for the static friction coefficient or for
the kinetic. Also, a few list a range of values.
Since the quality of the surfaces is not
mentioned, you should only use these readings as a
guide. It is best to measure the coefficients for your
specific materials and conditions of use to obtain
accurate values.
Coefficient of Sliding Friction (clean
surfaces) |
Aluminum |
Aluminum |
1.05 - 1.35 |
1.4 |
Aluminum |
Mild Steel |
0.61 |
0.47 |
Brake Material |
Cast Iron |
0.4 |
- |
Brake Material |
Cast Iron (wet) |
0.2 |
- |
Brass |
Cast Iron |
- |
0.3 |
Brick |
Wood |
0.6 |
- |
Bronze |
Cast Iron |
- |
0.22 |
Bronze |
Steel |
- |
- |
Cadmium |
Cadmium |
0.5 |
- |
Cadmium |
Mild Steel |
- |
0.46 |
Cast Iron |
Cast Iron |
1.1 |
0.15 |
Cast Iron |
Oak |
- |
0.49 |
Chromium |
Chromium |
0.41 |
- |
Copper |
Cast Iron |
1.05 |
0.29 |
Copper |
Copper |
1.0 |
- |
Copper |
Mild Steel |
0.53 |
0.36 |
Copper-Lead Alloy |
Steel |
0.22 |
- |
Diamond |
Diamond |
0.1 |
- |
Diamond |
Metal |
0.1 - 0.15 |
- |
Glass |
Glass |
0.9 - 1.0 |
0.4 |
Glass |
Metal |
0.5 - 0.7 |
- |
Glass |
Nickel |
0.78 |
0.56 |
Graphite |
Graphite |
0.1 |
- |
Graphite |
Steel |
0.1 |
- |
Graphite (in vacuum) |
Graphite (in vacuum) |
0.5 - 0.8 |
- |
Hard Carbon |
Hard Carbon |
0.16 |
- |
Hard Carbon |
Steel |
0.14 |
- |
Iron |
Iron |
1.0 |
- |
Lead |
Cast Iron |
- |
0.43 |
Leather |
Wood |
0.3 - 0.4 |
- |
Leather |
Metal (clean) |
0.6 |
- |
Leather |
Metal (wet) |
0.4 |
- |
Leather |
Oak (parallel grain) |
0.61 |
0.52 |
Magnesium |
Magnesium |
0.6 |
- |
Nickel |
Nickel |
0.7 - 1.1 |
0.53 |
Nickel |
Mild Steel |
- |
0.64 |
Nylon |
Nylon |
0.15 - 0.25 |
- |
Oak |
Oak (parallel grain) |
0.62 |
0.48 |
Oak |
Oak (cross grain) |
0.54 |
0.32 |
Platinum |
Platinum |
1.2 |
- |
Plexiglas |
Plexiglas |
0.8 |
- |
Plexiglas |
Steel |
0.4 - 0.5 |
- |
Polystyrene |
Polystyrene |
0.5 |
- |
Polystyrene |
Steel |
0.3 - 0.35 |
- |
Polythene |
Steel |
0.2 |
- |
Rubber |
Asphalt (dry) |
- |
0.5 - 0.8 |
Rubber |
Asphalt (wet) |
- |
0.25 - 0.75 |
Rubber |
Concrete (dry) |
- |
0.6 - 0.85 |
Rubber |
Concrete (wet) |
- |
0.45 - 0.75 |
Sapphire |
Sapphire |
0.2 |
- |
Silver |
Silver |
1.4 |
- |
Sintered Bronze |
Steel |
- |
- |
Solids |
Rubber |
1.0 - 4.0 |
- |
Steel |
Aluminum Bros |
0.45 |
- |
Steel |
Brass |
0.35 |
- |
Steel (mild) |
Brass |
0.51 |
0.44 |
Steel (mild) |
Cast Iron |
- |
0.23 |
Steel |
Cast Iron |
0.4 |
- |
Steel |
Copper Lead Alloy |
0.22 |
- |
Steel (hard) |
Graphite |
0.21 |
- |
Steel |
Graphite |
0.1 |
- |
Steel (mild) |
Lead |
0.95 |
0.95 |
Steel (mild) |
Phos. Bros |
- |
0.34 |
Steel |
Phos Bros |
0.35 |
- |
Steel (hard) |
Polythene |
0.2 |
- |
Steel (hard) |
Polystyrene |
0.3 - 0.35 |
- |
Steel (Mild) |
Steel (mild) |
0.74 |
0.57 |
Steel (hard) |
Steel (hard) |
0.78 |
0.42 |
Steel |
Zinc (plated on steel) |
0.5 |
0.45 |
Teflon |
Steel |
0.04 |
- |
Teflon |
Teflon |
0.04 |
- |
Tin |
Cast Iron |
- |
.32 |
Tungsten Carbide |
Tungsten Carbide |
0.2 - 0.25 |
- |
Tungsten Carbide |
Steel |
0.4 - 0.6 |
- |
Tungsten Carbide |
Copper |
0.35 |
- |
Tungsten Carbide |
Iron |
0.8 |
- |
Wood |
Wood (clean) |
0.25 - 0.5 |
- |
Wood |
Wood (wet) |
0.2 |
- |
Wood |
Metals (clean) |
0.2 - 0.6 |
- |
Wood |
Metals (wet) |
0.2 |
- |
Wood |
Brick |
0.6 |
- |
Wood |
Concrete |
0.62 |
- |
Zinc |
Zinc |
0.6 |
- |
Zinc |
Cast Iron |
0.85 |
0.21 |
In conclusion
Coefficient of friction for a number of
materials have been tabulated. These values apply only
to hard, clean surfaces sliding against each other.
Since various experimental parameters are not listed,
considerations should be made in using these tabulated
values because they may not directly relate to your
application. |