Effect of hygrothermal aging on the mechanical, thermal, impact, and surface wettability properties of carbon/flax intra-ply hybrid composites for motorboat propeller applications

This research investigates the durability and performance degradation of carbon/flax fiber-reinforced intra-ply hybrid polymer composites (CFFRPCs) intended for marine propeller applications under hygrothermal aging conditions. CFFRPCs were fabricated using the Vacuum Assisted Resin Infusion Molding (VARIM) process and subjected to hygrothermal aging for periods ranging from 0 to 60 days. The post-aging effects were systematically evaluated through flexural, impact, thermomechanical, thermal, surface wettability, and microstructural characterizations. Results reveal that prolonged hygrothermal exposure significantly impairs the composite properties. The bending modulus decreased by approximately 34% after 60 days, while the glass transition temperature and thermal conductivity also showed marked reductions. These changes are attributed primarily to moisture absorption, which promotes matrix plasticization and swelling, weakens fiber/matrix interfacial adhesion, and facilitates interfacial debonding and microstructural damage. Thermal stability analysis further confirmed a systematic decline in decomposition temperatures, indicating moisture-induced deterioration of the matrix–fiber system. Furthermore, surface wettability was increased with the water contact angle decreasing from 63° to 51°. Despite these degradations, the hybrid composite maintained high impact toughness, with approximately 96% retention, highlighting the contribution of the carbon/flax hybrid architecture to damage tolerance. SEM analysis provided further evidence of the aging-induced degradation through delamination, fiber pull-out, and fiber-bundle debonding, which correlated with the observed macroscopic property losses. Overall, although hygrothermal aging reduces the thermal and mechanical stability of CFFRPCs, their relatively high retention of impact performance suggests potential for marine applications, particularly when appropriate moisture-resistant surface protection is employed.