The Tribological Properties of Steel on Polyoxymethylene Interfaces

摘要

The tribological characteristics of a steel pin on injection molded polyoxymethylene homopolymer disk system were studied under various conditions. Samples were tested under externally-applied normal loads ranging from 20 to 160 N. Under dry, room temperature conditions, friction coefficients were found to display two distinct regions. Low friction coefficients occurred at low loads, rising to approximately 0.7 (static) and 0.6 (dynamic) at loads above 60N. This phenomenon is attributed to an increase to the contribution of the ploughing friction mechanism as loads increased. At these increased loads, the pin penetrates deeper through the injection mold-induced skin layer and into the softer core material. As loads fell below 60 N the pin does not significantly penetrate the disk during the test, and the adhesive mechanism dominates the tribological properties. Further testing was conducted using lithium soap thickened, low viscosity, synthetic hydrocarbon grease to understand the effects of lubrication on the interface. 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 were also found to display a linear dependence on the load. This indicated that the interface transitioned from the elastohydrodynamic lubrication regime at lower loads to the mixed lubrication regime and a behavior closer to dry contact at increased loads.;Further testing was conducting in temperatures ranging from 22 to 160 °C under externally-applied normal loads ranging from 20 to 80 N. Under this range of temperatures and loads, the friction coefficients displayed a linearly increasing dependence on the load. The load dependence is attributed to an enhanced contribution of the ploughing friction mechanism at higher loads, as with previous testing conducted at room temperature. The average coefficient of friction was also observed to decrease from 0.08 at 22 °C to 0.05 at 50 °C, and to then rise to 0.07 at 160 °C. The drop occurring between 22 and 50 °C was caused by a decrease in modulus of elasticity, attributed to the rise in molecular mobility with increased available thermal energy. The further decrease in modulus occurring at increased temperature, however, allowed the penetration of the pin asperities to increase much more significantly, thus increasing the amount of material displacement to initiate frictional motion.