The use of Atmospheric Pressure Plasma Jet (APPJ) technology for surface treatment of carbon fabrics is investigated to estimate the increase in the fracture toughness of carbon-fiber composite materials. Nitrogen and a nitrogen–hydrogen gas mixture were used to size the carbon fabrics by preliminarily optimizing the process parameters. The effects of the APPJ on the carbon fabrics were investigated by using optical and chemical characterizations. Optical Emission Spectroscopy, Fourier Transform Infrared-Attenuated Total Reflection, X-ray Photoelectron Spectroscopy and micro-Raman spectroscopy were adopted to assess the effectiveness of ablation and etching effects of the treatment, in terms of grafting of new functional groups and active sites. The treated samples showed an increase in chemical groups grafted onto the surfaces, and a change in carbon structure was influential in the case of chemical interaction with epoxy groups of the epoxy resin adopted. Flexural test, Double Cantilever Beam and End-Notched Flexure tests were then carried out to characterize the composite and evaluate the fracture toughness in Mode I and Mode II, respectively. N₂/H₂ specimens showed significant increases in G_IC and G_IIC, compared to the untreated specimens, and slight increases in P_max at the first crack propagation.

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 (${\mathrm{G}}_{IC}$) 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 ${\mathrm{N}}_2$-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 (${\mathrm{G}}_{IC,ai}$) relative to the untreated baseline, confirming that plasma-induced surface chemistry translates into a measurable enhancement of delamination resistance in quasi-static opening conditions.