UNIVERSITÀ DEGLI STUDI DI ROMA “ROMATRE” DIPARTIMENTO DI INGEGNERIA Mechanical and Optical Behaviour of Coated and Non-coated Al-1%Si Tethers for the Electric Solar Wind Sail Technology PhD student: Giuditta Montesanti Tutor: Prof. Edoardo Bemporad Doctorate Dissertation 2017 1 2 1 INDEX Abstract .................................................................................................................................................................... 5 Acknowledgements ................................................................................................................................................ 6 List of figures........................................................................................................................................................... 7 List of tables .......................................................................................................................................................... 11 1. Introduction .................................................................................................................................................. 12 2. The ESAIL technology – state of the art .................................................................................................. 14 2.1. Summary ................................................................................................................................................ 14 2.2. Concept ................................................................................................................................................. 14 2.3. Theory .................................................................................................................................................... 16 2.4. E-sail system ......................................................................................................................................... 21 2.5. Possible space missions ....................................................................................................................... 23 2.6. Current E-sail projects ........................................................................................................................ 25 3. Motivations .................................................................................................................................................... 27 4. Requirements for the tether material ......................................................................................................... 29 4.1. Summary ................................................................................................................................................ 29 4.2. Main tether requirements .................................................................................................................... 29 4.3. Tensile strength .................................................................................................................................... 30 4.4. Thermal equilibrium and the α/ε parameter .................................................................................... 33 4.5. Tether material options ....................................................................................................................... 38 4.6. Tether coatings ..................................................................................................................................... 43 4.6.1. Coating requirements .................................................................................................................. 43 4.6.2. Previous studies on tether coatings ........................................................................................... 46 4.6.3. E-sail coating production constraints ....................................................................................... 47 5. E-sail tether characteristics .......................................................................................................................... 49 5.1. Summary ................................................................................................................................................ 49 5.2. Al-1%Si wire selected in the ESAIL project .................................................................................... 49 5.3. State of the art ...................................................................................................................................... 51 5.3.1. Al-Si systems ................................................................................................................................. 51 5.3.2. Al-1%Si system ............................................................................................................................ 55 5.4. Bare tether: microstructural and resistivity analysis ........................................................................ 61 5.4.1. Microstructural properties .......................................................................................................... 61 5.4.2. Resistivity tests of tether wire .................................................................................................... 71 5.5. Tether coating activities ...................................................................................................................... 73 5.5.1. ESAIL project coating ................................................................................................................ 73 3 5.5.2. Coatings produced in this work ................................................................................................. 77 5.5.3. Microstructural properties of the coatings ............................................................................... 79 6. Experimental activity: mechanical tests on the bare tether .................................................................... 82 6.1. Summary ................................................................................................................................................ 82 6.2. Introduction .......................................................................................................................................... 82 6.3. Mechanical tests set-up........................................................................................................................ 82 6.3.1. Determination of the initial length of the sample ................................................................... 88 6.3.2. Verification of sliding for the tests at high temperature ........................................................ 93 6.4. Test parameters .................................................................................................................................... 93 6.5. Results .................................................................................................................................................... 94 6.5.1. Tensile tests to failure ................................................................................................................. 94 6.5.2. Load/unload test ......................................................................................................................... 99 6.5.3. Creep test .................................................................................................................................... 100 6.5.4. Strain rate sensitivity test .......................................................................................................... 103 6.5.5. Dynamic test ............................................................................................................................... 105 6.6. Effect of testing on the microstructure (TEM images) ................................................................ 110 7. Experimental activity: optical measurements of the tether’s surface .................................................. 115 7.1. Summary .............................................................................................................................................. 115 7.2. Absorbance ......................................................................................................................................... 115 7.3. Emissivity ............................................................................................................................................ 117 7.4. α/ε ratio ............................................................................................................................................... 120 8. Discussion ................................................................................................................................................... 123 9. Conclusions ................................................................................................................................................. 130 List of publications ............................................................................................................................................. 132 Bibliography ......................................................................................................................................................... 133 Appendix A – Coatings details ......................................................................................................................... 144 Appendix B – Tensile test data ........................................................................................................................