PI-encapsulated spherical filler structures enhance microwave absorption performances of the flame-sprayed BaFe12O19/CS/PI coatings

Developing efficient microwave-absorbing coatings via thermal spraying is usually hindered by the thermal demagnetization and oxidation of fillers. Herein, a low-temperature flame spraying strategy is proposed to fabricate Nd-doped BaFe 12 O 19 /nano-carbon spheres (CS)/polyimide (PI) coatings. By systematically controlling the feed rate of the starting feedstock, we precisely modulate filler dispersion and coating microstructure evolution, thereby inducing significant transitions in the dominant microwave attenuation mechanisms. At high feed rates, limited CS addition promotes localized surface plasmon resonance (LSPR) and metallization effects within the surface-dense layer, while excessive CS loading induces an overconnected network and prominent skin effect, impairing microwave absorption (MA) in both cases. Conversely, under low feed rates, favorable thermodynamic and kinetic conditions enable the formation of PI-encapsulated spherical filler structures in the surface region. This unique structure effectively suppresses LSPR and the skin effect, enhances polarization losses, prevents excessive conductivity rise (mitigating impedance mismatch), and thus optimizes over MA. Consequently, the high-feed-rate coating delivers a minimum reflection loss ( RL min ) of−39.1 dB and an effective absorption bandwidth ( EAB ) of 3.7 GHz at a thickness of 4.3 mm. In contrast, the coating with PI-encapsulated spherical filler structures achieves a better RL min of −52.3  dB and a wider EAB of 6.1  GHz at 2.1  mm. Furthermore, the coatings demonstrate outstanding adhesion, achieving Class 0 in the cross-cut testing. Scratch testing demonstrates excellent cohesion and interfacial robustness without catastrophic delamination. The results would give critical mechanistic insight into establishing scalable technical route for engineering flame-sprayed microwave absorbers with simultaneously enhanced structure-function integration and long-lasting mechanical performance.