Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Development of nanocomposite coatings for spacecraft components

Abstract Space is known as a very aggressive and hostile environment and its degradative effects on spacecraft materials can compromise the success of the entire mission. A few types of space weather impacts on spacecrafts have been detected and the main ones are for example: space-plasma that causes surface charging on the structures with consequently biasing of instrument and physical damages; microparticles and space debris which can cause abrasions on surfaces and structural damages; UV radiation, galactic cosmic rays and solar particle events that lead to a thermal, electrical, optical and structural integrity degradation of materials and many damages in the electronic components; large gradients of temperature when, for example, space vehicles are illuminated or not by sunlight lead to a degradation of polymeric materials as well as the presence of atomic oxygen, especially in LEO orbit, that corrodes the exposed polymeric material surfaces. It is therefore necessary to provide and find solution to protect and properly isolate the spacecraft components from the external space environment. Multifunctional coatings could play a key-role in protecting and safeguarding the various spacecraft components, of different nature, from the surrounding environment. For example, it’s well known that the non-linear nature of the MLI details, such as its shape, finishing details, grounding hardware and perforation patterns, as well as the nonlinear response of metallic contact points, contribute to passive intermodulation products. For these reasons, ESA has recently funded research projects (Artes 5.1 and 5.2) to find a solution for this problem. One of the most promising concepts is the use of frequency selective surfaces as layers of the MLI. This is accomplished by using nanostructured carbon-based films on membranes, which can also improve the protection of the payload by the MLI (and in general by flexible membranes) with respect to space radiations and impact events. Multifunctional films containing carbon nanoparticles are currently investigated also to create sensors for monitoring the radiation absorbed by astronauts during extra vehicular activities (EVA) or as new and advanced grounding systems to mitigate plasma-induced spacecrafts charging. Further novel space-based applications of carbon-based multifunctional films are future membrane reflector spacecraft. These ones are a promising key technology to deliver cost-effective space-based applications such as communication antennae, optical telescopes and solar energy collectors. The main objectives of my Ph.D. research are to design and realize carbon-based nanocomposite coatings with tailored multifunctional properties for spacecraft components. The research was focused on the development and design of nanocomposite films containing carbon nanoparticles, namely carbon nanotubes and graphene nanoplatelets, which have unique multifunctional properties. The selected carbon nanoparticles were added to different types of aerospace-grade polymer matrices at several concentrations. Nanocomposite films will be fabricated on different types of substrates like Mylar sheets, carbon-fibers reinforced polymer (CFRP) composite laminates or metal substrates that are widely used in spacecraft sub-systems. Such multifunctional coatings were fabricated on one side with the aim to protect and safeguard the various spacecraft components, of different nature, from the surrounding hostile space environment, especially from ultraviolet radiations in C band (UV-C) and electrically charged particles coming from space-plasma, and on the other side to achieve thermal performance of the unit increased over time, by increasing the heat transfer coefficients, resulting in an ideal design of heat exchangers widely used in the aerospace field to control, for example, the temperature of on-board electronic components. The development of polymeric nanocomposites films with tailored excellent and multifunctional mechanical, electrical, thermal and hydrophobic properties depends on the properties and geometrical features of the nanofillers, CNTs or graphene, their grade of dispersion within the polymeric matrix as nano-reinforcements, their interaction with the polymeric matrix and the alignment of the nanofillers in the matrix as well as the fabrication process of films. For these reasons, the definition of the carbon nanoparticles, their concentration and dispersion process in the polymer matrices, the investigation and optimization of different deposition techniques such as spin-coating, spray-coating, drop-casting and bar-coating, were important preliminary steps of my PhD research, in order to optimize the wanted multifunctional properties of the film. This Ph.D. research can be summarized in 5 research activities. The first one concerns the analysis and the investigation of the effects of space-abundant UV-C radiation on the surface (hydrophobic and electrical) properties of carbon-based nanocomposite films deposited on flexible Mylar substrates by spin-coating process. The second one is about the study of the spray-coating deposition process and the investigation of the role of its working parameters in setting the morphology and the electrical performance of MWCNT-based nanocomposite films. The spray-coating technique, as opposed to the previous used spin-coating one, allows to fabricate larger polymeric coatings in terms of covered area on planar and not substrates, becoming an attractive method for large scale production of coatings. In particular, the fabrication of uniformly electrically-conductive coatings on Mylar substrate can allow the mitigation of electromagnetic interferences and plasma-induced spacecraft charging, and so this aspect it was assessed. The third study is the design of nanocomposite coatings by bar-coating process on carbon/fibers reinforced polymer (CFRP) composite structures for electrostatic charge (ESC) build-up mitigation for spacecraft and with the aim to realize new and innovative advanced grounding systems for charging mitigation as well. The fourth research activity concerns the application of the electrical resistance tomography (ERT) technique to the detection of surface conductivity changes induced by UV exposure. This technique in combination with the fabricated UV-sensitive coatings containing DNA-functionalized graphene can provide a health monitoring method for composite materials and structures that are exposed to damaging levels of UV radiation. At last, the main idea of the fifth study is to fabricate hydrophobic and, in the same time, thermal conductive efficient nanocomposite coatings applied on aluminum substrate that can reduce the filmwise condensation encouraging the dropwise one leading to increased thermal performances resulting in an ideal design of heat exchangers, used for example in the aerospace sector to control the temperature of on-board electronic components.