3.1.1. Nanomechanical Properties of Fiber-Reinforced PEEK

The components of the fiber-reinforced materials may have a great influence on the mechanical properties. The nanomechanical properties of five kinds of PEEK materials were studied in this work, and their results are exhibited in Figure 2. The nanoindentations in pure PEEK, CF/PEEK, and GF/PEEK show different load–displacement curves.

In Figure 2, the depth of carbon fiber and glass fiber were both smaller than the PEEK with the same load, which proved that the hardness of the fiber was higher than the PEEK. Furthermore, the depth of the fiber-reinforced PEEK matrix was smaller than the pure PEEK, and the influence of the indentation depth was more significant with an increased fiber mass fraction. The hardness and modulus could be calculated based on the Olive-Pharr method [20], and the results are shown in Figure 3. The hardness and modulus of the PEEK matrix and fiber are exhibited in Table 2. As shown in Figure 3 and Table 2, the hardness and modulus of carbon/glass fiber far exceeded that of the PEEK matrix. The hardness and modulus of the PEEK matrix were enhanced by the fiber due to its excellent mechanical properties, and the reinforcement was improved with the increased mass fraction of fiber.

**Figure 2.** Load–depth (*P*–*h*) curves for (**a**) carbon-fiber-reinforced PEEK (CF/PEEK) materials and (**b**) glass-fiber-reinforced PEEK (GF/PEEK) materials from the nanoindentation tests with a maximum load of 8 mN. The *P*–*h* curves of the pure PEEK, the carbon fiber, and the glass fiber are displayed for comparative analysis.

**Figure 3.** The average hardness and modulus of the pure PEEK and fiber-reinforced PEEK materials.



3.1.2. Tensile Mechanical Properties of Fiber-Reinforced PEEK

The tensile load–displacement curves of the pure PEEK and fiber-reinforced PEEK materials are shown in Figure 4. The tensile load–displacement curves demonstrated the different deformation phases of fiber-reinforced PEEK materials. In Figure 4, the load of pure PEEK increased with the increase in displacement in the elastic phase. When the load attained the maximum tensile load, the pure PEEK had a necked phenomenon and the load decreased to a constant value until the material failed at the fracture point. The ultimate tensile strength was the value of the maximum tensile load, and the tensile curves proved that the pure PEEK is a ductile material.

The tensile curves of CF/PEEK are represented in Figure 4b. In the elastic phase, the load increased with the displacement. However, necking of the CF/PEEK material occurred, and it fractured immediately when the load attained the ultimate tensile stress. The tensile length of the CF/PEEK fracture was smaller than that of pure PEEK. The tensile length of the material fracture was smaller and the ultimate tensile stress was higher with the increased carbon fiber mass fraction. The tensile curves proved that the CF/PEEK is a brittle material.

Figure 4c demonstrates the tensile curves of GF/PEEK. Similarly, the tensile load increased in the elastic phase until the ultimate tensile stress was attained. The tensile length of the material fracture and ultimate tensile stress became smaller as the glass fiber mass fraction increased. The GF/PEEK is a brittle material. The ultimate tensile stress of five kinds of PEEK materials is exhibited in Table 3. The CF/PEEK had the greatest tensile strength.

**Figure 4.** Tensile load–displacement curves of dog-bone-shaped specimens loaded in tension with tensile speeds of 1 mm/min for various materials: (**a**) PEEK; (**b**) carbon-fiber-reinforced PEEK with 10% fiber mass fraction (CF10/PEEK); (**c**) carbon-fiber-reinforced PEEK with 30% fiber mass fraction (CF30/PEEK); (**d**) glass-fiber-reinforced PEEK with 10% fiber mass fraction (GF10/PEEK); (**e**) glass-fiber-reinforced PEEK with 30% fiber mass fraction (GF30/PEEK).


**Table 3.** The ultimate tensile strength of the pure PEEK and fiber-reinforced PEEK materials.
