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.