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

## 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.

### Surface roughness

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.

### Test setup

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.

### Testing environment

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.

### 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.

## 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)

#### Kinetic

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 - -
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
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 -
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) 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

## Summary

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

Ron Kurtus' Credentials

### Websites

Engineer's Handbook - Coefficient of Friction

RoyMech (UK) - Friction Factors

Friction Resources - Extensive list

### Books

The following books are available from Amazon.com.

#### Basic

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

Do you have any questions, comments, or opinions on this subject? If so, send an email with your feedback. I will try to get back to you as soon as possible.

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