Effect of stroke length on friction and wear of self-lubricating polymer composites during dry sliding against stainless steel at high contact pressures

The increased control of hydropower plants (i.e. shift from water level to primary control) leads to an increased number of load cycles on critical components such as bearings in hydropower turbines. Despite having shorter sliding amplitudes, this may result in a longer accumulated sliding distance that reduce the useful life of the bearings. In this study, the effect of stroke length on the tribological performance of two self-lubricating polymer composites, commonly used for bearings in hydropower turbines, during dry sliding against stainless steel is investigated. The reciprocating tests are carried out under relevant conditions, i.e. high pressures and long sliding distance, corresponding to years of operation of a hydropower turbine. The worn polymer and stainless-steel surfaces are examined using 3D optical surface profilometer and SEM/EDS to study the wear and friction mechanisms. The results show an increasing wear rate with increased stroke length for both bearing materials, especially when the stroke length is longer than the length of the polymer pin. The thermoset show the same trend for the frictional behaviour and it is attributed to decrease in coverage by transfer layers and solid lubricants at the sliding interface as well as increase in abrasive wear of the stainless steel. Meanwhile, the highest friction is observed at the shortest stroke length for the thermoplastic and the lowest at the intermediate stroke. Surface analysis reveals higher abrasive wear of the stainless-steel counter surface at the longest stroke length for both bearing materials due to lower wear particle entrapment. It can be concluded that changes in sliding amplitude have a significant influence on the tribological performance of the two polymer composites sliding against stainless steel.