Investigating interfacial properties vs interphase thickness in a thermoplastic composite

Abstract

Thermoplastic composites offer exceptional characteristics—particularly weldability, superior toughness, and potential for rapid out-of-autoclave processing—that make them highly attractive for diverse applications. The temperature history during their manufacturing plays a critical role in shaping key microstructural features, including fiber–matrix interfacial properties, matrix crystallinity, and interphase morphology. These characteristics, in turn, determine the composite's macroscale mechanical performance. In carbon fiber-reinforced low-melt polyaryletherketone (LM-PAEK™) composites, the influence of processing conditions was systematically examined by producing three distinct sample types through automated fiber placement and post-processing: (1) fast cooling followed by cold crystallization, (2) controlled cooling from melt at 2 °C/min, and (3) fast cooling at rates exceeding 10,000 °C/min. Interfacial mechanical properties and interphase size were characterized using cyclic nanoindentation push-out tests and force-modulation atomic force microscopy, revealing that specimens cooled at 2 °C/min exhibit an interphase region approximately three times thicker than that of rapidly cooled specimens, with enhanced interfacial fiber–matrix strength and fracture toughness. These findings highlight the importance of controlling the interphase thickness in thermoplastic composites.