Metalization of polymer-based composite structures by cold spray
Creators: name:  F. DELLORO 
name:   A. CHEBBI 
name:   H. PERRIN 
name:   G. EZO'O 
name:   A. BASTIEN 
name:   J. ASCANI 
name:   A. TAZIBT 
Publisher: name: Proceedings of  the 9th European Conference for Aerospace Sciences. Lille, France, 27 June - 1 July, 2022
Year: 2022
Subjects: STRUCTURES
MATERIALS
POLYMER
COMPOSITE

description: Polymer-based composite structures are extensively used in aero-space manufacturing due to their exceptional ability to combine high mechanical properties with low mass densities. Nevertheless, some critical issues need to be solved for their even broader diffusion. Among these, electrical and thermal conductivities are key for several applications, in space environment as well as in atmospheric flight. A possible solution is to add a conductive layer on the skin of these materials. The cold spray process, capable of deposing relatively thin and conductive coatings on a variety of materials in complete automation, is one of the candidates to produce such conductive coatings. This process uses powders as a feed-stock material and a gaz (typically nitrogen or helium) to accelerate them to supersonic speeds. As its name suggests, in cold spray temperature is relatively low, especially the one seen by the substrate, making it possible to be applied even to thermosensitive materials. Its application to polymer-based composite structures, with a particular focus on lightning strike protection [1] for middle range airplanes, is the aim of the CO3 project (EU Grant agreement: ID831979). Lightning strikes can be very dangerous for polymer-based composite structures, producing serious physical damage effects such as erosion, ablation or even the entire explosion of the structure. Actually, the most widespread solution for lightning strike protection is the application of metallic meshes, available in different materials and with different mass per unit surface to adapt to the different zones. In many parts of the airplane, metallic meshes are still manually laid up. Since composite manufacturing is following the road to full automation, especially for cost reduction, the metalization should follow as well. The present study reports recent results obtained in the framework of the CO3 project. The most critical point, concerning cold spray on polymer based materials, is to assure particle adhesion and coating build-up without damaging the substrate. A narrow window for viable particle velocity and temperature seems to exist, as a function of substrate and powder compositions, as shown by previous works [2-7]. Cold spray is sensitive to the substrate material chosen. In this study different types of substrate were tested: epoxy-based long carbon fiber composites, PEEK matrix with short or long carbon fibers, pure PEEK and pure PA. Different cold spray systems exists and are mainly divided in two families: high pressure and low pressure ones. Both solutions were tried. Several innovative strategies to improve coating adhesion and build-up were tried. i) Co-curing of a protective polymer film before high pressure cold spraying, helping particle implantation and, consequently, coating build-up. ii) Low pressure cold spraying of metal-polymer mixed powders, promoting coating adhesion and preventing fiber damage. iii) Supercritical Nitrogen Deposition technology, as a surface preparation technique capable of mechanically anchor metallic particles into the polymer before high or low pressure cold spraying. Copper coatings by high pressure cold spray, after the application of a protective film, seemed the most promising solution for thermoset based composites. For thermoplastic-based materials, low pressure cold spray of Al-PEEK mixed powders without any additional treatment was retained as the most interesting solution explored up to now. Adhesion strength and electrical conductivity were measured for all produced coatings. The most promising coatings underwent lightening strike tests in harsh conditions and performed better that the reference solution based on a metallic mesh.