Effect of desizing treatment on carbon fiber properties and performance in carbon–carbon composite rocket nozzle

This study investigates the influence of thermal desizing on the surface, structural, and mechanical properties of PAN-based carbon fibers for carbon–carbon composite applications. Fibers were heat-treated in nitrogen between 25 and 900 °C and characterized using SEM, AFM, Raman spectroscopy, XPS, TOF-SIMS, contact angle measurements, and tensile testing. Effective removal of the epoxy sizing layer occurred at approximately 400 °C, producing a clean surface with minimal damage and the lowest roughness. Higher temperatures (700-800 °C) increased roughness and defect density due to surface degradation and carbonaceous residue formation, while partial surface cleaning reappeared at 900 °C. Raman analysis confirmed that the bulk fiber structure remained largely unchanged, indicating predominantly surface-related modifications. Surface chemistry evolved from epoxy-dominated to graphitic sp 2 -rich above 400 °C, whereas TOF-SIMS detected residual epoxy species up to ∼ 520 °C. Tensile tests revealed moderate strength reduction after desizing and significant degradation above 600 °C. Composites fabricated with fibers desized at 500 °C exhibited improved performance, with interlaminar shear strength increasing by up to 44 %. The optimal desizing window was identified as 400-500 °C, providing a balance between sizing removal, surface quality, and mechanical performance for composite rocket nozzle applications.