The coefficient of friction between two surfaces is the ratio of the force required to move one over the other to the force pressing the two together. Thus, if F is the minimum force needed to move one surface over the other, and W is the force pressing the surfaces together, the coefficient of friction μ is given by μ = F/W. A greater force is generally needed to initiate movement from rest than to continue the motion once sliding has started. Thus, the static coefficient of friction μ(static) is usually larger than the sliding or kinetic coefficient μ(sliding).
This table gives characteristic values of both the static and sliding coefficients of friction for a number of material combinations. In each case Material 1 is moving over the surface of Material 2. All values refer to room temperature unless otherwise indicated. It should be emphasized that the coefficient of friction is very sensitive to the condition of the surface, so that these values represent only a rough guide.
Column definitions for the table are as follows.
Column heading | Definition |
Material 1 | Name of moving material |
Material 2 | Name of stationary material |
Conditions | Type of lubrication or other special condition |
μ(static) | Coefficient of friction for initiation of motion |
μ(sliding) | Coefficient of friction once motion has started |
Material 1 | Material 2 | Conditions | μ(static) | μ(sliding) |
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Metals | ||||
Hard steel | Hard steel | Dry | 0.78 | 0.42 |
Hard steel | Hard steel | Castor oil | 0.15 | 0.081 |
Hard steel | Hard steel | Steric acid | 0.005 | 0.029 |
Hard steel | Hard steel | Lard | 0.11 | 0.084 |
Hard steel | Hard steel | Light mineral oil | 0.23 | |
Hard steel | Hard steel | Graphite | 0.058 | |
Hard steel | Graphite | Dry | 0.21 | |
Mild steel | Mild steel | Dry | 0.74 | 0.57 |
Mild steel | Mild steel | Oleic acid | 0.09 | |
Mild steel | Phosphor bronze | Dry | 0.34 | |
Mild steel | Cast iron | Dry | 0.23 | |
Mild steel | Lead | Dry | 0.95 | 0.95 |
Mild steel | Lead | Mineral oil | 0.5 | 0.3 |
Mild steel | Brass | Dry | 0.35 | |
Cast iron | Cast iron | Dry | 1.10 | 0.15 |
Aluminum | Aluminum | Dry | 1.05 | 1.4 |
Aluminum | Mild steel | Dry | 0.61 | 0.47 |
Brass | Mild steel | Dry | 0.51 | 0.44 |
Brass | Mild steel | Castor oil | 0.11 |