Thickness-dependent dielectric evolution and vertically porous engineering of flame-sprayed WPU/MWCNTs/CS microwave-absorbing coatings

Thick microwave-absorbing coatings are essential for electromagnetic stealth and protection applications. However, during thermal spraying processes, the microstructural evolution induced by increasing coating thickness and cumulative thermal exposure significantly alter the dielectric response and absorption performance – a phenomenon that has not been systematically clarified. This study systematically examines the thickness-dependent dielectric behavior of TPU/MWCNTs coatings fabricated via flame spraying, revealing that absorption performance deteriorates sharply beyond a critical thickness of 50 μm. Specifically, as the coating thickness increases from 50 μm to 138 μm, the minimum reflection loss ( RL min ) decreases from −24.8 dB to −14.1 dB, and the effective absorption bandwidth ( EAB ) narrows from 5.2 GHz to 1.0 GHz. To address this issue, an innovative porous structure engineering strategy is proposed, wherein WPU replaces TPU and a water/ethanol co-solvent is employed to promote the formation of vertical pores within the coating. This approach significantly enhances the connectivity of conductive fillers along the thickness direction, effectively balancing polarization loss and conductive loss while optimizing both impedance matching and attenuation. The optimized porous coating achieves an exceptional RL min of −50 dB, an EAB of 5.2 GHz, and a maximum RCS of 29.8 dBm 2 . This structural design strategy not only mitigates the performance degradation in thick coatings but also provides a viable route for fabricating lightweight, broadband, and highly efficient microwave-absorbing coating.