*4.1. The Influence of Fiber Types on Lapping Machinability of Fiber-Reinforced PEEK Materials*

The tensile strength, hardness, and elastic modulus of CF/PEEK materials were larger than the pure PEEK due to the extremely larger hardness and modulus of carbon fiber. The hardness and modulus of glass fiber also far exceeded that of pure PEEK, resulting in improvements in the hardness and modulus of GF/PEEK materials. In contrast, the average length and diameter ratio of glass fibers was too high, which resulted in an easier concentration and formation of defects inside the PEEK matrix and may reduce the tensile strength [21].

As shown in Figure 5, the MRRs of fiber-reinforced PEEK materials were smaller than that of the pure PEEK. During the lapping process, the interaction between abrasive particles and the workpiece could be divided into two categories. The first category was the interaction between abrasive particles and the PEEK matrix, and the second category was the interaction between abrasive particles and reinforcing fibers. The contact between the abrasive particles and the workpiece surface was irregular. Some abrasive particles were in contact with the PEEK matrix, and the others were in contact with the fibers. It can be observed in Table 2 that the hardness of carbon fiber or glass fiber far exceeded that of the pure PEEK. The high hardness of carbon fiber or glass fiber made it more difficult to remove than the pure PEEK during the lapping process, resulting in smaller MRRs of the fiber-reinforced PEEK materials than that of the pure PEEK. In the lapping process, the abrasive particle was coarse and exhibited poor height uniformity. Therefore, the surface roughness significantly decreased, resulting in the quick removal of the material. A rapid reduction in surface roughness would occur when the abrasives were courser. When the abrasive paper with the mesh size of #1000 was applied, the abrasive grains were finer. This improved the high equivalence more, and the pressure of a single abrasive particle on the fiber-reinforced PEEK materials became smaller. The MRR was then reduced, and the surface roughness tended to be better and constant. As shown in Figures 6 and 7, the PEEK had poorer surface quality than the fiber-reinforced PEEK materials with the same mesh size abrasive paper lapping. The lapped surface morphology of CF30/PEEK and GF30/PEEK are shown in Figure 8. The pure PEEK is a ductile material, and the fiber-reinforced PEEK is a brittle material (Figure 4). In Figure 8, there were many built-up edges and delamination on the PEEK matrix surface due to the ductile performance and low strength of the PEEK materials [22]. In the early period of lapping, there was only friction between the workpiece and abrasive particles with no material removal in the workpiece surface, resulting in ductile and elastic deformation. As the abrasive cutting process continued, the PEEK matrix was pressed by the abrasive particle and formed the scratches on the surface [23,24]. It can be observed in Figure 8 that there were many scratches and built-up edges on the lapped surface.

In Figure 8b, the scratch also occurred on the carbon fiber surface, but the scratch depth was shallower than the pure PEEK due to its high hardness. During the lapping process, the material removal methods of carbon fiber included cracks, scratches, and desquamations from the matrix [25]. The concaves formed by fiber desquamation were covered by the PEEK matrix as the lapping process continued; therefore, the fiber-reinforced PEEK had better surface quality than the pure PEEK with the same mesh size abrasive paper lapping.

In Figure 6, the CF/PEEK attained better surface quality than the GF/PEEK with the same fiber mass fraction. On the one hand, in Table 2, the hardness of carbon fiber was higher than the glass fiber. With the same mesh size abrasive paper lapping, the carbon fiber was more difficult to remove than the glass fiber, and the scratches on the carbon fiber were shallower than the glass fiber. Moreover, the average diameter and length of glass fiber were larger than that of carbon fiber, and in Figure 8c, the carbon fiber concaves formed by fiber desquamation were easier to cover by the PEEK matrix, resulting in better surface quality and a smaller MRR.

On the other hand, the reinforcement of fiber on the PEEK matrix varied with the fiber types. In Figure 3, the hardness and modulus of CF30/PEEK were higher than that of GF30/PEEK. Therefore, deformation of CF30/PEEK was more difficult, and the scratches of CF/PEEK would be shallower than the GF/PEEK. The CF/PEEK can attain better surface quality, whereas the CF/PEEK matrix attained higher hardness, which made it more difficult to be removed than the glass fiber, the MRR of CF/PEEK was smaller than that of GF/PEEK.

**Figure 8.** Surface morphology lapped with the #1000 mesh size silicon carbide abrasive paper of various fiber types of reinforced PEEK materials, (**a**) pure PEEK; (**b**) carbon-fiber-reinforced PEEK with the 30% fiber mass fraction; (**c**) glass-fiber-reinforced PEEK with the 30% fiber mass fraction.
