Ultra-high molecular-weight polyethylene (UHMWPE) composites reinforced with Graphene Oxide (GO), Nanodiamonds (ND), and Short Carbon Fibres (SCF) are characterised for their mechanical performance in tensile and short-term creep tests. A methodology to separate and analyse the materials’ viscoelastic (VE) and viscoplastic (VP) responses is applied and evaluated. The results show a clear dependence of the performance on size scale/morphology of the reinforcements. All composites show time-dependent VP responses that can be expressed by Zapas model and fit the experimental data with high accuracy. The analysed VE strains and creep compliance curves reveal the nonlinear stress-dependent VE behaviour of all composites at all tested creep stresses. Combining multiscale reinforcements results in an improvement that surpasses that of individual reinforcements. The results of this work offer valuable input for the design and selection of polymer-based materials in demanding applications where prolonged use under service conditions is critical to their performance.

An UHMWPE-based multiscale composite containing graphene oxide, nanodiamonds, and short carbon fibres has shown excellent performance under distilled water lubrication. However, it is crucial to evaluate its tribological performance under conditions which more accurately represent the final application. In this study, the tribological performance of the developed UHMWPE-based multiscale composite is evaluated and compared with neat UHMWPE under different lubricating conditions: no lubricant (dry), in seawater (SW) and in an environmentally acceptable lubricant (EAL). While neat UHMWPE displays a lower friction and wear in dry conditions, the multiscale composite performs better under SW and EAL lubrication. A maximum reduction in friction coefficient of 77% and specific wear rate of 88% are obtained in SW. Under EAL lubricated conditions, the multiscale composite has a maximum reduction in specific wear rate of up to 75%. 

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.

UHMWPE has exhibited excellent performance when used as contact surfaces in tribological contacts. Traditionally, only UHMWPE grades, with narrow particle size and molecular weight distribution, have been deemed suitable for such applications. Now, various UHMWPE grades are available that are different from each other based on their particle size and molecular weight distribution. The question of whether the particle size of UHMWPE affects its performance and properties presents a research gap. The present study attempts to address this question. Additionally, the effect of processing of the UHMWPE is studied. It is observed that although minor differences were observed in the properties of the various grades of UHMWPE, they are inadequate to conclusively determine that the particle size and processing effect the properties and performance of the material.

To mitigate the effects of downstream lubricant spillage from hydroelectric power plants, environmentally friendly lubricants are required. For the sustainable operation of oil-free bearings, the development of high performance bearing materials is crucial. In this study, the tribological performance of PPS and UHMWPE-based composites, incorporating various reinforcements, such as graphene oxide, is evaluated and compared with five commercial materials. Experiments were performed under different lubricating conditions; Dry, water, and using a glycerol-based environmentally adaptive lubricant (EAL). The use of water inhibited an adequate transfer film, which increased wear for most materials. EAL lubrication showed a significant reduction in friction (up to 98%) when compared to dry conditions. The experimentally developed PPS composite provided superior tribological properties, especially under water-lubricated conditions.

In this work, the effect of hygrothermal aging on friction and wear of water lubricated, Ultra High Molecular Weight Polyethylene (UHMWPE) hybrid composites were evaluated. Graphene Oxide (GO), Nano Diamonds (ND) and Short Carbon Fibers (SCF) were used as reinforcements as they previously exhibited promising improvements in the tribological behavior of UHMWPE in water-lubricated sliding contacts. Hygrothermal aging and pin-on-disc tribological experiments were performed to evaluate the response of the UHMWPE composites. It was observed that the friction and wear of the composites were not significantly affected by the aging conditions, which was attributed to the structural integrity of the newly developed UHMWPE based hybrid composites.