Multiscale Control of Short Carbon Fiber Reinforced Polyetheretherketone Composites: Decoupling Analysis Reveals the Dominant Role of Low‐Density Interfacial Phases in Achieving High Strength

ABSTRACT Short carbon fiber reinforced polyetheretherketone (SCF/PEEK) composites exhibit excellent mechanical properties and processability, making them suitable for load‐bearing aerospace and automotive structures. However, a systematic understanding of how processing parameters control microstructure evolution—particularly interfacial structure—and mechanical performance remains limited. In this study, high‐resolution micro‐computed tomography (μ‐CT) combined with mold flow simulation enabled accurate prediction of fiber orientation. The evolution of the low‐density transcrystalline phase at the interface was thoroughly investigated as a key focus. It revealed the deterioration mechanisms of interfacial properties and tensile strength. Processing effects on fiber orientation and interfacial performance were decoupled, and the concept of “effective fiber orientation” was proposed to explain the dominant role of interfacial properties in mechanical behavior. Furthermore, process design was optimized, and post‐processing was applied. Overall fiber orientation was maintained in the optimal range, and the low‐density interfacial phase was reduced to its engineering minimum (approximately 3%). With only 20% SCF content, the composites achieved a tensile strength of 297 MPa, representing a 25.98% improvement. This work clarifies microstructural evolution across the processing window and provides guidance for high‐performance SCF/PEEK processing and interfacial optimization.