Space materials characterization and development of standoff bonding process

Abstract

In this thesis, chemical and physical characterization of polymeric and composite materials suitable for space applications is presented. In addition, the effect of several critical parameters in standoff bonding is assessed and analyzed. Space materials and processes must withstand harsh environmental conditions such as extreme thermal cycling, radiation effects, plasma, debris and more. In particular, large temperature gradients can impact how structures and related materials behave due to changes in properties such as the glass transition temperature, the coefficient of thermal expansion, the storage and loss moduli etc. These properties must be considered when designing a spacecraft since unexpected values can cause the deformation or even breakage of materials, undermining their structural stability. However, the data provided by the manufacturer is often not precise or complete enough, so dedicated testing is needed. To investigate the properties of some of these materials, thermal analysis was performed. Nano-enabled materials were included in order to evaluate the effect of added fillers on the basic properties. Results show that a trade-off must be considered between such properties and the benefit of having additional functionalities such as thermal and electrical conductivity. To assess the effects of critical parameters in standoff bonding process, tensile tests were performed for pristine and thermally cycled samples. For the latter, the aim was to simulate the approximate temperature cycles a spacecraft will encounter between -100°C and 90°C sustained during several orbits in Low Earth Orbit (LEO). Despite the fact that standoff bonding is an integral part of the thermal management system of space missions – by ensuring that the Multi-Layer Insulation (MLI) blankets stay in place – there is a lack of dedicated literature and the bonding is often poorly designed. This results in failures during qualification phases as the standoffs de-bond, causing long and costly delays that could have been avoided if more data was available. The aim of this standoff bonding assessment is to study how different adhesives, surface treatments and bondline affect the resulting bonding strength. The adhesives considered were epoxy, graphene nano-enabled epoxy and polyurethane; the surface treatments under study were no treatment, isopropyl alcohol (IPA) cleaning and fiber glass pen sanding and the bondline thicknesses selected were 120 μm, 250 μm, 500 μm. Results confirm the expected higher strength of epoxies vs polyurethane. Further, thermal cycling tended to improve the bonding strength of most epoxy and nano-enabled epoxy samples, while polyurethane seemed to not be so affected by temperature excursions.