Large-scale tests on friction and wear of engineering polymers for material selection in highly loaded sliding systems

Abstract

Tribological properties of polymer materials are traditionally tested on small-scale test rigs because of cost-efficiency and flexibility. For highly loaded applications (e.g., sea lock doors, retaining walls, positioning systems) however, large-scale tests should be performed for estimating the frictional behaviour and wear lifetime of different engineering polymers more accurately under nearly working conditions. In present paper, two filled polymer grades covering a broad range of mechanical properties polyethylene terephtalate (PET)/teflon (PTFE) and ultra high molecular weight polyethylene (UHMWPE)/carbon are slid against high alloy steel, stainless steel and epoxy resin, in order to make a selection for its use as bearing material under dry and lubricated conditions. A general thermal model is evaluated for present test rig. The coefficients of friction on large-scale tests are lower than those obtained from small-scale testing. Polymer wear rates are calculated from weight loss and compared to dimensional measurements (thickness reduction) including elastic deformation, thermal expansion and creep. Microscopic observation of the worn surfaces shows different wear mechanisms of PET/PTFE after sliding against stainless steel, developing a transfer layer onto the polymer surface, and after sliding against high alloy steel, causing polymer transfer onto the steel surface. The better adherence of a polymer transfer film onto a steel counterface is explained by higher attractive forces resulting from its high surface energy, while there is little adherence on stainless steel counterfaces in accordance with its lower surface energy and lower friction. Wear of PET/PTFE after sliding against stainless steel is partly caused by the incorporation of steel particles. After sliding of UHMWPE/carbon, no wear debris was observed as its higher toughness allows for tearing of the surface without particle detachment. The high surface toughness and abscence of a transfer film results however in higher friction.