## 1 Introduction  
The interface of composite materials is the heart of composite materials. There is a significant difference between the physical and chemical properties or the mechanical properties of the reinforcing fibers and the matrix resin itself [1]. Therefore when the materials are combined, the nature of the interface between the dissimilar materials can have a profound impact on the mechanical properties of the composite material. The physical properties of composite materials are often described by the rule of mixtures, which represent the average physical properties of the reinforcement and the matrix resin. However, in practical applications, situations arise when the rule of mixtures is not followed. This is because when an external energy is applied to the composite material, it is transferred from the matrix to the reinforcement, affecting the final physical properties of the interface between them rather than the intrinsic properties of both the reinforcement and the matrix. In addition, each matrix or reinforcement forms strong internal chemical bonds, and their interface forms weak physical bonds. Because of this, a force delivered from outside is dependent on the extent of the physical bonding [2; 3].  
In general, composite materials, which do not have any surface treatment of carbon fibers, are known to show a low degree of interlaminar shear strength (ILSS) or critical stress intensity factor (KIC). This is due to the fact that carbon fiber, which is used as reinforcement, has a low wettability level with the matrix and the surface itself is smooth, and thus the interfacial bonding strength is weak [4]. Therefore, the interfacial bonding strength of carbon fiber-reinforced composite material can be improved by two methods. (i) first removing impurities on the surface of the carbon fiber with surface treatment methods such as vapor phase oxidation, liquid phase oxidation, anodic oxidation, and plasma treatment. After that the number of functional groups (carboxyl, carbonyl, and hydroxyl) enhancing the bonding force with the matrix, is increased [5]; (ii) the interfacial bonding strength can be improved be adding trace amounts of other materials such as SiC [6] and TiO\({}_{2}\)[1] between the carbon fibers and the matrix.  
In this research, carbon fiber-reinforced composite materials were produced by dispersing HNTs and expected to enhance the interfacial bonding strength between the carbon fibers and the matrix, and epoxy resin using an ultrasonic homogenizer. In order to analyze the mechanical behavior in the presence of added HNTs to carbon fibers and the matrix, first the mechanical behavior of the HNT/matrix-based composites was observed by dispersing HNTs in the matrix. Based on this, the mechanical properties were evaluated in accordance with the interfacial bonding strength according to the addition of particles with varying HNT content.  
Also, mode II interlaminar fracture toughness was measured and evaluated to investigate the fracture phenomenon at the interface of carbon fiber reinforced composite with nanoparticles. In order to investigate the interfacial reinforcement effect of HNTs, nanoparticles were dispersed only in epoxy resins, and mechanical properties were evaluated to compare the reinforcing effect in the resin and the theoretical and experimental properties of the layered composite material.