Effect of cryogenic treatment on mechanical, thermal and microstructural characteristics of PEEK reinforced with CNT and graphene nanofillers

Polyether ether ketone (PEEK) is a premier high-performance semi-crystalline thermoplastic widely employed in aerospace, biomedical and cryogenic structural applications. Although carbon-based nanofillers particularly multi-walled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) are known to enhance the bulk properties of PEEK, the coupled influence of nanofiller reinforcement and prolonged cyclic cryogenic exposure on PEEK remains incompletely characterized. The present study systematically investigates the mechanical, thermal and microstructural evolution of neat PEEK, PEEK–CNT (0.5 wt%) and PEEK–Graphene (0.5 wt%) nanocomposites fabricated by hot compression moulding and subjected to ten cyclic deep cryogenic treatment (DCT) cycles at −196°C in liquid nitrogen, representing 70 days of cumulative exposure. The results demonstrate a significant enhancement in mechanical properties after cryogenic aging. The ultimate tensile strength was improved after cryogenic treatment and PEEK-CNT exhibited highest flexural strength and modulus. Thermal stability improved while DSC and XRD confirmed improved crystallinity, glass/melting temperatures and molecular packing. SEM showed a dense fracture morphology with enhanced crack bridging in CNT composites.