Delamination Resistance Enhancement in Unidirectional Carbon Fiber Fabrics by Nitrogen Atmospheric Pressure Plasma Jet Surface Treatment

The surface activation of industrial-grade unidirectional (UD) carbon fiber (CF) textiles by nitrogen-based Atmospheric Pressure Plasma Jet (APP Jet) treatment is investigated for application in CF UD/bio-epoxy composite laminates. A systematic parameter screening campaign, guided by Optical Emission Spectroscopy (OES) of the plasma source, identifies the suitable operating window. The effects of the APP Jet treatment on the UD carbon fabric are characterized by micro-Raman spectroscopy and Field Emission Scanning Electron Microscopy with Energy-Dispersive Spectroscopy (FESEM-EDS). Surface analysis indicates a substantial reduction of the surface coverage associated with the thermoplastic binder and sizing residues, a controlled increase in graphitic disorder, and a modification of the surface elemental composition characterized by nitrogen enrichment, consistent with plasma-induced surface functionalization, without evidence of significant morphological damage to the fibers. Mode I interlaminar fracture toughness (GIC) was assessed by Double Cantilever Beam (DCB) testing on vacuum-infused UD CF/bio-epoxy laminates according to the ASTM D5528 standard. Two batches were tested: one adopting the CF UD with 0.1 mm as fabric thickness, and the other 0.2 mm thick. The N2-treated specimens show a statistically significant improvement of +19.4% and +36.1%, respectively, for the 0.1 mm and 0.2 mm series, in mean propagation toughness (GIC,ai) relative to the untreated baseline, confirming that plasma-induced surface chemistry translates into a measurable enhancement of delamination resistance in quasi-static opening conditions.