A green PI-SiO₂ aqueous sizing coating for tailoring the interphase of quartz fiber/PEEK composites toward enhanced mechanical performance and electromagnetic transparency

Quartz fiber (QF)/polyether ether ketone (PEEK) composites are promising for high-temperature wave-transparent applications. However, their performance is severely limited by weak interfacial bonding and interfacial polarization. In this work, a green and thermally stable polyimide-silica (PI-SiO₂) aqueous sizing coating was developed to tailor the fiber/matrix interphase. The PI component enhances interfacial compatibility with the PEEK matrix through molecular chain entanglement, while silane-modified SiO₂ nanoparticles introduce reactive functional groups and nanoscale roughness, enabling interface anchoring and mechanical interlocking. As a result, a dense and gradient interphase with reduced defects is constructed. At an optimal composition (1 wt% PI and 1.5 wt% SiO₂), the interlaminar shear strength, flexural strength, and impact strength increased by 29.72%, 43.27%, and 67.60%, respectively, along with a 21.36% reduction in wear rate. The thermal decomposition temperature increases by 6.54%, accompanied by a 10.03% increase in char yield at 800 °C. More importantly, the dielectric constant is reduced to ∼3.0 with a dielectric loss below 0.02, resulting in enhanced electromagnetic wave transmission in the 2–18 GHz range. This improvement is attributed to the suppression of Maxwell-Wagner-Sillars polarization and defect-induced scattering due to the engineered interphase. This work demonstrates an environmentally friendly and scalable sizing coating strategy for simultaneously improving the mechanical, thermal, tribological, and electromagnetic properties of high-performance thermoplastic composites, providing new insights into interphase design for multifunctional coatings and advanced structural materials.