Graphene as Solid Lubricant in Polymer Composites : With Application in Hydropower Bearings

Abstract: With a rising global demand for green energy production, hydropower plays a key role in securing a reliable and sustainable energy supply. Hydropower is the largest and most efficient renewable energy source, with an ever-increasing capacity. To enable this high efficiency and moderate the turbine output, regulating surfaces including guide vanes and runner blades, can be adjusted. The frequent start-and-stop cycles, as well as the large variation in turbine output levels introduces harsh sliding conditions in the turbine bearing systems. Premature failure of the self-lubricating polymeric bearings is currently a major limiting factor for the reliability of hydropower systems. Consequently, there is an urgent need for new high performance bearing materials with a significantly enhanced service life. The currently used commercial bearing materials primarily use polytetrafluorethylene (PTFE) as solid lubricant. However, due to environmental concerns related to the production and use of PTFE, alternative solid lubricants are required. Graphene has been identified as potential solid lubricant with a great friction and wear performance at the nanoscale. At macro-scale, the introduction of graphene and its derivatives has not yet led to similar low friction properties as PTFE when used as solid lubricant in non-polar polymers, such as, ultrahigh molecular weight polyethylene (UHMWPE). In this thesis, graphene and its derivatives is evaluated as solid lubricant in polymer composites. Different graphene derivates are characterised compared and evaluated with respect to their tribological performance under dry and lubricated sliding. Furthermore, with a poor interface often observed between graphene derivatives and thermoplastic polymer matrices such as and UHMWPE, different methods of surface functionalization were explored to enhance the adhesion and stress transfer between the graphene and matrix. The tribological properties of the resulting composites were analysed in detail. Additionally, two multiscale reinforced composites based on UHMWPE and polyphenylene sulfide (PPS) were processed and evaluated with respect to commercial grade bearing materials. The friction and wear performance of these composites were characterised under varying sliding conditions including contact pressures of up to 40 MPa, simulating conditions as found in hydropower turbines.The results show a surprising increase in sliding friction when introducing the different graphene derivates, in comparison to the neat polymer matrix. Characterisation of the pin surface highlighted the presence of stick-slip which can be correlated to a reduction in degree of crystallinity and plastic deformation of the polymer at the sliding surface. By using surface functionalization with hydrophobic silanes, a well-defined interface between the chemically expanded graphite (CEG) and polymer matrix was successfully created. This effect was confirmed by fracture surface analysis and an increase in storage modulus with respect to the non-functionalized CEG. Sliding tests furthermore indicated significant reduction in sliding friction at a low CEG content, lower than both the non-functionalized CEG composites and the neat polymer. Evaluation of the two in-house processed multiscale reinforced composites and commercial bearing materials revealed a low dry sliding friction and wear for the commercial materials. However, when introducing water as lubricant, friction and wear increased dramatically in the absence of a well-developed transfer film. The PPS based composites performed exceptionally well under water lubrication, with a low coefficient of friction of 0.04. Furthermore, the specific wear rate was a factor of 3 lower than the best performing commercial material, confirming the potential of this novel multiscale reinforced thermoplastic composite for highly loaded hydropower bearings.