Sol-gel layers and electroless deposition coatings for lightning strike protection of CFRP composites

Abstract

With the aim of mitigating the disadvantages of the current metal meshes and foils, two different metallization strategies have been explored to study their potential as lightning strike protection (LSP) of carbon fiber reinforced polymer (CFRP) composites. In the first one, a sol-gel coating system was designed and later filled with self-synthesized copper nanowires (CuNW) with the aim of producing percolated networks of conductive particles. An ethanol diluted glycidylpropyl trimethoxysylane (GPTMS) and tetramethyl orthosilicate (TMOS) combination was found as the most indicated sol-gel system for creating homogeneous and adherent coatings. Extremely high aspect ratio CuNW were synthesized from very few chemicals in aqueous medium and were later incorporated to sol-gel. The formation of CuNW bundles in the mixture revealed problems of compatibility of the particles with the sol-gel. The bad interaction between CuNW and GPTMS was improved by switching the dispersion medium from water to ethanol. During application, the CFRP was spray-coated producing adherent layers with a certain degree of conductivity, but significantly lower than the commercial protections. However, the CuNW-filled sol-gel approach presented a good improvement potential since several issues were found to be limiting its performance. Secondly, electroless deposition of copper was performed on the CFRP laboratory samples producing continuous and highly conductive 1 to 2 μm thick layers. The layer growth, the morphology of the samples and other physical characteristics were studied. After, the development of the strategy led to the production of bigger samples that were used to study their performance under emulated lightning strikes. The samples were subjected to 40 kA A-component discharges presenting significantly less damage than the unprotected composite, as the ultrasonic inspection revealed. An approximation exercise led to think that a future 7 μm thick electroless coatings could withstand the strike without critical damage. In addition, interesting relationship between the charge transfer capabilities of the coatings and the damage was found. During the C-component emulation, a continuous current of 30 A was injected superficially for 1 s which produced leakage of currents towards the fibers that heated up rapidly. The potential of electroless copper as LSP was demonstrated and future works are foreseen for improving the performance of the layers.