Tribological behaviour and transfer layer development of self-lubricating polymer composite bearing materials under long duration dry sliding against stainless steel

Abstract

Fibre-reinforced thermosets and thermoplastics containing PTFE are commonly used in marine and hydropower applications due to their self-lubricating ability in both dry and water lubricated contacts. The constant development of such composite bearings over the last decades make them even more attractive in these fields of application, due to their low maintenance cost and long service life. Their tribological performance is usually estimated by extrapolation of short, accelerated tests, which may induce risks of inaccurate or even incorrect interpretation. In this work, reciprocating dry sliding behaviour of these materials against stainless steel is investigated under typical hydropower conditions. The aim is to study the wear and friction behaviour and the development of transfer layers during long sliding tests, corresponding to years of operation. In order to mimic operational shutdowns, the tests are stopped every 20 h and the counter surfaces are examined using 3D optical interferometry and SEM to study development of surface topography and transfer layers. Test results show that the wear rates of both materials decrease significantly with time. For the thermoplastic, COF decreases with time due to accelerated material transfer observed after 80 h. In contrast, much less material transfer is observed for the fibre-reinforced thermoset, which shows its highest transfer amount after 20 h. Surface analysis indicates severe abrasive wear of the counter surface caused by the wear debris from steel and reinforcements in the material and is accompanied by simultaneous COF increase for the thermoset. Cross-sectional analysis reveals thinner transfer layers and higher wear of the steel in the middle of the wear tracks, where the sliding speed and number of load cycles are highest. These findings contribute to a more accurate interpretation of results obtained than that of short time tests regarding the tribological performance of these materials. As a direct conclusion, the selection of counter surface material can be crucial.