Metallization of Carbon Fibre Reinforced Composites by Lay-up on Cold Sprayed Coatings and by GRIP Metal™ as Bond Coat for Cold Spray

Abstract

The aerospace industry improves safety, reliability and performance of aircrafts through greater incorporation of advanced materials. Carbon fibre reinforced composites (CFRCs), being the most prevalent materials among all, gradually replace many components originally made from aluminium. CFRCs gained their popularity thanks to their high strength-to-weight ratios. Although being strong and light, these materials exhibit vulnerabilities in terms of erosion and limited electrical conductivity. In-flight aircrafts are often subjected to foreign objects such as debris and hail, and to lighting strikes. Damage to critical CFRC components can both cause danger to passengers and put aircraft temporarily out of service. These issues are currently solved by costly methods such as gluing metallic sheets for erosion protection, and embedding copper mesh for lighting strike in relevant areas of the CFRCs. The purpose of this research was to investigate two potential alternative methods for metallizing, thus protecting CFRC against erosion and lightning strikes. The first approach features the use of a lay-up technique referred to as “Reverse Lay-up Method”. The objectives of this approach were, first, to develop a reliable demoulding process, then a dense and crack-free coating on CFRCs. The second approach, referred to as the “GRIP Metal™ as Bond Coat Method”, features the use of GRIP Metal™ technology as bond coat for cold spray. The objectives of this approach were to characterize the GRIP Metal™ products and to assess their integration into CFRCs. In the Reverse Lay-up Method, a simple and reliable demoulding process was developed whereby an extra length of CFRC, protruding one edge of the mould, was added to the composite component’s original dimensions. The protrusion served as a cantilever for facilitating the demoulding process. In term of microstructure, the results revealed that for given surface preparation and spray parameters, copper powder of dendritic morphology could not produce satisfying results due to the presence of microcracks in both as-sprayed and demoulded coatings. Desired coatings were achieved using copper powder of spherical morphology; the as-sprayed coatings were dense and crack-free, and remained undamaged when demoulded along with CFRC. In GRIP Metal™ as Bond Coat Method, the products were found to be inconsistent in hook height and curvature for some configurations. In term of general consolidation of metal-CFRC, it was found that the hooks of GRIP Metal™ induced fibre distortion leading to the formation of resin-rich zones and porosities. The occurrence, size and concentration of the defects were unpredictable. Samples from both metallization techniques were tested and compared in term of metal-CFRC adhesion strength. The results indicate that some samples of GRIP Metal™ as Bond Coat Method performed better than samples of Reverse Lay-up Method. Overall, GRIP Metal™ as Bond Coat Method offers higher metal-CFRC adhesion with trade-off in CFRC consolidation, whereas Reverse Lay-up Method offers lower metal-CFRC adhesion but intact consolidation quality.