The basic structure of carbon fiber is graphite microfiber structure, which is a graphite layer structure composed of condensed polycyclic aromatic hydrocarbons arranged along the fiber axis, with a width of 60-100nm. The graphite layers are stacked together to form microfibers, which are composed of several or dozens of microfibers to form the original fibers. The original fibers are stacked and entangled with each other to form a network like planar structure of graphite. The original fibers are arranged along the axis of the fibers to form a macroscopic carbon fiber monofilament.

The modulus of fibers is determined by the neatness of the arrangement of microfibers along the fiber axis. The high modulus fibers themselves have a relatively high degree of arrangement of graphite microfibers along the fiber axis, and there are many twisted and entangled original fibers that bind to each other, resulting in high crosslinking density of microfibers. Carbon fibers with high crosslinking density also have high mechanical strength and modulus. The structure of carbon fiber itself is not uniform, with a distinction between skin and core. The graphite crystals in the carbon fiber cortex are large in size and tightly arranged along the fiber axis; The microcrystalline structure of the core layer is small in size, disordered in arrangement, and has many voids. Therefore, the modulus of the carbon fiber outer layer is about twice that of the core layer, and the disordered structure of the core layer has a certain impact on the mechanical properties of carbon fiber.

The bonded carbon atoms in the graphite microcrystalline structure on the surface of carbon fibers exhibit chemical inertness, and polar groups only exist in the amorphous region, structurally incomplete areas, and defect areas of the crystal lattice on the surface. Causing carbon fiber to exhibit chemical inertness and poor wettability with the resin matrix, which affects the application of its resin based composite materials. Modifying the surface of carbon fiber is the key to solving this problem. The essence of surface modification is to change the surface state of the bonded parts, including removing surface pollutants and impurities, improving surface chemical composition, increasing surface free energy, changing surface morphology, etc., in order to improve the interfacial bonding strength.

Plasma treatment improves the bonding performance of carbon fiber composite materials

Plasma treatment is the process of ionizing gases such as oxygen, air, argon, nitrogen, etc. into atoms, molecules, ions, electrons, and other substances containing metastable and excited states, bombarding the surface of carbon fibers, increasing their roughness and generating polar groups, improving their wettability, and enhancing the adhesion of resin to fibers.

Figure 1-1 shows the schematic diagram of the effect of oxygen plasma treatment on pollutants and chemical groups at the CFRP bonding interface. After plasma treatment, the excited high-energy active oxygen ions break the molecular chains of surface residues, causing them to ionize and excite into smaller molecular chains, or vaporize and free them from the surface in the form of C radicals, CO2, H2O, and other molecules, effectively removing carbon pollutants, moisture, dust, and grease from the surface structure, and weakening the influence of weak interface layers on bonding performance. For silicon containing pollutants that are difficult to dissolve in alcohol, the conversion from organic silicon to inorganic silicides occurs through the extensive cleavage of Si-C. The broken Si bonds combine with excited oxygen ions to form easily adhesive SiO2 particles that adhere to the surface of the sample. In addition, while removing surface pollutants, oxygen-containing functional groups such as hydroxyl (- OH), carbonyl (C=O), and carboxyl (O-C=O) are also generated. Active functional groups contribute to the formation of hydrogen bonds and covalent bonds with adhesives, resulting in stronger interfacial adhesion than van der Waals interactions. These changes all contribute to promoting better infiltration of the adhesive on the surface of CFRP, facilitating better bonding between CFRP and adhesive, promoting the transition of joint failure mode from interfacial failure to cohesive failure, and thereby improving the strength of the bonded joint.

Figure 1.1 Schematic diagram of the effect of oxygen plasma treatment on pollutants and chemical groups at the CFRP bonding interface

Principle of plasma treatment to improve adhesion performance

The plasma treatment technology improves the interfacial adhesion performance between materials and adhesives mainly through the following three ways. Firstly, there is surface treatment, where plasma acts on the surface of CFRP to induce a series of physical and chemical changes. Utilizing the active particles and high-energy radiation contained within, small molecule volatile substances are formed by reacting and colliding with organic pollutant molecules on the material surface, which are removed from the surface to achieve a treatment effect; Next is surface etching, where the material interface is bombarded by plasma energy, resulting in the formation of small pores and capillary effects that create unevenness or bumps, increasing the bonding surface area between the adhesive and the material; Finally, surface activation occurs when plasma gas ionization produces active particles, mainly ions, excited atoms, electrons, and free radicals. The generated active ions have strong reactivity and interact with polar functional groups at the interface, introducing active species groups at the interface and undergoing implantation reactions, introducing substances that increase the content of interface elements.

Schematic diagram of plasma treatment enhanced carbon fiber interface adhesion performance

Compared with other carbon fiber surface modification methods, plasma treatment has several advantages:

1) High processing efficiency, short-term plasma treatment can improve the chemical inertness of carbon fiber surface, increase the proportion and roughness of active groups, and improve the wettability of fibers.

2) Plasma treatment only affects the physical and chemical properties within tens or hundreds of nanometers on the surface of carbon fibers, and will not damage the mechanical properties of carbon fibers.

3) The process of plasma treatment is green and environmentally friendly, without the involvement of chemical reagents, and there are no three wastes generated during the treatment process.

4) The selection of different reaction atmospheres and gas composition ratios for plasma treatment will have different treatment effects on carbon fibers. Choosing the appropriate reaction atmosphere based on the different usage environments can enable composite materials to meet the performance requirements. It can be seen that plasma cleaning technology has good research prospects.