Carbon fiber reinforcements have revolutionized various industries, including aviation, automotive, and mechanical engineering, due to their exceptional strength-to-weight ratio and other remarkable properties such as high thermal stability, conductivity, self-lubrication, and corrosion resistance. Since their commercial introduction in the 1960s, carbon fiber composites have been pivotal in reducing the weight of vehicles and equipment, thereby enhancing efficiency and performance across numerous applications, from wind turbines and aerospace to fuel cells and electromagnetic shielding materials.

However, despite these advantages, a significant challenge persists: the poor interfacial adhesion between carbon fiber surfaces and polymer matrices. This issue arises from the intrinsic hydrophobic and chemically inert nature of carbon fibers, which hampers the necessary bonding with polymers. Addressing this challenge is critical for advancing the development and performance of carbon fiber-reinforced composites.

Researchers have dedicated significant efforts to improving the physicochemical interactions at the fiber/matrix interface. Effective interfacial adhesion requires strong van der Waals and hydrogen bond forces during composite processing, as well as higher interfacial adhesion energy compared to the cohesion energy of the matrix. Enhancing the mechanical properties of carbon fiber composites hinges on overcoming these interfacial adhesion barriers.

Carbon fibers possess crystallized graphitic basal planes with non-polar surfaces, resulting in chemical inertness due to high-temperature carbonization and graphitization during manufacturing. This inertness, coupled with surface lipophobicity and smoothness, leads to weak bonding with matrix materials. To mitigate this, various surface modification techniques have been developed to improve fiber/matrix adhesion and stress transfer efficiency at the interface.

Surface modification methods for carbon fibers can be categorized into three main types: wet chemical modifications, dry modifications, and multi-scale modifications. Wet chemical methods include the application of polymer sizings, acid treatments, and electrochemical modifications, all aimed at enhancing wettability and bonding capabilities. Dry methods encompass plasma treatments, high-energy irradiation, and thermal treatments, which modify the fiber surface to improve adhesion properties. Multi-scale modifications involve the use of nanoparticles, carbon nanotubes, or graphene through techniques like electrophoretic deposition (EPD), chemical vapor deposition (CVD), and dip coating. These approaches increase the surface energy and roughness of carbon fibers, promoting better mechanical interlocking with polymer matrices and, consequently, stronger interfacial adhesion.

The simultaneous action of physical adsorption and chemical interaction is essential for achieving effective fiber/matrix adhesion. Traditional methods for carbon fiber surface modification, categorized as oxidative and non-oxidative treatments, have shown significant improvements in interfacial properties. However, these methods often come with the drawback of reduced single fiber strength due to the creation of surface flaws that serve as stress concentration points.

In the current landscape, where traditional fiber treatment methods are well-established and widely used in industrial applications, it is imperative to explore and adopt newly developed treatment methods. These advanced methods offer the potential for improved performance of both fibers and composites, meeting the evolving demands of research and industry.

This review highlights the major carbon fiber surface modification techniques, emphasizing their impact on the strength properties of carbon fibers and their composites with polymeric matrices. It underscores the importance of selecting and optimizing appropriate modification methods to achieve excellent interfacial and composite properties while maintaining adequate single fiber strength.

By systematically covering the advancements in wet chemical, dry, and multi-scale modifications, this article provides a comprehensive overview of the current state of carbon fiber surface enhancements. It aims to guide researchers and industry professionals in understanding and implementing the most effective strategies for improving interfacial adhesion and overall composite performance.