Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 08 , 2025

Abstract

The mechanical performance of carbon fiber–reinforced epoxy composites is often constrained by insufficient fiber–matrix interfacial adhesion, a consequence of the chemically inert and relatively smooth surface of as-received carbon fibers. This work synthesizes and reinterprets a 2025 study on electrochemical oxidation of T700 PAN-based carbon fibers in an ammonium bicarbonate electrolyte, focusing on how process parameters govern surface structure, chemistry, wettability and, ultimately, composite-scale properties in tension and compression.

Using an 8 wt% NH₄HCO₃ solution at 35 °C, the authors systematically vary current density (0.1–0.5 A·cm⁻²) and treatment time (30–300 s), identifying an optimal window at 0.2 A·cm⁻² for 100 s. Under these conditions, single-fiber strength retention remains high (~96%), while the fiber surface develops a 200–300 nm modified layer characterized by pronounced hierarchical roughness and a strong increase in oxygen-containing functional groups. Atomic force microscopy shows the arithmetic roughness Ra rising from 8.2 to 45.7 nm, while XPS reveals oxygen content increasing from 7.2 to 22.5 at.% and the fraction of C–O / C=O / O–C=O species more than doubling.

The resulting change in surface energy (42.3 → 68.7 mJ·m⁻², with polar contribution increasing by ~234%) leads to improved wettability and more effective chemical bonding with the E-51 epoxy matrix. At composite level, interfacial shear strength increases by about 60%, and failure mode shifts from interfacial debonding to matrix-dominated fracture. Unidirectional VARTM laminates exhibit tensile strength and modulus gains of ~14%, while compression strength and modulus improve by ~26% and ~11%, respectively, accompanied by a higher damage initiation stress and narrower kink bands.

The findings position electrochemical oxidation in mild aqueous electrolytes as a controlled and environmentally friendlier alternative to traditional chemical oxidation, offering a scalable route to enhanced interface performance in aerospace and automotive carbon/epoxy systems. Design implications and remaining challenges in process scale-up and long-term durability are discussed.

Keywords

electrochemical oxidation; T700 carbon fiber; ammonium bicarbonate treatment; interfacial shear strength; hierarchical surface roughness; oxygen-containing functional groups; epoxy composite; tensile and compressive performance; fiber–matrix adhesion

1. Introduction

Carbon fiber–reinforced epoxy (CF/EP) composites combine high specific stiffness and strength with good fatigue resistance and processing flexibility, making them central to aerospace and automotive structures. Yet, their full potential is often limited by the fiber–matrix interface. Commercial PAN-based carbon fibers are manufactured with smooth, graphitic surfaces that are chemically inert and poorly wettable. As a result, stress transfer from matrix to fibers is not fully optimized, especially under complex loading states such as compression and shear.

Conventional surface treatments—gas-phase oxidation, chemical baths, or plasma—aim to introduce functional groups and roughness to strengthen this interface. However, they frequently suffer from drawbacks: harsh chemicals and waste streams, non-uniform etching, or difficult process control. Electrochemical oxidation in aqueous electrolytes offers a compelling alternative. Operating at moderate temperatures and controlled current densities, it can selectively functionalize the outermost layer of the fiber while limiting damage to the load-bearing core.

The study analysed here implements such an electrochemical process on T700 PAN-based carbon fibers. By mapping current density and treatment time against surface chemistry, topography and mechanical response, it establishes a process window that maximizes interfacial enhancement while preserving fiber strength. The resulting composite performance, particularly under compression, confirms the central role of the engineered interface.

2. Materials, Electrochemical Treatment and Composite Processing

2.1 Carbon fibers and epoxy matrix

The reinforcement is a commercial T700 PAN-based carbon fiber:

• 12K tow, nominal tensile strength ~4.9 GPa.

Before treatment, the sizing is removed by immersion in acetone at 60 °C for 24 h, followed by rinsing and drying at 80 °C, producing a clean graphitic surface accessible to electrochemical modification.

The matrix is a standard bisphenol-A epoxy:

• Resin: E-51 epoxy,
• Hardener: T31 polyamine, mixed at 100:31 mass ratio.

Cure schedule for composites follows a two-step cycle: 2 h at 80 °C, then 4 h at 150 °C, with a ramp of 2 °C·min⁻¹.

2.2 Electrochemical setup and electrolyte

The electrochemical treatment is carried out in a three-electrode cell:

• Working electrode: desized carbon fiber tow,
• Counter electrode: graphite plate,
• Reference: saturated calomel electrode.

The electrolyte is an 8 wt% aqueous solution of ammonium bicarbonate (NH₄HCO₃) maintained at 35 ± 1 °C. This mild alkaline medium offers a less hazardous alternative to strong acids or oxidizing salt baths.

The external DC power supply controls current density in the range 0.1–0.5 A·cm⁻², while treatment times are varied between 30 and 300 s. The total electrochemical intensity can thus be described by the product of current density and time (J·t), which serves as a convenient measure for correlating surface modification with mechanical response.

2.3 Composite fabrication and mechanical testing

After treatment, the carbon fiber tows are rinsed, dried and used to fabricate unidirectional laminates by VARTM:

• Fiber alignment: unidirectional along the panel length.
• Panel dimensions: 250 × 25 × 2 mm.
• Vacuum-assisted resin infusion ensures wet-out of the treated fibers.

Mechanical characterization includes:

• Single-fiber tensile tests (ASTM D3379) to quantify strength retention,
• Interfacial shear strength (IFSS) via microdroplet tests (ASTM D2344-like),
• Composite tensile tests (ASTM D3039),
• Composite compression tests (ASTM D695),
• Surface analyses (SEM, AFM, XPS, FTIR, Raman),
• Wettability via contact-angle measurements and Owens–Wendt surface energy analysis.

This combination allows a direct link between electrochemical parameters, surface state and composite performance.

3. Optimization of Electrochemical Treatment

3.1 Process window: balancing oxidation and strength retention

Time-dependent XPS measurements under different current densities show that surface oxygen content increases rapidly at short times, followed by a plateau. At low current densities (e.g., 0.1 A·cm⁻²), oxidation proceeds more slowly, requiring longer exposures to reach a similar oxygen level; at higher densities (0.3–0.5 A·cm⁻²), oxygen builds quickly but the risk of excessive etching and fiber damage rises.

Single-fiber tensile data provide the critical constraint. When treatment time is fixed at 100 s and current density is varied:

• Up to 0.2 A·cm⁻², mean tensile strength decreases only slightly, from ~4.9 GPa to ~4.7 GPa, retaining about 96% of the original value.
• Above 0.2 A·cm⁻², strength drops sharply, reflecting deeper or more aggressive attack of the load-bearing shell.

By overlaying oxygen content and strength retention, the authors identify 0.2 A·cm⁻² for 100 s as the optimal condition, where surface modification is maximized without significant loss of fiber integrity.

3.2 Modified layer thickness

Cross-sectional SEM imaging shows that the electrochemically modified region is confined to an outer shell of roughly 200–300 nm. The internal core retains its original structure, explaining the high level of strength retention despite substantial surface oxidation and roughening.

This depth-limited modification is crucial: it ensures that the new functional and topographic features are located where stress transfer occurs, while the high-modulus, high-strength fiber core remains largely unaffected.

4. Surface Morphology: Hierarchical Roughness for Mechanical Interlocking

4.1 SEM observations

Scanning electron microscopy highlights the dramatic change in surface morphology:

• Untreated fibers exhibit relatively smooth surfaces with faint longitudinal ridges inherited from manufacturing.
• Treated fibers display a hierarchical pattern consisting of:
  • Primary grooves and micro-pits aligned roughly with the fiber axis, with characteristic dimensions of 0.5–1 µm,
  • Superimposed nano-scale asperities and etch features in the 50–200 nm range.

These micro- and nano-features are uniformly distributed along the fiber length, indicating controlled anodic dissolution rather than sporadic pitting. Such a hierarchy dramatically increases the available area and provides geometric undercuts for mechanical interlocking with the cured epoxy.

4.2 AFM roughness metrics

Atomic force microscopy quantifies this transformation:

• Arithmetic roughness Ra rises from 8.2 ± 0.5 nm to 45.7 ± 2.3 nm (~4.6-fold increase),
• RMS roughness Rq from 10.5 ± 0.6 to 52.4 ± 2.8 nm,
• Maximum peak height Rp and valley depth Rv both more than quadruple,
• Overall height Rt from 30.1 ± 1.8 to 140.1 ± 5.2 nm,
• Surface area ratio Sdr grows from 1.05 to 1.38, signifying ~31% more actual surface area compared to the projected area.

In composite terms, the epoxy now flows into a more complex landscape, anchoring itself mechanically as well as chemically. This dual anchoring underpins the large increase in interfacial shear strength observed later.

5. Surface Chemistry and Wettability: Building a Reactive, Polar Interface

5.1 XPS, FTIR and Raman: oxygen-containing groups and disorder

XPS reveals that electrochemical treatment significantly alters surface composition:

• Overall oxygen content climbs from 7.2 to 22.5 at.%.
• Deconvolution of the C1s peak shows:
  • C–C/C–H contribution drops from 85.3 to 65.2%,
  • C–O increases from 8.7 to 18.4%,
  • C=O from 4.2 to 10.8%,
  • O=C–O from 1.8 to 5.6%.

These changes indicate the formation of hydroxyl, carbonyl and carboxyl groups on the fiber surface. FTIR spectra reinforce this interpretation, with intensified bands near 1720 cm⁻¹ (C=O), ~1250 cm⁻¹ (C–O) and broad OH stretching around 3400 cm⁻¹.

Raman spectroscopy shows an increase in the D/G ratio from 0.85 to 1.2, consistent with higher defect density and edge sites created by oxidation. These defects act as anchoring points for functional groups, and in turn this chemically active skin is poised to interact strongly with epoxy and curing agents.

5.2 Surface energy and contact angles

Contact-angle measurements using water and diiodomethane, combined with Owens–Wendt analysis, demonstrate a marked increase in surface energy:

• Total surface energy γ rises from 42.3 ± 1.4 to 68.7 ± 2.0 mJ·m⁻².
• The dispersive component γᵈ grows from 33.8 to 40.3 mJ·m⁻².
• The polar component γᵖ increases much more strongly, from 8.5 ± 0.5 to 28.4 ± 1.3 mJ·m⁻² (~234% increase).
• The polarity index γᵖ/γ thus nearly doubles, from 0.20 to 0.41.

Water contact angle decreases from 72.5° to 45.8°, and diiodomethane from 48.3° to 32.6°, confirming improved wetting by both polar and non-polar liquids.

For epoxy processing, this means that the resin can spread more easily across individual filaments and penetrate interfilament spaces, while polar and reactive functionalities near the surface provide sites for covalent or strong secondary bonding with the epoxide and amine groups.

6. Mechanical Properties: From Fiber Scale to Composite Response

6.1 Single-fiber strength

Single-fiber tensile tests show that the optimized treatment window (0.2 A·cm⁻², 100 s) preserves fiber integrity:

• Mean tensile strength decreases slightly from 4.9 ± 0.3 to 4.7 ± 0.4 GPa, corresponding to ~96% strength retention.

This confirms that the electrochemical process has primarily reshaped and functionalized the outer shell without compromising the core, a key requirement for any surface treatment intended for high-performance composites.

6.2 Interfacial shear strength and failure modes

Microdroplet tests quantify the impact on interfacial shear strength (IFSS):

• IFSS increases from 45.3 ± 2.8 to 72.6 ± 3.2 MPa, a ~60% improvement.
• The coefficient of variation decreases from 11.5% to 8.4%, indicating a more uniform interface.
• The Weibull modulus rises from 8.9 to 12.3, reflecting tighter strength distribution and more reliable interfacial performance.

The failure mode distribution shifts markedly:

• Untreated fibers: ~85% failures occur at the interface, with limited matrix cracking.
• Treated fibers: only ~25% interfacial failures, while ~65% fail cohesively within the epoxy.

This transition from interfacial to cohesive failure is a direct manifestation of the strengthened bonding. It demonstrates that the interface, once the weakest link, now surpasses the matrix in shear resistance.

6.3 Tensile properties of unidirectional laminates

In tension along the fiber direction, the treated and untreated laminates differ significantly:

• Untreated composite:
  • Strength: 1100 ± 30 MPa
  • Modulus: 70 ± 2 GPa
  • Strain to failure: 1.8 ± 0.1%
• Treated composite:
  • Strength: 1250 ± 35 MPa
  • Modulus: 80 ± 2 GPa
  • Strain to failure: 2.0 ± 0.1%

The gains—about 14% increase in strength and modulus—arise from more efficient load transfer and reduced premature interfacial debonding. Fractography of tensile specimens reinforces this: untreated panels show extensive fiber pull-out and clean interfaces, while treated panels exhibit more cohesive matrix fracture and shorter fiber pull-out lengths.

6.4 Compressive properties and kink-band behaviour

Compression is particularly sensitive to interface quality due to the role of micro-buckling and shear instability at the fiber–matrix boundary. The electrochemically treated composites show substantial improvements:

• Compressive strength: 1250 ± 45 → 1580 ± 52 MPa (~+26%),
• Compressive modulus: 128.5 ± 3.8 → 142.3 ± 4.2 GPa (~+11%),
• Strain at maximum load: 1.15 ± 0.08 → 1.42 ± 0.09%,
• Damage initiation stress: 985 ± 38 → 1290 ± 45 MPa.

Microscopic examination of kink bands reveals smaller kink angles in the treated composites (≈15°) compared to untreated ones (≈23°), indicating better constraint of fiber rotation and more stable compressive response.

These results underscore that strengthening and roughening the interface are particularly effective strategies for improving compression-dominated performance—critical for stiffeners, frames, pressure vessels and compression-after-impact scenarios.

7. Mechanistic Picture and Design Implications

7.1 Mechanism summary

The overall mechanism linking process to performance can be summarised as:

1. Electrochemical oxidation in a mild ammonium bicarbonate electrolyte, under an optimized J–t window, creates:
   • A shallow oxidized shell (~200–300 nm),
   • Hierarchical micro/nano roughness,
   • Elevated defect density and oxygen-containing functional groups.
2. These changes produce a high-energy, polar, topographically complex surface that:
   • Promotes resin wetting and infiltration,
   • Enables chemical and polar bonding with epoxy and amine groups,
   • Provides mechanical keying through micro-pits and grooves.
3. Under mechanical loading:
   • Shear transfer at the interface improves; debonding is suppressed,
   • Tension: fibers carry load more uniformly; failure shifts towards matrix-dominated modes,
   • Compression: micro-buckling is delayed; kink bands form at higher stress and with smaller angles, increasing compressive strength and stiffness.

7.2 Comparative advantages and practical considerations

Compared with conventional chemical oxidation and some plasma methods, this electrochemical route offers:

• Better control via adjustable current density and time,
• Use of a mild, aqueous electrolyte with reduced environmental impact,
• Depth-limited modification, protecting fiber core properties,
• A process concept that can be adapted to continuous tow or fabric lines.

Remaining challenges include:

• Engineering large-scale treatment baths with uniform current distribution for wide tows,
• Integrating treatment with re-sizing steps in existing fiber production or prepreg lines,
• Assessing long-term durability, including fatigue, environmental aging and hot/wet exposure.

8. Conclusions

A carefully optimized electrochemical oxidation in ammonium bicarbonate solution can transform the surface of T700 PAN-based carbon fibers into a highly rough, oxygen-rich, and wettable interface layer while retaining approximately 96% of the original fiber tensile strength. The resulting hierarchical roughness and enriched functional group population significantly strengthen the fiber–epoxy interface, as evidenced by a 60% increase in interfacial shear strength and a shift from interfacial to cohesive failure in microdroplet tests.

At the laminate level, unidirectional VARTM composites fabricated with treated fibers show notable gains in tensile and compressive properties, including enhanced stiffness, strength and damage initiation stress. These improvements stem from more efficient load transfer and improved resistance to micro-buckling, confirming that the engineered interface is now better matched to the intrinsic capability of the fiber.

Electrochemical treatment in mild aqueous media thus emerges as a promising, more sustainable alternative to traditional oxidative methods for tailoring fiber surfaces in aerospace and automotive composites. Future work on scaling, coupling with sizing chemistries, and exploring fatigue and environmental performance will determine how quickly this approach can transition from laboratory optimization to industrial practice.