Development of silicon carbide fiber-reinforced silicon oxycarbide composites for low-observable unmanned aerial vehicle exhaust nozzles via filament winding, and polymer infiltration and pyrolysis

Unmanned combat aerial vehicles require lightweight, stealth-capable exhaust systems. However, traditional metallic nozzles increase radar detectability and reduce range, while advanced composites offer high performance but are expensive. Therefore, to improve the operational range and survivability of unmanned combat aerial vehicles, a lightweight, high-temperature-resistant, oxidation-resistant, and low-observable composite exhaust nozzle is developed to replace conventional metallic straight-type nozzles. The nozzle features a double serpentine shape to reduce radar and infrared signatures and is manufactured as a monolithic structure using the filament winding process, accommodating the complex geometry and large size (length: 1.8 m, width: 0.8 m). The exhaust nozzle consists of a ceramic matrix composite made of silicon carbide fibers and a silicon oxycarbide matrix, which absorbs and scatters radio frequency signals while withstanding prolonged exposure to high-temperature (700 °C) oxidizing environments typical of engine exhaust gases. The polysiloxane resin used to produce the silicon oxycarbide matrix poses significant challenges owing to its low tackiness and high viscosity variations depending on the presence of nanoparticles, making filament winding difficult. These challenges are addressed by optimizing resin viscosity and winding pattern design. As a result, the tensile strength of the composite specimens fabricated with the optimized viscosity increases by 228.03% before pyrolysis and 97.68% after pyrolysis, compared with that of the non-optimized specimens. In addition, the density and tensile strength of the composite processed via three cycles of polymer infiltration and pyrolysis increased by 13.08% and 80.37%, respectively, compared to those of the non-densified composite. High-temperature oxidation and flame tests demonstrate exceptional thermal and oxidative stability. Furthermore, when compared with carbon fiber-reinforced ceramic matrix composites, the developed composite exhibits a permittivity at least two levels lower and a reflection loss below −7 dB within the frequency range of 9.3–10.9 GHz, underscoring its superior electromagnetic stealth performance. Manufacturing process of high-temperature-resistant, oxidation-resistant, and low-observable composite exhaust nozzle. • Developed lightweight SiC/SiOC composites for UAV exhaust nozzle applications. • Optimized the viscosity of resin for filament winding of complex nozzle shapes. • Superior tensile strength and density were achieved through densification cycles. • Excellent oxidation and flame resistance under extreme conditions. • Validated low-dielectric properties for reduced radar cross-section performance.