Advanced Multifunctional Metamaterial Coatings for Corrosion Protection and Color Coding

For aerospace applications light weight and corrosion protection of metal surfaces is essential. However, color coding for easy identification of parts and safety must be also implemented, while keeping in mind the progress towards sustainability through advanced materials selection for lightweighting and removal of hazardous chemicals. In this context advanced coatings for structural colors are on the rise because they provide a suitable technology that aims to replace the traditionally used dyes and pigments that are contaminant, age and are easily degraded by corrosive environments. Structural coloring shows remarkable advantages since it is generated by building photonic nanostructures with minimal foot-print, light-weight, non-contaminating elements and virtually no ageing, moreover these coatings include protection against corrosion due to the inherent use of dielectrics. However, to achieve high performance most of the structural color approaches include nanolithography, or thick dielectric layers, and finally all of them include the deposition of an optically thick metallic back mirror as a first step, usually made from noble metals, that makes industrial implementation for everyday objects, especially large ones, costly and highly impractical.

In this work, we propose a minimalist original approach, based on integrating the usual industrial metal surfaces of as an active part of the photonic color structure using a thin film metamaterial dielectric-based structure. In this context we demonstrate successful structural coloring and corrosion protection on stainless-steel, in substitution of the usual noble metal (Au, Ag) back mirror. Inspired by traditional Fabry-Perot cavities, we have designed and fabricated multifunctional thin film coatings formed by a 3-layer metamaterial structure (dielectric/semimetal/dielectric) that are directly deposited by a plasma -based technique on the metal surface and enables its coloring and protective functions [1]. For the dielectric layer we have chosen aluminum oxide (Al₂O₃) due to its high thermal stability and resistance to corrosion in a wide pH range (4 to 9). As a proof of concept, we have designed and fabricated these coatings on aluminum and stainless-steel. As the central part of the structure we have selected ultrathin layers (10 nm) of the semimetal bismuth (Bi) because recently we have demonstrated that is a sustainable material that is able to generate vivid, well defined structural colors with excellent angular robustness, while keeping the total thickness below 250 nm [2,3]. We will show the excellent performance of these multifunctonal metamaterial coatings, and we will illustrate the underlying physics behind this approach by providing simulations of the reflectance of these novel structures. This ingenious and effective approach eliminates the need of the back-mirror, reduces the fabrication steps and simplifies the overall fabrication process, critical towards industrial aerospace applications.

[1] A. Caño et al. Appl. Surf. Sci. 640, (2023) 158236.
[2] S. Adhikari, et al. Small 19 (2023) 202206003.
[3] F. Chacon-Sanchez et al, Adv Opt Mater 12 (2024) 2302130.