The Frictional Properties of Plastics

The frictional properties of a group of linear polymers have been investigated at loads ranging from a few milligrams to several kilograms at slow speeds of sliding At light loads the coefficient of friction tends to increase but at loads above 100 g the coefficient of friction is generally constant and is almost independent of the size and shape of the surfaces From the results obtained at loads above 100 g a tentative theory to explain the origin and magnitude of the friction has been developed The theory is similar to that of Bowden and Tabor for the friction of metals When a plastic slides on a harder metal strong adhesion occurs between the surfaces and shearing takes place within the bulk of the plastic rather than at the interface The frictional force is essentially equal to the product of the area of contact and the bulk shear strength of the plastic The experiments also show that the area of contact is proportional to the applied load so that the plastic behaves as if it possesses an effective yield pressure which is constant. The coefficient of friction is thus primarily determined by the bulk properties of the plastic and is equal to the ratio of the shear strength to the effective yield pressure of the plastic. When a plastic slides on a softer metal shearing occurs within the metal and the friction is primarily determined by the bulk properties of the metal itself rather than those of the plastic. Examination of the surface damage produced during sliding shows marked transfer of the softer to the harder material

The frictional behaviour of polytetrafluoroethylene differs from that of the other plastics in that the adhesion during sliding is small and shearing apparently takes place preferentially at the interface However, when this plastic slides on a sufficiently soft metal marked adhesion and transfer of the metal to the plastic occurs.