The Coefficient of Friction (COF) test is used to determine the friction characteristics of materials ranging from lubricants to films to household items (ceramic tiles). This is usually determined by the ease with which two surfaces (often of different materials) slide against each other. It is the maximum value of the friction force divided by the normal force. The dimensionless value of COF is the ratio of the force required to slide the surface to the force perpendicular to the surface. Low COF indicates that the surfaces are smooth, eg less resistant to a sliding motion. The polymers used for film manufacturing are typically dry tested against a sled with a calibrated load.
The general results available for the coefficient of friction are:
Static friction or coefficient
Static friction is the force that holds a stationary object to the point that it begins to move. Thus, the constant COF is related to the force restricting the motion of a stationary object 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 the static friction goes away, the kinetic friction follows and is the force holding the regular motion back. This kinetic COF is related to the force restricting the motion of an object sliding on a relatively smooth, hard surface. It is calculated by finding the average load during the test and dividing it by the weight of the sled. Please note that the static peak force should not be included in the average force.
The components supplied for a Coefficient of Friction test include a sled of known mass that is connected via a cable to a nosepiece attachment.
A support plate is fixed to the bottom nosepiece and the nosepiece attachment and cable from the sled are passed under the freewheel mounted on the support plate and attached to the top nosepiece. Once all appropriate fixtures have been tightened, the friction test is ready to be performed. This fixture is compatible with the ST series of benchtop material testing machines.
Tinius Olsen offers force measurement equipment and material testing machines for friction testing. All testing hardware is fully complemented by our Horizon software.