Coefficient of Friction (COF) is the maximum value of the frictional force divided by the normal force.

COF is used in a variety of materials from lubricants, to films to household items (ceramic tiles) to determine the frictional characteristics of a material. It is generally determined to be the easy by which wto surfaces (often of different materials) slide against each other. The dimensionless value of the COF is the ratio of the force required to slide the surface to the force perpendicular to the surface. A low COF indicates that the surfaces are smoother, e.g. less resistant to a sliding motion. Polymers used for film production are usually tested dry against a sled with a calibrated load.

Common results available from Lloyd Instruments™ materials testing systems for COF are:

• Static friction or coefficient

Static friction is the force that holds back a stationary object up to the point that it just starts to move. Thus, the static COF concerns the force restricting the movement of an object that is stationary on a relatively smooth, hard surface. It is calculated by finding the initial peak force required to move the sled and dividing the value by the weight of the sled.

• Kinetic friction or coefficient

Once static friction is overcome, kinetic friction follows and is the force holding back regular motion. This kinetic COF concerns the force restricting the movement of an object that is sliding on a relatively smooth, hard surface. It is calculated by finding the average load during the test and dividing this by the weight of the sled, which holds the other material. Please note that the staic peak force should not be included in the average force.

Note: Friction is defined as the force between two adjoining surfaces. Slip is the opposite of friction so a high COF gives a low skip factor.

COF testing can be carried out on a range of Lloyd Instruments material testing machines together with  NEXYGENPlus data analysis software when more comprehensive testing results are required.

Common standards for determining COF

• ASTM D1894

Typical graph showing coefficient of friction test.

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