Friction and wear of acetal: A matter of scale

Most studies on polymer tribology are traditionally performed on small-scale test specimens. However, to obtain data relevant for practical design of polymer parts used in high load/low sliding velocity systems one must simulate real working conditions as close as possible on laboratory scale. A large-scale test rig has presently been used for determination of friction and wear behaviour of a commercial polyoxymethylene homopolymer (POM-H). Test results are compared to those obtained on a small-scale cylinder-on-plate configuration, investigating possibilities for extrapolation. For small-scale tests, a transition is found from pure adhesive/abrasive wear to deformation and softening when the calculated bulktemperature exceeds 90 °C, corresponding to stabilisation in friction. Overload conditions occur at higher temperatures due to the lack of polymer transfer. Softening and melting of large-scale polymer samples allow for huge transfer films, providing low to extremely low friction and stabilisation in wear rates. Although a thermal extrapolation model with a macroscopic geometry factor is evaluated, friction and wear rates cannot be estimated from small-scale tests because of differences in wear mechanisms, influenced by transfer ability, creep and contact area.