*4.2. The Influence of Fiber Mass Fraction on Lapping Machinability of Fiber-Reinforced PEEK Materials*

In Figures 2 and 3, the hardness and modulus of the PEEK matrix improved significantly as the fiber mass fraction increased. The more carbon fiber or glass fiber mass fraction, the more the fiber was widely distributed on the PEEK matrix surface, which improved the hardness and modulus of fiber-reinforced PEEK materials. In Figure 4, the fiber-reinforced PEEK materials performed as brittle materials due to the brittleness of carbon and glass fiber. The tensile length of the fracture point decreased with the fiber mass fraction. The carbon fiber could improve the ultimate tensile strength of CF/PEEK. However, the length and diameter ratio of glass fiber was too large to form the defects inside the PEEK, resulting in the decrease in the ultimate tensile strength. Furthermore, the ultimate tensile strength decreased as the fiber mass fraction increased.

In Figures 5 and 7, the fiber-reinforced PEEK materials that had more fiber mass fraction could attain a smaller MRR and a better surface quality. In terms of MRRs of the same fiber type reinforced with PEEK of different fiber mass fractions, the more fiber mass fraction, the wider the interaction area between the abrasive particles and the fiber would be. Furthermore, the hardness of the PEEK matrix would be improved with an increased fiber mass fraction, resulting in the decreasing trend of the MRR due to the high hardness of the fiber.

Figure 9 shows the surface morphology of different carbon fiber mass fraction reinforced PEEK materials. As shown in Figure 2a, the nanoindentation curves of CF30/PEEK were above the CF10/PEEK, demonstrating that CF30/PEEK has a better ability to resist

deformation. The scratches on the CF30/PEEK matrix were shallower than the CF10/PEEK. On the other hand, the carbon fiber could attain better surface quality than the PEEK matrix. Therefore, the improvement effect on the surface quality was more significant with an increased carbon fiber mass fraction.

**Figure 9.** Surface morphology lapped with the #1000 mesh size silicon carbide abrasive paper of carbon-fiber-reinforced PEEK materials with different carbon fiber mass fractions, (**a**) carbon-fiberreinforced PEEK with 10% carbon fiber mass fraction; (**b**) carbon-fiber-reinforced PEEK with 30% carbon fiber mass fraction.

The lapped surface morphology of GF10/PEEK and GF30/PEEK are shown in Figure 10. In Figures 2b and 3, the GF30/PEEK had superior mechanical properties to the GF10/PEEK. In Figure 10a, the scratches on the GF10/PEEK matrix were wider and more densely distributed, which led to poor surface quality, and there was tearing damage and material accumulation near the glass fiber concaves due to the ductile performance of the PEEK matrix. In Figure 10b, the glass fiber was distributed wider and the mechanical properties of PEEK matrix were improved significantly more than the CF10/PEEK. The scratches became shallow and narrow. The tearing damage phenomenon was greatly weakened and the surface quality improved.

**Figure 10.** Surface morphology lapped with the #1000 mesh size silicon carbide abrasive paper of glass-fiber-reinforced PEEK materials with different glass fiber mass fractions, (**a**) glass-fiberreinforced PEEK with 10% glass fiber mass fraction; (**b**) glass-fiber-reinforced PEEK with 30% glass fiber mass fraction.
