## 4 Results and Discussion
### Effect of the Sliding Speed on the Friction and Wear Characteristics  
Figures 19 and 20 compare the SEM image and three-dimensional topography of the wear surface of the hemispherical-pit surface friction pair under the rotation speed of 1000 r/min and 1750 r/min (corresponding to the sliding speeds of 1.57 m/s and 2.75 m/s), exploring the effect of the sliding speed on the friction and wear characteristics. Figure 17: EDS analysis of the wear surface of the 316L bottom specimen (\(P\) = 300 N, \(n\) = 1500 r/min): (**a**) energy spectrum diagram; (**b**) surface scanning diagram. Figure 19: SEM of the wear surface of hemispherical-pit surface friction pair at different speeds (\(P\) = 300 N): **(a)** CF/PEEK surface, \(n\) = 1000 r/min; **(b)** 316L surface, \(n\) = 1000 r/min; **(c)** 316L pit, \(n\) = 1000 r/min; **(d)** CF/PEEK surface, \(n\) = 1750 r/min; **(e)** 316L surface, \(n\) = 1750 r/min; and **(f)** 316L pit, \(n\) = 1750 r/min. Figure 18: EDS analysis of the debris in a hemispherical pit on the 316L bottom specimen after wear (\(P\) = 300 N, \(n\) = 1500 r/min): **(a)** SEM image; **(b)** energy spectrum diagram. It is generally known that the total friction coefficient \(\mu\) in the case of boundary lubrication and mixed lubrication includes two parts: the fluid friction coefficient \(\mu_{L}\) and the solid friction coefficient \(\mu_{S}\). As concluded from Figures 9 and 10, the hemispherical-pit surface friction pair is in the hydrodynamic lubrication state in the rotation speed range from 1000 r/min to 1750 r/min. A continuous seawater lubricating film separates the two friction surfaces, and no solid friction coefficient \(\mu_{S}\) is generated at this time. The total friction coefficient \(\mu\) is only determined by the fluid friction coefficient \(\mu_{L}\), and its value is \(\eta dv/dz\), which is produced by the internal friction of the fluid. The velocity gradient \(d\omega/dz\) is low at a low sliding speed of 1.57 m/s (\(n\)=1000 r/min); thus, the friction coefficient is low. The low sliding speed also brings a low specimen contact temperature, because the seawater can wash away the debris and cool down the friction surface in a timely manner. As illustrated in Figures 19a-c and 20a,b, the grooves on the CF/PEEK and 316L wear surfaces are even, and are obviously generated during the running-in period; some original machining tool marks still can be seen on the 316L wear surface. Surface plastic deformation causes oxygen in the air to diffuse to the deformed layer. Oxygen first reaches saturation on the surface, and then successively diffuses to the interior of the surface layer, and the content of oxygen gradually decreases from the outside to the inside. At low speed friction, the oxidation degree of the 316L surface is not high, and the main components are oxygen iron solid solution and granular oxide FeO (Figure 19b) [32]. The oxide film adhering to the CF/PEEK surface generates cracks and falls off under cyclic contact stress (Figure 19a). The adhesive wear makes the wear rate of the specimens high. As the sliding speed increases to 2.36 m/s (\(n\) = 1500 r/min), the velocity gradient increases and the friction coefficient increases. The increased speed lengthens the sliding stroke and raises the intensity of the relative movement between the friction surfaces, which increases the frictional heat generation and the contact temperature of the specimen. As illustrated in Figures 15 and 18, with the increase of the temperature, the grooves on the CF/PEEK and 316L wear surfaces are even but not obvious, the plastic deformation and oxidation degree of the 316L surface are enhanced, and the oxide film is regenerated on the surface, the main component of which is lamellar oxide Fe\({}_{2}\)O\({}_{3}\) (Figure 15f) [32].