Sustainable Surface Functionalization of Lightweight Materials : Cerium Oxide Nanoparticles Replacing Chromium in Anodic Coatings and Carbon Nanomaterials for Lightning Strike Protection

Abstract: Aviation accounts for 2-3% of the carbon dioxide emitted globally. One way to reduce emissions is to develop and introduce sustainable, functional, lightweight materials and coatings that increase the lifetime and fuel efficiency of aircraft. The main lightweight materials used in the aerospace industry today are aluminum alloys and carbon fiber reinforced plastic composites. In the work presented in this licentiate thesis, a new sustainable alternative for the replacement of toxic hexavalent chromium in a low energy and chemical consumption sealing procedure of anodized aluminum alloys is suggested (paper I and II). An alternative to the conventional metal mesh used as lightning strike protection for composite structures used today is also presented. The proposed solution adds considerably less weight and has a possibility to reduce the CO2 emission from aviation (paper III).   Aluminum alloys as well as composites both exhibit high strength-to-weight ratios but come with individual drawbacks. Fiber reinforced plastics exhibit limited electrical conductivity, which is why additional protection is needed to avoid severe damage following a lightning strike. Aluminum alloys have instead the disadvantage of being susceptible to corrosion and surface protection is required to prolong the materials lifetime and to avoid devastating failures. Anodization, formation of a porous aluminum oxide coating, is the most common choice of surface treatment. This is often followed by closure of the pores through a sealing procedure. Both processes have up until recently been performed in large, energy consuming tanks with highly toxic solutions containing hexavalent chromium which must be replaced to reduce the environmental impact.  In paper I, the environmentally friendly tartaric sulfuric acid has been used as anodization electrolyte and cerium oxide nanoparticles have been investigated as a promising alternative for sealing. Cerium-based and hydrothermal sealing (immersion in hot water), individually and combined, were investigated. The morphological and chemical composition were studied by means of scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The investigation confirmed the growth of cerium oxide nanoparticles throughout the coating and closing of the pores by hydrothermal sealing. A corrosion immersion test revealed a superior corrosion resistance of surfaces treated with the two-step sealing process compared to plain anodized, cerium or hydrothermally sealed surfaces.  In paper II, the potential use of an aerosol-based wet thin film coating technique called nFOG for cerium sealing as a low chemical and energy consumption alternative to traditional bath-type sealing was investigated. Characterizations of the morphology and composition reveal cerium oxide nanoparticles evenly distributed within the porous coating by the nFOG technique. The new application of the nFOG method was also shown to provide anti-corrosion properties comparable to bath-type sealing. This wet coating technique has the potential to replace chromium and reduce the environmental impact in the treatment process.  Furthermore, the limited electrical conductivity of carbon fiber reinforced plastics can be circumvented by loading the polymer matrix of the composite structure (commonly epoxy) with highly conductive carbon nanomaterials. In paper III, graphene nanoplatelets and carbon nanotubes were loaded into the epoxy. Simulated lightning strike tests showed an improved damage tolerance for the loaded composites compared to composites prepared with plain epoxy. The results suggest that a combination of graphene nanoplatelets and carbon nanotubes increases the damage tolerance by carrying the resulting high electric current from a lightning strike.   In conclusion, the application of cerium oxide nanoparticles and carbon nanomaterials moves the aerospace industry towards a sustainable fuel efficiency using functional, lightweight materials and coatings.