Carbon fiber (CF) composites feature high specific strength, excellent high-temperature resistance, low coefficient of thermal expansion and other superior properties, making them core structural materials for aerospace vehicles and high-temperature industrial components. Nevertheless, the disordered graphite structure on carbon fiber surfaces leads to poor wettability with resin matrices, which severely restricts the full performance of composite materials. As a non-contact and low-energy surface engineering technology, plasma cleaning utilizes high-energy particles generated by gas-discharge plasma to induce physical etching and chemical reactions on carbon fiber surfaces. It can regulate surface nanostructures and introduce polar functional groups without impairing the intrinsic properties of carbon fibers, thereby modifying their surface physicochemical characteristics and boosting the interfacial performance of composites.

Effects of Air Plasma Cleaning Power on Carbon Fiber Surface Morphology

One functional mechanism of plasma modification lies in the bombardment and etching of fiber surfaces by high-speed particles (ions, electrons, etc.) within plasma. This process effectively alters the micro-geometric morphology of fibers and tunes their surface physical properties to meet specific application requirements.

Scanning Electron Microscopy (SEM) was adopted to observe carbon fiber surface morphologies under different treatment conditions. Figure 1.1 reveals that untreated pristine carbon fibers exhibit smooth surfaces without grooves, a feature determined by their manufacturing process. The precursor fibers of carbon fibers are fabricated via dry jet-wet spinning, resulting in smooth, groove-free fiber surfaces, which creates a strong demand for surface modification treatment.

Figure 1.1 SEM images of carbon fibers after air plasma cleaning
(a) Untreated; (b) 100 W; (c) 200 W; (d) 300 W; (e) 400 W

As a critical parameter of modification, plasma cleaning power regulates etching efficiency on fiber surfaces by altering plasma density and electron temperature. Distinct differences can be observed in the micro-morphology of carbon fibers treated under varying power settings in Figure 1.1. Minor protrusive structures emerge at partial sites on fiber surfaces treated at 100 W. With increased treatment power, the evolution of surface microstructures becomes more prominent: the quantity of protrusions rises sharply and distributes uniformly across fibers, indicating that higher power facilitates surface structural reconstruction. However, slight surface delamination occurs at 400 W, which is attributed to excessive energy input that breaks surface chemical bonds and triggers partial exfoliation of the surface layer.

The interfacial bonding strength between carbon fibers and resin matrices is governed by surface morphology, roughness, surface active functional groups, surface energy and other factors. Irregular fiber surfaces enhance mechanical interlocking between carbon fibers and resins, improving interfacial bonding strength and facilitating stress and heat transfer across the interface.

Effects of Air Plasma Cleaning Power on Carbon Fiber Surface Roughness

A comprehensive analysis of air plasma treatment on carbon fiber surface morphology and roughness is conducted based on Figure 2.2 and Table 2.1. Untreated carbon fibers have a roughness of 41.03 nm with relatively smooth surfaces marked only by faint striations, and tiny protrusions merely appear in isolated local regions. All air plasma treated samples display increased surface roughness to different degrees. When treated at 200 W, fiber roughness rises to 114.13 nm, accompanied by abundant evenly distributed protrusive structures. Experimental results demonstrate a positive correlation between carbon fiber roughness and air plasma power within the test power range.

Increased roughness provides stable bonding sites for carbon fiber-resin integration and promotes physical interlocking to strengthen interfacial adhesion. Conversely, excessively high roughness hinders resin spreading and infiltration on fiber surfaces, triggering pore defects at the interface and degrading composite material performance.

Figure 2.2 AFM images of carbon fibers after air plasma treatment
(a) Untreated; (b) 100 W; (c) 200 W; (d) 400 W

Table 2.1 Surface roughness of carbon fibers after air plasma treatment 

Effects of Air Plasma Cleaning Power on Chemical Composition of Carbon Fiber Surfaces

X-ray Photoelectron Spectroscopy (XPS) was employed to analyze the surface chemical composition of carbon fibers treated with air plasma at different power levels. Full-scan XPS spectra of carbon fiber surfaces (Figure 3.3) present three characteristic peaks at binding energies of 285 eV, 399 eV and 532 eV, corresponding to C1s, N1s and O1s respectively. Peak intensities of all three elements change notably after air plasma cleaning, and elemental content variations are summarized in Table 3.2.

Prior to plasma treatment, carbon is the dominant surface element accounting for 86.46%, followed by oxygen at 10.85% and nitrogen at the lowest proportion of 2.69%, with an O/C ratio of 0.125. After 100 W air plasma treatment, the carbon proportion drops from 86.46% to 83.67%, nitrogen content decreases slightly from 2.69% to 1.98%, while oxygen content rises from 10.85% to 14.35%, lifting the O/C ratio to 0.172. At 200 W, carbon content further declines to 76.18%, oxygen increases to 18.67% and nitrogen reaches 5.15%, yielding a maximum O/C ratio of 0.245. When power is elevated to 300 W, carbon content rebounds to 82.32%, oxygen falls to 16.00% and nitrogen drops to 1.68%, with the O/C ratio decreasing to 0.194. At the maximum power of 400 W, carbon content stands at 86.34%, oxygen reduces to 11.49% and nitrogen is measured at 2.17%, with the O/C ratio falling to 0.133.

Overall, carbon content follows a trend of initial decline followed by a rise, reaching its minimum value of 76.18% at 200 W. Oxygen content rises first then falls, peaking at 18.67% under 200 W treatment. Nitrogen content shows no consistent monotonic trend, while the O/C ratio also increases to a maximum at 200 W before declining.

Figure 3.3 Full-scan XPS spectra of carbon fiber surfaces after air plasma treatment
(a) Untreated; (b) 100 W; (c) 200 W; (d) 300 W; (e) 400 W

Table 3.2 Surface elemental contents of carbon fibers after air plasma treatment

The above data verifies that air plasma cleaning elevates surface oxygen content on carbon fibers, with both oxygen proportion and O/C ratio maximized at 200 W. This phenomenon can be explained as follows: at 100 W, the concentration of active particles in plasma is low, limiting the number of oxygen atoms bonded to fiber surfaces. When power increases to 200 W, the density of active plasma particles surges to generate abundant active sites, facilitating the attachment of more oxygen atoms onto fiber surfaces. Further power escalation leads to excessively high plasma density and energy, which breaks oxygen-containing functional groups on fiber surfaces and ultimately reduces surface oxygen content.

In air plasma, the introduction of oxygen atoms, nitrogen atoms and active free radicals alters the distribution of chemical functional groups on carbon fiber surfaces. Deconvoluted C1s XPS spectra of carbon fibers treated at different plasma powers are displayed in Figure 4.4. The C1s spectrum of untreated fibers can be split into four functional group peaks assigned to -C-C-, -C-N-, -C-O- and -O-C=O, at binding energies of 284.8 eV, 285.3 eV, 286.7 eV and 288.8 eV respectively. All functional groups undergo noticeable changes after air plasma cleaning, with detailed data listed in Table 4.3. Figure 4.4 clearly illustrates obvious fluctuations in peak areas of specific functional groups as treatment power rises, proving a strong correlation between plasma power and surface functional group composition.

Pristine fiber surfaces mainly consist of 65.49% -C-C-, 18.28% -C-N-, 8.97% -C-O- and 7.26% -O-C=O groups, with a polar/non-polar group ratio of 0.53. According to the table, 100 W air plasma treatment slightly reduces -C-C- content by approximately 2.2%, while -C-O- and -O-C=O proportions increase marginally by 1.2% and 3.2% respectively, pushing the polar/non-polar group ratio up to 0.58. When power rises from 100 W to 200 W, surface -C-C- content drops by roughly 11.5% compared with the untreated group; -C-O- content rises from 8.97% to 16.64% and -O-C=O content increases moderately from 7.26% to 10.84%, driving the polar/non-polar group ratio sharply up to 0.85.

Figure 4.4 Deconvoluted C1s XPS spectra of carbon fiber surfaces after air plasma treatment
(a) Untreated; (b) 100 W; (c) 200 W; (d) 300 W; (e) 400 W

Table 4.3 Surface functional group contents of carbon fibers after air plasma treatment

It can be concluded that air plasma cleaning at all tested power levels increases the proportion of polar functional groups. Under 200 W treatment, high-energy oxygen-containing active particles bombard fiber surfaces, cleaving surface chemical bonds and forming new covalent bonds with oxygen species to generate abundant oxygen-containing functional groups on fibers. Nevertheless, at 300 W and 400 W, -C-O- content declines from the 200 W peak of 16.64% to 10.84% and 13.59% respectively, while -O-C=O content drops from 10.84% to 5.84% and 3.74% correspondingly. This confirms that excessive plasma etching at ultra-high power destroys surface oxygen-containing functional groups, consistent with the conclusions drawn from elemental composition analysis.