## 4 Results and Discussion
### Analysis of the Specim Contact Temperature  
The specimen contact temperature variation of different surface friction pairs with the test time at different speeds is presented in Figure 11. The contact temperature of all of the bottom specimens was adjusted from room temperature to a stable value during the initial stage, and remained stable until the end of tests. The adjustment time and the stable value interval of the smooth surface were, respectively, 5000 s and 54-62 \({}^{\circ}\)C, and those of the hemispherical-pit surface were 3000 s and 44-58 \({}^{\circ}\)C. The frictional heating raises the specimen surface temperature, which corresponds to the rise in the initial stage of the temperature curve. The specimen will dissipate its heat in the form of thermal radiation when its temperature is higher than that of the surrounding seawater. As the temperature increases, the heat dissipation rate gradually accelerates. Therefore, the subsequent heating curve gradually slows down. When the frictional heat generation and thermal radiation reach equilibrium, the temperature curve enters a stable stage. Compared with Figure 8,  
Figure 10: Stribeck curve.  
Figure 9: Curve dependence of the average friction coefficient on the rotation speed for different friction pairs.  
the change trend of the specimen's contact temperature curve is similar to that of the friction coefficient curve. When the friction coefficient curve presents a large oscillation during the initial 500-1000 s at 1750 r/min, the temperature curve corresponds with a large overshoot under the same stage and rotation speed. The friction heat is generated in the outermost layer of the surface, and gradually spreads inward and downward with the contact point as the center. The isothermal surface of each contact point at a certain depth inside the surface will converge into a common isothermal surface, and the temperature gradually decreases along the depth direction. Once the surface temperature comes up to a certain critical value, the lubrication film fails, causing adhesion and destruction, and the friction coefficient and the wear loss rise sharply [32]. That is, the maximum overshoot of the temperature curve and the corresponding oscillation peak of the friction coefficient curve appear.  
Figure 12 shows the curve relationship between the average specimen contact temperature of different surface friction pairs and the speed. In the test time of 10,800 s, the higher the speed, the longer the sliding stroke of the friction pair, and the greater the friction heat and temperature rise. Therefore, the average specimen contact temperatures of these two surface friction pairs both rise with the growth of the rotation speed. The hydrodynamic lubrication effect increases the bearing capacity of the seawater film and keeps the friction interfaces from contacting directly with each other, making the average specimen contact temperature of the hemispherical-pit surface lower than that of the smooth surface.  
Figure 11: Variation in the specimen contact temperature with the test time for different friction pairs: (**a**) smooth surface; (**b**) hemispherical-pit surface.  
Figure 12: Curve dependence of the average specimen contact temperature on the rotation speed for different friction pairs.