Friction Coefficients for Common Materials and Surfaces

Overview

Friction coefficients describe the resistance to sliding motion between two contacting surfaces. The coefficient of friction (μ) is a dimensionless ratio of the friction force to the normal force pressing the surfaces together. Two distinct types exist: static friction (μₛ), which resists the onset of motion, and kinetic or sliding friction (μₖ), which acts once relative motion is underway. Static coefficients are always greater than or equal to kinetic coefficients—more force is needed to start moving an object than to keep it moving.

Surface conditions such as lubrication, roughness, temperature, and contamination significantly influence the measured coefficient. The values in engineering references represent typical ranges; actual applications require testing under representative conditions.

Key Formulas

Laws of Friction

The source's laws of friction are preserved here in practical engineering form:

• Friction acts tangentially at the contact surface and opposes impending or actual relative motion.
• The friction force is proportional to the normal force between the surfaces.
• For dry sliding surfaces, the friction force is commonly modeled as independent of the apparent contact area.
• Static friction resists the start of motion; kinetic or sliding friction acts after motion begins and is usually lower.
• The coefficient depends strongly on material pairing, surface finish, lubrication, contamination and temperature.

Friction Force

$F_f = \mu \cdot N$

The friction force equals the friction coefficient multiplied by the normal force between the surfaces.

Normal Force on a Horizontal Surface

$N = m \cdot a_g$

For an object resting on or moving across a horizontal surface, the normal force equals the object's weight.

Combined Friction Force (Horizontal, Gravity-Loaded)

$F_f = \mu \cdot m \cdot a_g$

Kinetic Energy of a Moving Body

$E_k = \frac{1}{2} m v^2$

Braking Distance (Constant Friction)

$d = \frac{v^2}{2 \mu a_g}$

The distance required to bring a vehicle to a stop under friction braking, derived by equating kinetic energy to the work done by friction.

Variables

Friction Force Calculator

Static vs. Kinetic Friction — Illustrative Examples

Worked Example — Braking Distance

A car with mass 2000 kg travels at 100 km/h (27.78 m/s) on a wet road with a kinetic friction coefficient of 0.2.

The kinetic energy is:

$E_k = \frac{1}{2}(2000)(27.78)^2 \approx 771{,}605 \text{ J}$

The friction braking force is:

$F_f = 0.2 \times 2000 \times 9.81 = 3{,}924 \text{ N}$

The required stopping distance is:

$d = \frac{E_k}{F_f} = \frac{771{,}605}{3{,}924} \approx 197 \text{ m}$

Friction Coefficients for some Common Materials and Materials Combinations

The following table is restored from the original source page to preserve the complete reference data, including ranges and blank cells where the source does not provide a value.

Engineering Notes

• Static coefficients always exceed kinetic values. A larger force is required to initiate motion than to sustain it. This difference can be 2× or more for some material pairs (e.g., cast iron on cast iron: μₛ = 1.1 vs. μₖ = 0.15).
• Lubrication dramatically reduces friction. Grease or oil can lower the coefficient by 50–90 % compared with dry surfaces and makes friction nearly independent of the surface materials involved.
• Surface condition matters. Temperature, humidity, oxide layers, and surface roughness all alter measured coefficients. Values tabulated here apply to clean, controlled laboratory conditions unless otherwise noted.
• Pressure effects. At low to moderate pressures, friction is proportional to normal force and independent of contact area. At extreme pressures, friction increases sharply and seizure can occur.
• Velocity effects. At very low sliding speeds, friction is essentially speed-independent. As speed increases, kinetic friction generally decreases.
• Tire–road friction. A dry asphalt coefficient around 0.72 drops to roughly 0.2 on wet surfaces, which is why braking distances can increase by a factor of three or more in rain.
• Safety factors. Design for friction-dependent systems (brakes, clutches, conveyor belts) should include appropriate safety margins and account for coefficient variation over the component's service life.

References

• Engineering ToolBox — Friction Coefficients