Components of novel propulsion systems in electric aviation contain materials defined as critical by the European Union: electric motors contain Rare Earth Elements neodymium and dysprosium, and state-of-the-art batteries materials like lithium and cobalt. Such critical raw materials have high supply risks but are crucial for the economy. The demand of all these materials is forecasted to increase drastically in the coming years, which means that the rate of supply might no longer suffice to fill the demand. Circular economy principles have been suggested as a solution. Materials recovered from end-of-life components can secure supplies and reduce the environmental impact of products.

In this research, the implementation of circular economy approaches to address critical raw material demand in electric aviation is studied. According to the developed models, the material demand is negligible in comparison to other industries until 2050. However, as the electric aircraft technologies are still in development, there is a lot of uncertainty around the demand.

Beyond 2050, components will start reaching their end-of-life stage. In this case, a circular strategy considered feasible for electric motors is remanufacturing. After the Rare Earth Element magnets in the motors become obsolete, they can be recycled to recover the critical raw materials. Both hydrogen decrepitation and a combination of hydro- and pyrometallurgical processes can be used to regain materials for magnets in aviation or other applications.

Although circular economy strategies will not be able to significantly reduce the primary material demand in electric aviation by 2050, these can still lower the environmental impacts from production. Additionally, well-established circular practices could address the material demand more substantially in the future, after 2050, if electric technologies are more widely adopted then.

This thesis explores the potential of sustainable resins in composite manufacturing, focusing on bio-based benzoxazine resins and bio-based epoxy derived from brown algae. The research investigates the feasibility of integrating these sustainable alternatives into composite structures. Bio-based benzoxazine resins were synthesized in-house, utilizing m-guaiacol, furfurylamine, sesamol, paraformaldehyde, Jeffamine T403, and DDS, while a commercial bio-based epoxy, phloroglucinol triepoxy (PHTE), was employed. The study involves comprehensive material analysis and testing, including an examination of chemical properties and mechanical performance. The findings reveal contrasting outcomes for the two types of bio-based resins. Bio-based benzoxazine resins exhibited challenges, as curing was not achieved, preventing their combination with composites. In contrast, phloroglucinol triepoxy, the bio-based epoxy from brown algae, emerged as a promising candidate for sustainable composite investigations. This resin demonstrated excellent mechanical properties, although its high viscosity and reactivity posed challenges for traditional manual manufacturing techniques. Furthermore, resin characterization unveiled a high glass transition temperature (Tg) system, especially noteworthy given the use of an aliphatic curing agent and monofunctional reactive diluent that lowers crosslinking density. This opens the door to its application in composites across various industries.

Coating deposition by Physical Vapour Deposition (PVD) on high strength steels is an important research project at TATA Steel - IJmuiden. The research is aimed at replacing the conventional hot-dip galvanisation process to obtain a defect-free coating without affecting their well-engineered properties. However, the annealing treatment of the steel, performed to obtain these properties, affects the coating adhesion properties. This is a result of the selective oxidation process, which forms external oxides of alloying elements at the surface. Therefore, a pre-treatment process is required to remove these surface oxides from the steel strips before they are coated. Plasma sputtering is currently being
used for the pre-treatment to remove any contaminants and surface oxides from the steel. To obtain a good coating adhesion, a relatively large amount of surface oxides needs to be removed by sputtering. Also, the removed material is known to contaminate the vacuum chamber in the PVD deposition line, for which frequent maintenance of the chamber might be necessary. Thus, an additional
surface pre-treatment step was investigated in this study to reduce the sputtering process inside the vacuum as much as possible. In the present work, the effect of both direct current electrolytic alkaline cleaning and sulphuric acid etching on the surface of DP800 steel was investigated. Two different baths were considered for this purpose; a 27 g/L NaOH bath with some additive at 60°C and a 50 g/L H₂SO₄ bath at 25°C and 50°C. A current density of 1.5 A/dm² was applied during the electrolytic cleaning for which both cathodic and anodic polarisation methods were investigated. Also, a range of acid etching times (10s to 120s) was investigated for the given concentration and temperatures of the acid bath to study its effect on the surface. The effect of adding a corrosion inhibitor into the acid bath on the rest of the coating deposition process was also investigated. Various surface characterisation techniques and wettability tests were performed to study the changes in morphology and composition of the surface and their effect on the coating adhesion properties of the treated samples. Finally, coating adhesion tests were performed after zinc deposition to investigate the adhesion performance of the steel after the pre-treatment steps. Initial surface analysis during electrolytic alkaline cleaning showed that the anodic polarisation was more effective than cathodic polarisation of the sample, as the latter tends to reduce the surface wettability by additional deposits of iron fines over the surface. A subsequent acid etching provided a reduction in the minimum required sputter intensity to obtain a good adhesion from 2300 kJ/m² to about 800 kJ/m². A further reduction was achieved to a sputter intensity of only 214 kJ/m² after retarding the effects of surface reoxidation by vacuum sealing the samples. Acid etching at 25°C provided bad coating adhesion at lower etching times, attributed to the partial dissolution of surface oxides and absence of an initial grain roughening. Good coating adhesion was either obtained at higher etching times or by increasing the temperature of the acid bath to 50°C. Addition of a corrosion inhibitor was considered impractical as high sputter intensities (> 321 kJ/m²) was required to remove the adsorbed inhibitor molecules from the surface. Thus, a reduction in the required sputter intensity was achieved by more than a factor of 10 after acid etching, only if the effects of surface reoxidation during the transfer time between acid etching and entering the PVD installation can be minimized.

Alkaline water electrolysis will become increasingly important for supplying the world with sufficient renewable energy, and with raw material for the chemical and pharmaceutical industry. Zero Emission Fuels B.V. (ZEF), a technology start-up based in the Netherlands, is developing a small scale alkaline electrolysis cell (AEC), which is integrated in a methanol producing micro-plant. The challenge of this project is to look into the use of polymers for the encapsulation of the ZEF AEC. The conditions of the ZEF AEC are not to be neglected. The system will run in a 30wt% KOH solution at a temperature of 90 °C and a pressure of 50 bar. Not many polymers will be able to withstand these conditions for a desired lifetime of 20 years. Using CES EduPack, a selection of 30 potentially suitable polymers has been made, from which high density polyethylene (HDPE), 40% glass-reinforced polyphenylene sulfide (PPS-40%gf) and polysulfone (PSU) are further investigated. Based on a literature review and a simple KOH ageing test, HDPE is found unsuitable for application in the ZEF AEC. PPS-40%gf and PSU have been subjected to durability testing for a range of different conditions involving a variety of KOH concentrations (15, 30 and 45wt%), and two oxygen partial pressures (O2 at 5 bar and air with pO2=20%) at different temperatures (90, 120 and 170 °C). The purpose of the ageing experiments is to give a better understanding of the effect of these parameters on the structure and integrity of the polymer; and to eventually be able to acquire a lifetime prediction. Extensive characterisation of the exposed samples has been carried out using different techniques, including weight measurement, tensile testing, DMA, creep-recovery testing, DSC, FTIR, XRD and SEM. After 12 weeks of ageing, it is found that glass-filled polymers are unsuitable for application in a strong alkaline solution at elevated temperatures, due to the dissolution of the glass fibres, which leads to a reduction in mechanical and barrier properties. However, the PPS matrix and PSU are found to be resistant to thermo-oxidative and chemical degradation in the tested ageing conditions. Only subtle changes in mechanical, visco-elastic and thermal behaviour are observed, which can be assigned to the effects of physical ageing. Due to the undesirable brittle nature of PPS, it can be concluded that PSU is the most promising candidate for the long-term application in alkaline electrolysis.

This master thesis investigates the effect of surface morphology of Selectively Laser Melted Ti-6Al-4V on fatigue performance. The literature study performed here iden- tifies roughness as one of three major causes for the decreased performance of the additive manufactured alloys (along side porosity and residual stresses). In order to eliminate the two other causes for poor fatigue, and isolate the surface roughness, a hot isostatic pressing (HIP) treatment was performed on every sample. Luckily, a first set of tests proved that chemical treatments, specifically electropolishing and plasma- polishing, greatly improve the surface quality. A second series tests (rotating bending fatigue tests) concluded that, although specific roughness parameter such as Ra reach lows of 0.2μ there was no statistically sound relation between fatigue performance and an improved surface roughness. Upon further investigation of the specimens, it was discovered through CT scans, that their porosity was in fact much closer to that of non HIP treated specimens. This master thesis illustrates once more that the effect of surface roughness on SLM Ti-6AL-4V alloys cannot be properly identified if the bulk material shows internal defects.

The autonomous filling of creep-induced grain-boundary cavities by tungsten-rich precipitates has been studied for Fe-W alloys at various temperatures, loading conditions and creep rupture times. To this aim, scanning electron microscopy, energy-dispersive spectroscopy and X-ray nano-tomography were used. Additionally, the effect of local plastic deformation, followed by ageing processes, on the surface precipitation in these alloys has been studied by means of scanning electron microscopy, together with energy-dispersive spectroscopy. Finally, a diffusion-based finite element analysis has been performed to study cavity filling for different combination of diffusion mechanisms. From scanning electron micrographs it can be confirmed, that Fe-W alloys show damage-selective precipitation of the Laves phase (Fe2W). Furthermore, the filling ratios of individual cavities are determined and it is found, that the autonomous healing strongly depends on the duration of the loading at of elevated temperature. From the quantitative analysis of the size and shape of the precipitates and cavities it is found, that the orientation of grain boundaries to the stress direction has an influence on the morphology of the induced-cavities, as well as, the filling ratio. The grain-boundary cavities orientated favourably for the cavity growth show low filling ratios, due to the fact, that their propagation proceeds with a higher rate than the solute transport. The critical volume of a cavity, that can be fully filled with solute atoms, is found to be of the order of 10 μm3, while the average filling ratio for Fe-W alloys is found to be around 50% for the sample loaded to 100% of its lifetime. Moreover, the scanning electron micrographs of the indentation-deformed samples subjected to ageing revealed surface precipitates (Fe2W), which existence has to be further researched. The hardness measurements of Fe-W samples showed that after 200 hours of ageing at 580oC, the strength of the alloy is only reduced if the material has been pre-strained, but it remains constant for the solution heat treated specimens. This result shows, that the process of dislocation recovery dominates over work-hardening only in the case of a high dislocation density. Finally, the finite element analysis revealed, that during the diffusive cavity filling of an equilibrium-shaped void, three different diffusion profile regimes are plausible. Namely, a diffusive filling dominated by volume diffusion, which occurs for a relatively small spacing between neighbouring cavities and is independent of the ratio of bulk and grain-boundary diffusion. Secondly, a diffusive filling governed by volume diffusion with contributing grain-boundary diffusion, which occurs for relatively low ratios of bulk and grain-boundary diffusion. And ultimately, a diffusive filling governed by grain-boundary diffusion, which occurs for relatively high ratios of bulk and grain-boundary diffusion.