## 3 Results and Discussion  
### Composite Material Performance Testing  
#### 3.1.1 SEM and Contact Angle Testing  
SEM images of the surface morphology of the carbon fibers in CF, CF-D, CF1, CF2, CF3, and CF4 are shown in Figures 2 and 3. From Figure 2, it can be seen that there is a small amount of epoxy residue on the surface of the carbon fibers. Due to the effect of the epoxy, the original carbon fibers show hydrophilicity, with a contact angle of 47.2\({}^{\circ}\) with deionized water. The original carbon fibers become de-slurried after acetone treatment. Carbon fiber axial grooves appear on the surface, making the surface relatively smooth, as shown in Figure 2. This indicates that the de-slurring process is relatively thorough, returning the surface of the carbon fiber to its initial structure, transitioning from hydrophilic to extremely hydrophobic. From Figure 2, it can be seen that a large number of multi-walled carbon nanotubes are attached to the surface of the carbon fiber in CF1, and the surface exhibits a reticular structure with increased roughness. Due to the incorporation of the multi-walled carbon nanotubes, CF1 presents hydrophilicity. From Figure 2, it can be seen that a large number of multi-walled carbon nanotubes with small amounts of nanohydroxyapatite are attached to the surface of the carbon fibers in CF2. Due to the addition of hydroxyapatite, the hydrophilicity of CF2 reaches the optimum, and the contact angle is only 19.6\({}^{\circ}\). From Figure 2, it can be seen that a very small number of multi-walled carbon nanotubes with large amounts of nanohydroxyapatite are attached to the surface of the carbon fibers in CF3, which is probably due to the fact that a large number of multi-walled carbon nanotubes attach to nanohydroxyapatite with an increase in the HA mass fraction. It is possible that when the HA mass fraction increases, a large number of multi-walled carbon nanotubes attach to the nanohydroxyapatite material. From Figure 2, it can be seen that a large amount of nanohydroxyapatite is attached to the surface of the carbon fibers in CF4, which leads to an enhancement in hydrophilicity. It can be inferred that many multi-walled carbon nanotubes attach to nanohydroxyapatite particles with an increase in the HA mass fraction. Table 2 shows the surface energy data of CF, CF-D, CF1, CF2, CF3, and CF4, from which it can be seen that the surface energy data of CF2 reaches the maximum of about 357 times, thus indicating that nanohydroxyapatite and multi-walled carbon nanotubes can significantly increase the surface energy of the composites. Increasing surface energy can lead to an increase in the bonding strength between the composites and liquids. It can also improve the wettability between the composites and polyetheretherketone (PEEK) wettability. Bond strength enhancement between carbon fiber and polyetheretherketone is primarily due to two reasons: (1) the attachment of hydroxyapatite to the surface of carbon fiber can enhance interface adhesion, simultaneously improving mechanical locking and interaction with the polyetheretherketone material; (2) the addition of multi-walled carbon nanotubes can regulate the fracture toughness of the interfacial region, slowing down therate of crack expansion and improving the toughness of the composite material, as well as its fatigue life. Moreover, the incorporation of multi-walled carbon nanotubes can result in excellent interfacial interactions with polyetheretherketone, leading to improvements in the composites' mechanical properties under various loading conditions, such as tensile, bending, and compression, and increasing the bonding strength with polyetheretherketone. Overall, with the incorporation of hydroxyapatite and multi-walled carbon nanotubes, the surface roughness of the carbon fibers increases, and the bond strength between the carbon fibers and polyetheretherketone is enhanced, resulting in an improvement in both the high performance and multifunctional properties of the composites. Figure 2: Cont.