Coefficient of Friction Testing: Procedure, Application, Benefits, and Challenges

Coefficient of friction testing is a key part of material selection and quality control for some applications. An important area for the coefficient of friction testing is in plastic and paper packaging materials (such as LLDPE), as it affects how easily packages can be stacked and handled. There are different ways to measure the coefficient of friction, but all essentially involve measuring the force while moving one item relative to another. This can be used to measure both static friction and kinetic friction.

The article below covers each of these aspects in more detail.

What Is the Coefficient of Friction Testing?

Coefficient of friction (CoF) testing is a means of measuring how easily a material will slip. CoF testing is a way of quantifying the slipperiness or the grip provided by a material. This testing is commonly carried out on plastic films and paper products used for packaging, but can also be applied to a wide range of other materials for other applications. For plastic film testing, a weighted sled is caused to move over a flat plane. The coefficient of friction is the ratio of the force applied in the direction of motion (horizontal) to the force perpendicular to that acting vertically down, (i.e., weight). It is usually measured film to film (both the sled and the plane have the material being tested), but can also be measured between the film and another material, such as glass or steel.

How Does Coefficient of Friction Testing Work?

Coefficient of friction testing essentially works by causing a weight (sled) to slide over a flat surface (plane) and measuring the force during that sliding. Different geometric arrangements can be used to create this movement. The most widely used is a horizontal plane, which can be fitted to a universal testing machine. The plane can be stationary with a moving sled, or the sled can be stationary against a moving plane. The material to be tested is typically used to cover the surfaces of both the sled and the plane. As they are caused to move relative to each other, the horizontal force on the sled is measured accurately.

Another arrangement for testing the coefficient of friction is the inclined plane method. This uses a plane that can be tilted, with a stationary sled that rests on top of it. The angle of the plane is increased until it is steep enough that the sled moves. The angle is used to calculate a coefficient of friction. Different materials may not be suited to these arrangements, and so others do exist, such as a stationary sphere on a rotating disc.

What Requires Coefficient of Friction Testing?

Coefficient of friction testing is applied to a wide range of materials and situations but is particularly relevant whenever friction is an important part of the proper operation of the material. It can be applied to plastic films and paper products used for packaging, to ensure that they can be handled and stacked without sliding. It is also useful for testing the plastic film used in electric vehicle (EV) batteries. When the batteries are constructed, multiple cells are assembled with plastic films between them, and the coefficient of friction of that film is an important parameter to ensure that the batteries remain intact and perform well. Materials, such as disc brakes and floor tiles — for which friction is a key part of what dictates good performance — require coefficient of friction testing.

How Is the Coefficient of Friction Measured?

The coefficient of friction of a material is measured by determining the force required for a weight to move while resting on the material. This measurement is carried out according to a standardized procedure that is specific to the type of material being measured — for instance, ISO 8295 or ASTM D1894 for plastic films. These standards have either a moving sled (weighted block) and a stationary plane, or a stationary sled and a moving plane. As the sled and plane move relative to each other, the force on the sled is measured and recorded. However, there are some other methods and measuring devices for coefficient of friction, such as an inclined-plane apparatus, that increases the angle of the inclined plane until the sled slips.

Is Coefficient of Friction Testing Applicable for Ceramic Coating?

Yes, coefficient of friction testing is applicable for ceramic coating materials. Ceramic coatings can be applied to either decrease the coefficient of friction (such as in bearing applications) or to increase the coefficient of friction (for non-slip ceramic tiles). However, a different testing apparatus is typically used to measure the coefficient of friction for coated ceramics — a ball and disc machine. With this arrangement, a ball (typically steel) is pressed with a known load onto a rotating (or reciprocating) disc of the test material. Coefficient of friction testing can be used to establish the optimum composition of a ceramic coating for the desired application.

What Is the Coefficient of Friction Testing Procedure?

To test the coefficient of friction, the following procedure is followed:

1. Select the material that is to be tested. Depending on the details of the standardized test, you may need to prepare two pieces — one for the sled and another for the plane. The material should be clean and without creases or blemishes.
2. Prepare the testing equipment. The CoF tester must be on a flat, completely level surface. The various test parameters will need to be entered into the tester, such as drag speed and applied load.
3. Run the test, with the sled moving relative to the flat plane while the force is recorded.
4. Measure the normal force. It is important to confirm the normal force, the force acting perpendicular to the moving surface. This is essentially the weight of the sled.
5. Calculate the coefficient of friction. The CoF is simply the drag force (frictional force) divided by the normal force (weight acting down). Typically, the average drag force over the test required to keep the sled moving is used in the calculation of the coefficient of friction.

What Are the Benefits of Coefficient of Friction Testing?

There are multiple benefits to carrying out coefficient of friction testing such as: 

1. Is commonly used as a quality control tool. In this way, it ensures that the end user’s experience of the material is consistent and reliable, including how easily that material slides.
2. Depending on the application, safety may be enhanced with a less slippery surface (such as floor tiles) or with a more slippery surface (such as airbags). Testing the coefficient of friction of these materials allows for improvements to be made.
3. New materials are developed, or tweaks can be made to a material’s composition, to target improved properties. Where friction is an important property, CoF testing is invaluable in optimizing that property.

What Are the Challenges of Coefficient of Friction Testing?

No testing is perfect; there are always limitations and challenges in obtaining meaningful results. Some challenges with coefficient of friction testing are described below:

1. Is not an inherent physical property of a material, like density or melting point. Rather, it is the product of multiple microscopic interactions and conditions, and therefore does not communicate the nature of the frictional behavior but oversimplifies it into a single number.
2. The arrangement of the coefficient of friction test may not effectively represent the real-world application. For instance, the contact area may be a different size or shape, the perpendicular pressure may be different, and the speed of movement may be significantly faster or slower. A standardized CoF test will not replicate these conditions.
3. Environmental factors such as temperature and humidity can affect the performance of some materials.
4. In some applications, lubrication is expected to be used. Controlling for this lubrication can be very difficult in a test environment, thereby introducing uncertainties.

What Is the Use of Coefficient of Friction Testing?

The primary use of coefficient of friction testing, particularly for packaging materials, is to predict the difficulty in shipping. Although not a direct measure, it indicates how a material will be handled in industrial and automated shipping processes. The coefficient of friction affects how well packages of the material will stack. It also affects how these packages will behave on conveyors, or while being handled by forklifts and other vehicles.

A secondary use of CoF testing is for quality control. For the reasons above, it is necessary to control packing materials within a certain reliable specification. Therefore, CoF testing can be carried out regularly to ensure the material will behave exactly as the user expects. 

What Are Industry Applications of Coefficient of Friction Testing?

Various industries and applications rely on a predictable coefficient of friction. Some are explained below:

1. Automotive: Various automotive components work on friction, such as disc brakes, tires, and clutches. The materials used in these components are optimized for their frictional force.
2. Packaging: It is important to understand the friction of packaging materials, as it affects how well packages can be stacked, handled, and moved. If they slide too easily, it can be hazardous and damaging to the packages.
3. Textiles: Different fabrics naturally have very different friction coefficients (consider silk compared with denim). The friction of fabrics is important to understand in clothing but also in industrial textiles, such as seat covers.
4. Construction: Particularly with flooring materials, the coefficient of friction is a crucial property for safety to avoid injury due to slipping.
5. Sports Equipment: The friction of material is essentially the “grip” that material provides, and therefore is a key part of the design of clubs, balls, and shoes.

What Are Different Types of Friction?

There are essentially two types of friction:

1. Static: Static friction is the force that needs to be overcome between two surfaces before there can be relative movement between the surfaces. It is called static friction because it is when the surfaces are at rest relative to each other. The static friction force to be overcome before moving is higher than the frictional force once there is movement. In CoF testing, the coefficient of static friction is calculated as the peak force on the sled just before it starts to move against the plane, divided by the weight of the sled.
2. Kinetic: Kinetic friction (also termed dynamic friction) is the force between two surfaces while remaining in motion relative to each other. This is the force that continues to act in the opposite direction to the direction of movement, restricting that movement. Kinetic friction is lower than static friction — there is less resistance to movement once that movement has begun. In CoF testing, the coefficient of kinetic friction is measured as the average frictional force acting on the sled while moving during the test, divided by the weight of the sled.

What Are the Factors That Affect Coefficient of Friction?

As with any material testing, several factors can affect the coefficient of friction testing. The main factors are explained below:

1. Any oil, detergents, or other contaminants on the sample will directly affect the coefficient of friction measurement. The sample must be confirmed to be free of any contamination, including dust or dirt. The sample should also not be wrinkled or damaged.
2. The result will be affected if the testing table is not perfectly level. This is something that needs to be checked and confirmed regularly.
3. Environmental factors such as temperature and humidity should be within a controlled range, and not fluctuate widely during testing.

What Is the Role Coefficient of Friction in Material Section for Different Applications?

The coefficient of friction plays a pivotal role in material selection for some applications. For some uses the friction coefficient is a key part of the suitability of the material, such as for disc brakes. For others, the CoF may not be the most important parameter, but it may be important for practical reasons. For example, with packaging material, if the CoF is too low, the packages may slide too easily and be difficult or unsafe to handle. It is therefore vital to understand the coefficient of friction for the materials being considered to be certain that a material matches the requirements of the application.

What Is the ASTM Standard of Coefficient of Friction?

The ASTM standard for testing the coefficient of friction is ASTM D1894. It is specifically for testing thin plastic films and sheets. It uses a universal testing machine, with a horizontal plane attachment and a sled of known weight. The testing standard requires that both the plane and the sled are fitted with specimens of the same material to be tested — it does not allow for CoF testing of dissimilar materials.

To learn more, see our full guide on What is ASTM.

What Is the ISO Standard of Coefficient of Friction?

The ISO standard for measuring the coefficient of friction is ISO 8295. This standard is specifically for application in plastic film and sheeting, using a horizontal plane and sled (which can be performed on a universal testing machine). ISO 8295 is a little broader than the equivalent ASTM standard, as it provides for testing the coefficient of friction of a material against a dissimilar material, i.e., the plane material can be glass or steel.

What Do the Coefficient of Friction Results Mean?

A coefficient of friction result is simply the frictional force that needs to be overcome during movement divided by the downward-acting force of the test sled. This means that a smaller coefficient of friction is measured when a smaller frictional force is present, i.e., a low coefficient of friction means that there is less friction experienced by that material and that it slides easily. Conversely, a high coefficient of friction indicates a high frictional force, and that the tested material does not slip easily.

What Is the Role of Coefficient of Friction Testing for Safety and Compliance?

Coefficient of friction testing plays a role in ensuring safety and compliance. Some critical applications rely on friction to operate safely. For instance, road surfaces should allow tires to grip, and flooring materials should not be slippery, even if they get wet. In applications like these, a certain minimum acceptable coefficient of friction should be adhered to. CoF testing is therefore also necessary to ensure compliance. Regular testing against the required standard will ensure that the accepted safety standards are maintained.

Is Coefficient of Friction Testing Accurate?

Yes, coefficient of friction testing is considered to be accurate. Although some factors can affect the results obtained by the testing, these factors can generally be controlled or accounted for. These include aspects such as the temperature at which the test is undertaken, and the cleanliness and integrity of the material samples. So long as effort is taken to carry out the test carefully and according to the details of the agreed standard, the results can be considered accurate.

Is the Coefficient of Friction Test Known Globally?

Yes, the coefficient of friction test is known globally. There are multiple international standards for coefficient of friction testing, with the ISO and ASTM standards being the most widely used. However, there are also standards from other international organizations such as TAPPI (the Technical Association of the Pulp and Paper Industry) for paper and cardboard packaging materials. Coefficient of friction testing allows for internationally standardized testing and interpretation of results.

Summary

This article presented coefficient of friction testing, explained it, and discussed its procedure and various benefits. To learn more about coefficient of friction testing, contact a Xometry representative.

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