Carbon-fibre composites have transformed aircraft, automobiles, wind turbines and sporting equipment by combining low weight with exceptional strength. Yet the material has a stubborn weakness: the boundary where carbon fibres meet a plastic matrix can fail long before the fibres themselves break. A new study reports a water-based surface treatment that strengthens this critical zone by coating carbon fibres with a bio-inspired adhesive and two sizes of silica nanoparticles. The researchers say the multiscale architecture increased interlaminar shear strength by 71 per cent in thermoplastic composite laminates, while also making the nanoscale interphase stiffer and better able to dissipate mechanical energy.

The work, published in the Journal of Materials Science, targets a problem that becomes especially important when fibres are recovered from discarded composite components. Recycling carbon-fibre composites can preserve much of the fibre’s intrinsic strength, but the treatments used to remove the original polymer often alter or strip away surface chemistry that helps the new matrix adhere. Carbon itself is strong and chemically resistant, but its relatively inert surface does not always bond efficiently with thermoplastic polymers. When adhesion is poor, loads cannot move smoothly from the matrix into the fibres. Instead, cracks can form and spread along the interface, producing delamination and premature failure.

The researchers’ solution begins with dopamine, a small molecule inspired by the chemistry of mussel adhesive proteins. In mildly alkaline water, dopamine undergoes oxidation and self-polymerisation, forming a thin, adherent coating known as polydopamine, or PDA. The coating can attach to a wide range of surfaces through a combination of covalent and non-covalent interactions, including hydrogen bonding, π–π interactions and coordination with certain chemical groups. On carbon fibres, PDA acts as both a surface modifier and a chemical anchor. It introduces reactive and polar groups that can interact more strongly with a surrounding polymer, while also providing binding sites for inorganic particles.

The team used this adhesive layer to graft silica nanoparticles onto the fibres. Rather than applying particles of a single size, the researchers combined smaller particles measuring roughly 30–60 nanometres with larger particles in the 100–200 nanometre range. This bimodal design is intended to create a more complex surface across several length scales. Smaller particles can occupy gaps and increase nanoscale roughness, while larger particles produce more substantial protrusions and irregularities. The resulting coating was described as intermittent rather than as a completely continuous shell, a configuration that can preserve access to the fibre surface while creating mechanical features that the polymer matrix must flow around and grip.

The treatment was carried out using an aqueous process, an important detail for possible manufacturing scale-up. Water-based surface modification avoids some of the volatile organic solvents and energy-intensive equipment associated with plasma treatments, aggressive oxidation or specialised deposition methods. The method also allows the particle size and surface coverage to be adjusted. According to the study, the dopamine layer anchored the silica particles with high coverage, producing a deliberately engineered transition between the carbon fibre and the thermoplastic matrix. That transition, or interphase, is not simply a passive boundary: its chemistry, stiffness and ability to deform can determine how stresses are distributed throughout the composite.

To test whether the coating damaged the fibres or improved their performance, the researchers conducted single-fibre tensile tests. These measurements are sensitive to changes in the fibre’s own strength because the specimens contain no large-scale laminate structure to obscure the result. The treated fibres showed consistent mechanical gains, indicating that the aqueous grafting process did not merely reinforce the interface at the expense of the carbon filaments. This distinction matters for recycled fibres, which may already contain defects or have experienced thermal and chemical damage. A surface treatment that produces stronger laminates but weakens individual fibres could deliver little practical benefit; the reported results suggest that the modification avoided that trade-off under the tested conditions.

The team also used atomic force microscopy nanoindentation to probe the interphase at nanometre-scale resolution. In this technique, a sharp probe presses into the surface while the instrument records the force and displacement, allowing researchers to estimate local stiffness and deformation behaviour. The measurements revealed nanoscale stiffening after treatment. At the laminate level, dynamic mechanical analysis showed a more complicated response: the interphase became stiffer but also more dissipative. This combination is valuable because a purely rigid interface can transfer loads efficiently yet concentrate stresses and encourage brittle crack propagation. An interphase that can dissipate energy, by contrast, may absorb part of the mechanical work through molecular motion, local deformation and frictional processes before a crack advances.

The most striking result came from interlaminar shear strength testing, which probes how effectively adjacent composite layers resist sliding apart. The laminate containing the bimodal silica–polydopamine coating exhibited a 71 per cent increase in ILSS compared with the relevant untreated system, according to the authors. The improvement is attributed to three linked mechanisms. First, chemical activity from PDA and silica-related surface groups can increase interactions with the thermoplastic matrix. Second, polymer chains may become more entangled or constrained near the modified fibre surface, raising resistance to interfacial separation. Third, the nanoparticles create a rough, uneven landscape that promotes mechanical interlocking: the matrix must deform around the particles rather than peeling cleanly from a smooth fibre.

The multiscale nature of the coating may be central to its performance. A single layer of large particles could leave voids, while very small particles alone might provide insufficient topographical depth to resist crack growth. Together, the two particle populations can increase contact area and create a hierarchy of obstacles to interfacial failure. As a crack attempts to travel along the fibre–matrix boundary, it may be forced to deviate around particles, consume more energy and encounter regions with different local stiffness. The intermittent arrangement could also help prevent the interphase from becoming excessively brittle or blocking polymer impregnation. However, the study’s proposed mechanisms remain interpretations of the measured behaviour rather than a complete molecular picture; detailed studies of fracture surfaces, long-term durability and environmental ageing will be needed to determine how each contribution changes under real service conditions.

The findings arrive as manufacturers seek ways to make lightweight composites more sustainable without sacrificing reliability. Carbon fibres require substantial energy to produce, and end-of-life composite parts are difficult to recycle because fibres are embedded in durable polymers. Reusing recovered fibres in thermoplastic matrices offers potential advantages: thermoplastics can be softened and reshaped, and their processing may support repair or remanufacture more readily than permanently cross-linked thermosets. Yet recycled fibres often have inconsistent surfaces, variable lengths and uncertain sizing chemistry. A scalable coating that works on original, recycled and reused fibres could help standardise the interface before consolidation. The authors’ use of water, dopamine and silica is therefore significant not because it makes carbon fibre intrinsically stronger, but because it addresses the fragile junction that determines whether the fibre’s strength can be exploited.

The study does not establish that the treatment is ready for commercial production. Industrial composites face challenges absent from laboratory coupons, including rapid coating of continuous fibre tows, uniform nanoparticle distribution, drying and wastewater management, compatibility with different thermoplastics and resistance to heat, moisture, fatigue and impact. Nanoparticles can also affect melt viscosity and processing conditions if they detach or accumulate. The 71 per cent ILSS gain is a result for the tested laminate and coating formulation, not a universal prediction for every carbon fibre composite. Even so, the research offers a practical design principle: engineer the fibre–matrix interphase as a graded, mechanically active region rather than treating it as a simple adhesive layer. By combining mussel-inspired chemistry with particles spanning multiple size scales, the approach could provide a route toward tougher, more repairable and more recyclable carbon-fibre composites.

Subject of Research: Multiscale silica nanoparticle and polydopamine surface architectures for strengthening the interface between carbon fibres and thermoplastic composite matrices

Article Title: Multiscale silica nanoparticle/polydopamine surface architectures on carbon fibre surface for improving interfacial properties of thermoplastic composites

Article References: Lyu, Y., Li, J., Yu, C. et al. “Multiscale silica nanoparticle/polydopamine surface architectures on carbon fibre surface for improving interfacial properties of thermoplastic composites.” Journal of Materials Science (2026). Original research article

Image Credits: AI Generated

DOI: 10.1007/s10853-026-13562-6

Keywords: carbon-fibre composites, polydopamine, silica nanoparticles, thermoplastic composites, interfacial adhesion, recycled carbon fibre, interlaminar shear strength, sustainable materials

Tags: advanced composite material durabilitybio-inspired adhesive coatings for compositesCarbon fiber composite reinforcementimproving fiber-matrix adhesion in thermoplasticsinterlaminar shear strength enhancementmultiscale nanoparticle surface treatmentnanoscale energy dissipation in composite materialsnanostructured interphase in fiber-reinforced plasticsrecycling of carbon-fiber compositessilica–polydopamine coatingsthermoplastic composite interface strengtheningwater-based surface modification for composites