Listing of the coefficient of sliding friction values for a variety of hard clean materials - Succeed in Understanding Physics. Key words: Physical Science, sliding, static, kinetic, surface roughness, clean, oxidation, lubricated, greased, engineering, measurements, scientific, School for Champions. Copyright © Restrictions
Coefficient of Friction Values for Clean Surfaces
by Ron Kurtus (2 November 2005)
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?
This lesson will answer those questions. Useful tool: Units Conversion
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.
Other effects like oxidation of a metal surface, dirt, water or grease can dramatically change the coefficient of friction for the given materials.
Effect of oxidation
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.
Need to know surface conditions
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.
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)
|Brake Material||Cast Iron||0.4||-|
|Brake Material||Cast Iron (wet)||0.2||-|
|Cast Iron||Cast Iron||1.1||0.15|
|Diamond||Metal||0.1 - 0.15||-|
|Glass||Glass||0.9 - 1.0||0.4|
|Glass||Metal||0.5 - 0.7||-|
|Graphite (in vacuum)||Graphite (in vacuum)||0.5 - 0.8||-|
|Hard Carbon||Hard Carbon||0.16||-|
|Leather||Wood||0.3 - 0.4||-|
|Leather||Oak (parallel grain)||0.61||0.52|
|Nickel||Nickel||0.7 - 1.1||0.53|
|Nylon||Nylon||0.15 - 0.25||-|
|Oak||Oak (parallel grain)||0.62||0.48|
|Oak||Oak (cross grain)||0.54||0.32|
|Plexiglas||Steel||0.4 - 0.5||-|
|Polystyrene||Steel||0.3 - 0.35||-|
|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|
|Solids||Rubber||1.0 - 4.0||-|
|Steel (mild)||Cast Iron||-||0.23|
|Steel||Copper Lead Alloy||0.22||-|
|Steel (mild)||Phos. Bros||-||0.34|
|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|
|Tungsten Carbide||Tungsten Carbide||0.2 - 0.25||-|
|Tungsten Carbide||Steel||0.4 - 0.6||-|
|Wood||Wood (clean)||0.25 - 0.5||-|
|Wood||Metals (clean)||0.2 - 0.6||-|
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.
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Resources and references
Friction Resources - Extensive list
The following books are available from Amazon.com.
Complete Idiot's Guide To Physics by Johnnie T. Dennis; Alpha (2003) $18.95
What Is Friction? (Ages 4-8) by Lisa Trumbauer; Children's Press (CT) (2004) $4.95
Friction Science and Technology (Mechanical Engineering Series) by Peter J. Blau; Marcel Dekker Pub. (1995) $89.95
Physics of Sliding Friction (NATO Science Series E:) by B.N. Persson, E. Tosatti; Springer Pub. (1996) $358.00
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Coefficient of Friction Values for Clean Surfaces