## 1 Introduction  
Carbon-fiber-reinforced composites exhibit excellent strength and stiffness and are gradually replacing existing metal-based materials owing to the development of modern production methods and technologies. Carbon-fiber-reinforced polymer (CFRP) composites exhibit superior properties compared to metal-based materials. Hence, CFRP composites are widely used in the automobile, sporting goods, and aerospace industries. Among the composite material fabrication methods, laminated carbon-fiber-reinforced composites are the most important and convenient composite fabrication methods since the development of prepregs and binders. Delamination is the primary mode of failure for laminated polymer composites. In recent years, studies on enhancing the resistance to delamination have been undertaken [1, 2]. Delamination decreases the stiffness and strength of the composite and is not visible in composite materials until complete failure occurs. Therefore, it is vital to improve the interlaminar fracture toughness of composite laminates. Interleaving, which is one of the methods to improve interlaminar mechanical properties, has been used to prevent crack propagation in fiber-reinforced composite materials since the 1990s. Polycarbonate, polystyrene, and other thermoplastic resins were nano-interleaved by electrospinning and placed between the ply interface to reduce microcracks during delamination [3]. The interleaves made from Nylon-66 nanofibers increased the mode I interlaminar fracture toughness of the composites [4; 5]. In another study, Nylon-66 nanofibers were interleaved in CFRPs using electrospinning to increase the threshold impact energy and reduce the rate of impact damage growth [6]. An amorphous thermoplastic film and a nitride-rubber-modified epoxy were applied to graphite/epoxy composites to improve the mode II interlaminar fracture toughness [7]. The effects of interleaving polyimide (PI) and polypropylene (PP) nets, manufactured using a carding machine, on the interlaminar fracture toughness and impact performance of CFRPs were studied [8]. Veils based on polyethylene-terephthalate (PET), polyphenylene-sulfide (PPS), and polyamide-12 (PA 12) fibers were used as interlayers in unidirectional non-crimp fabric and 5-harness satin weave carbon fiber/epoxy laminates [9]. Non-woven carbon tissues with different fiber lengths were interleaved to enhance both the delamination resistance and electrical conductivity of CFRP [10]. Carbon nanotubes (CNTs) were widely used in many studies as nano-reinforcements for composite materials. Multiple researchers studied the effect of CNTs on the fracture toughness of polymer matrices [11; 12; 13]. The viscosity of polymer resins increases rapidly, even if a small amount of CNTs is added directly. Thus, it is difficult to uniformly disperse the CNTs in polymer resins using conventional composite manufacturing processes [14; 15; 16]. Several techniques were developed to overcome these limitations. [PERSON] et al. proposed the idea of growing CNTs on the surface of carbon fibers to improve adhesion with polymer resins [17]. Aligned CNTs are grown at high temperatures and then transferred to a graphite/epoxy prepreg [18]. The CNT/epoxy film cast using three-roll milling improved the mode II interlaminar fracture toughness [19]. Electrospinning was used to fabricate CNT/polymer nanofiber-based interlayers to improve the flexural performance, mode II toughness, and impact properties [20]. Multi-wall carbon nanotube interlayers were fabricated using a spraying technique to enhance the mixed-mode I/II interlaminar fracture toughness [21]. CNT veils were fabricated using chemical vapor deposition and a direct spinning process. The CNT veils were interleaved with a woven carbon fiber/epoxy composite to improve the interlaminar properties [22]. Our study demonstrated that the CNT buckypaper improved the mode I and mode II interlaminar fracture toughnesses of carbon fiber composites without damaging the in-plane mechanical properties.