## 4 Results and Discussion
### Effect of Micro-Hemispherical Pit-Textured Surfaces on Friction and Wear Behaviors  
Figures 14 and 15 describe the wear surface morphology of the smooth-surface friction pair and the hemispherical-pit surface friction pair under the working condition of \(P\) = 300 N and \(n\) = 1500 r/min. Compared with the original surface morphology in Figure 2, the surface of the upper specimen CF/PEEK becomes bright, the grooves become small, and the observable carbon fibers in the matrix become more numerous and long (Figure 14a,b and Figure 15a,b). There are slight circular scratches, obvious grooves along the sliding direction, and blue oxidation on the surface of the bottom specimen of 316L (Figure 14d,e and Figure 15d,e). This, in combination with the analysis in Figures 8 and 11, indicates that the micro-asperities on the CF/PEEK and 316L surfaces contacted and embedded each other under a normal load. When the upper and bottom specimen slid relatively, the micro-asperities collided fiercely, which corresponds to the high friction coefficient and specimen contact temperature during the running-in period. Due to the hardness of CF/PEEK being lower than that of 316L, the micro-asperities on its surface were gradually ground flush by those on the 316L surface, then its surface material was preferred to be pushed, generated plastic flow, and ploughed out new grooves that changed the original machining tool marks. This microscopic cutting process is termed as a plowing effect. A blue color appeared in the grooves of the 316L in Figure 14e, indicating that chemical oxidation occurred during the friction process. The exfoliated CF/PEEK surface material and 316L oxide were caught between the friction surfaces, resulting in abrasive wear. Abrasive wear caused the matrix material of the upper specimen to wear, such that carbon fibers were exposed in larger sections, and caused the grooves on the bottom specimen surface to become wide and deep. When the surfaces of the friction pair slid relatively, the bonding node formed by the adhesion effect underwent shear fracture, and the CF/PEEK surface material and 316L oxide film were captured, filled  
Figure 14: Wear surface morphology of the smooth-surface friction pair (\(P\) = 300 N, \(n\) = 1500 r/min): (**a**) photograph of CF/PEEK, (**b**) LSCM image of CF/PEEK, (**c**) SEM image of CF/PEEK, (**d**) photograph of 316L, (**e**) LSCM image of 316L, and (**f**) SEM image of 316L. and bonded by the grooves on the match surface, migrating from one surface to the other, which caused adhesive wear. Figures 16 and 17 show the EDS analysis of the wear surface of the hemispherical-pit surface friction pair under the working conditions of \(P=300\) N and \(n=1500\) r/min, which verify the occurrence of oxidative wear and adhesive wear. The elemental energy spectrum analysis of the upper specimen CF/PEEK in Figure 16a shows that the mass fraction of O, Mg and Fe increase to 37.6%, 18.41% and 16.69%, respectively. Its surface scanning analysis in Figure 16b shows that the highlighted areas of the elements O, Mg and Fe are basically the same. This indicates that, in the rubbing, the oxidation film which formed on the surface of the bottom specimen of 316L was worn off and bonded to the surface of the upper specimen of CF/PEEK. Combined with the surface scanning diagram of the elements Na and Cl in Figure 16b, it was concluded that the crystalline substance on the right of the SEM image is crystalline salt separated from the seawater. Adhesive wear causes the surface material to transfer bidirectionally. The elemental energy spectrum analysis of the bottom specimen of 316L in Figure 17a shows that the mass fraction of element C increased from less than 0.03% to 12.19%, which should originate from the carbon fiber in the upper specimen CF/PEEK. Element O, with a mass fraction of 30.65%, was observed on the 316L wear surface, of which the source corresponds to the oxide generated during rubbing.