Enhancing Mechanical Performance of Microwave Absorbing Coatings by Controlled Fluorosilane Modification of Absorbents

Microwave absorbing coatings must simultaneously achieve strong electromagnetic performance and robust mechanical durability. While high absorbent loading enhances microwave absorption, it typically compromises flexibility and adhesion, and existing approaches generally improve only one of these mechanical properties at the expense of the other. Herein, we present a strategy that concurrently enhances flexibility and preserves adhesion strength through controlled surface modification of FeSiAl absorbents with triethoxy-1<i>H</i>,1<i>H</i>,2<i>H</i>,2<i>H</i>-tridecafluoro-<i>n</i>-octylsilane (FOTS) at an optimized dosage of 2 wt %. The grafted FOTS layer lowers particle surface energy, reduces interfacial friction, and promotes uniform dispersion in the epoxy matrix, enabling the coating to pass a 25 mm mandrel bend test, significantly outperforming unmodified coatings, which crack even on a 70 mm mandrel. Crucially, residual hydroxyl groups on the modified particle surface maintain chemical compatibility with the resin, sustaining a high adhesion strength of 5.32 MPa, comparable to that of the unmodified reference (5.30 MPa). The improved dispersion also enhances electromagnetic homogeneity and effective filler utilization, leading to better impedance matching and a slight increase in magnetic permeability. As a result, the optimized coating achieves a reflection loss of -18.8 dB at an exceptionally low microwave frequency of 0.15 GHz (sub-1 GHz range). This work overcomes the long-standing trade-off between flexibility and adhesion in high-loading absorber systems by redefining the filler-matrix interface not as a rigid bond but as a tunable, multifunctional zone. The approach offers a scalable pathway toward durable, high-performance microwave absorbing coatings for demanding applications in radar stealth, flexible electronics, and electromagnetic compatibility.