Para‑aramid fiber is a new‑type high‑performance fiber material featuring high orientation and high crystallinity. It exhibits outstanding properties including fatigue resistance, high‑temperature resistance, corrosion resistance, high modulus and high tensile strength. As is well‑known, the mechanical properties of fiber‑reinforced composites depend on the reinforcing fiber, resin matrix and the interfacial bonding between them. Owing to the high crystallinity of para‑aramid fiber, its surface is smooth with chemically inert structures. This weakens the mechanical bonding between resin matrix and fiber surface, and directly limits load transfer across the interface. Therefore, surface modification of para‑aramid fiber is necessary to increase surface roughness, strengthen bonding with resin matrix, and improve the physical‑mechanical performance of composites.

Common modification approaches for aramid fibers are chemical‑based methods, such as polymerization modification, chemical surface grafting and chemical etching. Chemical modification delivers long‑lasting effects with remarkable improvement in surface roughness and interfacial shear strength. Nevertheless, it may damage fiber surface structures and degrade mechanical performance, and waste chemical reagents also bring environmental hazards. Plasma modification addresses the above drawbacks. It generates high‑energy electrons, ions and neutral particles, which collide with fiber surfaces to trigger a series of physical and chemical reactions. This alters polar functional groups and surface roughness without impairing fiber mechanical properties or causing environmental pollution.

Analysis of Plasma Effects on Fiber Surfaces

Morphology Observation

Scanning Electron Microscopy (SEM) was adopted to characterize the etching degree on fiber surfaces. Figure 1‑1 presents SEM micrographs of para‑aramid fiber surfaces treated with three plasma gas sources. As shown in the images, fibers treated by nitrogen plasma display the fewest pits and grooves. By contrast, argon plasma induces considerable surface etching. The argon‑nitrogen dual‑plasma treatment yields the most severe etching effect. Etching degree governs mechanical interlocking between fiber and matrix. Higher etching degree corresponds to better fiber‑matrix interfacial performance. Hence, argon‑nitrogen dual‑plasma delivers the optimal surface modification performance for para‑aramid fibers.

Figure 1‑1 SEM micrographs of aramid fiber surfaces modified by three gas sources
(a) Nitrogen plasma treatment; (b) Argon plasma treatment; (c) Argon‑nitrogen dual‑plasma treatment

Atomic Force Microscopy (AFM) was used to observe surface morphological variations of para‑aramid fibers before and after dual‑plasma treatment. Figure 1‑2(a) and (b) show fiber surface morphologies prior to and after treatment respectively, confirming that argon‑nitrogen dual‑plasma can modify micro‑scale surface topography. The untreated fiber surface is relatively smooth. Inherent to para‑aramid fiber structure, shallow grooves run parallel along the fiber axis, with an average surface roughness of 80.3 nm. After modification, surface morphology changes significantly: the quantity and depth of protrusions and grooves increase substantially, raising average surface roughness to 254 nm. Such changes benefit fiber‑matrix interfacial performance, as rougher surfaces provide more contact points and facilitate stronger mechanical interlocking between fiber and matrix.

Figure 1‑2 AFM images of para‑aramid fibers before and after argon‑nitrogen dual‑plasma treatment

FT‑IR Spectroscopy Analysis

Fourier‑transform infrared spectroscopy was employed to analyze chemical structures of para‑aramid fibers before and after dual‑plasma treatment. Spectral data were collected within the wavenumber range of 400 cm⁻¹ to 4000 cm⁻¹. Figure 1‑3 compares infrared spectra of untreated para‑aramid fibers and samples treated by argon‑nitrogen dual‑plasma, argon plasma and nitrogen plasma. For dual‑plasma modification, argon and nitrogen work synergistically. Nitrogen molecules collide with high‑energy carrier gas Ar molecules during discharge and produce excited‑state active nitrogen atoms. Meanwhile, argon atoms exert strong etching effects, inducing intensive surface etching and generating abundant functional groups on fiber surfaces.
The absorption peak at 3310.3 cm⁻¹ corresponds to N‑H stretching vibration, indicating the formation of ‑NH₂. The peak at 1647.2 cm⁻¹ originates from C=C stretching vibration, associated with ‑COOH‑ groups. Signals at 818.2 cm⁻¹ verify the generation of ‑OH groups.

Figure 1‑3 FT‑IR spectra of untreated para‑aramid fibers and fibers treated by argon‑nitrogen dual‑plasma, argon plasma and nitrogen plasma

Nitrogen‑plasma‑modified samples lack the synergistic effect from high‑energy argon carrier gas. Only ‑NH₂ is formed at 3310.3 cm⁻¹, accompanied by weak etching and low polar surface free energy. After argon‑plasma treatment, in‑situ‑generated free radicals undergo auto‑oxidation upon exposure to ambient air, yielding a smaller quantity of functional groups. Polar ‑COOH‑ emerges at 1647.2 cm⁻¹ and ‑OH appears at 818.2 cm⁻¹. Since argon is chemically more inert than nitrogen, higher energy input is required to produce excited‑state argon atoms, which possess stronger etching capability than nitrogen atoms. Accordingly, argon‑nitrogen dual‑plasma achieves the best modification outcome.

Summary: Compared with conventional materials, para‑aramid fiber is a high‑performance fiber material characterized by high orientation and crystallinity, featuring fatigue resistance, high‑temperature resistance and other merits. It is widely used in aerospace, automotive engineering and ballistic protection industries. However, its high crystallinity and relatively inert chemical structure weaken interfacial bonding between fiber surface and resin matrix. Plasma treatment is therefore applied to para‑aramid fiber surfaces to improve bonding strength and surface roughness. On one hand, bombardment by atoms and electrons introduces new polar functional groups onto fiber surfaces. On the other hand, plasma etching creates grooves and elevates surface roughness, so as to enhance interfacial adhesion between fiber and matrix.