Tribology of self-lubricating polymer composites for hydropower applications

Abstract: The ongoing global climate crisis and its impacts calls for urgent actions, such as significant reduction of CO2 emissions by introducing more renewable energy. This has led to a rapid integration of variable power sources, such as wind and solar power, into the electricity systems. These sources have an unpredictable output and requires more active control of the power output in order to compensate fluctuations in supply. The hydropower plants shoulder a large portion of the regulation and balancing duty in many power systems and are therefore subjected to a higher number of start/stop cycles and significantly higher number of load cycles than before. This result in a longer accumulated wear distance and harsher operating conditions for critical components such as sliding bearings for the guide vanes and the turbine blades. As a result, in recent years several bearing failures have been reported in the turbines operating under these more fluctuating conditions.Despite the severity of the issue, and the additional maintenance and/or replacement costs involved, only a limited number of studies has been reported dealing with wear and friction behaviour of self-lubricating polymer composite bearing materials used for hydropower applications. These studies often lack thorough surface analysis where the governing friction and wear mechanisms have been investigated. In addition, there is a lack of knowledge regarding the influence of various operating conditions on the tribological performance. Hence, there is insufficient knowledge to estimate the actual effect of the increased control and how to optimize the operating conditions in order to reduce friction and wear.Therefore, the aim of the present work is to increase the knowledge and understanding of how different parameters such as sliding speed, contact pressure, stroke length, test duration, and counter surface topography influence the tribological performance, and its governing mechanisms, of different types of self-lubricating polymer composite bearing materials commonly used in hydropower applications.To achieve this aim, a systematic tribological characterization has been carried out to study the influence of the abovementioned parameters on the tribological performance of self-lubricating polymer composite bearing materials sliding against stainless steel. The tests have been carried out in dry sliding using different linear reciprocating flat-on-flat configurations and long test duration. The governing friction and wear mechanisms have been thoroughly investigated by various surface analysis techniques, such as 3D optical interferometry, SEM, and EDS. In addition, material characterization of the polymer composites was carried out using a range of analytical techniques to study macro- and micro-structure as well as composition to aid the interpretation of the tribological behaviour.The results show that the investigated parameters have a significant influence on friction, wear, and formation of transfer layers. The formation of transfer layers is a transient process involving continuous build-up and break-down. The friction and wear mechanisms for the polymer composites are also transient processes and need to be studied over long durations to enable accurate predictions of their long-term tribological behaviour. Surprisingly, under some operating conditions the stainless steel counter surface showed clear signs of abrasive wear caused by micro-cutting and micro-ploughing. This is caused by interaction with work hardened and oxidised steel wear debris as well as reinforcement and filler particles from the polymer composites.The parametric study showed that the coefficient of friction is decreasing with increased contact pressure due to higher concentrations of solid lubricants in the sliding interface. The wear rates showed an increasing trend with increased sliding speed due to thermal softening of the polymer composite materials. This was particularly pronounced at lower contact pressures, as a result of reduced availability of solid lubricants. The influence of stainless steel counter surface topography revealed that too smooth steel surfaces result in higher friction and more wear of the steel, while rougher steel surfaces have a negative effect on the wear of the polymers. The effect of steel surface lay orientation with respect to the sliding direction differed between the polymer composites and was highly influenced by the initial surface roughness of the steel surface. However, for higher surface roughness, all polymer composites showed reduced friction with perpendicular lay compared to parallel due to thicker transfer layers. The results showed increasing wear rate with increased stroke length, especially when the stroke length is longer than the length of the polymer pin. This was accompanied by increased wear of the steel surface at the longest stroke length due to reduced entrapment of wear particles. The influence of stroke length on friction behaviour differed between the polymer composite materials.In summary, by optimizing the operating conditions for the self-lubricating polymer composite bearing materials in hydropower applications, it is possible to both save energy and prolong the useful lifetime of the bearings. Furthermore, the obtained data may be useful for selection of bearing materials for given operating conditions to ensure improved tribological performance of the bearings.