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Moving towards a Green Economy, there is a growing demand to use environmentally friendly tribological systems that has resulted in industries turning towards new materials 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, recyclability, low friction, high resistance to wear, corrosion, and chemical solutions are suitable candidates for demanding tribological applications. In research carried out at the Lulea and Ljubljana on numerous polymers, Ultra High Molecular Weight Polyethylene (UHMWPE) has been observed to perform well under water-lubricated conditions. However, if these polymers, including UHMWPE, are used in their pure/unflled 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 comxiii bined 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 lubricated tribological contacts are developed and evaluated for their mechanical, thermal and tribological properties. The research starts with evaluating the infuence o f p article size, molecular weight, and processing of various UHMWPE grades on their thermomechanical properties and tribological performance. It is found that all the diferent UHMWPE materials display similar thermomechanical properties and tribological performance. Based on the information gathered and after selecting one UHMWPE grade, various composites containing carbon-based reinforcements such as Nanodiamonds (ND), Graphene Oxide (GO) and Short Carbon Fibres (SCF) in diferent q uantities ( 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 signifcantly 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 signifcant duration, the Multiscale composite’s integrity, structure and tribological performance are not afected n e gatively. F or c o ntinued r e search a n d d e velopment t o wards 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 stifness degradation. In general, the addition of reinforcements is observed to improve the time-dependent behaviour. More specifcally, t he M ultiscale c omposite d isplays t he h ighest r esistance to creep and stifness 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 verify their tribological behaviour under the relevant tribological conditions. This includes higher contact pressure and diferent lubrication c onditions, i ncluding starved ( dry), seawater 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 fnal 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 affect its thermomechanical p roperties and tribological performance. A synergistic efect is o btained i n t he M ultiscale c omposite by the successful inclusion of all the fllers. It e xhibits a 2 1% l ess c oefcient of fri ction value and 15% lower specific wear rate compared to unfilled UHMWPE under DI water lubrixiv 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 viscoplastic strain model for UHMWPE composites are extracted, and the behaviour of multiscale composites for long-term performance is predicted. Under seawater lubrication, a maximum reduction of 77% in friction coefcient and 88% in specifc 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.