Coefficient of Friction Testing

Coefficient of friction testing measures the ease with which two surfaces in contact are able to slide past one another. There are two different values associated with the coefficient of friction – static and kinetic. Static friction must be overcome in order to initialize motion between the two surfaces, and kinetic friction is the resistance to sliding once the surfaces are in relative motion. Coefficient of Friction (COF) is the maximum value of the frictional force divided by the normal force. Because surface cleanliness is so important to gathering reliable data, appropriate measures must be taken to ensure freedom from dirt, grease, and other contaminants.

 Coefficient of Friction (COF) testing is used for a variety of materials from lubricants, to films to household items (ceramic tiles) to determine the frictional characteristics of a material. 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 for COF are:

Static friction or coefficient

Static friction is the force between two bodies in contact that resists the initiation of sliding motion of one over the other.. Therefore, the static COF is the force limiting the movement of an object that is stationary on a fairly 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

When static friction has been overcome, kinetic friction comes into play and is the force that holds back normal motion. This kinetic COF concerns the force limiting the movement of an object that is sliding on a fairly 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..
Friction: Friction is defined as the resistance that one surface or object encounters when moving over another.


ASTM D1894 - 11e1 Standard Test Method for Static and Kinetic Coefficients of Friction of Plastic Film and Sheeting 



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