Moving towards a Green Economy, there is a growing demand to use environmentally friendly tribological systems that has resulted in industries turning towards new mate-rials and water-based lubrication to satisfy their needs. Considering the low viscosity of water, tribological contacts lubricated with it are likely to operate in boundary/mixed lubrication regime for relatively long periods. Naturally, the most critical attributes of contact materials for water lubricated tribological systems are that they should have low friction and high wear resistance under these boundary lubricating conditions, which will inevitably be met during start-up, running, and shut down of a tribological operation.High performing thermoplastics that possess excellent mechanical properties, re-cyclability, low friction, high resistance to wear, corrosion, and chemical solutions are suitable candidates for demanding tribological applications. In research carried out at the Luleå University of Technology on numerous polymers, Ultra High Molecular Weight Polyethylene (UHMWPE) has been observed to perform well under water-lubricated con-ditions. However, if these polymers, including UHMWPE, are used in their pure/unfilled state as tribological material in water-lubricated applications, mixed wear and friction performance with unsatisfactory service life has been obtained. One way to improve the properties and performance of a polymer is by adding reinforcements/fillers. The combined addition of micro and nano reinforcement materials to create novel multiscale polymer-based composites has shown great potential in this regard.In this thesis, UHMWPE based multiscale polymer composites for water lubri-cated tribological contacts are developed and evaluated for their mechanical, thermal and tribological properties. The research starts with evaluating the influence of particle size, molecular weight, and processing of various UHMWPE grades on their thermomechani-cal properties and tribological performance. It is found that all the di˙erent UHMWPE materials display similar thermomechanical properties and tribological performance.Based on the information gathered and after selecting one UHMWPE grade, var-ious composites containing carbon-based reinforcements such as Nanodiamonds (ND), Graphene Oxide (GO) and Short Carbon Fibres (SCF) in di˙erent quantities (wt%) are manufactured. The Multiscale composite containing all the reinforcement materials, i.e. UHMWPE (89wt%) + GO (0.5wt%) + ND (0.5wt%) + SCF (10wt%), shows the best tribological performance. The oxidation and degradation temperatures are significantly delayed, indicating an improvement in service life. To gain a better insight into their service life, the developed composites are subjected to accelerated hygrothermal ageing. It is found that even after ageing at elevated temperature and humidity for a significant duration, the Multiscale composite’s integrity, structure and tribological performance are not a˙ected negatively. For continued research and development towards utilising such composites in practical applications, their time-dependent properties are evaluated. Viscoelasticity (VE) and viscoplasticity (VP) are analysed in short-term creep tests. In addition, supporting loading/unloading tests are conducted to evaluate sti˙ness degrada-tion. In general, the addition of reinforcements is observed to improve the time-dependent behaviour. More specifically, the Multiscale composite displays the highest resistance to creep and sti˙ness degradation.Furthermore, for better understanding of the performance of such composites in hydropower applications and to get them closer to real-world use, it is essential to ver-ify their tribological behaviour under the relevant tribological conditions. This includes higher contact pressure and di˙erent lubrication conditions, including starved (dry), sea-water and Environmentally Acceptable Lubricant (EAL). In tribological tests conducted with this premise, the performance of the Multiscale composite is found to be dependent on the type of lubrication used. As the final study in this thesis, the developed Multiscale composite is compared with other developed and commercial materials. It is observed that its tribological performance under demanding conditions is on par with the rest of the materials studied.To summarise the findings from all the studies; The particle size, molecular weight or processing of UHMWPE is found not to a˙ect its thermomechanical properties and tribological performance. A synergistic e˙ect is obtained in the Multiscale composite by the successful inclusion of all the fillers. It exhibits a 21% less coeÿcient of friction value and 15% lower specific wear rate compared to unfilled UHMWPE under DI water lubri-cation. The extended service life of the Multiscale composite is evident from its delayed oxidation and degradation temperatures and ability to retain tribological performance even after undergoing hygrothermal ageing. A maximum of 77% and 70% improvement in modulus and stress at yield, respectively, is witnessed. The parameters for the viscoplas-tic strain model for UHMWPE composites are extracted, and the behaviour of multiscale composites for long-term performance is predicted. Under seawater lubrication, a max-imum reduction of 77% in friction coeÿcient and 88% in specific wear rate is obtained for the multiscale composite, compared to neat UHMWPE. Wear is reduced by 75%for the same under EAL lubrication. All these results and outcomes contribute towards the development of novel UHMWPE-based multiscale composites for water lubricated applications.

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.