Geometry-Associated Mechanical Trends of Graphene Oxide and Carbon Nanotube SPEEK Sizings in CF/PEEK Commingled Yarn Laminates

The interfacial region dictates the load-transfer efficiency and mechanical performance of continuous carbon fiber/polyetheretherketone (CF/PEEK) thermoplastic composites. Nanofiller geometry represents a promising route to tailor fiber–matrix interphases, yet it remains challenging to decouple geometric effects from confounding variables including surface chemistry, defect density, and filler dispersion. In this work, sulfonated poly(ether ether ketone) (SPEEK) sizing modified by one-dimensional (1D) carbon nanotubes (CNT) and two-dimensional (2D) graphene oxide (GO) was deposited onto carbon fibers to construct geometry-differentiated interphases. A Box–Behnken response-surface methodology was adopted to identify the optimal hot-pressing processing window. Low-porosity laminates with consistent fiber volume fractions were fabricated to minimize void-induced property scatter. Mechanical characterizations reveal that both GO and CNT modified sizings substantially improve interlaminar shear strength and flexural/tensile performance relative to desized reference samples. SEM fractography confirms a transition from pure adhesive interfacial debonding toward mixed cohesive/adhesive failure upon nanofiller incorporation. Halpin–Tsai semi-quantitative estimation demonstrates that the direct stiffening contribution from the intrinsic rigidity of nanofillers is negligible. We therefore propose a working hypothesis that the enhanced mechanical properties arise from geometry-associated interphase modification, where nanofiller morphology alters polymer chain mobility and local crystallization behavior at the fiber–matrix boundary. This screening study highlights the potential of nanofiller geometric design for interphase engineering in CF/PEEK composites, while also clarifying current limitations: geometric, chemical, and dispersion effects cannot be fully isolated under the present experimental framework, and dedicated interfacial characterization is required in future work to validate the proposed interphase mechanisms.