Static and Dynamic Friction Characteristics of a Steel on Polyoxymethylene Interface Under Dry and Lubricated Contact Conditions

Abstract

The static and dynamic friction characteristics of a steel pin on injection molded polyoxymethylene homopolymer disk system were studied under both lubricated and dry contact conditions. Samples were tested at externally-applied normal loads ranging from 20 to 160 N. Under dry test conditions, friction coefficients displayed two distinct regions with very low friction coefficients at low loads and rising to approximately 0.7 (static) and 0.6 (dynamic) at loads above 60N. This phenomenon is attributed to an enhanced contribution of the ploughing friction mechanism at higher loads. As load increases, the pin penetrates through the injection mold-induced skin layer and into the core. At loads lower than 60 N, however, the pin does not significantly penetrate the disk during the test and the adhesive mechanism dominates the tribological properties. Additional tests were performed in order to determine the effects of a lithium soap thickened, low viscosity, synthetic hydrocarbon grease. The average static and dynamic friction coefficients for the lubricated interface were found to be 0.031 ± 0.01 and 0.027 ± 0.01, respectively. The friction coefficients exhibited a linear dependence on the load. This result indicates a shift from the more optimal elastohydrodynamic lubrication regime at lower loads to a mixed lubrication regime and a behavior closer to dry contact at higher loads. Results are interpreted in light of the principal static and dynamic friction mechanisms.