In order to improve the composition distribution and flame retardancy of composites, the effects of silicon dioxide (SiO2) coatings with different contents on physical properties of lead (Pb) powders and composites were investigated in this research. SiO2 coated Pb powders (SiO2@Pb) with contents of 0, 0.237, 0.486, 0.683, 0.967 wt% were synthesized by a sol-gel method, then mixed with boron carbide (B4C) powders and boron phenolic resins (BPRs) to prepare SiO2@Pb/B4C/BPRs composites by molding. SiO2 coating on the surface of Pb powders in flakes or islands increases the specific surface area and oxidation temperature of the SiO2@Pb powders. For the SiO2@Pb/B4C/BPRs composites, the composition uniformity of composites is improved due to the reduction of the true density difference value between fillers (Pb, B4C), which is beneficial for the physical properties of the composites. Furthermore, the mechanical properties and thermal conductivity increase with the addition of SiO2 content, achieving a maximum value at 0.237 wt%, and then decrease gradually with a further increase of SiO2 content. Moreover, SiO2 coatings improve the limit oxygen index (LOI) of the composites and reduce the cracks of composites after burning. Composites with the SiO2 content of 0.486 wt% have optimal comprehensive physical properties, where the tensile strength, bending strength, impact toughness are 42.5 MPa, 72.4 MPa, 6.5 kJ m-2, respectively and the LOI is 41.8%.
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