Chopped Basalt Fiber Reinforced Silicone‐Phenolic Composite With Superior Specific Modulus and Thermal Stability via Hollow Glass Microsphere‐Based Density Control

ABSTRACT Phenolic resin (PR) is extensively employed in aerospace thermal protection systems owing to its excellent flame retardancy, high char yield, and low cost. To enhance its modulus and thermal stability while simultaneously reducing density, this study fabricated an isotropic composite utilizing silicone‐modified PR as the matrix, co‐reinforced with hollow glass microspheres (HGMs) and chopped basalt fibers (BFs). The incorporation of BFs significantly increased the specific modulus and thermal stability of the composite. Concurrently, HGMs effectively reduced the composite's density and further enhanced both its specific modulus and thermal stability. The optimized composite achieved a density of 1.07 g·cm ⁻³ , a specific modulus of 3.46 MN·m·kg ⁻¹ , and a residual mass of 82.9% at 800°C, corresponding to changes of −10.8%, +239.2% and +33.5%, respectively, compared to the neat resin. The co‐reinforced composite demonstrates significant potential for application in lightweight thermal protection systems.