Fig. 1. Surface morphology (a) and particle size distribution (b) of HMS.

Fig. 2. Schematic diagram of the preparation process of the epoxy-based coating.

Fig. 3. FTIR spectra of fully cured epoxy resin (a), HMS (b), and epoxy-based coating (c) with 31 wt.% HMS.

Fig. 4. Hardness of coatings with different content HMS (a) Vickers hardness, (b) Rockwell hardness (HR15Y).

Fig. 5. Elastic modulus (a) and bonding strength (b) of coatings with different content HMS.

Fig. 6. Cross-sectional SEM images of the samples with different content of HMS (a) 7.5 wt.%, (b) 16 wt.%, (c) 31 wt.% and (d) porosity of coatings.

Fig. 7. Density of coatings with different content HMS.

Fig. 8. Vickers hardness of coatings as a function of porosity.

Fig. 9. Rockwell hardness of coatings as a function of porosity.

Fig. 10. Bonding strength of coatings as a function of porosity.

Fig. 11. Tensile fracture surface SEM images of the samples with different content of HMS (a) 7.5 wt.%, (b) 16 wt.%, (c) 31 wt.%, and (d) fracture surface porosity of coatings.

Fig. 12. Tensile fracture surface of the coating with 7.5 wt.% HMS: (a) SEM enlargement image and (b) interface between the spherical shell and epoxy-resin matrix.

Fig. 13. Particle diameter differential distribution (a) and particle diameter cumulative distribution (b) of 16 wt.% HMS-epoxy sample polish section and fracture section.

Fig. 14. Schematic diagram of (a) original coating, (b) polish cross-sectional, and (c) tensile fracture surface.

Fig. 15. Performances comparison between epoxy-based coatings and aluminum-based coatings. (a) adhesive strength and (b) lightweight.

Fig. 16. Radar map of comprehensive performance comparison between epoxy-based coatings and aluminum-based coatings.