*3.2. Lapping Processing Properties of Fiber-Reinforced PEEK*

The material removal rate (MRR) reflects the degree of material removal difficulty during the lapping process. Taking the lapping experiment with abrasive paper of #240 mesh size as an example, Figure 5 shows the MRR of pure PEEK and fiber-reinforced PEEK materials. It can be seen in Figure 5 that the maximum MRR was of pure PEEK with the value of 17.4 μm/min. The MRR of CF10/PEEK and CF30/PEEK were 10.3 and 8.5 μm/min, respectively, whereas the MRR of GF10/PEEK and GF30/PEEK were 10.6 and 9.8 μm/min, respectively. As per the data, the MRR during the lapping process showed a decreasing trend when the fiber mass fraction increased.

**Figure 5.** The material removal rates of the pure PEEK and the fiber-reinforced PEEK.

The surface roughness reflects the quality of the lapping process. The results of surface roughness after the lapping process are shown in Figures 6 and 7. The surface roughness of fiber-reinforced PEEK showed a downward trend with the particle size of the abrasive paper decreasing. When the mesh sizes of abrasive paper were small (#180, #240, #320), the lapped surface roughness was poor, and in addition, the downward trend of the surface roughness was obvious. When the abrasive paper of the #1000 mesh size was applied, the surface roughness improved and the downward trend flattened.

As shown in Figure 6a, with the same mesh size of the abrasive paper, the surface roughness of CF10/PEEK was lower than the GF10/PEEK, which was the same trend between the CF30/PEEK and GF30/PEEK. The CF/PEEK showed better lapping machinability than the GF/PEEK.

**Figure 6.** Comparative analysis on surface roughness of fiber types: (**a**) The surface roughness of the pure PEEK, CF10/PEEK, and GF10/PEEK, (**b**) the surface roughness of the pure PEEK, CF30/PEEK, and GF30/PEEK.

Figure 7 compares the surface roughness of different fiber mass fraction reinforced PEEK materials with the same type of fiber. As shown in Figure 7a, the surface roughness of CF30/PEEK was lower than that of CF10/PEEK. Similarly, the surface quality of GF30/PEEK was better than that of GF10/PEEK. The surface quality of fiber-reinforced PEEK improved as the fiber mass fraction increased. As per the data of MRR and surface roughness during the lapping process, the carbon fiber and glass fiber could improve the surface quality but decrease the MRR.

**Figure 7.** Comparative analysis on surface roughness of the fiber mass fraction: (**a**) The surface roughness of the pure PEEK, CF10/PEEK, and CF30/PEEK, (**b**) the surface roughness of the pure PEEK, GF10/PEEK, and GF30/PEEK.

#### **4. Discussion**
