## 4 Results and Discussion
### Analysis of the Wear Rate  
Wear rate is a key parameter to characterize wear characteristics, and the calculation formula in this study is as follows:  
\[W=\frac{\Delta m}{\rho pL}(mm^{3}/MPa\cdot m) \tag{2}\]  
where \(\Delta m\) is the specimen mass loss (the mass before the test minus the mass after the test), \(\rho\) is the specimen density, \(p\) is the normal pressure, and \(L\) is the total sliding distance. It is worth mentioning that the normal pressure is used in this formula to replace the normal load. The distribution of the pits on the hemispherical-pit surface means that its actual contact area is not equal to that of the smooth surface. Therefore, it is more accurate to use the volume worn under the unit pressure per unit length to characterize the wear rate.  
The wear rate of the specimens with the smooth surface and hemispherical-pit surface at varying rotation speeds is illustrated in Figure 13. The wear rate of CF/PEEK is about 150 times that of 316L, because its hardness is low. The wear rate of the upper specimen at most rotation speeds is negative, and that of the bottom specimen is positive, indicating that the mass of the most upper specimen increased, and that of most bottom specimen decreased after the test. The abrasive wear cuts the surface material, causing the specimen's mass to decrease. The adhesive wear migrates the surface material, causing the specimen's mass to decrease when the material is sheared, and causing the specimen's mass to increase when the surface is made up of adhered material. For the upper specimen CF/PEEK, the mass lost by the 316L surface roughness peak cutting is less than the mass gained through being adhered by the 316L oxide film, so the mass increased after the test. For the bottom specimen of 316L, due to its density being higher, the mass of the oxide film shed during the friction process is much more than the mass of the surface material adhered from CF/PEEK.  
As the rotation speed climbs, the wear rate of the specimen has a trend to increase. The upper specimen wear rate and the bottom specimen wear rate were both positive, and greatly increased at 1750 r/min. This, in combination with Figures 8 and 11, indicates that the friction coefficient and the friction heat increased sharply in the running-in period, leading to an excessively high local specimen temperature, which softened the surface material and made it easier to peel off. The hemispherical-pit surface shows a higher wear rate than the smooth surface for both CF/PEEK and 316L at 1750 r/min. This should be because the higher speed means that the turbulent flow in the pits is heavier, and the wear debris falling into the pits are rolled up and taken out of the pits again, resulting in a more serious three-body wear between the friction surfaces.  
This wear rate is one order of magnitude lower than that of the friction pair in which the upper specimen is made of 316L and the bottom specimen is made of CF/PEEK under the same working conditions, based on the comparative analysis of the test results in [33]. This pin-on-disc friction is apt to migrate the surface material on the disc to the pin, which  
Figure 13: Specim near rate for different friction pairs: (**a**) upper specimen CF/PEEK, and (**b**) bottom specimen 316L.  
was concluded in the previous study [33; 34]. The material 316L was used to make the bottom specimen in this test due to its higher hardness, and the surface material was not easy to plough-cut and fell off in the friction process. Compared with using CF/PEEK as the bottom specimen, the abrasive wear was reduced, and the anti-wear and drag-reduction effect were greatly improved.