## 5 Conclusions  
This paper used a combination of numerical simulation and experimental exploration to analyze the effect of hemispherical-pit textured surfaces and the sliding speed on the lubrication mechanism and friction and wear characteristics of the CF/PEEK and 316L stainless steel friction pair under seawater lubrication conditions. We concluded as follows:  
1. When the upper specimen rotates relative to the bottom specimen, seawater between the friction surfaces follows the movement, forms a vortex ring flow in the hemispherical pit of the bottom specimen, uses the convergent gap to generate a hydrodynamic effect, produces the bearing capacity, and realizes fluid lubrication. For the smooth-surface friction pair as the compared group, the sliding speed of the upper specimen is only transferred downwards layer by layer in the thickness direction of the seawater film, and no hydrodynamic effect is produced.
2. The hemispherical pits diminish the abrasive wear during the friction process by storing the wear debris, and the main wear forms of the hemispherical-pit surface friction pair are oxidative wear and adhesive wear. The friction coefficient of the hemispherical-pit surface friction pair is 0.018-0.027, and the specimen contact temperature is 40.2-55.1 \({}^{\circ}\)C. The effect of drag reduction and anti-wear is significantly improved compared with the smooth-surface friction pair under the same working conditions.
3. The hemispherical-pit surface friction pair is in the hydrodynamic lubrication state at rotation speeds ranging from 1000 r/min to 1750 r/min. The total friction coefficient is only determined by the fluid friction coefficient, which increases with the increase of the sliding velocity gradient. As the sliding speed increases, the specimen contact temperature climbs, and the oxidation reaction gradually becomes full. Oxidative wear and adhesive wear alternately play a dominant role in the friction, and the wear rate first decreases and then increases sharply.
4. As the friction progresses and the temperature rises, the surface material of the upper specimen of CF/PEEK undergoes plastic deformation, falls off, and transfers to the surface of the bottom specimen of 316L stainless steel. The temperature rise also induces the oxidation of the 316L stainless steel surface, and a continuous transfer film and oxide film are gradually formed on the surface. The film avoids the direct contact of the friction surface and realizes the self-lubrication of the textured surfaces. Therefore, the friction coefficient of the upper CF/PEEK-bottom 316L friction pair is 41.9-54.5% lower than that of the upper 316L-bottom CF/PEEK friction pair.  
**Author Contributions:** Conceptualization, Y.L. and D.G.; methodology, Y.L.; software, Z.Z. and J.G.; validation, Y.S. and Z.Z.; formal analysis, Y.L. and Z.Z.; investigation, Y.L. and J.G.; resources, Y.L.; data curation, J.G.; writing--original draft preparation, Y.L. and J.G.; writing--review and editing, D.G., Y.S., Z.Z. and J.Z.; supervision, J.Z.; project administration, D.G.; funding acquisition, Y.L. and D.G. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the National Natural Science Foundation of China, Grant No. 52005428, and was supported by the Natural Science Foundation of Hebei Province, Grant No. E2020203107 and Grant No. E2021203099.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** All data generated or analyzed during this study are included in this article.  
**Conflicts of Interest:** The authors declare no conflict of interest.