What is the coefficient of friction?

The coefficient of friction is a unit-less measure of the amount of friction between two materials or surfaces. The power of this friction force depends on the materials in contact, as some material have higher coefficients than others.

For example, a hockey puck sliding on ice is slowed by friction. On the other hand, rubber on asphalt has a high coefficient. If both surfaces are equal, the coefficient of friction will be the same.

When measuring coefficient of friction, there are two types of surface friction to consider: dynamic (kinetic) and static.

What is static coefficient of friction?

Coefficient of static friction is the friction force acting between two objects when there is no relative motion. Think about a box that remains at rest on a ramp, or a car parked on a hill. In order for motion to take place, that static coefficient of friction must be overcome.

The formula for the coefficient of static friction is μs = Fs /N.

The μs represents the coefficient of static friction, with Fs the maximum static frictional force (force required to move the sled) and N the force of gravity acting on the sample and test sled.

What is dynamic coefficient of friction?

Also known as kinetic friction, dynamic coefficient of friction (DCOF) is used to calculate friction forces created between two objects in relative motion. A good example is the force experienced by the wheels of a moving bicycle.

The formula for the dynamic coefficient of friction is μs = Fd/N.

The μs represents the coefficient of static friction, with Fd the average dynamic frictional force, and N the force of gravity acting on the sample and test sled.

How do you measure the coefficient of friction?

Measuring coefficient of friction is done using a Coefficient of Friction Tester or Friction Tester / Slip Tester for short. The coefficient of friction measurement device will have a sled of a known / controlled size and weight, and either a load cell or means to angle the bed. There are different types of coefficient of friction measurement device that we’ve discussed in detail below.

Coefficient of friction measurement device types

Horizontal Plane (Flat Bed) Friction Tester - Measures the dynamic Fd/N & dynamic coefficient of friction Fs/N. A flat bed is required and a sled is dragged across the bed and material being tested. There is a load cell in between sled and the motor that measures the force required to keep moving the sled over the material and the initial force required to move it.

Inclined Plane Coefficient Friction Tester - This simple low cost instrument only measures static coefficient of friction Fs/N. To perform the test, the bed raises and the angle that the sled breaks away from the material is recorded. This is either converted automatically or manually into a static COF value.

Universal Testing Machine / Tensile Tester – Another option is to use a universal testing machine alongside a flat plane coefficient of friction measurement device. This is ideal if you would like to perform other tests such as tensile or compressive strength, but the downside is the friction tests won't be as accurate or repeatable as using a dedicated COF tester. However, we do see lots of companies using this type of device.

What are typical applications for measuring coefficient of friction?

Compact or advanced friction testers can be used, depending on your application. A simple, easy-to-use basic COF tester is best suited to shop floor and quality control laboratories. For R & D, and more demanding QC testing, you should opt for an advanced COF tester with powerful software that offers a complex analysis of results.

If static COF is all you need, an Inclined Plate Coefficient Friction Tester is ideal for performing easy and quick tests.

Typical applications for measuring coefficient of friction include:

Plastic packaging films
Packaging board
Corrugated board
Ink manufacture and development
Carton printers
Evaluating the friction of connectors and touchscreens in electronics.
Determining the friction levels necessary for tyres to achieve optimal grip and traction on roads.
Optimising surface interactions for athletic performance in sports equipment.

What affects the coefficient of friction?

The coefficient of friction depends on various factors, including:

Material properties – Elasticity, hardness, etc.
Surface finish - The rougher the surface, the more friction is created.
Environmental factors - Humidity, temperature of the material, etc.
The amount of force applied.
Wear and contamination of the surfaces.

Because of these factors, different materials have different coefficient of friction, even with the same surface finish.

What does a high coefficient of friction mean?

A high coefficient of friction means the frictional force is stronger than the normal force, so greater force is required for sliding to occur. For example, rubber on pavement (the materials don’t slide past each other easily).

What does low coefficient of friction mean?

A lower coefficient of friction means the better and more easily two surfaces slide over each other. Ice on steel is the archetypal example.

What material has the highest coefficient of friction?

No material has the distinction of having the highest coefficient of friction. However, materials such as leather, rubber, and wood are known for having high coefficients of friction.

What material has the lowest coefficient of friction?

With a coefficient of friction of 0.02, the ceramic alloy BAM (Boron, Aluminium, Magnesium) has an incredibly slippery feel and the lowest known coefficient of friction.

How AML Instruments can help with the accuracy of your coefficient of friction testing

Whether you need your coefficient of friction machine calibrating or a new coefficient of friction instrument to assess the static and kinetic friction of materials, our experts can ensure you’re getting precise and repeatable measurements of frictional properties.

Friction can be a powerful force for good, so contact us today to see how we can work together to optimise your coefficient of friction testing.

Friction / Slip Testing Why & How To Measure Coefficient of Friction